CHAPTER 23
Antibacterial Agents and Resistance
The ability to direct therapy specifically at a disease-causing infectious agent is unique to
the management of infectious diseases. Its initial success depends on exploiting
differences between our own makeup and metabolism and that of the microorganism in
question. The mode of action of antimicrobials on bacteria is the focus of this chapter.
The continued success of antibacterial agents depends on whether the organisms to which
the agent was originally directed develop resistance. Resistance to antibacterial agents is
also addressed here.
ANTIBACTERIAL AGENTS AND THERAPY
I. GENERAL CONSIDERATIONS
1. Clinically effective antimicrobial agents all exhibit selective toxicity toward the
parasite rather than the host
2. Ideally, selective toxicity is based on the ability of an antimicrobial agent to
attack a target present in bacteria but not humans
A. Definitions
1. Antibiotic—antimicrobials of microbial origin, most of which are produced
by fungi or by bacteria of the genus Streptomyces.
2. Antimicrobial, antimicrobic—any substance with sufficient antimicrobial
activity that it can be used in the treatment of infectious diseases.
3. Bactericidal—an antimicrobial that not only inhibits growth but is lethal to
bacteria.
4. Bacteriostatic—an antimicrobial that inhibits growth but does not kill the
organisms. The host defense mechanisms are ultimately responsible for
eradication of infection.
5. Chemotherapeutic—a broad term that encompasses antibiotics,
antimicrobials, and drugs used in the treatment of cancer. In the context of
infectious diseases, it implies the agent is not an antibiotic.
6. Minimal inhibitory concentration (MIC)—a laboratory term that defines
the lowest concentration (µg/mL) able to inhibit growth of the microorganism.
7. Resistant—organisms that are not inhibited by clinically achievable
concentrations of an antimicrobial agent.
8. Sensitive—term applied to microorganisms indicating that they will be
inhibited by concentrations of the antimicrobic that can be achieved clinically.
9. Spectrum—an expression of the categories of microorganisms against which
an antimicrobial is typically active. A narrow-spectrum agent has activity
against only a few organisms. A broad-spectrum agent has activity against
organisms of diverse types (e.g., Gram-positive and Gram-negative bacteria).
10. Susceptible—term applied to microorganisms indicating that they will be
inhibited by concentrations of the antimicrobic that can be achieved clinically.
B. Sources of Antimicrobial Agents
1. Antibiotics are synthesized by molds or bacteria
2. Production in quantity is by industrial fermentation
3. Chemicals with antibacterial activity are discovered by chance or as the result
of screening programs
4. Naturally occurring antimicrobics can be chemically modified
C. Spectrum of Action
1. Spectrum is the range of bacteria against which the agent is typically active
2. Broad-spectrum agents inhibit both Gram-positive and Gram-negative species
II. SELECTED ANTIBACTERIAL AGENTS
A. Antimicrobials That Act on Cell Wall Synthesis
1. Cross-linking of peptidoglycan is the target of β-lactams and glycopeptides
a. β-Lactam Antimicrobials - Penicillins
1. A β-lactam ring is part of the structure of all β-lactam antimicrobics
2. Interfere with peptidoglycan cross-linking by binding to transpeptidases called
PBPs
3. Penicillins, cephalosporins, monobactams, and carbapenems differ in terms of
the structures fused to the β-lactam ring
4. β-Lactam antimicrobics kill growing bacteria by lysing weakened cell walls
5. Resistance to staphylococcal and Gram-negative β-lactamases determines
spectrum
6. Penetration of Gram-negative outer membrane is often limited
7. Broad-spectrum penicillins penetrate the outer membrane of some Gramnegative bacteria
8. Some penicillins are inactivated by staphylococcal penicillinase
i. Cephalosporins
1. Cephalosporins are penicillinase resistant
2. Shifting between first- and third-generation cephalosporins gives a wider
Gram-negative spectrum
3. Second- and third-generation cephalosporins have less activity against Grampositive bacteria
4. First-generation cephalosporins inhibit Gram-positive bacteria and a few
Enterobacteriaceae
5. Second-generation cephalosporins are also active against anaerobes
6. Third-generation cephalosporins have increasing potency against Gramnegative organisms
7. Ceftriaxone and cefotaxime are preferred for meningitis
8. Ceftazidime is used for Pseudomonas
9. Fourth-generation cephalosporins have enhanced ability to penetrate outer
membrane
ii. Carbapenems
1. Carbapenems imipenem and meropenem have the broadest spectrum of all
β-lactam antibiotics
2. Spectrum due to the combination of easy penetration of Gram-negative and
Gram-positive bacterial cells and high level of resistance to β-lactamases
iii. Monobactams
1. Aztreonam has a spectrum limited to aerobic and facultatively anaerobic
Gram-negative bacteria, including Enterobacteriaceae, P. aeruginosa,
Haemophilus, and Neisseria
2. Poor affinity for the PBPs of Gram-positive organisms and
iv. β-Lactamase Inhibitors
1. β-lactams with little or no antimicrobial activity but are capable of binding
irreversibly to β-lactamase enzymes and, in the process, rendering them
inactive
2. Clavulanic acid, sulbactam, and tazobactam must be hydrolyzed by a βlactamase before becoming effective inactivators of the enzyme
3. Combinations of one of these inhibitors with an appropriate β-lactam
antimicrobial agent protects the therapeutic agent from destruction by many βlactamases and significantly enhances its spectrum
v. Clinical Use
1. Drugs of choice for infections by susceptible organisms because of their
low toxicity and bactericidal action
2. Of great value in the prophylaxis of many infections
3. Third-generation cephalosporins agents of choice in the treatment of
undiagnosed meningitis
b. Glycopeptide Antimicrobials
1. Two agents, vancomycin and teicoplanin, belong to this group
2. Glycopeptide antimicrobics bind directly to amino acid side chains
B. Inhibitors of Protein Synthesis
a. Aminoglycosides
1. Aminoglycosides must be transported into cell by oxidative metabolism
2. Not active against anaerobes
3. Ribosome binding disrupts initiation complexes
4. Newer agents bind to multiple sites
5. No entry into human cells
6. Gentamicin and tobramycin are the major aminoglycosides
7. Spectum includes P. aeruginosa
8. Renal and vestibular toxicity must be monitored
9. Broad spectrum and slow development of resistance enhance use
10. Often combined with β-lactam antimicrobics
b. Tetracyclines
1. Tetracyclines block tRNA attachment
2. Activity is bacteriostatic
3. Broad spectrum includes some intracellular bacteria
4. Orally absorbed but chelated by some foods
5. Dental staining and enamel damage to permanent teeth limits use of
older generation tetracyclines in children
c. Chloramphenicol
1. Chloramphenicol blocks peptidyl transferase
2. Diffusion into body fluid compartments occurs readily
3. Marrow suppression and aplastic anemia are serious toxicities
4. Use is sharply restricted
d. Macrolides
1. Erythromycin, azithromycin, and clarithromycin, affect protein
synthesis at the ribosomal
2. Ribosomal binding blocks translocation
3. Erythromycin is active against Gram-positives and Legionella
4. Azithromycin and clarithromycin have enhanced Gram-negative
spectrum
e. Clindamycin
1. Spectrum is similar to macrolides with addition of anaerobes
2. May mitigate toxin production
f. Oxazolidinones
1. Linezolid is the most widely used of a new class of antibiotics that act
by binding to the bacterial 50S ribosome
2. Activity against Gram-positive bacteria resistant to other agents
g. Streptogramins
1. Quinupristin and dalfopristin are used in a fixed combination known as
quinupristin-dalfopristin
2. Inhibit protein synthesis by binding to different sites on the 50S
bacterial ribosome
3. Useful against vancomycin-resistant enterococci
C. Inhibitors of Nucleic Acid Synthesis
a. Quinolones
Fluorinated derivatives are now dominant
Inhibition of topoisomerase blocks supercoiling
Fluoroquinolones have a broad spectrum, including Pseudomonas
Well distributed after oral administration
b. Folate Inhibitors
1. Bacteria must synthesize folate that humans acquire in their diet
2. Competition with PABA disrupts nucleic acids
3. Major use is urinary tract infections
i.
Trimethoprim-Sulfamethoxazole
1. Trimethoprim inhibits the activity of bacterial dihydrofolate
reductase, which catalyzes the conversion of folate to its reduced
active coenzyme form
2. When combined with sulfamethoxazole, a sulfonamide,
trimethoprim leads to a two-stage blockade of the folate pathway
3. Dihydrofolate reductase inhibition is synergistic with sulfonamides
4. Trimethoprim-sulfamethoxazole (TMP-SMX) has a much broader
and stable spectrum than either of its components alone
5. Activity against common bacteria and some fungi
c. Metronidazole
1. Antibacterial action requires reduction of the nitro group under
anaerobic conditions,
2. Reduction products induce breaks in DNA strands.
3. Action requires anaerobic conditions
d. Rifampin
1. Rifampin binds to the β-subunit of DNA-dependent RNA polymerase,
which prevents the initiation of RNA synthesis
2. Active against most Gram-positive bacteria and selected Gram-negative
organisms but not members of the Enterobacteriaceae
3. Clinically useful property is its antimycobacterial activity, which
includes Mycobacterium tuberculosis and other
D. Antimicrobials Acting on the Outer and Cytoplasmic Membranes
1. The polypeptide antimicrobial agents polymyxin B and colistin bind to the cell
membranes of susceptible Gram-negative bacteria and alter their permeability
2. Use is now limited to topical applications
Antimicrobial Resistance
I. SUSCEPTIBILITY AND RESISTANCE
1. Minimal inhibitory concentrations (MICs) must be below achievable blood
levels to be considered susceptible
2. Clinical experience must validate in vitro data
3. Susceptible bacteria are inhibited at achievable nontoxic levels, resistant strains
are not
4. Borderline isolates are called intermediate
5. Pharmacologic properties such as absorption, distribution, and metabolism
affect the usefulness of antimicrobials
II. LABORATORY CONTROL OF ANTIMICROBIAL
THERAPY
1. Bacteria are tested against antimicrobials over a range of concentrations
2. Selection considers susceptibility, pharmacology, and clinical experience
3. Penetration inside cells may be important
A. Dilution Tests
1. Dilution tests determine the MIC directly by using serial dilutions of the
antimicrobial agent in broth
2. MIC endpoint is the lowest concentration that inhibits growth
B. Diffusion Tests
1. Antimicrobial in disks to produces a circular concentration gradient
2. Inhibition zone is a measure of the MIC
C. Automated Tests
1. Instruments are now available that carry out rapid, automated variants of the
broth dilution test determining endpoints by turbidimetric or fluorometric analysis
2. Possible to generate MICs in as little as 4 hours
D. Molecular Testing
1. Molecular methods detect resistance genes
E. Bactericidal Testing
1. Quantitation of the % bacteria killed determines the minimal bactericidal
concentration (MBC)
F. Antimicrobial Assays
1. Pharmacologic monitoring is necessary in some situations
III. BACTERIAL RESISTANCE TO ANTIMICROBIALS
1. Resistance has eroded the effectiveness of many agents
2. Resistance and virulence are separate properties
A. Mechanisms of Resistance
a. Exclusion
1. Cell wall and outer membrane are barriers to antimicrobics
2. Outer membrane protein porins restrict access to interior
3. Active transported required for some
4. Efflux pumps push antimicrobials back out
b. Altered Target
1. If the target is altered in a way that decreases its affinity for the
antimicrobial, the inhibitory effect will be proportionately decreased
2. Binding affinity for enzymes and ribosomes can change
3. Multiple binding sites reduces chances for resistance
4. PBPs are altered transpeptidases
5. Altered PBPs have reduced affinity for β-lactams
6. Penicillins and cephalosporins are affected to the same degree
7. Pneumococci and MRSA have altered PBPs
8. New enzymes can alter bacterial targets
9. Mutation or acquisition of a new enzyme is possible
c. Enzymatic Inactivation
1. Enzymes may disrupt or chemically modify antimicrobials
2. Bacterial enzymes break open the β-lactam ring
3. β-Lactamases have variable activity against β-lactam substrates
4. May be exoenzymes or act in periplasmic space
5. Extended-spectrum β-lactamases (ESBLs) have broad activity against
cephalosporins
6. Weak β-lactamase producers are still considered resistant
7. Resistance to amino-glycosides is through enzymes that acetylate,
adenylate, or phosphorylate the aminoglycoside molecule
8. Chemically modified aminoglycosides do not bind to ribosomes
B. Genetics of Resistance
a. Intrinsic Resistance
1. Permeability barriers or enzyme production may be intrinsic
2. Inducible enzymes may have broad spectrum
b. Acquired Resistance
1. Conjugation and transposition are most important
i. Mutational Resistance
1. Mutations in structural or regulatory genes can confer resistance
2. Mutations are usually low-frequency
ii. Plasmids and Conjugation
1. Resistance genes on plasmids (R plasmids) can determine
resistance to one antimicrobial or to several that act by different
mechanisms
2. A single bacterial cell may contain more than one distinct
plasmid and/or multiple copies of the same plasmid
3. Plasmid conjugation allows multidrug resistance
4. Species may carry multiple or no plasmids
5. Conjugation genes and host range enhance plasmid spread
iii. Transposons and Transposition
1. Transposons resistance genes move between chromosomes and
plasmids
2. Transposition and conjugation combine for resistance spread
iv. Other Genetic Mechanisms
1. Transduction is limited by specificity of bacteriophages
2. Importance of transformation is unknown
C. Epidemiology of Resistance
1. Clinical use of antimicrobials is typically followed by resistance
2. Preexisting resistant strains are selected by antimicrobial use
3. Resistance often appears after many years of use
4. Antimicrobial use creates selection for resistance
5. Overuse increases resistance risk for patients and population at large
6. Antimicrobials in animal feeds increase the resistant population
7. Outbreaks have been traced from patients back to farms
Selection of Antibacterial Agents
I. Empiric Therapy
1. Empiric therapy is that based solely on clinical findings
2. Probable etiology and susceptibility statistics guide initial selection
3. Narrow versus broad spectrum is influenced by clinical severity
II. Specific Therapy
1. Isolation of the causative agent allows specificity
2. Susceptibility tests provide final guide
3. Combinations of antimicrobials may be synergistic
III. Prophylaxis
1. High-risk exposures (plague, anthrax) merit prophylaxis
2. Some surgical procedures benefit from a few hours of prophylaxis
3. Intrapartum prophylaxis prevents neonatal group B streptococcal (GBS) disease