Study Notes: Antimicrobial Drugs and Drug Resistance (Chapter 20)

Controlling Microbial Growth in the Body: Antimicrobial Drugs

Key Terms and Definitions

Understanding antimicrobial drugs requires familiarity with several important terms:

• Selective toxicity: The ability of a drug to target and kill microbial cells without harming host cells.

• Synthetic drugs: Chemically manufactured drugs used to treat infections, as opposed to those derived from natural sources.

• Antibiotics: Naturally occurring substances produced by microorganisms (such as bacteria or fungi) that inhibit or kill other microbes.

• Broad Spectrum Antibiotic: An antibiotic effective against a wide range of microbial species, both Gram-positive and Gram-negative bacteria.

• Narrow Spectrum Antibiotic: An antibiotic effective against a limited group of microbes, often targeting specific families or genera.

Characteristics of an Ideal Antimicrobial Drug

Effective antimicrobial drugs possess several desirable properties:

• Selective toxicity to microbes, sparing host cells.

• No hypersensitivity reactions in the host (minimal allergic responses).

• Solubility to penetrate body tissues and reach infection sites.

• Low likelihood of resistance development by microbes.

• Inexpensive to produce and purchase.

• Stable without requiring refrigeration.

• Easy administration (oral, injection, topical).

• Pleasant taste for oral drugs, improving patient compliance.

Targets of Antimicrobial Drugs

Antimicrobial drugs act on specific targets within microbial cells. The evolutionary distance between prokaryotes and eukaryotes allows for selective toxicity:

• Cell wall: Drugs like penicillin and bacitracin inhibit cell wall synthesis, effective against bacteria (which have peptidoglycan cell walls).

• Ribosomes: Drugs such as streptomycin and tetracycline target bacterial 70S ribosomes, sparing eukaryotic 80S ribosomes.

• Nucleic acid synthesis: Rifampicin inhibits RNA synthesis in bacteria.

• Enzymes: Sulfa drugs block essential bacterial enzymes (e.g., folic acid synthesis).

• Plasma membrane: Polymyxin B and nystatin disrupt membrane integrity, leading to cell death.

Example: Penicillin targets bacterial cell wall synthesis, making it highly effective against Gram-positive bacteria but ineffective against organisms lacking cell walls, such as Mycoplasma or viruses.

Additional info: The diversity of antibiotic structures allows for many different mechanisms of action and sites of attack. Most antimicrobials are effective against prokaryotes due to their unique cellular features, while viruses are difficult to target because they rely on host cell machinery.

Comparative Table: Drug Targets in Microbial Groups

Microbial Group	Main Drug Targets	Effectiveness of Antimicrobials
Bacteria (Prokaryotes)	Cell wall, 70S ribosomes, nucleic acid synthesis, enzymes, plasma membrane	High
Fungi, Protozoa, Helminths (Eukaryotes)	80S ribosomes, unique cell wall components (fungi), metabolic pathways	Moderate
Viruses	Few unique targets; rely on host cell machinery	Low

Additional info: Eukaryotic pathogens (fungi, protozoa, helminths) share more similarities with human cells, making selective toxicity more challenging. Viruses are especially difficult to target because they use host cell processes for replication.

Drug Resistance in Microbes

Drug resistance is a major concern in antimicrobial therapy. Resistance arises in microbial populations, not in the host, through natural selection:

• Microbial populations contain genetic diversity; some individuals may carry resistance genes.

• In the absence of antibiotics, resistance genes may be disadvantageous (extra metabolic cost).

• When antibiotics are present, sensitive microbes die, and resistant ones survive and multiply.

• Resistance genes can spread via conjugation (gene transfer between bacteria), increasing resistance in both pathogens and normal flora.

• Disturbance of normal flora can lead to superinfection (overgrowth of resistant organisms or opportunistic pathogens).

Example: Overuse of antibiotics in hospitals can lead to the emergence of multidrug-resistant bacteria, such as Staphylococcus aureus (MRSA).

Preventing and Reducing Drug Resistance

While resistance cannot be completely prevented, it can be reduced by careful management:

• Use antibiotics only when necessary: Avoid prescribing antibiotics for viral infections (e.g., bronchitis).

• Complete the full course: Patients should take all prescribed antibiotics, even if symptoms improve, to ensure all pathogens are eliminated.

• Combination therapy: For infections requiring long-term treatment (e.g., tuberculosis, HIV), use multiple drugs simultaneously. This reduces the chance of resistance developing to all drugs at once.

Example: Tuberculosis treatment involves a combination of four antibiotics: Isoniazid (INH), Rifampin (RIF), Pyrazinamide (PZA), and Ethambutol (EMB) or Streptomycin (SM).

Summary Table: Strategies to Reduce Drug Resistance

Strategy	Explanation
Judicious antibiotic use	Prescribe only when necessary; avoid for viral infections
Complete prescribed course	Ensures elimination of all pathogens, prevents survival of resistant strains
Combination therapy	Multiple drugs reduce likelihood of resistance to all agents

Additional Academic Context

• Superinfection: A secondary infection caused by the overgrowth of resistant organisms after normal flora are disrupted by antibiotics.

• Natural selection: The process by which environmental pressures (such as antibiotics) favor the survival and reproduction of resistant microbes.

• Equation for natural selection:

$\text{Frequency of resistance alleles after selection} = \frac{\text{Number of resistant individuals surviving}}{\text{Total number of individuals surviving}}$

Additional info: Proper antibiotic stewardship is essential to limit the spread of resistance and preserve the effectiveness of existing drugs.