Antimicrobial Therapy

Important words and concepts from Chapter 13, Black, 1999 (3/28/2003):

by Stephen T. Abedon (abedon.1@osu.edu) for Micro 509 at the Ohio State University

Course-external links are in brackets

Click [index] to access site index

Click here to access text’s website

(1) Chapter title: Antimicrobial Therapy

(a) Antimicrobial therapy is the treatment of infectious disease using, typically, chemotherapeutic agents that either kill microbes or otherwise interfere with microbial growth

(b) "Infectious disease claimed the lives of about one in every 100 U.S. residents per year as late as 1900 but only about one in every 300 in 1990. Although antimicrobial agents still don't save all patients, they have drastically lowered the death rate from infectious disease. A period of increased infectious diseases could return, however, if patients and the medical community fail to protect the effectiveness of antimicrobial agents. As many pathogens develop resistance to available antimicrobial drugs, our ability to fight infectious diseases is dwindling." (p. 340, Black, 1999)

(c) [“It is said that the discovery and use of antibiotics and immunization procedures against infectious disease are two developments in the field of microbiology that have contributed about twenty years to the average life span of humans in developed countries where these practices are employed. While the greater part of this span in time is probably due to vaccination, most of us are either still alive or have family members who are still alive because an antibiotic conquered an infectious disease that otherwise would have killed the individual. If we want to retain this medical luxury in our society we must be vigilant and proactive: we must fully understand how and why antimicrobial agents work, and why they don't work, and realize that we must maintain a stride ahead of microbial pathogens that can only be contained by antibiotic chemotherapy.” (Microbiology Webbed Out)]

(d) [“In 1922, Alexander Fleming, a bacteriologist in London, had a cold. He was not one to waste a moment and consequently used his cold as an opportunity to do an experiment. He allowed a few drops of his nasal mucus to fall on a culture plate containing bacteria. He was excited to find some time later that the bacteria near the mucus had been dissolved away. Fleming showed that the antibacterial substance was an enzyme, which he named lysozyme—lyso because of its capacity to lyse bacteria and zyme because it was an enzyme… Fleming found that tears are a rich source of lysozyme. Volunteers provided tears after they suffered a few squirts of lemon—an ‘ordeal by lemon.’ The St. Mary’s Hospital Gazette published a cartoon showing children coming for a few pennies to Fleming’s laboratory, where one attendant administered beatings while another collected their tears! Fleming was disappointed to find that lysozyme was not effective against the most harmful bacteria. But seven years later, he did discover a highly effective antibiotic, penicillin—a striking illustration of Pasteur’s comment that chance favors a prepared mind.” (Lubert Stryer, 1995, Biochemistry Fourth Edition, pp. 207-8)]

(e) [antimicrobial therapy (Google Search)] [Antimicrobial Chemotherapy (complex site with nice overview of subject)] [therapeutic category index (this is an amazing list of antimicrobials all linked to extensive descriptions including discussions of mechanisms of action) (Lexi-comp, Inc. / Emedline)] [index]

(f)

CHEMOTHERAPEUTIC ANTIMICROBIALS

(2) Chemotherapy (chemotherapeutic agent, drug)

(a) Chemotherapy is the use of chemical substances to treat disease

(b) To be effective, a chemotherapeutic agent (i.e., a drug) must combat the disease (e.g., poison a pathogen) to a greater extent than that drug poisons the host

(d) [chemotherapy -cancer, "chemotherapeutic agent" -cancer (Google Search)] [index]

(3) Antimicrobial agent

(a) An antimicrobial agent is a chemotherapeutic agent used to treat the underlying cause of infectious disease, i.e., by inhibiting microbial growth and microbial survival

(b) [“Although the immune system efficiently and regularly protects us from microorganisms intent on upsetting the balance between themselves and their host, there are times when it cannot cope, especially when it is confronted with invasion by rapidly growing microorganisms. In these and other situations, antibiotics which kill microorganisms or inhibit their growth give the immune system the time it needs to produce a favourable outcome for the host, avoiding damage and in some cases potential death of the host.” “Bactericidal agents are generally more effective than bacteriostatic agents, but bacteriostatic agents can be extremely beneficial since they provide time for the normal defences of the host to kill the microorganisms. Knowledge of whether the action of an antibiotic is bactericidal or bacteriostatic means that the potential outcome of using combinations of antibiotics can be predicted.” (Antimicrobial Chemotherapy)]

(i) Modes of action

(ii) Source (e.g., various microbes such as Streptomyces spp.)

(iii) Mechanism of production (e.g., antibiotics versus synthetic drugs versus semisynthetic drugs)

(iv) Toxicity / side effects

(v) Spectrums of activity

(vi) Evolved or inherent organismal resistance

(d) [antimicrobial agent (Google Search)] [index]

(4) Antibiotic

(a) An antibiotic is "a chemical substance produced by microorganisms which has the capacity to inhibit the growth of bacteria and even destroy bacteria and other microorganisms in dilute solution." (emphasis mine) (p. 341, Black, 1999)

(b) [“Penicillium and Cephalosporium: produce Beta-lactam antibiotics: penicillin, cephalosporin, and their relatives. ¶ Actinomycetes, mainly Streptomyces species: produce tetracyclines, aminoglycosides (streptomycin and its relatives), macrolides (erythromycin and its relatives), chloramphenicol, ivermectin, rifamycins, and most other clinically-useful antibiotics that are not beta-lactams. ¶ Bacillus species, such as B. polymyxa and Bacillus subtilis produce polypeptide antibiotics (e.g., polymyxin and bacitracin), and B. cereus produces zwittermicin. ¶ These organisms all have in common that they live in a soil habitat and they form some sort of a spore or resting structure. It is not known why these microorganisms produce antibiotics but it may rest in the obvious: affording them some nutritional advantage in their habitat by antagonizing the competition… Antibiotics tend to be rather large, complicated, organic molecules and may require as many as 30 separate enzymatic steps to synthesize. The maintenance of a substantial component of the bacterial genome devoted solely to the synthesis of an antibiotic leads one to the conclusion that the process (or molecule) is important, if not essential, to the survival of these organisms in their natural habitat. ¶ Most of the microorganisms that produce antibiotics are resistant to the action of their own antibiotic, although the organisms are affected by other antibiotics, and their antibiotic may be effective against closely-related strains.” (Microbiology Webbed Out)]

(c) [“In the majority of situations in which antibiotics are used, a "best guess" procedure is followed. A doctor makes a provisional diagnosis that a patient has a bacterial infection which requires treatment. Depending on the type of infection there will be a short list of bacteria most likely to be causing that infection. Depending on the type of bacteria there will be an antibiotic most likely to successfully treat that infection. The doctor is then in the position to write a prescription for that antibiotic. There are inherent risks in following this course of action. ¶ "Best guess" treatment is not always successful or appropriate as many bacteria have unpredictable susceptibilities to antimicrobial agents. The susceptibilities or resistances of unusual or hospital acquired causes of infection invariably need to be determined to help guide the selection of the most appropriate antimicrobial agent. Alternatively it could be said that the activity of different antibiotics towards these bacteria (needs) to be determined.” (Antimicrobial Chemotherapy)]

(d) [antibiotic (Google Search)] [what the heck is an antibiotic? (Jack Brown – University of Kansas] [classification of antibiotics, brief overview of structures and characteristics of select antibiotics (Antimicrobial Chemotherapy)] [general characteristics of antibiotics] [antibiotics (see Table 4 on this page for a nice summary of antibiotic types, specific examples, their sources, and their modes of action) (Microbiology Webbed Out)] [index]

(e) [Fungus-growing ants use antibiotic-producing bacteria to control garden parasites (an article published in Nature and presented in its entirety)]

(5) Synthetic drug

(a) Contrast antibiotic with synthetic drug: Synthetic drugs are substances, some of which can act identically to antibiotics, but which are synthesized in the laboratory rather than by a microorganism

(b) ["synthetic drug" and antibiotic (Google Search)] [index]

(6) Semisynthetic drug

(a) The middle ground between a synthetic drug and an antibiotic is an antimicrobial agent that is produced by chemically modifying a natural product, e.g., the chemical modification of an antibiotic or its precursor

(b) [semisynthetic drug (Google Search)] [index]

ANTIBIOTIC EFFICACY

(7) Selective toxicity

(a) The ability of an antimicrobial to harm a pathogen without harming the host is termed selective toxicity

(b) [“The single most important characteristic [of an antimicrobial agent] is selective toxicity, meaning that the antibiotic is far more toxic to the microorganism than to the host. A drug that disrupts a microbial function not found in eucaryotic animal cells often has a greater selective toxicity and a higher therapeutic index.” (Antimicrobial Chemotherapy)]

(d) Instead, selective toxicity refers to the range between the dose necessary to inhibit pathogen growth and the dose at which the host is harmed

(e) We can quantify selective toxicity in terms of

(i) The therapeutic dosage level

(ii) The toxic dosage level

(iii) The chemotherapeutic index

(f) [selective toxicity (Google Search)] [index]

(8) Therapeutic dosage level

(a) This is the dose at which pathogen growth is inhibited

(b) Ideally, at this dosage the antimicrobial is not toxic to the host

(c) Note that a number of factors influence whether a therapeutic dosage level may be established and then maintained at the site of infection (both quotes from p. 663 of Prescott, Harley, and Klein, 1996. Microbiology Third Edition. Wm C. Brown Publishers):

(i) “The drug must actually be able to reach the site of infection. The mode of administration plays an important role. A drug such as penicillin G is not suitable for oral administration because it is relatively unstable in stomach acid. Some antibiotics… are not well absorbed from the intestinal tract and must be injected intramuscularly or given intravenously… Even when an agent is administered properly, it may be excluded from the site of infection. For example, blood clots or necrotic tissue can protect bacteria from a drug, either because body fluids containing the agent may not easily reach the pathogens or because the agent is absorbed by materials surrounding it.”

(ii) “The chemotherapeutic agent must exceed the pathogen’s MIC (minimum inhibitory concentration) value if it is going to be effective. The concentration reached will depend on the amount of drug administered, the route of administration and speed of uptake, and the rate at which the drug is cleared or eliminated from the body. It makes sense that a drug will remain at high concentrations longer if it is absorbed over a long period and excreted slowly.”

(d) [therapeutic dosage (Google Search)] [index]

(9) Toxic dosage level

(a) This is the dose at which the host is harmed

(b) Many antibiotics can be toxic (often extremely so) in numerous ways

(c) See the discussion of individual antibiotics on pages 355-on in your text as well as in the following figures (no need to memorize all antibiotics):

(i) Figure 13.13, Selected antibacterial drugs

(ii) Figure 13.15, Selected antifungal, antihelminthic, antiviral, and antiprotozoan drugs

(d) [toxic dosage (Google Search)] [index]

(10) Chemotherapeutic index

(a) Ideally, the therapeutic dosage level is significantly lower than the toxic dosage level

(b) The ratio of toxic dosage level to the therapeutic dosage level is termed the chemotherapeutic index (more specifically: “…the chemotherapeutic index is defined as the maximum tolerable dose per kilogram of body weight, divided by the minimum dose per kilogram body weight, that will cure the disease.” p. 342, Black, 1999)

(d) Anti-cancer chemotherapeutics are examples of drugs (though not antimicrobials) that typically have low chemotherapeutic indices; this is because cancer cells so closely resemble normal body cells that it is difficult to poison the cancer cells without poisoning the body as well

(e) A broadly useful antibiotic will have a high chemotherapeutic index

(f) Typically this is accomplished by the chemotherapeutic drug attacking a pathogen molecule or metabolic pathway that is not also present in or used by the host

(g) Note that drugs with low chemotherapeutic indices when taken internally may still be acceptable for topical use (e.g., bacitracin)

(h) Other drugs with low chemotherapeutic indices are still employed internally because they represent the only drugs available to treat various infections (e.g., vancomycin)

(i) [chemotherapeutic index (Google Search)] [index]

(11) Spectrum of activity

(a) Not all antimicrobials inhibit the growth of all microbial pathogens

(b) In fact, not one antimicrobial inhibits the growth of all microbial pathogens

(c) Instead, just as viruses have host ranges, antimicrobials have spectrums of activity, that range of pathogen types a given antimicrobial is active against

(d) We can distinguish antimicrobial agents into those that have a broad spectrum of activity and those that have narrower spectrums of activity

(e) See Figure 13.1, The spectrum of antibiotic activity

(f) See Table 13.1, The spectrum of activity of selected antimicrobial agents

(g) [spectrum of activity (Google Search)] [index]

(12) Broad spectrum of activity

(a) An antimicrobial drug that is effective against a large variety of microorganisms is said to have a broad spectrum of activity

(b) An example of an antimicrobial with a broad spectrum of activity would by one that is effective against both Gram-negative and Gram-positive bacteria

(c) See Figure 13.1, The spectrum of antibiotic activity

(d) Advantages of using a broad-spectrum antibiotic are a high likelihood of efficacy against an unidentified pathogen

(e) [broad spectrum of activity (Google Search)] [index]

(13) Normal flora (normal microbiota)

(a) Disadvantages of using a broad-spectrum antibiotic are a high likelihood of the drug also destroying the friendly/helpful bacteria making up an individual's normal microbial flora, i.e., the non-pathogenic microorganisms normally found associated with a host

(b) "Because they have such a wide spectrum of activity, [tetracyclines] destroy the normal intestinal microflora and often produce severe gastrointestinal disorders." (p. 360, Black, 1999)

(c) [normal microflora (MicroDude)] [index]

(14) Superinfection

(a) Knocking out these non-pathogenic bacteria can lead to disease (e.g., diarrhea, Clostridium difficile-associated colitis, Candida vaginal yeast infections, etc.)

(b) Normal flora can compete with pathogenic bacteria (microbial antagonism), thus preventing disease; removing these flora can thus make an individual more susceptible to subsequent disease

(d) This is particularly a problem in hospital settings due to the common occurrence in those settings of readily superinfecting pathogens

(e) A means of combating superinfection is essentially normal-flora replacement therapy

(f) [superinfection, superinfection and antibiotic, Candida superinfection, Candida infection, C. difficile superinfection, C. difficile colitis (Google Search)] [index]

(15) Narrow spectrum of activity

(a) A narrow-spectrum antibiotic is effective against only a relatively small subset of bacteria

(b) Use of a narrow-spectrum antibiotic allows an avoidance of some of the destruction of normal flora associated with antibiotic use

(c) Penicillin is an example of an antibiotic possessing a relatively narrow spectrum of activity, acting particularly against Gram-positive bacteria (i.e., ones with cell walls but lacking outer membranes)

(d) Disadvantages include a requirement before treatment can commence for pathogen identification and, in some cases, identification of pathogen antibiotic susceptibility

(e) [narrow spectrum of activity (Google Search)] [index]

SPECIFIC ANTIBIOTICS

(16) Modes of action (mechanism of action)

(a) "Like other medicines, antimicrobial agents are sometimes used simply because they work, without our always knowing how they work. Many people's lives have been saved by medicines whose actions at the cellular level have never been understood. However, it is always desirable to know the mode of action of an agent. With that knowledge, effects of actions on patients can be better monitored and controlled, and ways of improving them may be found." (p. 342, Black, 1999)

(b) “For an antibiotic to affect the growth of a microbial cell it must (i) enter the cell and reach the site of action, (ii) bind to a target molecule involved in an essential cell process, (iii) markedly inhibit this process. An antibiotic can be bactericidal or bacteriostatic. A bactericidal effect occurs when the antibiotic interaction results in an irreversible disruption or binding whereas a bacteriostatic effect involves lower affinity binding and as such is reversible when the antibiotic is removed from the environment.” (Antimicrobial Chemotherapy)

(i) Inhibition of cell wall synthesis

(ii) Disruption of cell membrane function

(iii) Inhibition of protein synthesis

(iv) Inhibition of nucleic acid synthesis (i.e., inhibition of replication of genetic material or transcription)

(v) Action as antimetabolites

(d) See Figure 13.2, Modes of action

Supplemental Table (Antimicrobial Chemotherapy) (antibiotic names/categories linked to descriptions) (see web page this table is found on)
Site of action	Antibiotic
Cell wall (peptidoglycan) synthesis	Glycopeptides B-lactams
Replication or transcription of genetic material	Quinolones Nitroimidazoles Rifampicin
Protein synthesis	Aminoglycosides Chloramphenicol Macrolides
Cell membrane functions (fungi)	Polymixins
Antimetabolites	Diaminopyrimidines

(e) [antibiotic "mode of action", antibiotic "mechanism of action" (Google Search)] [therapeutic category index (this is an amazing list of antimicrobials all linked to extensive descriptions including discussions of mechanisms of action) (Lexi-comp, Inc. / Emedline)] [index]

(17) Inhibition of cell wall synthesis

(a) Antibiotics that inhibit cell wall synthesis work because:

(i) Most eubacteria have peptidoglycan-based cell walls (while mammals do not)

(ii) Successful cell-wall synthesis by these bacteria is impossible in the absence of peptidoglycan synthesis

(iii) In the absence of a cell wall most eubacteria are susceptible to osmotic lysis

(iv) Actively growing bacteria treated with cell-wall-synthesis inhibitors can lose cell-wall integrity and are thus subject to osmotic lysis

(b) Animals lack a structure that is equivalent to the bacterial peptidoglycan-based cell wall (thereby explaining the selective toxicity of those drugs)

(18) Penicillin

(a) An example of an inhibitor of cell wall synthesis is penicillin

(b) Note that penicillin is more active against Gram-positive cell walls due to the lack of an outer membrane in these bacteria

(c) However, there exist derivatives of penicillin that display significant activity against Gram-negative organisms (e.g., amoxycillin, ampicillin, ticarcillin, piperacillin, carbenicillin) including, particularly, Pseudomonads [penicillins (Antimicrobial Chemotherapy)]

(d) Other cell wall synthesis inhibitors include (need not memorize list):

(i) Ampicillin

(ii) Bacitracin

(iii) Carbapenems

(iv) Cephalosporin

(v) Methicillin

(vi) Oxacillin

(vii) Vancomycin

(e) See b-lactamase discussion, below

(f) See Figure13.3, Inhibition of cell wall synthesis by penicillin

(g) See Figure 13.11, Penicillins

(h) [“The first antibiotic, penicillin, was discovered in 1929 by Sir Alexander Fleming who observed inhibition of staphylococci on an agar plate contaminated by a Penicillium mold. World War II (and the inevitable bacterial infections that occurred in war-related wounds) was an important impetus to study the chemotherapeutic value of penicillin. Penicillin became generally available for treatment of bacterial infections, especially those caused by staphylococci and streptococci, about 1946. Initially, the antibiotic was effective against all sorts of infections caused by these two Gram-positive bacteria. It is important to note that a significant fraction of all human infections are caused by these two bacteria (i.e., strep throat, pneumonia, septicemia, skin infections, wound infections, scarlet fever, toxic shock syndrome). Penicillin had unbelievable ability to kill these bacterial pathogens without harming the host that harbored them… Resistance to penicillin in some strains of staphylococci was recognized almost immediately after introduction of the drug. (Resistance to penicillin today occurs in as many as 80% of all strains of Staphylococcus aureus). Surprisingly, Streptococcus pyogenes (Group A strep) have never fully developed resistance to penicillin and it remains a reasonable choice antibiotic for many types of streptococcal infections. Interestingly, penicillin has never been effective against most Gram-negative pathogens (e.g. Salmonella, Shigella, Bordetella pertussis, Yersinia pestis, Pseudomonas) with the notable exception of Neisseria gonorrhoeae. Gram-negative bacteria are inherently resistant to penicillin because their vulnerable cell wall is protected by an outer membrane that prevents permeation of the penicillin molecule.” (Microbiology Webbed Out)]

(i) [penicillin (Google Search)] [index]

(j) A sampling of penicillin web sites: [penicillin structures and function (this is a student project with a nice collection of images, but it has an annoying backgroun) (Molecular Modeling Course – Middlebury College)] [what the heck is penicillin] [penicillin derivatives] [penicillin derivatives] [penicillin allergy skin testing] [informed drug guide: penicillin] [informed drug guide: amoxycillin] [penicillins] [penicillins (advertisement)] [1940’s advertisement for penicillin] [penicillin use in horses and equines] [invasive penicillin-resistant pneumococcal infections: a prevalence and historical cohort study] [penicillin, the wonder drug (note: has an annoying background)] [prevalence of penicillin-resistant Streptococcus pneumoniae -- Connecticut, 1992-1993] [penicillin: the drug of choice for treating group A streptococcal pharyngitis] [index]

(19) Cephalosporin

(a) Cephalosporins are structurally related to penicillins but isolated from a different organism (Cephalosporium rather than Penicillium)

(b) See b-lactamase discussion, below, for a comparison of structures

(c) “Although cephalosporins usually are not the first drug considered in the treatment of an infection, they are frequently used when allergy or toxicity prevents the use of other drugs. But because cephalosporins are structurally similar to penicillin, some patients who are allergic to penicillin may also be sensitive to the cephalosporins. Nevertheless, cephalosporins account for one-fourth to one-third of the pharmacy expenditures in American hospitals, mainly because they have a fairly wide spectrum of activity, rarely cause serious side effects, and can be used prophylactively in surgical patients. Unfortunately, they are often used when a less expensive and narrower-spectrum agent would be just as effective.” (p. 358, Black, 1999)

(d) See Figure 13.11, Penicillins

(e) [cephalosporin (Google Search)] [cephalosporin antibiotics success story] [informed drug guide: ceftriaxone] [commonly prescribed oral cephalosporins] [cephalosporins] [cephalosporins and vitamin K metabolism] [cephalosporines (advertisement)] [index]

(20) Vancomycin

(a) Vancomycin is yet another cell-wall-synthesis inhibitor (plus has additional modes of action), though bears little structural resemblance to penicillins or cephalosporins; i.e., versus (vancomycin and methicillin, respectively)

(b) Vancomycin is used against methicillin-resistant Gram-positive cocci, particularly Staphylococcus as well as Streptococcus and Enterococcus

(c) [vancomycin, vancomycin -resistant -resistance (Google Search)] [vancomycin tutorial (Pharmacotherapy of Infectious Diseases)] [index]

(21) Disruption of cell membrane function

(a) Polymixins are antibiotics that act by disrupting the Gram-negative outer membrane (they additionally serve to inhibit the toxic effects of endotoxin though this comes at costs associated with their low chemotherapeutic indices)

(b) ["disruption of cell membrane" and antibiotic, polymixin (Google Search)] [index]

(22) Inhibition of protein synthesis

(a) Antibiotics that inhibit protein synthesis take advantage of the fact that the bacterial ribosome and the eucaryotic ribosome differ structurally; consequently, there exist chemicals that can inhibit bacterial translation but not eucaryote translation

(b) The one caveat is that the mitochondria ribosome is structurally similar to the eubacteria ribosome; this gives antibiotics that inhibit protein synthesis a potential for toxicity

(23) Tetracycline

(a) Tetracycline acts against the bacterial ribosome, inhibiting protein synthesis

(b) Since bacteria all possess similar ribosomes (all eubacteria, at least), tetracycline serves as a broad-spectrum antibiotic; "Tetracyclines have the widest spectrum of activity of any antibiotic." (p. 360, Black, 1999)

(c) In terms of side effects, in addition to inhibiting mitochondrial translation (protein synthesis, above), tetracycline displays side effects due to additional interactions with tissues including forming complexes with Ca⁺⁺ (calcium) ions that can result in a discoloration of forming teeth (p. 360 of Black)

(d) Other protein synthesis inhibitors include (need not memorize list):

(i) chloramphenicol

(ii) erythromycin

(iii) gentamycin

(iv) neomycin

(v) streptomycin

(e) [tetracycline (Google Search)] [informed drug guide: erythromicin] [index]

(24) Inhibition of nucleic acid synthesis

(a) Some antibiotics inhibit bacterial RNA synthesis (particularly rifampin, which is a rifamycin)

(b) Inhibition of nucleic acid synthesis, however, is especially relevant in anti-virals, e.g., anti-HIV reverse transcriptase inhibitors (though note that your text considers these as antimetabolites since they act as nucleic acid analogs rather than as inhibitors of nucleic acid polymerases)

(c) ["inhibition of nucleic acid" and antibiotic, rifampin (Google Search)] [rifamycins (Antimicrobial Chemotherapy)] [index]

(25) Action as antimetabolites

(a) Sulfanilimide is an antimetabolite that inhibits bacterial folic acid synthesis (a B vitamin) from PABA (another B vitamin)

(b) [antibiotic antimetabolite, sulfanilimide (Google Search)] [index]

CAVEATS

(26) Side effects

(a) As noted, side effects are consequences of drug use that affect the host for the worse

(b) Side effects may be distinguished into

(i) Toxicities, the inability of a drug to completely distinguish host physiology from pathogen physiology

(ii) Allergies

(iii) Normal flora disruptions

(27) Antibiotic resistance

(a) The down side of antimicrobial use, other than the more-immediate side effects, is that these substances serve as a selective evolutionary force for microbes that are antimicrobial resistant

(b) This works two ways

(i) One way is when antimicrobials eliminate all bacterial species (within a community, i.e., your microflora) except those that are inherently not susceptible (a problem especially prevalent when using broad-spectrum antibiotics); see superinfection, above

(ii) A second way is when antimicrobials select for not-susceptible members of otherwise susceptible populations (individual species) of microorganisms

(i) Evasion

(ii) Chromosomal antibiotic resistance (mutation-mediated antibiotic resistance)

(iii) Extrachromosomal antibiotic resistance (acquired antibiotic resistance)

(d) For simplicity, consider these three means of attaining resistance particularly in terms of the second mechanism of resistance noted above, i.e., resistance that develops within a single population (species) of microorganisms

(e) [“Remember that a single bacterial colony on a plate consists of a billion or a thousand million cells, 10⁹ in mathematical shorthand. Although most of these cells will be sensitive to the action of a particular antibiotic at the concentration used, there will always be a very small number that are not, maybe only one cell maybe ten cells. This is because spontaneous mutation toward resistance to a single antibiotic generally occurs with a frequency lower than 10^-7. ¶ These mutant cells occur without any special impetus and are greatly outnumbered by the sensitive cells. They are not a problem until the antibiotic is used to prevent bacterial growth i.e., to treat an infection. Growth of the sensitive bacteria will then be blocked but the resistant cells will continue to grow unless they are destroyed by natural host defence mechanisms. Over an extended period of exposure to the antibiotic, and in a debilitated patient with poor immune function however, eventually the whole (pathogen) population may be made up of resistant cells.” “There are three general types of resistance mechanisms. (1). Inactivation of the antibiotic by hydrolysis (B-lactams) or covalent modification (aminoglycosides). (2). Limitation of access to the target by reduced entry into cell (aminoglycosides, B-lactams, tetracyclines) or by increased efflux (tetracyclines, erythromycin, fusidic acid). (3). Alteration or modification of the target to a less sensitive form (B-lactams, trimethoprim).” (go to page) (Antimicrobial Chemotherapy)]

(f) [antibiotic resistance (Google Search)] [mechanisms of antibiotic resistance (includes three nice animated gifs) (Antimicrobial Chemotherapy)] [bacterial resistance to antibiotics, bacterial resistance to antibiotics (these are two modestly different pages found on the same site) (Microbiology Webbed Out)] [general characteristics of antibiotics] [the effects of under-usage of antibiotics on bacteria] [index]

(28) Evasion

(a) The organism may enter or be present in an antimicrobial-resistant state such that all members of a population are destroyed by the antimicrobial except those that happen to be in the resistant state (e.g., such as endospores)

(b) For example, not-growing bacteria are not sensitive to penicillin but will eventually display sensitivity unless they persist in the resistant state

(c) “Nongenetic resistance occurs when microorganisms such as those that cause tuberculosis persist in the tissues out of reach of antimicrobial agents. If the sequestered microorganisms start to multiply and release their progeny, the progeny are still susceptible to the antibiotic.” (p. 347, Black, 1999)

(d) [antibiotic evasion (Google Search)] [index]

(29) Chromosomal antibiotic resistance (mutation-mediated antibiotic resistance)

(a) The organism may become mutated such that the site of action of the antimicrobial is no longer affected by the antimicrobial (e.g., a mutation that affects ribosome structure; e.g., resistance to erythromycin)

(b) Such resistant mutants are typically resistant to only a single type of antibiotic

(c) Since the normal structure is coded by a gene that resides on the bacterial chromosome, the resistance genes (properly called alleles) are mutated versions of normal, chromosomal bacterial genes

(d) ["chromosomal resistance" and antibiotic, erythromycin (Google Search)] [index]

(30) Extrachromosomal antibiotic resistance (acquired antibiotic resistance)

(a) Extrachromosomal resistance is associated with resistance (R) plasmids

(b) Typically extrachromosomal resistance involves an inactivation of the antibiotic or a prevention of entry rather than a change in the structure of the antibiotic target

(c) Extrachromosomal resistance, also typically, does not involve the mutation within a given bacteria to antibiotic resistance but instead the acquisition of resistance plasmids from other bacteria

(d) ["extrachromosomal" and antibiotic, "acquired antibiotic resistance", erythromycin (Google Search)] [index]

(31) b-lactamase

(a) [“The beta lactam antibiotics (penicillins and cephalosporins) inhibit the last step in peptidoglycan synthesis, the final cross-linking between between peptide side chains, mediated by bacterial carboxypeptidase and transpeptidase enzymes.” (Microbiology Webbed Out)]

(b) One mechanism of extrachromosomal resistance to penicillin and its derivatives is the production of an enzyme called b-lactamase

(c) The b-lactamase cleaves the four-member (N-C-C-C) b-lactam ring found in active penicillin, thus inactivating the drug (the squares in the figure below)

(d)

(e) See Figure 13.7, The effect of b-lactamase on penicillin

(f) See Figure 13.11, Penicillins

(g) [lactamase, lactamase and "antibiotic resistance" (Google Search)] [b-lactamase structures (3-D witch Chime manipulation, etc.) (Structural Classification of Proteins)] [inhibition of peptidoglycan synthesis (includes structure of the normal substrate that b-lactam resembles) (Antibiotics, Disinfection, and Sterilisation – Nottingham Trent University)] [index]

(h) [carboxypeptidase and… antibiotic, lactam, penicillin (Google Search)]

(i) [transpeptidase and… antibiotic, lactam, penicillin (Google Search)]

(32) First-line, second-line, third-line drugs

(a) A drug that is found to effectively treat a given bacterial infection does not necessarily remain effective indefinitely since resistance can evolve in the treated populations

(b) If resistance occurs, then this "first-line drug" (i.e., first-used drug) is no longer effective for treating this organism (or, at least, populations which are resistant)

(c) Ideally, when resistance develops there will exist a second drug, or second-line drug that is capable of treating bacterial infections that are resistant to the first-line drug

(d) Note that there typically is nothing that prevents a bacterial population that is resistant to the first-line drug from developing resistance to the second-line drug

(e) Again, ideally, when resistance develops to the second-line drug, there will exist a third-line drug that can still effectively eliminate the infection

(f) Note that the existence of "n+1"-line drugs is not guaranteed and there exists the potential for medicine to run out of drugs that are effective against all strains of all pathogens

(g) An ability to kill a pathogen is not the only criteria used in choosing an antibiotic since such factors as cost and toxicity are also highly relevant; often first-line drugs are employed because of relatively low toxicities, low costs, or high availabilities

(h) Contrast "n"-line drug with "n"-generation drug, where the former represents the replacement of the use of one drug with the use of a different one while the latter refers to the development of new variations on old drugs

(i) [first-line antibiotic (Google Search)] [index]

(33) Cross resistance

(a) Very often resistance to one antibiotic will result in resistance to other, similar antibiotics

(b) For example, development of resistance to penicillin often will result in cross-resistance to various penicillin derivatives

(c) On the other hand, new versions of old drugs are often developed (through chemical modification) particularly so that old mechanisms of resistance will not be effective against the new versions of the drug

(d) [antibiotic "cross resistance" (Google Search)] [index]

ENHANCING ANTIBIOTIC EFFICACY

(34) Limiting drug resistance

(a) Three mechanisms may (and ought to be) applied to limit the evolution of drug resistance among pathogens

(i) Antibiotics should be employed only when necessary; e.g., antibiotics should be employed only when there is reasonable potential for efficacy (i.e. treating viral infections with antibacterial agents is unwarranted except under unusual circumstances where they are employed prophylactically against secondary infections)

(ii) Antibiotics can be employed such that high concentrations of drug is maintained over long periods (i.e., taking all of one's pills over the prescribed duration of a treatment)

· See Figure 13.8, Effects of premature termination of antibiotic treatment

(iii) Antibiotics may be employed together such that a bacterium that achieves resistance to one antibiotic will not necessarily achieve simultaneous resistance to the second antibiotic; additionally, two antibiotics administered simultaneously may be capable of effecting synergism

(b) [limiting antibiotic resistance, limiting drug resistance (Google Search)] [principles of antibioitic use (and avoidance of abuse), best guess therapy (Antimicrobial Chemotherapy)] [preventing antibiotic resistance (MicroDude)] [index]

(35) Preventing antibiotic resistance (supplemental discussion)

(a) The following discussion is based on a table from p. 53 of Levy (Levy, S. B., 1998. The challenge of antibiotic resistance. Scientific American. March:46-53) and consists of rules of thumb that health professionals ("physicians") and consumers can follow to minimize the evolution of antibiotic resistance among pathogens, as well as to minimize the disruption of microbial communities by antibiotic (and other antimicrobial) use:

(b) Physicians (can do):

(i) Wash hands thoroughly between patient visits

(ii) Do not accede to patients' demands for unneeded antibiotics

(iii) When possible, prescribe antibiotics that target only a narrow range of bacteria

(iv) Isolate hospital patients with multidrug-resistant infections

(v) Familiarize yourself with local data on antibiotic resistance

(c) Consumers (can do):

(i) Do not demand antibiotics (from physicians)

(ii) When given antibiotics, take them exactly as prescribed and complete the full course of treatment; do not hoard pills for later use

(iii) Wash fruits and vegetables thoroughly; avoid raw eggs and undercooked meat, especially in ground form

(iv) Use soaps and other products with antibacterial chemicals only when protecting a sick person whose defenses are weakened. This is

· to minimize the disruption of normal, "good", microbial communities (both on and off the body)

· to avoid selecting for resistance among these normal microbial community members

· to prevent the inadvertent selection for communities consisting, unnaturally, solely of naturally resistant bacterial types, which themselves may become emergent pathogens

(v) The basic premise is that our bodies do a pretty good job of resisting infection by the vast majority of microbes which dominate our environment, so why consciously change the mix of microbes in our environment?

(d) Treatment with more than one drug:

(i) An additional means by which antibiotic resistance can be prevented is to treat bacterial infections with more than one drug simultaneously

(ii) So long as resistance to the two drugs is achieved only through different means, treatment with more than one drug simultaneously can make it much more difficult for a bacterium to survive via resistance stemming simply from the occurrence of fortuitous mutations that convey resistance; this is because mutations occur at an only constant, relatively low rate

(iii) The bacterial population size necessary to achieve a given mutation is basically the inverse of the mutation rate (if the mutation occurs in one in every million bacteria, then it will take approximately one million bacteria for the mutation to occur in a given population)

(iv) If a second mutation is necessary to achieve resistance to a second drug, then the odds of coming up with both mutations is the product of the odds of coming up with each mutation singly; similarly, the population size necessary to achieve with reasonable probability both mutations is equal to the product of the population size necessary to come up with each mutation singly

(v) If one in one million bacteria are necessary to see one mutation or the other, but not both, then the population size necessary to see both with high probability is one million times one million or 10¹²

(vi) This effect is also the explanation for why two or more anti-cancer or anti-HIV chemotherapeutics are typically given more or less simultaneously

(36) Combinations of antimicrobial agents

(a) “At times it is necessary to use a combination of antibiotics. These times include:

(i) treating a life-threatening infection

(ii) preventing the emergence of resistance

(iii) treating a mixed infection

(iv) enhancing antibacterial activity

(v) using lower concentrations of a toxic drug”

(b) (above quoted in its entirety from Antimicrobial Chemotherapy)

(c) For more on the benefits of using antimicrobial agents in combinations see below: multiplicative killing, synergy, and antagonism

(d) [“Jawetz's laws make the following predictions: (i) a combination of a bacteriostatic agent with a bactericidal agent will be Antagonistic, (ii) a combination of a bacteriostatic agent with a bacteriostatic agent will be Additive or Indifferent, (iii) a combination of a bactericidal agent with a bactericidal agent will be Synergistic. This is a very rough guide and should always be confirmed by laboratory tests.” (see page) (Antimicrobial Chemotherapy)]

(e) [combinations of antimicrobial agents (Google Search)] [index]

(37) Multiplicative killing (the sum of its parts, indifferent killing, additive killing)

(a) Using two antimicrobials simultaneously (and ones to which cross-resistance does not occur) can result in greater killing than when just one drug is used alone

(b) Stated mathematically (and using easy to understand numbers), if antibiotic A typically kills all but one in 100 bacteria in a population while antibiotic B also typically kills all but one in 100 bacteria in a population, then the simultaneous administration of both antibiotic A and antibiotic B should result in the killing of all but one in ten-thousand bacteria in a population

(i) i.e., 100 x 100 = 10,000 = sum of the parts = indifferent = additive = “multiplicative killing”

(ii) this assumes no cross-resistance

(d) Stated another way, should a bacterium develop resistance to antibiotic A (i.e., the one in 100 bacteria, above), they will still have only a one in 100 chance of simultaneously developing resistance to antibiotic B (that is, starting with an antibiotic A resistant population, then we would expect one in 100 bacteria to be resistant to both antibiotic A and antibiotic B; starting with a population that is resistant to neither drug then we would expect only one in 10,000 bacteria to be resistant to both drugs)

(e) This use of multiple drugs to achieve multiplicative killing is also employed during anti-cancer chemotherapy as well as anti-HIV chemotherapy

(f) Note that, in general, this building up of mutations incrementally rather than simultaneously is how evolution works; by not using drugs simultaneously one is simply giving microorganisms a better shot at developing antibiotic resistance to available drugs via normal evolutionary mechanisms (i.e., mutation followed by selection)

(38) Synergism (greater than the sum of its parts)

(a) Synergism is used to describe when the efficacy of two antibiotics administered simultaneously is greater than the expected sum of their individual effects

(b) That is, positive synergism is a situation where the whole (efficacy) is greater than the sum of the parts

(39) Antagonism (negative synergy, less than the sum of its parts)

(a) Synergism, or even expected levels of multiplicative killing do not always occur

(b) This is because certain drugs when mixed together can inhibit each other's efficacy

(c) Antagonism can occur, for example, when a bacteriostatic agent (e.g., tetracycline) is mixed with an antimicrobial that requires cell growth for efficacy (e.g., penicillin)

(d) Antagonism is negative synergy, i.e., the whole (efficacy) is less than the sum of the parts

(e) [antibiotic antagonism (Google Search)] [index]

(40) Determining microbial sensitivities to antimicrobial agents

(a) "Microorganisms vary in their susceptibility to different chemotherapeutic agents, and susceptibilities can change over time. Ideally, the appropriate antibiotic to treat any particular infection should be determined before any antibiotics are given. Sometimes an appropriate agent can be prescribed as soon as the causative organism is identified from a laboratory culture. Often tests are needed to show which antibiotic kills the organisms." (p. 351, Black, 1999)

(b) [“The activity of an antimicrobial is dependent on its concentration. Some idea of the effectiveness of a chemotherapeutic agent can be obtained from determining the minimal inhibitory concentration or MIC. The MIC is the lowest concentration of a drug that prevents growth of a particular pathogen. ¶ Different microorganisms may test sensitive or resistant to a particular antibiotic depending on the concentration of antibiotic used in the test. A microrganism is either sensitive or resistant to a particular antibiotic at a particular concentration. ¶ The concentration of an antibiotic at a site of infection depends on many factors including the dose, the route of administration, the absorption, the extent of protein binding, the rate of metabolism and the rate of excretion. A drug must reach a concentration at the site of infection above the pathogen's MIC to be effective.” (MIC discussion) (Antimicrobial Chemotherapy)]

(d) [antibiotic "microbial sensitivity" (Google Search)] [index]

ADDITIONAL ISSUES AND AGENTS

(41) Nosocomial infections and drug resistance

(a) Nosocomial, i.e., hospital acquired infections will be considered in more detail in the subsequent chapter; however, your text discusses drug resistance in the context of nosocomial infections on pages 369 and 372

(b) "First, despite efforts to maintain sanitary conditions, a hospital provides an environment where sick people live in close proximity and where many different kinds of infectious agents are constantly present and easily spread. Second, hospitalized patients tend to be more severely ill than outpatients; many have lowered resistance to infection because of their illnesses or because they have received immunonsuppressant drugs. Finally, and most importantly, hospitals typically make intensive use of a variety of antibiotics. Because many infections are being treated and different antibiotics are used, organisms resistant to one or more of the antibiotics are likely to emerge. The resistant strains can readily spread among patients. ¶ Treatment of resistant infections creates a vicious cycle. If an antibiotic can be found to which an organism is susceptible, that drug can be used to treat the infection. However, some strains of the organism that are resistant to the new antibiotic may then proliferate and require treatment with another new drug. A recurrent cycle in which new antibiotics are used and the organisms subsequently develop resistance to them is established."

(42) Antifungal agents

(a) "Antifungal agents are being used with greater frequency because of the emergence of resistant strains and an increase in the number of immunosuppressed patients, especially those with AIDS. Because fungi are eukaryotes and thus similar to human cells, antifungal treatment often causes toxic effects. At less toxic levels, many systemic fungi infections are slow to respond. Furthermore, laboratory tests are not available to determine appropriate susceptibility and therapeutic levels. Despite these difficulties, numerous effective drugs are now becoming available, many without prescription." (p. 363, Black, 1999)

(b) That is, fungi are physiologically a lot like us, so antifungal chemotherapeutic indices tend to be low (i.e., the same thing that is toxic to a fungi tends to be toxic to us) and anti-fungal treatments are not nearly as advanced/well-developed as antibacterial treatments

(43) Antiprotozoan agents (supplemental discussion)

(a) [informed drug guide: metronidazole] [informed drug guide: chloroquine] [index]

(b) [antiprotozoan (Google Search)] [index]

(44) Antiviral agents

(a) "Until recent years no chemotherapeutic agents effective against viruses were available. One reason for the difficulty in finding such agents is that the agent must act on viruses within cells without severely affecting the host cells. Currently available antiviral agents inhibit some phase of viral replication, but they do not kill the viruses." (p. 366, Black, 1999)

(b) That is, viral infections, since they use our own cells are physiologically a lot like us so chemotherapeutic indices tend to be low (i.e., the same thing that is toxic to a virus tends to be toxic to us)

(c) Note that the same kind of observation can be made of anti-cancer chemotherapeutics (as well as anti-fungal, anti-protozoal, and anti-helminth drugs)

(d) One particularly successful antiviral agent is acyclovir which is a nucleotide (guanine) analog that acts particularly against herpes viruses

(e) A large and increasing number of antivirals (nucleotide analogs, protease inhibitors) have been developed against HIV

(f) [antiviral (Google Search)] [strategies for antiviral therapy based on the retroviral life cycle] [informed drug guide: acylovir] [what the heck does a protease inhibitor have to do with HIV?] [index]

(45) Natural antimicrobials (supplemental discussion)

(a) "Herbs and spices flavor and tenderize meat, but they also serve a more evolutionarily signifcant purpose---killing contaminating bacteria, claims Paul Sherman, an evolutionary biologist at Cornell University in Ithaca, New York. Sherman and colleague Jennifer Billing looked at patterns of spice use in 4164 traditional meat recipees from 31 countries. Onion, black and white pepper, garlic, lemon juice, hot peppers, and ginger proved among the most popular. When they combed the literature to determine what herbs and spices had been shown to have antibacterial effects, they found that most are 'really powerful antibiotics,' Sherman reported last month at the annual meeting of the Animal Behavior Society in College Park, Maryland. Garlic, onion, allspice, and oreganon killed all the bacteria they were tested against, including Salmonella and Staphylococcus. Others, such as hot peppers, destroyed at least 75% of their bacterial targets. The researchers say their case is bolstered by the fact that the hotter the climate---and thus the more danger of food spoilage---the more spices are used in a cuisine. Conversely, some spices low in antibiotic properties, such as celery seed, are not much used in southern cuisines. Comments Zuleyma Tang-Martinez, an ethologist at the University of Missouri, St. Louis, 'Most people think the only reason we use spices is because of the taste, but [Sherman] has gone beyond that.'" (Holden, C., 1997. Antibiotic basis for spice use. Science 277:321)

(b) [natural antimicrobials, natural antimicrobial, bacteriocins, colicins (Google Search)] [phage therapy, Immunology I: Basic Principles of Specific Immunity & Immunization (MicroDude)] [Mrs. Field’s chicken soup recipe (advertisement)] [index]