Supplemental Lecture (98/11/21 update) by Stephen T. Abedon (


(1)   Chapter title: Control of Microbial Growth

(a)    A list of vocabulary words is found toward the end of this document

(b)   Found at this site are additional pages of possibly related interest including: [sterilization and disinfection] [specific antimicrobials]

(c)    An index to all of the vocabulary words found on this site also exists [index]

(2)   Overview (of the Control of Microbial Growth)

(a)    Antimicrobials are things that kill microbes.

(b)   Germicides are antimicrobials.

(c)    Disinfectants are germicides that are not capable of sterilizing, typically because they fail to kill endospores, some viruses, and such organisms as Mycobacterium tuberculosis.

(d)   Antiseptics are disinfectants that are used on living tissues.

(3)   Microbial death [exponential death]

(a)   Exponential death:

(i)                  When microorganisms are killed using some method of control of microbial growth , they tend to display exponential death.

(ii)                That is, they die at some fractional rate per unit time.

(b)   If 50% of the microorganisms in a population die every minute, then:

(i)                  after 2 minutes there will be 25% still alive

(ii)                after 3 minutes there will be 12.5% still alive

(iii)               after 4 minutes there will be 6.25% still alive

(iv)              etc.

(c)    This can be graphed as a straight line on semi-logarithmic graph paper

(d)   "A definite proportion of the organisms die in a given time interval. . . The total number of organisms present when disinfetion is begun affects the length of time required to eliminate them. We can state a second principle: The fewer organisms present, the shorter time needed to achieve sterility. Thouroughly cleaning objects before attempting to sterilize them is a practical application of this principle. Clearing objects of tissue debris and blood is also important because such organic matter impairs the effectiveness of many chemical agents." (p. 333, Black, 1996)

(4)   Stasis [bacteriostasis, static, bacteriostatic]

(a)    The term stasis is used as a suffix as in bacteriostasis.

(b)   It implies the inhibition of microbial growth by means other than killing.

(c)    Reversible inhibition:

(i)                  A prominant distinguishing feature of static agents is that, upon removal following relatively short periods of treatment, microbial growth can very rapidly return.

(ii)                This is opposed to the case with killing agents where growth may return, but only as a consequence of a reestablishment of a population from those few individuals not-yet-killed.

(d)   However, even only reversible inhibition can lead to death if applied for long enough.

(5)   -cidal [bactericidal]

(a)    An agent which actually kills is given the suffix -cidal as in:

(i)                  germicidal

(ii)                bactericidal

(iii)               sporicidal

(iv)              fungicidal

(v)                virucidal

(vi)              amoebicidal

(vii)             spermicidal

(viii)           etc.

(b)   Not reversible:

(i)                  A "-cidal" agent either damages the microorganism or interacts permanently with it.

(ii)                Except in terms of the grow back of not yet killed individuals, a "-cidal" agent is not considered to act reversibly.

(6)   Sterilization

(a)    Literally the removal of all microbes from a surface or a volume.

(b)   Various methods:

(i)                  Methods of sterilization include:

(1)   moist heat

(2)   dry heat

(3)   filtration (to some extent)

(4)   ionizing radiation

(5)   UV radiation

(ii)                Note that for the most part chemical germicides are not efficient at effecting sterilization.

(iii)               Exceptions include the various aldehydes and gaseous chemosterilizers.

(7)   Disinfection

(a)    "Disinfection means reducing the number of pathogenic organisms on objects or in materials so that they pose no threat of disease." (p. 332-333, Black, 1996)

(b)   Disinfection is the destruction of all microorganisms except endospores and viruses

(8)   Antisepsis

(a)    Antisepsis is the disinfection of living tissues such as human skin and, especially, wounds.

(9)   Asepsis

(a)   Successful antisepsis:

(i)                  Asepsis is the product of proper aseptic technique

(ii)                Asepsis is the prevention of microbial contamination (exclusion of microbes)

(b)   Degrees of asepsis:

(i)                  surgical asepsis

(ii)                medical asepsis

(10)           Surgical asepsis

(a)   Complete asepsis:

(i)                  The goal of surgical asepsis is the exclusion of all microorganisms through aseptic technique

(ii)                Surgical asepsis is employed during most or all invasive procedures

(11)           Medical asepsis

(a)    The goal of medical asepsis is the exclusion of all pathogenic microorganisms through aseptic technique

(12)           Degerming

(a)    Degerming is the removal of microorganisms from the skin, i.e., as in cleaning the skin prior to injections

(13)           Sanitization

(a)    Sanitization is the cleaning of pathogenic microorganisms from public eating utensils and objects such as that done by the kitchen of a restaurant

(14)           Antimicrobial

(a)    An antimicrobial is a substance (chemical or physical) that can prevent microbial growth either by some static action or by the outright killing of microbes

(b)   Non-specific categories of antimicrobials include:

(i)                  germicides

(ii)                disinfectants

(iii)               antiseptics

(15)           Germicide

(a)    Germicides are antimicrobial substances or physical agents that kill

(b)   Exceptional breadth of action:

(i)                  Note, that, contrasting the definition of disinfection , some germicides are active against endospores and viruses

(ii)                That is, the definition of germicide is broader than that of disinfectant

(c)    Germicides are also known for the specific microorganisms they kill, e.g.,

(i)                  bacteriocide

(ii)                sporicide

(iii)               fungicide

(iv)              virucide

(v)                amoebicide

(vi)              spermicide

(16)           Disinfectant

(a)    A disinfectant is a disinfecting chemical or physical agent (typically the former), normally applied to non-living material

(b)   That is, disinfectants are typically chemical germicides which are not capable of killing endospores, some viruses, or mycobacterium

(17)           Antiseptic

(a)    Antiseptics are a subset of disinfecting chemical agents, ones which are applied to living material, and used to effect antisepsis

(b)   "A few agents are suitable as both disinfectants and antiseptics, although most disinfectants are too harsh for use in delicate skin." (p. 332, Black, 1996)

(18)           Disinfectant efficacy

(a)   Making disinfectants work better:

(i)                  Under the following conditions, disinfectants generally tend to work better:

(1)   at warm temperatures

(2)   when applied over long periods

(3)   when used on surfaces

(4)   when surfaces are clean

(5)   when cells are growing

(6)   against gram-positive bacteria

(7)   when endospores are absent

(ii)                The converse, of course, is also true: When these conditions are not met, disinfectants work less well

(iii)               Bottom line: Germicides are not necessarily as effective as you would hope they would be

(b)   Be aware/read the label:

(i)                  It is always best to know under what conditions the disinfectant you are using works and make sure the chosen disinfectant is used under those conditions

(ii)                That is, be aware and read the instructions which come with the disinfectant

(19)           Vocabulary [index]

(a)    Antimicrobial

(b)   Antiseptic

(c)    Antisepsis

(d)   Asepsis

(e)    Bacteriocidal

(f)     Bacteriostasis

(g)    Bacteriostatic

(h)    -cidal

(i)      Degerming

(j)     Disinfectant

(k)   Disinfectant efficacy

(l)      Disinfection

(m)  Exponential death

(n)    Germicide

(o)   Medical asepsis

(p)   Microbial death

(q)   Sanitization

(r)     Stasis

(s)    Static

(t)     Sterilization

(u)    Surgical asepsis

(20)           Practice questions [index]

(a)    Which of the following, sitting on a stainless steal table for a month, would you expect to die faster? [PEEK]

(i)                  Escherichia coli suspended in a drop of water

(ii)                E. coli suspended in pus

(iii)               an endospore

(iv)             Mycobacterium tuberculosis

(b)   Why should a microbiologist care about endospores even if the organism she works with does not sporulate? [PEEK]

(c)    A collection of methods of control of microbial growth whose goal particularly is the prevention of unintended transfer of pathogenic microorganisms is? [PEEK]

(i)                  Antisepsis

(ii)                medical asepsis

(iii)               surgical asepsis

(iv)              degerming

(d)   You treat a culture with a disinfectant for 5 minutes. You do everything perfectly under perfect conditions for killing an easily killed organism. Your rate of die off is 99% of the organisms per minute, and all cells present are equally susceptible. Assuming that only this organism is present, can you assume 100% sterility and why or why not? (hint, don't worry about whether there exist microorganisms against which this disinfectant is not active and don't invoke "if anything can go wrong it will" to answer this question) [PEEK]

(e)    Name three circumstances/conditions/microorganisms that will generally result in less efficient antimicrobial action. [PEEK]

(f)     Define antiseptic without using the term disinfectant or disinfection. [PEEK]

(g)    Distinguish degerming from sanitization. [PEEK]

(h)    Compare and contrast surgical and medical asepsis. [PEEK]

(i)      Your goal is to kill or remove all of the pathogenic, vegetative bacterial cells on the surface of an object, and to do so by employing an already chosen chemical agent which you are told is effective against pathogens, including mycobacterium. Name three things you can do (i.e., which are under your control) to increase the efficacy of your disinfection. [PEEK]

(j)     A general name for a chemical which can stop bacterial growth but not by a mechanism which involves an immediate killing the bacteria is a(n) __________ agent. [PEEK]

(k)   Though capable of killing most vegetative bacterial cells, __________ typically can't kill viruses or endospores and therefore are not dependable sterilizers. [PEEK]

(l)      Contrast disinfectant and antiseptic in terms of the materials they disinfect. [PEEK]

(m)  __________ is a genera of non-spore-forming bacteria which is typically highly resistant to disinfection. [PEEK]

(n)    True or False, a typical sterilizing agent kills by repeatedly damaging individual bacterial cells, resulting in a sudden, effectively simultaneous death of all the members of a given, pure culture of bacteria. [PEEK]

(21)           Practice question answers [index]

(a)    i, Escherichia coli suspended in a drop of water

(b)   Endospores are difficult to kill (sterilization schemes generally are deemed effective only if they are capable of killing endospores and endospores are often the most difficult to kill organisms in any sterilization scheme). If you fail to kill endospores they will germinate and contamination of your media, etc.

(c)    ii, medical asepsis

(d)   No, and it's not because disinfectants don't sterilize---we've taken care of that by stating that there is only one organism and that it is 100% susceptible---it's because there will still be fractionally 0.015 organisms still alive after 5 minutes of the total you started with, and if you start with enough organism, that will guarantee that at least some will still be alive.

(e)    not a surface which is being treated, if surface then not clean, cold temperatures, short duration of exposure, stationary phase cells, gram-negative cells, Mycobacterium tuberculosis.

(f)     Chemical antimicrobials that don't work against endospores or certain viruses, and which can be employed on living tissues.

(g)    The former is removal of microorganisms from a living surface, particularly the skin, while the latter is the removal of especially pathogenic microorganisms from inanimate objects, particularly those which come into contact with

(h)    Surgical asepsis is the exclusion of all microorganisms through aseptic technique. Medical asepsis is the exclusion only of pathogenic microorganisms.

(i)      (i) clean the surface of the object, (ii) increase the temperature of the object, (iii) let the disinfectant stand for a long period

(j)     bacteriostatic

(k)   disinfectants

(l)      antiseptics are a category of disinfectants which can be used on living tissue; disinfectants which are not antiseptics can be too harsh (i.e., cause too much additional damage) to be routinely used on living tissue such as skin

(m) Mycobacterium

(n)   False, this is not exponential death.

(22)           References [index]

(a)    Black, J.G. (1996). Microbiology. Principles and Applications. Third Edition. Prentice Hall. Upper Saddle River, New Jersey. pp. 332-352.

(b)   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. 167-188.