Supplemental Lecture (98/11/21 update) by Stephen T. Abedon (abedon.1@osu.edu)

 

(1)   Chapter title: Antimicrobial Drugs

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

(i)                  New anti-bacterial antibiotics are relatively simple to discover and isolate. An impressively large fraction of the known anti-bacterial antibiotics have been isolated from just one bacterial genera, Streptomyces spp. The challenge comes in finding effective antibiotics that are well tolerated by infected hosts such as humans. Nevertheless, a large number of effective anti-bacterial antibiotics have been discovered over the past 50 or so years. These antibiotics differ from each other:

(ii)                chemically

(iii)               in what microorganism they were isolated from

(iv)              in where in/on the microorganism they act

(v)                in the spectrum of microorganisms they are effective against

(vi)              in the degree to which otherwise susceptible microorganisms have evolved resistance

(vii)             in terms of their toxicity to the host

(b)   Found at this site are additional pages of possibly related interest including: [antimicrobial therapy]

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

(2)   Spectrum of activity

(a)    A given antibiotic has a range of microorganisms whose growth it is capable of inhibiting

(b)   Broad range and toxicity:

(i)                  This range may be narrow or broad

(ii)                The spectrum of activity is very unlikely to span too broadly and still maintain an acceptably low level of host toxicity

(iii)               For example, a hypothetical antibiotic which was effective against both bacteria and fungi would very likely also be toxic to humans since whatever site of action that both bacteria and fungi happen to share, in all likelihood is sufficiently well conserved by evolution that it is present in humans, too

(3)   Narrow spectrum of activity

(a)   Affects limited number of species:

(i)                  An antibiotic that affects an only limited number of microorganisms is said to have a narrow spectrum of activity

(ii)                For example, the spectrum of activity might include most gram-positive bacteria but few if any gram-negative bacteria

(b)   Penicillin is a narrow-spectrum antibiotic with just these properties

(4)   Broad spectrum of activity

(a)    A broad spectrum antibiotic is active against a broader spectrum of microorganisms than a narrow spectrum antibiotic

(b)   such an antibiotic is said to have a broad (or, at least, broader) spectrum of activity

(c)    Example: tetracycline:

(i)                  One such broad-spectrum antibiotic is tetracycline

(ii)                It is effective against most eubacteria

(iii)               Tetracycline has the broadest spectrum of antibiotics employed

(iv)              "The fact that tetracyclines have the widest spectrum of activity of any antibiotics is a two-edged sword. They are effective against many gram-positive and gram-negative bacterial infections and are suitable for treating rickettsial, chlamydial, mycoplasmal, and some fungal infections. But because they have such a wide spectrum of activity, they destroy the normal intestinal microbiota and often produce severe gastrointestinal disorders. Recalcitrant superinfections of tetracycline-resistant Proteous, Pseudomonas, and Staphylococcus, as well as yeast infections, also can result." (p. 375, Black, 1996)

(5)   Broad-spectrum vs. narrow-spectrum

(a)    A clear advantage to the use of broad-spectrum antibiotics is that there is less of a need (as compared with narrow-spectrum antibiotics) to identify the infecting pathogen with real certainty before commencing treatment

(b)   On the other hand, a broad-spectrum antibiotic will have a more profound effect on your normal flora, thus interfering with microbial antagonism

(6)   Superinfection

(a)    A superinfection results from a disruption of normal flora:

(i)                  can set the stage for more profound infection by the original pathogen should treatment be less than 100% effective

(ii)                can result in infection by opportunistic pathogens

(b)   Both are consequences referred to as superinfection

(7)   Sites of action

(a)    Sites on bacteria where antibiotics exert their negative influence on growth include:

(i)                  cell wall

(ii)                protein synthesis/ribosomes

(iii)               plasma membrane

(iv)              RNA synthesis

(v)                etc.

(b)   Examples include (respectively):

(i)                  penicillin, ampicillin, vancomycin, bacitracin, etc.

(ii)                tetracycline, chloramphenicol, erythromycin, etc.

(iii)               polymyxin B

(iv)              rifampin

(8)   Inhibition of cell wall synthesis

(a)   Mechanism:

(i)                  Antibiotics that inhibit cell wall synthesis work under two (correct) premises:

(1)   (most) eubacteria have peptidoglycan-based cell walls but mammals do not

(2)   growth under normal circumstances is impossible in the absence of peptidoglycan synthesis

(3)   Actively growing bacteria treated with cell-wall-synthesis inhibitors are thus subject to osmotic lysis

(b)   Gram negatives less susceptible:

(i)                  In addition, gram-negative bacteria generally are less susceptible to inhibitors of cell wall synthesis than are gram-positive bacteria

(ii)                In the former cell wall synthesis inhibitors fail to reach the cell wall because they are blocked by the gram-negative outer membrane

(c)    Examples:

(i)                  Penicillin is the classic example of an inhibitor of cell wall synthesis

(ii)                Other examples include:

(1)   ampicillin

(2)   bacitracin

(3)   carbapenems

(4)   cephalosporin

(5)   methicillin

(6)   oxacillin

(7)   vancomycin

(9)   Inhibition of protein synthesis

(a)   Ribosomal structural differences:

(i)                  Antibiotics that inhibit protein synthesis take advantage of the fact that the bacterial ribosome and the eucaryotic ribosome differ structurally

(ii)                Consequently, there exist chemicals that can inhibit bacterial translation but not eucaryote translation

(b)   Mitochondrial ribosomes:

(i)                  The one caveat is that the mitochondria ribosome is structurally similar (effectively identical?) to the eubacteria ribosome

(ii)                This gives antibiotics that inhibit protein synthesis a potential for toxicity (in addition to its other damaging effects, tetracycline complexes with Ca++)

(c)    There are a number of bacteriostatics among protein synthesis inhibitors, thus indicating a reversible intereference with ribosome function

(d)   Examples include:

(i)                  chloramphenicol

(ii)                erythromycin

(iii)               gentamycin

(iv)              neomycin

(v)                streptomycin

(vi)              tetracycline

(10)           Injury to plasma membrane

(a)    Antiobiotics that injure bacterial plasma membranes lead to cell death through leakage of cell contents and associated disruption of the cross-membrane potential (which essentially are ion concentration gradients)

(b)   Examples:

(i)                  Examples include polymixin B

(ii)                See also antifungals that bind sterols

(11)           Antifungals that bind sterols

(a)   Membrane structural differences:

(i)                  The composition of the fungal plasma membrane differs from the composition of the mammalian plasma membrane particular in terms of the presence or absence of certain kinds of sterols

(ii)                There exist antibiotics that, consequently, more effectively recognize fungal plasma membrane than mammalian plasma membrane

(b)   Examples include:

(i)                  amphotericin B

(ii)                ketoconazole

(iii)               miconazole

(iv)              nystatin

(12)           Other sites of antimicrobial activity

(a)   Other differences:

(i)                  Additional sites of antibiotic activity is against various enzymatic functions including:

(1)   DNA replication

(2)   transcription

(3)   etc.

(ii)                The key, as usual, is that the site interfered with be sufficiently different from that found mammals that there is a bias toward inhibition of microbial function

(b)   Examples include:

(i)                  ethambutol

(ii)                the fluoroquinolones

(iii)               isonizid

(iv)              the quinolines

(v)                rifampin

(vi)              sulfonamides

(13)           Antibiotic resistance

(a)   Microbial genes:

(i)                  There are three general mechanisms by which a microorganism may express resistance to an antibiotic

(ii)                All of these are consequences of microbial gene expression

(iii)               They include:

(1)   inactivation of the drug (e.g., enzymes)

(2)   barriers to contact with the drug (e.g., changes in plasma or other membrane permiability, or formation of capsules)

(3)   alteration of the site of drug activity (such as ribosomal proteins)

(b)   Plasmids:

(i)                  Antibiotic resistance genes are often carried on plasmids (i.e., R plasmids )

(ii)                Exceptional are mechanisms of resistance which prevent antibiotic binding through a change in the structure of the bacterial target molecule

(iii)               Since the target molecule typically is coded by chromosome-located genes, the resistance genes (actually, alleles) are also found on chromosome

(14)           Preventing antibiotic resistance

(a)    The following are quoted from p. 53 of Levy, 1998. They describe what health professionals ("physicians") and consumers can do to minimize the evolution of antibiotic resistance among pathogens, as well as to minimize the disruption of microbial communities by antibiotic (and other antimicrobials) 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 (this is to avoid agriculturally-sourced antibiotic residues)

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

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

(2)   to avoid selecting for resistance among these normal microbial community members

(3)   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 which 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 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 anti-cancer chemotherapeutics are typically given simultaneously

(15)           Antibiotic basis for spice use

(a)    "Herbs and spices flavor and tenderize meat, but they also serve a more evolutioarily 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, 1997)

(16)           Vocabulary

(a)    Antibiotic resistance

(b)   Antibiotic basis for spice use

(c)    Antifungals that bind sterols

(d)   Broad spectrum of activity

(e)    Broad vs. narrow spectrum of activity

(f)     Inhibition of cell wall synthesis

(g)    Inhibition of protein synthesis

(h)    Injury to plasma membrane

(i)      Narrow spectrum of activity

(j)     Preventing antibiotic resistance

(k)   Spectrum of activity

(17)           Practice questions [index]

(a)    Why, in general, does penicillin fail to inhibit the growth of gram-negative bacteria? (circle only one correct answer) [PEEK]

(i)                  presence of glycocalyx

(ii)                different ribosome structure

(iii)               presence of outer membrane

(iv)              different cell wall structure

(v)                all of the above

(vi)              none of the above

(b)   What would you expect the odds to be that a bacterium is resistant to two unrelated antibiotics? Why? [PEEK]

(c)    Antibiotics which inhibit the synthesis of cell walls are bactericidal because they weaken the cell walls sufficiently that they cause osmotic lysis. However, targeting of cell wall synthesis also means that only bacteria which are actually synthesizing cell walls are susceptible to inhibitors of cell wall synthesis. Thus, antibiotic treatment with a cell wall synthesis inhibitor can be expected to spare any and all cells which are not growing for whatever reason during the period of treatment. Inhibitors of protein synthesis, on the other hand, often are bacteriostatic. In terms of its interaction with the ribosome, describe a key property of an antibiotic which inhibits protein synthesis and that would result in its being bacteriostatic. [PEEK]

(d)   Describe an advantage and a disadvantage of employing a broad-spectrum antibiotic to treat an infection. [PEEK]

(e)    Describe two general physiological mechanisms by which antibiotic resistance may be achieved (I'm not asking for how, genetically, a bacterium might acquire antibiotic resistance). [PEEK]

(f)     "A patient with streptococcal sore throat takes penicillin for two days of a prescribed 10-day regime. Because he feels better, he then saves the remaining penicillin for some other time. After three more days, he suffers a relapse of the sore throat. Discuss the probable cause of the relapse." (p. 514, Tortora et al., 1995) [PEEK]

(g)    "What similar problems are encountered with antiviral, antifungal, antiprotozoan, and antihelminthic drugs?" (p. 513, Tortora et al., 1995) [PEEK]

(h)    Name two reasons for employing two antimicrobials simultaneously rather than just one, or two in serial. (from p. 513, Tortora et al., 1995) [PEEK]

(i)      Describe penicillin in terms of site of action, specificity, and relative breadth of its spectrum activity. [PEEK]

(j)     Name a negative consequence of prescription of a broad spectrum antibiotic. [PEEK]

(k)   Contrast penicillin and tetracycline in terms of sites of action and spectrum of activity. [PEEK]

(l)      Considering only spectrum of activity, what are the advantages and disadvantages of an antibiotic having a narrow spectrum of activity? [PEEK]

(m)  Why do you suppose it is so much more difficult to kill cancer cells and viruses than it is to kill most bacteria when employing chemotherapeutics? [PEEK]

(n)    Bacteria treated with a cell wall-synthesis inhibitor die because of _________. [PEEK]

(o)   _________ is a specific example of an antimicrobial that inhibits ribosome action. [PEEK]

(p)   _________ is a type of antibiotic resistance most likely coded by chromosomal genes (i.e., as opposed to plasmid-coded genes). [PEEK]

(i)                  inactivation of the drug

(ii)                change in plasma membrane permeability

(iii)               formation of capsule

(iv)              formation of barrier to contact with drug

(v)                alteration in target of drug activity

(q)   Microbial resistance to antibiotics and other antimicrobials may be prevented by all of the following except: [PEEK]

(i)                  hand washing by health-care workers

(ii)                habitual use of antimicrobial soaps and hand creams

(iii)               completing antibiotics course, even if you already feel good

(iv)              employ only narrow-spectrum antibiotics if possible

(v)                do not treat viral infections with antimicrobial antibiotics

(r)     Antibiotics that bind to the bacterial ribosome, but not the eucaryote ribosome, interfere with bacterial metabolism by inhibiting _________ synthesis (i.e., the normal catalytic function of both bacterial and eucaryotic ribosomes). [PEEK]

(18)           Practice question answers [index]

(a)    iii, presence of outer membrane. That is, the gram-negative outer membrane blocks penicillin access to the gram-negative cell wall.

(b)   If the odds that a given bacterium is resistant to antibiotic A are x and the odds that a given bacterium is resistant to antibiotic B are y, then the odds that a bacterium is resistant to both antibiotics is expected to be x*y. The answer to Why? is similarly statistical. It is because in most cases the resistance to one antibiotic is independent of (i.e., has not bearing on) the resistance to a second, unrelated antibiotic.

(c)    reversible ribosome binding

(d)   advantage = treatment before identification of pathogen. disadvantage = greater disruption of normal microbiota.

(e)    (i) enzymatic inactivation, (ii) change in cell envelope permeability, (iii) mutational change in site of antibiotic action.

(f)     The patient failed to kill all of the infecting streptococci.

(g)    They all must act against the eucaryotic (or biochemically eucaryotic) pathogen without harming the eucaryotic host.

(h)    inhibition of evolution of resistant pathogen strains, potentially lower dosage requirement, synergistic effects, selective broadening of spectrum (especially useful if diagnosis/pathogen identification is incomplete).

(i)      cell wall, gram-positive bacteria, relatively narrow.

(j)     Indiscriminate destruction of normal flora and superinfection.

(k)   Penicillin is a cell wall inhibitor which affects mostly gram positives. Tetracycline is a protein synthesis (ribosome) inhibitor which has a very broad spectrum of activity, encompassing almost all of the eubacteria.

(l)      The (an) advantage is that the antibiotic will have less of an effect on normal flora. The (a) disadvantage is that you will have to identify the pathogen more thoroughly prior to successful treatment.

(m)  Bacteria are distinctly different physiologically from a eucaryotic host. Chemotherapeutics consequently can distinguish between pathogen and host. Cancer cells, since they are derived from the body's own cells, are nearly identical physiologically to host cells and therefore very difficult to distinguish chemotherapeutically. Viruses utilize the host cellular machinery and consequently are about as difficult to distinguish from host tissue chemotherapeutically as are cancer cells. Another way to look at this is that bacteria contain many thousands of gene products that are different from host gene products while viruses and cancer cells may possess only a few gene products that are different from those of the host. For cancer cells, since they are derived directly from host cells, the differences are extremely subtle.

(n)    osmotic lysis

(o)   tetracycline, etc.

(p)   v, alteration in target of drug activity

(q)   ii, habitual use of antimicrobial soaps and hand creams

(r)     protein

(19)           References [index]

(a)    Black, J.G. (1996). Microbiology. Principles and Applications. Third Edition. Prentice Hall. Upper Saddle River, New Jersey. pp. 356-387.

(b)   Holden, C. (1997). Antibiotic basis for spice use. Science 277:321.

(c)    Levy, S.B. (1998). The challenge of antibiotic resistance. Scientific American. March:46-53.

(d)   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. 491-514.