Week 5a: Antimicrobial Drugs – Mechanisms, Spectrum, and Resistance

Antimicrobial Drugs

Introduction to Antimicrobial Drugs

Antimicrobial drugs are essential tools in medical microbiology for treating infections caused by bacteria, fungi, viruses, protozoa, and helminths. Their effectiveness depends on their ability to target microbial processes without harming the host.

• Chemotherapy: The use of drugs to treat a disease.

• Antimicrobial drugs: Agents that interfere with the growth of microbes within a host.

• Antibiotic: A substance produced by a microbe that, in small amounts, inhibits another microbe.

• Selective toxicity: A drug that kills harmful microbes without damaging the host.

The Spectrum of Antimicrobial Activity

Definitions and Concepts

The spectrum of activity describes the range of microorganisms an antimicrobial drug can affect. Drugs may be narrow-spectrum (targeting specific groups) or broad-spectrum (affecting a wide range).

• Narrow spectrum: Effective against specific types, e.g., Penicillin G for gram-positive bacteria.

• Broad spectrum: Effective against both gram-positive and gram-negative bacteria.

• Selective toxicity for gram-negative bacteria: Depends on the lipopolysaccharide layer and porin channels; large or lipophilic drugs cannot enter easily.

Advantages and Disadvantages:

• Broad-spectrum drugs are efficient but can destroy normal microbiota, leading to superinfection (overgrowth of opportunistic organisms).

The Action of Antimicrobial Drugs

Types of Drug Actions

Antimicrobial drugs can act in different ways to control microbial growth:

• Bactericidal: Kill microbes directly.

• Bacteriostatic: Prevent microbes from growing.

• Bacteriostasis: Relies on host defenses (e.g., phagocytosis, immunoglobulin production) to eliminate microbes.

Examples:

• Bacteriostatic: Chloramphenicol, Erythromycin, Clindamycin, Sulfonamides, Trimethoprim, Tetracyclines

• Bactericidal: Aminoglycosides, Beta-lactams, Vancomycin, Quinolones, Rifampin, Metronidazole

Overview of Antimicrobial Drug Classes

Antibacterials

• Inhibitors of cell wall synthesis: Penicillins, Carbapenems, Monobactams, Cephalosporins, Polypeptide antibiotics (Bacitracin, Vancomycin, Telavancin)

• Inhibitors of protein synthesis: Chloramphenicol, Aminoglycosides, Tetracyclines, Macrolides, Streptogramins, Oxazolidinones, Pleuromutilins

• Injuring plasma membrane: Lipopeptides (Daptomycin), Polymyxin B, Bacitracin

• Inhibitors of nucleic acid synthesis: Rifamycins, Quinolones, Fluoroquinolones

• Inhibitors of essential metabolites: Sulfonamides

Antifungals

• Affecting fungal sterols: Polyenes (Amphotericin B), Azoles (Imidazoles, Triazoles), Allylamines

• Affecting fungal cell walls: Beta-glucan inhibitors (Echinocandins, Caspofungin)

• Inhibitors of nucleic acids: Flucytosine

Antivirals

• Entry and fusion inhibitors (Maraviroc, Enfuvirtide)

• Uncoating, genome integration, and nucleic acid synthesis inhibitors (Amantadine, Raltegravir, Acyclovir)

• Interference with assembly and release of viral particles (Protease inhibitors: Saquinavir, Boceprevir, Telaprevir)

• Exit inhibitors (Zanamivir, Oseltamivir)

• Interferons (Alpha interferon, Imiquimod)

• HIV/AIDS: Nucleoside analogs (Zidovudine, Tenofovir), combination drugs (Atripla), non-nucleoside inhibitors (Nevirapine)

Anti-protozoan Drugs

• Quinine and Chloroquine (Malaria)

• Quinacrine (Giardiasis)

• Metronidazole (Trichomonas vaginalis, dysentery)

• Trinidazole, Nitazoxanide

Antihelminth Drugs

• Niclosamide (Tapeworm infections)

• Praziquantel (Tapeworms, alters plasma membrane permeability)

• Mebendazole and Albendazole (Intestinal helminthic infections)

• Ivermectin (Nematodes, mites, ticks, insects)

Mechanisms of Antimicrobial Drug Action

1. Inhibitors of Cell Wall Synthesis

These drugs prevent the synthesis of peptidoglycan, a key component of bacterial cell walls, leading to cell lysis. They are selectively toxic because human cells lack peptidoglycan.

• Penicillins: Contain a beta-lactam ring; prevent cross-linking of peptidoglycans.

• Carbapenems: Broad spectrum; e.g., imipenem + cilastatin (Primaxin).

• Monobactams: Synthetic; effective against gram-negative bacteria.

• Cephalosporins: Beta-lactam ring differs slightly; resistance is an issue.

• Polypeptide antibiotics: Bacitracin (topical), Vancomycin (MRSA), Telavancin (alternative to vancomycin).

• Antimycobacterial drugs: Isoniazid and ethambutol inhibit mycolic acid synthesis in Mycobacterium.

Penicillin Types Table

Type	Main Features
Natural	Cultures of Penicillium; Penicillin G (narrow spectrum); susceptible to penicillinases
Semisynthetic	Overcome disadvantages of natural product; part produced by mold, part chemically added
Penicillinase-resistant	Resistant to beta-lactamase; e.g., Methicillin (MRSA)
Extended spectrum	Gram-negative and Gram-positive; not resistant to penicillinases
Plus Beta-lactamase inhibitors	Combine with clavulanic acid; e.g., Augmentin

2. Inhibitors of Protein Synthesis

These drugs target bacterial ribosomes (70S), which differ from eukaryotic ribosomes (80S), allowing for selective toxicity. However, mitochondria also have 70S ribosomes, which can lead to side effects.

• Chloramphenicol: Binds 50S subunit; inhibits peptide bond formation; can cause bone marrow suppression.

• Aminoglycosides: Change shape of 30S subunit; cause misreading of mRNA; side effects include hearing loss and kidney damage.

• Tetracyclines: Interfere with tRNA attachment; broad spectrum; can suppress normal microbiota.

• Macrolides: Erythromycin (cannot penetrate gram-negative), azithromycin, clarithromycin.

• Streptogramins: Bind to 50S and unique ribosomal sites; expensive, many adverse effects.

• Oxazolidinones: Linezolid (MRSA); fully synthetic; binds 50S and interferes with 30S.

• Pleuromutilins: Topical use; origins in mushrooms.

3. Injuring the Plasma Membrane

Drugs in this class disrupt the integrity of the plasma membrane, causing leakage of cell contents and cell death.

• Lipopeptides: Daptomycin (gram-positive skin infections).

• Polymyxin B: Topical use for gram-negative infections.

• Bacitracin: Topical use, often combined with other antibiotics.

• Antifungal drugs: Amphotericin B, miconazole, ketoconazole (combine with sterols in fungal membranes).

4. Inhibitors of Nucleic Acid Synthesis

These drugs interfere with DNA replication and transcription, but their usefulness is limited by potential effects on mammalian DNA/RNA.

• Rifamycins: Inhibit mRNA synthesis; used in TB and leprosy; side effect: orange-red body fluids.

• Quinolones/Fluoroquinolones: Inhibit DNA gyrase; broad spectrum; e.g., ciprofloxacin, norfloxacin.

5. Inhibitors of Essential Metabolites

These drugs act as competitive inhibitors of enzymes, blocking metabolic pathways essential for microbial survival.

• Sulfonamides: Resemble PABA; block folic acid synthesis; bacteriostatic; used in combination with trimethoprim for synergistic effect.

Metabolic Pathway Inhibition Table

Drug	Target	Effect
Sulfonamides	PABA analog	Blocks folic acid synthesis
Trimethoprim	Dihydrofolic acid	Blocks further step in folic acid pathway

Antifungal Drugs

Agents Affecting Fungal Sterols

• Polyenes: Amphotericin B (toxic to kidneys; liposome formulation reduces toxicity)

• Azoles: Imidazoles (clotrimazole, miconazole), triazoles (fluconazole, itraconazole, voriconazole, posaconazole)

• Allylamines: Terbinafine, naftifine (combat resistance to azoles)

Agents Affecting Fungal Cell Walls

• Beta-glucan inhibitors: Echinocandins, caspofungin (target Aspergillus and Candida spp.)

Agents Inhibiting Nucleic Acids

• Flucytosine: Analog of cytosine; interferes with RNA and protein synthesis; selective toxicity due to fungal-specific conversion.

Other Antifungals

• Griseofulvin: Oral; binds keratin; blocks microtubule assembly.

• Tolnaftate: Topical; alternative to miconazole.

• Pentamidine: Binds DNA; used for Pneumocystis pneumonia and protozoan diseases.

Antiviral Drugs

Mechanisms of Action

• Entry and fusion inhibitors: Block initial steps of infection (e.g., Maraviroc, Enfuvirtide).

• Uncoating, genome integration, and nucleic acid synthesis inhibitors: Amantadine, Raltegravir, Acyclovir.

• Interference with assembly and release: Protease inhibitors (Saquinavir, Boceprevir, Telaprevir).

• Exit inhibitors: Zanamivir, Oseltamivir (block neuraminidase).

• Interferons: Alpha interferon, imiquimod (stimulate host defenses).

• HIV/AIDS: Nucleoside analogs (Zidovudine, Tenofovir), combination drugs (Atripla), non-nucleoside inhibitors (Nevirapine).

Anti-protozoan and Antihelminth Drugs

Anti-protozoan Drugs

• Quinine and Chloroquine: Malaria; act on Plasmodium stages.

• Quinacrine: Giardiasis; diiodohydroxyquin for intestinal amebic disease.

• Metronidazole: Parasitic protozoa and anaerobic bacteria.

Antihelminth Drugs

• Niclosamide: Tapeworm infections; inhibits ATP production.

• Praziquantel: Tapeworms; alters plasma membrane permeability.

• Mebendazole and Albendazole: Intestinal helminthic infections; inhibit microtubule formation.

• Ivermectin: Nematodes, mites, ticks, insects; paralysis and death of helminths.

Tests to Guide Chemotherapy

Diffusion Methods

Used to determine microbial susceptibility to drugs. Includes the Kirby-Bauer disk diffusion test and E test for minimum inhibitory concentration (MIC).

Broth Dilution Tests

• MIC (Minimum Inhibitory Concentration): Lowest concentration of drug that inhibits visible growth.

• MBC (Minimum Bactericidal Concentration): Lowest concentration that kills bacteria.

• Test bacteria are inoculated into broth with decreasing drug concentrations; wells without growth are cultured in fresh broth to determine bactericidal activity.

Broth Dilution Test Table

Drug	Growth Pattern
Doxycycline	Growth in all wells (resistant)
Sulfamethoxazole	Trailing end point (80% reduction in growth)
Streptomycin	No growth in any well (sensitive)
Ethambutol	Growth in fourth wells (equally sensitive to ethambutol and kanamycin)
Kanamycin	Growth in fourth wells

Antibiotic Resistance

Overview

Antibiotic resistance arises when microbes evolve mechanisms to withstand the effects of drugs. This is a major public health concern.

• Exposure to new antibiotics increases susceptibility of microbes.

• Surviving populations may become persister cells.

• Superbugs: Bacteria resistant to many antibiotics.

• Resistance genes are often on plasmids or transposons, facilitating transfer between bacteria.

Mechanisms of Antibiotic Resistance

• Enzymatic destruction of drug: Beta-lactamases degrade beta-lactam antibiotics (e.g., penicillins, cephalosporins).

• Prevention of penetration: Gram-negative bacteria restrict drug entry via cell wall and porins.

• Alteration of drug target site: Modifications in ribosomal proteins or penicillin-binding proteins (PBPs) prevent drug binding.

• Rapid ejection of drug: Efflux pumps expel antibiotics from bacterial cells.

Mechanisms Table

Mechanism	Example
Enzymatic destruction	Beta-lactamase in MRSA
Prevention of penetration	Porin modification in gram-negative bacteria
Alteration of target site	Modified PBPs in MRSA
Rapid ejection	Efflux pumps in gram-negative bacteria

Effects of Drug Combinations

Synergism and Antagonism

• Synergism: The effect of two drugs together is greater than the effect of either alone.

• Antagonism: The effect of two drugs together is less than the effect of either alone.

Summary Table: Major Classes of Antimicrobial Drugs

Class	Mechanism	Examples
Cell wall synthesis inhibitors	Block peptidoglycan synthesis	Penicillins, Cephalosporins, Carbapenems
Protein synthesis inhibitors	Target ribosomes	Tetracyclines, Aminoglycosides, Macrolides
Plasma membrane disruptors	Increase permeability	Polymyxin B, Daptomycin
Nucleic acid synthesis inhibitors	Block DNA/RNA synthesis	Rifampin, Quinolones
Metabolic pathway inhibitors	Block folic acid synthesis	Sulfonamides, Trimethoprim

Key Equations and Concepts

• MIC and MBC:

$\text{MIC} = \text{Minimum Inhibitory Concentration}$ $\text{MBC} = \text{Minimum Bactericidal Concentration}$

• Competitive inhibition: Antimetabolite competes with substrate for enzyme active site.

$\text{Rate} = k[\text{Enzyme}][\text{Substrate}]$

Additional info:

• Some slides reference figures and diagrams from standard microbiology textbooks (e.g., mechanisms of penicillinase, structure of acyclovir), which are not fully reproduced here but are described in context.

• Self-study sections (e.g., Diffusion Methods) should be supplemented by textbook reading for detailed protocols.