Antibiotics: Introduction and Historical Context

Definition and Importance

Antibiotics are chemical compounds produced by one species of microbe that can inhibit the growth or kill other microbes. Their discovery revolutionized the treatment of infectious diseases, drastically reducing mortality from minor infections since the 1940s.

• Antibiotics: Substances that target bacteria, not viruses.

• First recognized for medical use in the early 1940s.

Discovery of Antibiotics

The usefulness of molds was known to ancient civilizations, but modern antibiotic therapy began with the discovery of penicillin.

• Alexander Fleming discovered penicillin in 1929, but it was initially forgotten due to instability.

• Howard Florey rediscovered penicillin in 1940, leading to its widespread use.

• Gerhard Domagk discovered sulfa drugs, which are metabolized in the body to active forms (e.g., sulfanilamide).

Mechanisms of Antibiotic Action

Targets of Antibiotics

Antibiotics act by targeting specific microbial structures or processes not found in humans, ensuring selective toxicity.

• Cell wall synthesis (peptidoglycan): Penicillins, cephalosporins, bacitracin, vancomycin

• Protein synthesis: Chloramphenicol, tetracyclines, aminoglycosides, macrolides, lincosamides

• Cell membrane: Polymyxin, amphotericin B, imidazoles (antifungal)

• DNA synthesis: Quinolones (e.g., nalidixic acid)

• RNA synthesis: Rifampicin, actinomycin D

• Intermediary metabolism: Sulfonamides, trimethoprim

Selective Toxicity

Antibiotics must affect the infectious organism but not the patient. Selective toxicity is achieved by targeting microbial physiology absent in humans.

• Examples: Peptidoglycan (cell wall), differences in ribosome structure, unique metabolic pathways.

• Side effects may occur at high concentrations (e.g., chloramphenicol can harm bone marrow).

Spectrum of Activity

Broad vs. Narrow Spectrum

Antibiotics vary in their range of effectiveness.

• Broad-spectrum: Effective against many species.

• Narrow-spectrum: Effective against few or a single species.

• Most antibiotics are natural products, some are chemically modified or fully synthetic.

Types of Antibiotic Compounds

Bactericidal vs. Bacteriostatic

• Bactericidal: Kill the target organism.

• Bacteriostatic: Prevent growth, allowing the immune system to clear the infection.

• Many drugs only affect growing cells (e.g., cell wall synthesis inhibitors like penicillin).

Measuring Antibiotic Effectiveness

Minimal Inhibitory Concentration (MIC)

The MIC is the lowest concentration of an antibiotic that prevents visible growth of a microorganism.

• Determined by serial dilution and observation of growth.

• MIC may allow survival of non-growing cells.

Minimal Lethal Concentration (MLC)

The MLC is the lowest concentration of an antibiotic that kills the organism (no colonies form when plated).

Kirby-Bauer Disk Diffusion Test

This test assesses bacterial sensitivity to multiple antibiotics using disks on an agar plate.

• Cleared zones around disks indicate sensitivity (zone of inhibition).

• Diameter of the zone correlates with MIC values.

Drug Levels in Tissue

For effective treatment, drug concentration in tissue or blood must remain above the MIC.

• Achieved by multiple doses or higher doses.

Mechanisms of Antibiotic Resistance

Overview

Antibiotic resistance is a growing problem due to overuse and misuse of antibiotics, leading to multidrug-resistant strains.

• Over 80% of Streptococcus pneumoniae infections are penicillin-resistant in some countries.

Methods of Resistance

• Destroy the antibiotic: e.g., β-lactamase enzymes degrade penicillins.

• Add modifying groups: Enzymes modify antibiotics (e.g., aminoglycosides) via acetylation, phosphorylation, or adenylation.

• Modify the target: Alteration of target enzymes or ribosomes prevents antibiotic binding.

• Pump the antibiotic out: Efflux pumps remove antibiotics from the cell, often conferring multidrug resistance.

Beta-lactam Resistance

• β-lactamases cleave the β-lactam ring, inactivating penicillins and related drugs.

• Resistance genes may be carried on plasmids and transferred between bacteria.

Summary Table: Mechanisms of Action of Antimicrobial Agents

Peptidoglycan Structure and Antibiotic Targeting

Gram-Positive vs. Gram-Negative Bacteria

• Gram-negative: Direct cross-linking, thin peptidoglycan layer.

• Gram-positive: Peptide interbridge, thick peptidoglycan layer.

Cell Wall Synthesis Inhibitors

• Penicillins: Inhibit transpeptidation (crosslinking) via β-lactam ring.

• Vancomycin: Binds peptide ends, prevents crosslink formation.

• Cycloserine: Blocks formation of peptide for crosslink.

• Bacitracin: Blocks movement of disaccharide subunits across membrane.

Other Mechanisms of Action

DNA and RNA Synthesis Inhibitors

• Quinolones: Block DNA gyrase, prevent DNA replication.

• Rifampicin: Blocks bacterial RNA polymerase.

• Actinomycin D: Binds DNA, inhibits polymerase movement.

Protein Synthesis Inhibitors

• Macrolides, chloramphenicols, lincosamides: Bind large ribosomal subunit, block peptide transfer.

• Aminoglycosides: Prevent 30S and 50S subunits from binding.

• Tetracyclines: Bind small subunit, block aminoacyl-tRNA binding.

Disruption of Cytoplasmic Membranes

• Gramicidin: Forms cation channels, causing H+ leakage and cell death.

• Polyenes, azoles, allyamines, polymyxin: Incorporate into and damage cell membranes.

Antibiotic Resistance Mechanisms: Details

Destroying the Antibiotic

• β-lactamase enzymes degrade β-lactam antibiotics, conferring resistance.

Modifying the Antibiotic

• Enzymatic modification (acetylation, phosphorylation, adenylation) of aminoglycosides prevents their action on ribosomes.

Altering the Target

• Mutations in target enzymes or ribosomal proteins prevent antibiotic binding.

Drug Efflux

• Multidrug resistance (MDR) transporters pump antibiotics out of the cell, conferring resistance to multiple drug classes.

Summary

Antibiotics are essential tools in clinical microbiology, targeting specific microbial processes to treat infections. However, the rise of antibiotic resistance due to misuse and overuse poses a significant challenge, necessitating careful use and ongoing research into new therapies.