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Antimicrobial Chemotherapy
CHAPTER OVERVIEW
The control or the destruction of microorganisms that reside within the bodies of humans and other animals is
of tremendous importance. This chapter introduces the principles of chemotherapy and discusses the ideal
characteristics for successful chemotherapeutic agents (including the concept of selectively damaging the target
microorganism while minimizing damage to the host). The chapter also presents characteristics of some
commonly used antibacterial, antifungal, antiviral, and antiprotozoan drugs.
CHAPTER OBJECTIVES
After reading this chapter you should be able to:




discuss the various ways in which antimicrobial agents can damage pathogens while causing minimal
damage to the host
discuss the various factors that influence the effectiveness of a chemotherapeutic agent
discuss the increasingly serious problem of drug-resistant pathogens
discuss the modes of action and selectivity of antifungal, antiviral, and antiprotozoan agents
CHAPTER OUTLINE
I.
Introduction
A. Chemotherapeutic agents are chemical agents used to treat disease
B. Antibiotics are microbial products or their derivatives that kill or inhibit susceptible microorganisms
C. Synthetics—drugs that are not microbially synthesized
II. The Development of Chemotherapy
A. Paul Ehrlich (1904–1909)—aniline dyes and arsenic compounds
B. Gerhard Domagk, and Jacques and Therese Trefouel (1939)—sulfanilamide
C. Ernest Duchesne (1896) discovered penicillin, however, this discovery was not followed up
D. Alexander Fleming (1928) accidentally discovered the antimicrobial activity of penicillin on a
contaminated plate; however, follow-up studies did not show the drug would remain active in the
body long enough to be effective
E. Howard Florey and Ernst Chain (1939) aided by the biochemist, Norman Heatley, worked from
Fleming’s published observations, obtained a culture from him, and demonstrated the effectiveness
of penicillin
F. Selman Waksman (1944)—streptomycin; this success led to a worldwide search for additional
antibiotics, and the field has progressed rapidly since then
III. General Characteristics of Antimicrobial Drugs
A. Selective toxicity—ability to kill or inhibit microbial pathogen with minimal side effects in the host
1. Therapeutic dose—the drug level required for clinical treatment of a particular infection
2. Toxic dose—the drug level at which the agent becomes too toxic for the host (produces
undesirable side effects)
3. Therapeutic index—the ratio of toxic dose to therapeutic dose: the larger the better
B. Chemotherapeutic agents can occur naturally, be synthetic, or semisynthetic (chemical modifications
of naturally occurring antibiotics)
C. Drugs with narrow-spectrum activity are effective against a limited variety of pathogens; drugs with
broad-spectrum activity are effective against a wide variety of pathogens
D. Drug can be cidal (able to kill) or static (able to reversibly inhibit growth)
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E.
Minimal inhibitory concentration (MIC) is the lowest concentration of the drug that prevents growth
of a pathogen; minimal lethal concentration (MLC) is the lowest drug concentration that kills the
pathogen
IV. Determining the Level of Antimicrobial Activity
A. Dilution susceptibility tests—a set of broth-containing tubes are prepared; each tube in the set has a
specific antibiotic concentration; to each is added a standard number of test organisms
1. The lowest concentration of the antibiotic resulting in no microbial growth is the MIC
2. Tubes showing no growth are subcultured into tubes of fresh medium lacking the antibiotic to
determine the lowest concentration of the drug from which the organism does not recover; this
is the MLC
B. Disk diffusion tests
1. Disks impregnated with specific drugs are placed on agar plates inoculated with the test
organism; clear zones (no growth) will be observed if the organism is sensitive to the drug; the
size of the clear zone is used to determine the relative sensitivity; zone width also is a function
of initial concentration, solubility, and diffusion rate of the antibiotic
2. Kirby-Bauer method is most commonly used disk diffusion test; test results are determined
using tables that relate zone diameter to the degree of sensitivity
C. The Etest®
1. Especially useful for testing anaerobic microorganisms
2. Makes use of special plastic strips that contain a concentration gradient of an antibiotic; each
strip is labeled with a scale of MIC values; after incubation an elliptical zone of inhibition is
observed and its intersection with the strip is used to determine the MIC
V. Antibacterial Drugs
A. Inhibitors of cell wall synthesis are effective and selective because bacterial cell walls have unique
structures not found in eukaryotic cells
1. Penicillins—inhibit cell wall synthesis; many types have been identified or synthesized
including ampicillin, carbenicillin, and methicillin; they differ in spectrum of activity and
administration route but all have a -lactam ring that is crucial for activity; resistance is an
increasing problem, often due to penicillinase; some patients are allergic to these antibiotics
2. Cephalosporins—inhibit cell wall synthesis; broad spectrum of activity; they contain a lactam ring, but are not subject to degradation by penicillinase; they can be given to some
patients with penicillin allergies; they include cephalothin, cefoxitin, and ceftriazone
3. Vancomycin and teicoplanin—glycopeptide antibiotics that block peptidoglycan synthesis;
vancomycin is particularly important as the last line of defense against antibiotic-resistant
staphylococcal and enterococcal infections
B. Protein synthesis inhibitors exploit the differences between prokaryotic and eukaryotic ribosomes
1. Aminoglycosides—contain cyclohexane ring and amino sugars; includes kanamycin,
streptomycin, neomycin, and gentamicin; can be quite toxic to patients; act through
interference with 30S ribosomal subunit, causing mistranslation with damaged proteins
activating the envelope stress response and oxidative damage
2. Tetracyclines—contain a four-ring structure with side chains; very broad spectrum that
includes intracellular parasites and mycoplasmas; bacteriostatic
3. Macrolides—12- to 22-carbon lactone rings linked to sugars; broad spectrum similar to that of
penicillin; includes erythromycin and clindamycin; inhibit peptide chain elongation by binding
rRNA
4. Chloramphenicol—has a broad spectrum but is quite toxic; inhibits peptidyl transfer reaction
C. Metabolic antagonists are structural analogs of metabolic intermediates that act as antimetabolites,
inhibiting metabolic pathways; bacteriostatic
1. Sulfonamides or sulfa drugs—inhibit folic acid synthesis in bacteria (humans don’t synthesize
folic acid, so are not affected); resistance is increasing and many patients are allergic to these
drugs; includes p-aminobenzoic acid (PABA)
2. Trimethoprim—synthetic antibiotic that blocks folic acid production; broad spectrum often
combined with sulfa drugs
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D.
Nucleic acid synthesis inhibitors block enzymes of transcription and translation; generally not as
selectively toxic
1. Quinolones—synthetic drugs that inhibit bacterial DNA gyrase or topoisomerase II, thereby
disrupting replication, repair, and other processes involving DNA; broad spectrum; includes
nalidixic acid and ciprofloxacin (Cipro)
VI. Antifungal Drugs
A. Fungal infections are more difficult to treat than bacterial infections because the greater similarity of
fungi and host limits the ability of a drug to have a selective point of attack; furthermore, many
fungi have detoxification systems that inactivate drugs
B. Superficial mycoses are infections of superficial tissues and can often be treated by topical
application of antifungal drugs such as miconazole, nystatin, and griseofulvin, thereby minimizing
systemic side effects
C. Systemic mycoses are more difficult to treat and can be fatal; amphotericin B and flucytosine have
been used with limited success; amphotericin B is highly toxic and must be used with care;
flucytosine must be converted by the fungus to an active form, and animal cells are incapable of
this; some selectivity is possible, but severe side effects have been observed with both drugs;
posaconazole is effective against drug-resistant fungi and are less toxic
D. Subcutaneous mycoses (e.g., mycetomas) are treated with a mixture of therapies
VII. Antiviral Drugs
A. Selectivity is a problem because viruses use the metabolic machinery of the host
B. Antiviral drugs target specific steps of life cycle, including viral uncoating and DNA replication
(e.g., amantadine, vidarabine, acyclovir, cidofovir, and azidothymidine)
C. Anti-HIV drugs (e.g., AZT, ddI, 3TC) have four targets: nucleoside reverse transcriptase inhibitors
(NRTIs), nonnucleoside reverse transcriptase inhibitors (NNRTIs), protease inhibitors (block viral
polypeptide processing), and fusion inhibitors (block viral entry into cell); combinations of drugs
often used
D. Tamiflu is a neuraminidase inhibitor that is used to treat influenza
VIII. Antiprotozoan drugs
A. Mechanisms of action for antiprotozoan drugs are largely unknown; as protozoans and humans are
both eukaryotes, selective toxicity is difficult to achieve
B. Chloroquine and mefloquine—used to treat malaria; variety of mechanisms proposed
C. Artemisinin—anti-malarial drug used in traditional Chinese medicine
D. Metronidazole—used to treat Entamoeba infections; appears to interact with DNA
E. Atovaquone—an analog of ubiquinone that interferes with electron transport chain
IX. Factors Influencing the Effectiveness of Antimicrobial Drugs
A. Drug’s ability to reach the site of infection—this is greatly influenced by the mode of administration
(e.g., oral, topical, parenteral), but also can be influenced by exclusion from the site of infections
(e.g., blood clots or necrotic tissue protects bacterium)
B. Susceptibility of pathogen—influenced by growth rate and by inherent properties (e.g., whether or
not pathogen has target of the drug)
C. Factors influencing drug concentration in the body—must exceed the pathogen’s MIC for the drug
to be effective; this will depend on the amount of drug administered, the route of administration, the
speed of uptake, and the rate of clearance (elimination) from the body
D. Drug resistance has become an increasing problem
X. Drug Resistance
A. Mechanisms of drug resistance
1. Prevent entrance of drug (e.g., alter drug transport into cell)
2. Pump the drug out of the cell once it has entered (efflux pump)
3. Enzymatic inactivation of the drug—chemical modification of the drug by cellular enzymes
can render it inactive before it has a chance to damage the cell
4. Alteration of target enzyme or organelle—modification of the target so that it is no longer
susceptible to the action of the drug
5. Use of alternative pathways and increased production of the target metabolite have been used
by some organisms to minimize the effects of the drug
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B.
C.
The origin and transmission of drug resistance
1. Organisms that produce antibiotics must also develop resistance to those compounds
2. Spontaneous mutations in chromosomal genes; these are then inherited by progeny of the
resistant mutant
3. Transfer of R plasmids that contain resistance genes
4. Other genetic elements can carry one or more resistance genes
a. Composite transposons (e.g., Tn5, Tn9, Tn10, Tn21, Tn551, and Tn4001)
b. Integrons—genetic elements that contain a site into which genes can be inserted and an
integrase gene that allows for incorporation of the integron into the bacterial
chromosome; gene cassettes (a unit of genetic material that contains a set of resistance
genes; gene cassettes are usually a linear part of a transposon, plasmid, or bacterial
chromosome, and they are able to move resistance genes from one recombination site to
another) can be captured by integrons and spread to other sites and organisms
c. Conjugative transposons
Overcoming drug resistance
1. Several strategies can be used to discourage emergence of drug resistance (e.g., administration
of high doses, simultaneous treatment with more than one drug, limited use of broad-spectrum
antibiotics)
2. Drug resistance has become an increasing problem; new drugs are constantly being developed
and new treatment methods (e.g., phage treatment of bacterial infections) are being explored
TERMS AND DEFINITIONS
Place the letter of each term in the space next to the definition or description that best matches it.
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
____ 6.
____ 7.
____ 8.
____ 9.
____ 10.
____ 11.
____ 12.
____ 13.
The ratio of therapeutic dose to toxic dose
Compounds used in the treatment of disease that kill or
prevent the growth of microorganisms at
concentrations low enough to avoid undesirable
damage to the host
Chemotherapeutic agents that are natural products of
microorganisms
Activities of a chemotherapeutic agent that damage the
host either by inhibiting the same process in the host as
in the target organism or by damaging other processes
Describes an antibiotic that attacks many different
pathogens
Describes an antibiotic that is effective against only a
limited variety of pathogens
Susceptibility tests that involve the inoculation of a set
of dilutions of an antimicrobial agent with a test
microorganism
Term used to describe chemotherapeutic agents that
reversibly inhibit the growth of microorganisms
Term used to describe chemotherapeutic agents that
kill microorganisms
Lowest concentration of a drug necessary to prevent
the growth of a particular microorganism
Lowest concentration of a drug necessary to kill a
particular microorganism
An immune system chemical that inhibits virus
replication
Drugs that block the function of metabolic pathways
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____ 14. Small circular DNA
molecules that can exist
separately from the
chromosome or be
integrated into it
____ 15. Plasmids that bear one
or more resistance genes
____ 16. A set of resistance genes
that can be captured by
an integron and
transferred as a unit
____ 17. A genetic element that
has a site at which genes
can be inserted and a
gene for integrase, which
allows the element to
insert into other DNA
molecules
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
antibiotics
antimetabolites
broad-spectrum drug
chemotherapeutic agents
cidal
dilution susceptibility tests
gene cassette
integron
interferon
minimal inhibitory concentration
minimal lethal concentration
narrow-spectrum drug
plasmids
R plasmids
side effects
static
therapeutic index
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DRUGS AND THEIR DISCOVERERS
Match the following scientists with their discoveries.
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
____ 6.
____ 7.
____ 8.
Use of the arsenic compound Salvarsan as a treatment
for syphilis
Use of Prontosil Red (sulfanilamide) as a treatment for
streptococcal and staphylococcal infections
First discovered penicillin, but discovery was lost
Rediscovered penicillin, but did not pursue the
significance of it
Co-discoverer of the therapeutic value of penicillin
Co-discoverer of the therapeutic value of penicillin
Discovered streptomycin and stimulated intense search
for other antibiotics
Biochemist who helped with work demonstrating the
therapeutic value of penicillin
a.
b.
c.
d.
e.
f.
g.
h.
Chain
Domagk
Duchesne
Ehrlich
Fleming
Florey
Heatley
Waksman
IMPORTANT CHEMOTHERAPEUTIC AGENTS
For each of the drugs below, provide the requested information.
Chemotherapeutic
Agent
Acyclovir
Adenine arabinoside
(vidarabine)
Amantadine
Structural
Features
Mechanism of
Action
Amphotericin B
Aminoglycosides
(e.g., streptomycin)
AZT (zidovudine)
Cephalosporins
Chloramphenicol
Chloroquine
Griseofulvin
Macrolides (e.g.,
erythromycin)
Nystatin
Penicillin
Quinolones
Sulfonamides
Tetracyclines
Vancomycin
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Cidal/Static
Targeted Organism(s)
FILL IN THE BLANK
1.
2.
3.
4.
The __________ __________ is the ratio of the therapeutic dose to the toxic dose. The larger this ratio,
the greater the __________ __________ and the less likely it is that the chemotherapeutic agent will
cause __________ __________.
A number of useful drugs act as ____________; they block the functioning of metabolic pathways by
inhibiting key enzymes.
Antibiotics that are taken by a route other than by mouth are said to have a ____________ route of
administration.
The discovery of HIV and AIDS has heightened the urgency for discovering antiviral chemotherapeutic
agents. One of the first anti-HIV agents was ____________, which is an inhibitor of HIV reverse
transcriptase (RT). This drug is a __________ __________ of thymine nucleotides and therefore blocks
activity of RT and replication of HIV. Another HIV protein, HIV protease, also is a good target for
therapy. Today several __________ __________ __________ have been developed, including saquinvir,
indinavir, and ritonavir.
MULTIPLE CHOICE
For each of the questions below select the one best answer.
1.
2.
3.
The most selective antibiotics are those that
interfere with bacterial cell wall synthesis.
Why is this?
a. because bacterial cell walls have a
unique structure not found in eukaryotic
cells
b. because bacterial cell wall synthesis is
easy to inhibit, while animal cell wall
synthesis is more resistant to the actions
of the drugs
c. because animal cells do not take up the
drugs
d. because animal cells inactivate the
drugs before they can do any damage
Which of the following is not an
antiprotozoan drug?
a. chloroquine
b. metronidazole
c. pentamidine
d. amantadine
What is penicillinase?
a. an enzyme that modifies penicillins,
making them more potent
b. an enzyme that cleaves the beta-lactam
ring of penicillin, rendering it inactive
c. a semi-synthetic form of penicillin
d. none of the above
4.
5.
6.
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Which of the following is NOT a common
mechanism by which microorganisms
develop drug resistance?
a. enzymatic inactivation of the drug
b. exclusion of the drug from the cell
c. use of an alternative pathway to bypass
the drug-sensitive pathway
d. All of the above are common
mechanisms by which microorganisms
develop drug resistance.
Which of the following is used to discourage
the development of drug resistance?
a. sufficiently high drug doses to destroy
any resistant mutants that may have
arisen spontaneously
b. use of two drugs simultaneously with
the hope that each will prevent the
emergence of resistance to the other
c. avoidance of indiscriminate use of
drugs
d. All of the above are used to discourage
development of drug resistance.
What is the drug level required for the
clinical treatment of a particular infection
called?
a. therapeutic dose
b. toxic dose
c. therapeutic index
d. None of the above is correct.
7.
Which of the following is NOT a reason that
treatment of fungal infections generally has
been less successful than treatment of
bacterial infections?
a. Fungi use the metabolic machinery of
the host and therefore cannot be
selectively attacked.
b. Fungi are more similar to human cells
than are bacteria, and many drugs that
inhibit or kill fungi are toxic for
humans.
c. Fungi have detoxifying systems that
rapidly inactivate many drugs.
d. All of the above are reasons that
treatment of fungal infections has been
less successful than treatment of
bacterial infections.
8.
9.
Which of the following affects the size of the
clear zone in a Kirby-Bauer test?
a. the initial concentration of the drug
b. the solubility of the drug
c. the diffusion rate of the drug
d. All of the above are correct.
For which organisms is antibiotic
resistance becoming a major problem?
a. bacterial diseases
b. fungal diseases
c. Both (a) and (b).
d. Neither (a) nor (b).
TRUE/FALSE
____ 1.
A drug that disrupts a microbial function not found in animal cells usually has a lower therapeutic
index.
____ 2. Static agents do not kill infectious organisms and therefore are not useful as chemotherapeutic
agents.
____ 3. Protein synthesis inhibitors have a high therapeutic index because they can usually discriminate
between prokaryotic and eukaryotic ribosomes; however, their therapeutic index is not as high as
that of cell wall synthesis inhibitors.
____ 4. Sulfonamides and other drugs that inhibit folic acid synthesis have a high therapeutic index because
humans must obtain folic acid in their diets while microorganisms synthesize their own.
____ 5. The fungus Candida albicans is normally present in various parts of the body and can cause
problems (superinfection) when bacterial competition is eliminated by antibiotic treatment.
____ 6. Isoniazid is a narrow-spectrum antibiotic. However, it is considered useful because it is one of the
few drugs that are effective against tuberculosis.
____ 7. Drugs with highly toxic side effects are usually used only in life-threatening situations where
suitable alternatives are not available.
____ 8. There are few effective antiviral drugs because viruses use the metabolic machinery of their hosts,
making it difficult to identify a selective point of attack.
____ 9. One way in which organisms may exhibit resistance to a drug is to pump the drug out of the cell
immediately after it has entered.
____ 10. One approach to limiting the development of drug resistance is to use multiple drugs simultaneously
at high doses.
____ 11. One of the most serious threats to the successful treatment of disease is the spread of drug-resistant
pathogens.
CRITICAL THINKING
1.
Antibiotics are natural products of certain microorganisms. What advantages might these antibiotics
provide for the organisms that produce them?
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2.
Viruses generally use the metabolic machinery of their hosts. Therefore, they should present no selective
point of attack for potential antiviral drugs. Yet, recently there have been several antiviral drugs
developed that have a reasonably high therapeutic index. Explain.
ANSWER KEY
Terms and Definitions
1. q, 2. d, 3. a, 4. o, 5. c, 6. l, 7. f, 8. p, 9. e, 10. j, 11. k, 12. i, 13. b, 14. m, 15. n, 16. g, 17. h
Drugs and Their Discoverers
1. d, 2. b, 3. c, 4. e, 5. f, 6. a, 7. h, 8. g
Fill in the Blank
1. therapeutic index; selective toxicity; side effects 2. antimetabolites 3. parenteral 4. AZT; structural analog;
HIV protease inhibitors
Multiple Choice
1. a, 2. d, 3. b, 4. d, 5. d, 6. a, 7. a, 8. d, 9. c
True/False
1. F, 2. F, 3. T, 4. T, 5. T, 6. T, 7. T, 8. T, 9. T, 10. T, 11.T
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