Study Guide: Antimicrobial Drugs and Drug Resistance ~ Chp 20

Antimicrobial Drugs

I. Key Terms and Concepts

Antimicrobial drugs are agents used to treat infections by inhibiting or killing microorganisms. Understanding their types and properties is essential for effective clinical use.

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

• Synthetic drugs: Chemically synthesized compounds used to treat infections (not derived from natural sources).

• Antibiotics: Substances produced by microorganisms (or synthetically) that inhibit or kill other microbes.

• Broad Spectrum Antibiotic: Effective against a wide range of microbial species (both Gram-positive and Gram-negative bacteria).

• Narrow Spectrum Antibiotic: Effective against a specific group or species of microorganisms.

II. What Makes a Good Antimicrobial Drug?

Ideal antimicrobial drugs possess several important characteristics to ensure efficacy and safety.

• Selective toxicity: Targets the pathogen without damaging the host.

• No hypersensitivity in host: Should not provoke allergic reactions.

• Soluble: Must be able to penetrate body tissues and reach the site of infection.

• Low potential for resistance: Microbes should not easily develop resistance to the drug.

• Inexpensive: Affordable for widespread use.

• Stable: Does not require refrigeration; maintains potency during storage.

• Easily administered: Convenient routes of administration (oral, injection, etc.).

• Palatable: Tastes acceptable to patients (especially important for oral medications).

III. Targets of Antimicrobial Drugs

Antimicrobial drugs act on specific targets within microbial cells. The main targets in bacteria are:

Target	Example Drugs
Cell wall synthesis	Penicillin, Bacitracin
Ribosome (protein synthesis)	Streptomycin, Tetracycline
Nucleic acid synthesis	Rifampicin
Enzymes (metabolic pathways)	Sulfa drugs

Note: Eukaryotic pathogens (e.g., fungi, protozoa, helminths) have 80S ribosomes, while prokaryotes have 70S ribosomes. This difference is exploited by some antibiotics to achieve selective toxicity.

Viruses: Viruses are difficult to target because they use the host's metabolic machinery. Antiviral drugs often target unique viral enzymes or processes.

IV. Drug Resistance

Drug resistance is the ability of microbes to withstand the effects of antimicrobial agents. It is a major concern in clinical microbiology.

• Mechanism: Resistance arises in microbes, not in the host. It occurs by natural selection within large populations. When antibiotics are used, sensitive microbes are killed, but resistant ones survive and multiply.

• Genetic basis: Resistance genes may be present in a small fraction of the population. These genes can be transferred between microbes, especially via microbial conjugation (horizontal gene transfer).

• Selection pressure: The use of antibiotics creates a selective environment favoring resistant strains.

Dangers in Use of Antibiotics:

• Encourages resistance in pathogens and normal flora.

• Resistance genes can be transferred to other bacteria, including pathogens.

• Disturbance of normal flora can lead to superinfection (overgrowth of non-susceptible organisms).

Example: A study found that more than 60% of antibiotics prescribed in a hospital were either given in the wrong dosage or for conditions where they were not effective, contributing to resistance.

V. Preventing Development of Drug Resistance

While it may not be possible to completely prevent resistance, several strategies can reduce its development:

• Limit use of antibiotics: Only prescribe when necessary and for appropriate indications.

• Complete the full course: Patients should use all prescribed antibiotics for the full duration to ensure all pathogens are eliminated.

• Combination therapy: For certain infections (e.g., tuberculosis, HIV), use multiple drugs simultaneously to reduce the likelihood of resistance developing to all agents.

Example: In tuberculosis (TB) treatment, a combination of four drugs is used: Isoniazid (INH), Rifampin (RIF), Pyrazinamide (PZA), and Ethambutol (EMB) or Streptomycin (SM). The probability of a microbe developing resistance to all four drugs at once is extremely low.

VI. Additional Academic Context

• Equation for Selection Pressure: The rate at which resistance develops can be modeled as: $\text{Rate of resistance} = k \times [\text{Antibiotic use}] \times [\text{Microbial population}]$ where $k$ is a proportionality constant reflecting mutation and gene transfer rates.

• Superinfection: Occurs when the normal microbiota is disturbed, allowing overgrowth of resistant organisms or opportunistic pathogens.

• Horizontal Gene Transfer: Includes transformation, transduction, and conjugation, all of which can spread resistance genes among bacteria.

Summary Table: Broad vs. Narrow Spectrum Antibiotics

Type	Definition	Example
Broad Spectrum	Effective against a wide variety of bacteria	Tetracycline
Narrow Spectrum	Effective against specific bacteria	Penicillin G

Additional info: The above notes include inferred academic context and examples to ensure completeness and clarity for exam preparation.