Supplemental Lecture (98/05/09 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Bacterial Mechanisms of Pathogenicity
    1. A list of vocabulary words is found toward the end of this document
    2. A microorganism which is able to cause disease is said to be pathogenic. The degree to which disease is caused is the virulence (or pathogenicity) displayed by a pathogen. For many pathogens there is only an incomplete understanding of how infection leads to disease.
    3. It is important to understand that just because a host is exposed to a pathogen doesn't mean that the host will become infected by the pathogen nor that disease will result. The reasons that this is so tends to be shared by properties of both the host and the pathogen. We will focus on the pathogen in this lecture. Particularly, from entry through the induction of disease, the general principles of pathogenicity, with emphasis on bacterial pathogenicity, will be discussed here.
  2. Pathogenicity
    1. Definition:
      1. Pathogenicity is the ability or degree to which something can cause disease.
      2. Note that this term barely differs in its definition from that of virulence.
    2. Requirements/steps:
      1. To cause disease an organism typically must:
        1. enter into the host*
        2. adhere to tissue within the host**
        3. cause damage to the host in some manner
      2. *typically through a reasonably well defined portal of entry.
      3. **evasion of host defences.
  3. ID50 [infectious dose 50%]
    1. Infection never assured:
      1. Different organisms have different per-individual probabilities of successfully infecting a given host.
      2. For some pathogens very large number must be exposed to the host in order for infection to occur while for other pathogens relatively few individual microorganisms are necessary to induce disease.
    2. Measurements of pathogenicity:
      1. The infectious dose 50% is a measure of the likelihood of a host acquiring a disease given exposure to a pathogen.
      2. The likelihood that disease will occur increases as the number of pathogens gaining access to the host increases.
      3. The number of pathogens required to cause disease (or, at least, infection) in half of the exposed hosts is called the ID50.
    3. More than one:
      1. ID50s typically are 1, that is, it usually takes far more than just exposure to a single organism for a host to become infected.
      2. This is one reason why we can be surround by microorganisms and even pathogens and still remain healthy most of the time.
  4. LD50 [lethal dose 50%]
    1. The number of pathogens required to cause lethal disease in half of the exposed hosts is called an LD50.
  5. Adherence
    1. Attachment to host:
      1. Once entry has been accomplished, most microorganisms have mechanisms of host attachment, a.k.a., adherence.
      2. For bacterial pathogens, successful adherence is usually a necessary prerequisite for virulence and even infection.
    2. Such microbial structures as glycocalyx and fimbriae (i.e., attachment pili) are involved in adherence.
  6. Damage to the host
    1. During infection there are three general mechanisms by which a pathogen can induce damage to the host:
      1. direct damage
      2. via hypersensitivity reactions
      3. toxin-induced damage
  7. Direct damage
    1. Consequences of pathogen growth:
    2. Direct damage results from the means a pathogen utilizes to:
      1. adhere
      2. grow
      3. evade host defenses
    3. Direct damage is usually the more minor of the three general methods of pathogen induced damage.
  8. Hypersensitivity reaction
    1. A hypersensitivity reaction is an immune response that is excessive beyond the bounds of normalcy, to a point where it leads to damage (as with endotoxins) or is potentially damaging to the individual.
  9. Toxin [toxicity, toxemia, toxigenicity]
    1. Poisonous substances:
      1. Toxins are poisonous substances produced by microorganisms.
      2. About 220 bacterial toxins are known. About 40% disrupt plasma membranes.
    2. Toxins are categorized as:
      1. exotoxins
      2. endotoxins
    3. Toxicity is the degree to which a toxin is poisonous.
    4. Toxigenicity is the capacity of a microorganism to produce toxins.
    5. Toxemia is the presence of toxins in the blood.
  10. Toxin-induced damage
    1. "Action at a distance":
      1. Pathogens often produce substances for various reasons, generally referred to as toxins, that cause host damage plus disease.
      2. Toxins tend to be produced during a local infection but are released and do their damage more systemically.
    2. Lack of natural immune defense:
      1. Only very small amounts of toxin are often sufficient to cause disease.
      2. Consequently, recovery from a toxin-induced disease does not necessarily confer immunity.
      3. For some toxins which are detrimental only in relatively high concentrations (i.e., endotoxins), immunogenicity is lacking thus resulting in a lack of immunity upon recovery, though for different reasons (e.g., because the toxin does not induce an immune reaction regardless of concentration).
    3. Toxin-induced damage can include:
      1. fever
      2. cardiovascular disturbance
      3. destruction of blood vessels
      4. diarrhea
      5. disruption of the nervous system
      6. plasma membrane disruption
      7. inhibition of protein synthesis
      8. shock
  11. Exotoxin
    1. Exotoxins are proteins, often enzymes, produced inside of cells and which do their damage (as with endotoxins) only upon release from the cell.
    2. Exotoxins range in toxicity up to and including extremely lethal.
    3. Property of gram positives:
      1. Gram-positive rather than gram-negative bacteria tend to be producers of exotoxins.
      2. Note, however, that this is not an absolute rule.
    4. Many exotoxins have generalizable targets or sites of action. Thus there exist:
      1. cytotoxins
      2. enterotoxins
      3. neurotoxins
  12. Cytotoxin
    1. A cytotoxin is an exotoxin that disrupts host cells.
    2. Diseases in which cytotoxins play a role include:
      1. gas gangrene
      2. diphtheria
      3. scarlet fever
  13. Enterotoxin
    1. An enterotoxin is an exotoxin that disrupts the lining of the gastrointestinal tract.
    2. Various food poisonings and diarrhea are caused by enterotoxins.
  14. Neurotoxin
    1. A neurotoxin is an exotoxin that disrupts nerve cells.
    2. Botulism and tetanus are both caused by neurotoxins.
  15. Antitoxin
    1. Antitoxins are antibodies that bind to exotoxins, thus inactivating the exotoxin.
    2. Transfusion of antitoxin is a means of providing passive immunity.
  16. Toxoid
    1. A toxoid is a physically or chemically inactivated exotoxin.
    2. Toxoids are employed as vaccines, such as against tetanospasmin, since they induce a host immune response (i.e., antitoxin) but have limited or nonexistent toxicity.
  17. Endotoxin
    1. Lipid A portion of LPS:
      1. The lipid A portion of lipopolysaccharide (LPS) is also known as endotoxin.
      2. Endotoxins are relatively weak in terms of host damage induced, except given exposure to large quantities.
    2. Since LPS is found solely on gram-negative bacteria, endotoxins are also associated exclusively (or nearly so) with gram-negative bacteria.
    3. Consistent disease:
      1. Qualitatively, all endotoxins produce the same symptoms.
      2. This is because the symptoms are a consequence of the body reacting to the presence of endotoxin rather than endotoxins exerting some specific effect on the host.
      3. Host responses include:
        1. fever
        2. chills
        3. weakness
        4. aches
        5. shock*
        6. death
        7. *septic shock.
    4. Consequence of bacterial lysis:
      1. Endotoxins exert their effect only upon release from the bacterial cell envelope.
      2. This typically occurs only upon bacterial cell lysis.
    5. Note that bacterial cell lysis is the consequence of the action of some antibiotics and thus a temporary worsening of symptoms often accompanies the initiation of antibiotic therapy.
  18. Digestive system intoxication
    1. Toxin in food:
      1. A digestive system intoxication is a disease caused by a toxin (exotoxin) already present in food prior to ingestion.
      2. A digestive system intoxication is not a disease caused by microbial growth following ingestion (that would be a digestive system infection).
    2. Rapid onset/minimal fever:
      1. Digestive system intoxications are marked by rapid onset (often at most a few hours).
      2. Additionally, digestive system intoxications are not necessarily associated with a fever.
  19. Digestive system infection
    1. Live pathogen in food:
      1. A digestive system infection is a disease in which microbial growth occurs within the digestive system.
      2. This contrasts with an intoxication of the digestive system.
    2. Delayed onset/fever:
      1. Two key indications that a digestive system disease is an infection are:
        1. significant delay between exposure and symptoms*
        2. the presence of a fever
      2. *Many hours or even days.
      3. The delay occurs because it takes time for sufficient numbers of microorganisms to be present to cause significant damage/symptoms/disease.
  20. Botulin
    1. Botulin is the neurotoxin produced by Clostridium botulinum causing botulism.
  21. Tetanospasmin
    1. Tetanospasmin is the neurotoxin produced by Clostridium tetani causing tetanus.
  22. Vocabulary
    1. Adherence
    2. Antitoxin
    3. Cytotoxin
    4. Damage to the host
    5. Digestive system infection
    6. Digestive system intoxication
    7. Direct damage to the host
    8. Endotoxin
    9. Enterotoxin
    10. Exotoxin
    11. ID50
    12. Infectious dose 50%
    13. LD50
    14. Lethal dose 50%
    15. Mucous membrane portal of entry
    16. Neurotoxin
    17. Pathogenicity
    18. Toxin-induced damage
    19. Toxemia
    20. Toxicity
    21. Toxigenicity
    22. Toxoid
  23. Practice questions
    1. Which of the following genera is most infectious? (circle only one correct answer) (p. 405, Tortora et al., 1995) [PEEK]
      1. Escherichia (ID50 = 108 cells)
      2. Salmonella (ID50 = 105 cells)
      3. Shigella (ID50 = 2x102 cells)
      4. Treponema (ID50 = 5x101 cells)
      5. all of the above
      6. none of the above
    2. True or False, gram-positive bacteria toxin is at least plausibly called an exotoxin because it is produced intracellularly but acts extracellularly (circle True or False)? [PEEK]
    3. Staphylococcal food poisoning is caused by __________ (circle best answer) [PEEK]
      1. an enterotoxin
      2. a neurotoxin
      3. a cytotoxin
      4. Staphylococcus aureus infection
      5. all of the above
      6. none of the above
    4. Which of the following is as likely a product of a gram negative bacterial infection as a gram positive infection? (choose best answer) [PEEK]
      1. exotoxin
      2. antitoxin
      3. toxoid
      4. cytotoxin
      5. neurotoxin
      6. toxemia
    5. In order to cause disease, three steps of pathogen infection must occur, usually in a well defined order. Which of the following is not one of those steps? (choose best answer) [PEEK]
      1. entry into host
      2. adherence
      3. damage to the host
      4. exit from host
      5. all of the above
      6. none of the above
    6. A chemically or physically treated exotoxin which is now biologically but not immunologically inactivated is called a(n) _________? (one word answer) [PEEK]
    7. To cause disease a bacterial pathogen typically must (i) ____________, (ii) __________, and (iii) __________. [PEEK]
    8. Various diseases caused by the activity of bacterial toxins display symptoms including destruction of blood vessels, diarrhea, plasma membrane disruption, and disruption of the nervous system (though most diseases do not display all of these symptoms). The general category of toxins which cause these various diseases may be described as __________. (choose best answer) [PEEK]
      1. cytotoxins.
      2. enterotoxins.
      3. neurotoxins.
      4. endotoxins.
      5. antitoxins.
      6. exotoxins.
      7. toxoids.
    9. Bob, Matt, Mike, and Chris sit down to identical meals one day at the local Grease Hut. 24 hours later Bob has the runs like you would not believe, but Matt, Mike, and Chris are unaffected. Bob's almost positive that he's sick due to something he ingested. However, he's unsure whether he's justified in whipping out his elephant gun, riding his Harley through the glass front door the Grease Hut, and blowing everybody away (after he first gets off the john, of course). In terms of the etiology of Bob's digestive system disorder, why do you suppose that he is unsure about who to blame and, in general terms, what do you suppose is wrong with him? [PEEK]
    10. Contrasting with the chemical nature of endotoxins, exotoxins typically consist of __________. [PEEK]
    11. __________ are exotoxins which exert their toxic effect by damaging or killing cells found, depending on the specific toxin, in one or many of numerous locations throughout the body. [PEEK]
    12. Gas gangrene, diphtheria, and scarlet fever are all diseases caused by gram positive bacteria that additionally have what in common that they all don't have in common with the diseases tetanus or botulism? (note: I'm looking for an answer other than the latter two being caused by soil bacteria) [PEEK]
    13. Transfusion of __________ is a means of providing passive immunity against damage induced by an exotoxin. [PEEK]
    14. Define ID50. [PEEK]
    15. Why don't toxin-induced diseases necessarily induce immunity? [PEEK]
    16. Inoculation with a __________ is a means of providing vaccination against an exotoxin. [PEEK]
    17. Given a gram-negative bacterial infection, e.g., septicemia, what must typically occur before a host is exposed to significant amounts of endotoxin? [PEEK]
    18. What category of exotoxin is employed by the etiologies associated with the diseases, gangrene, diphtheria, and scarlet fever, but not by the etiologies associated with the diseases, tetanus or botulism? [PEEK]
    19. In addition to toxin-induced damage and direct damage, bacterial pathogens can also induce host damage, and therefore disease, by inducing __________. [PEEK]
  24. Practice question answers
    1. iv, Treponema (ID50 = 5x101 cells)
    2. True
    3. i, an enterotoxin
    4. vii, toxemia
    5. iv, exit from host. Exiting the host is only relevant with regard to pathogen dissemination. While this is highly relevant to pathogen evolutionary biology, it is not a prerequisite for the occurrence of disease on a per host basis. An example would be a pathogen which is defective in utilizing its portal of exit but is otherwise completely competent in accomplishing all of other pathogen phenotypes. Such an infection would be identical to a wild type infection except that no pathogen progeny would be shed by the host.
    6. Toxoid
    7. enter its host, attach to appropriate tissue, damage the host in some manner.
    8. vi, exotoxins.
    9. Bob probably forget to wash his hands and infected himself while eating (or, perhaps, at some other time). His problem is that he has a digestive system infection.
    10. proteins, enzymes, or exoenzymes.
    11. cytotoxins.
    12. They all cause disease which are induced by cytotoxins, i.e., exotoxins which induce cytological damage to host cells.
    13. antitoxin.
    14. The number of microorganisms which will result in successful infection in half (i.e., 50%) of the hosts exposed.
    15. Either the amount of toxin necessary to induce disease is too slight to induce an immune response against the toxin (exotoxin), or the toxin is not immunogenic regardless of concentration (endotoxin).
    16. Toxoid
    17. pathogen lysis
    18. cyotoxin
    19. a hypersensitivity reaction in the host.
  25. References
    1. Black, J.G. (1996). Microbiology. Principles and Applications. Third Edition. Prentice Hall. Upper Saddle River, New Jersey. pp. 400-405.
    2. Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 390-400.