Antibiotics and Antibiotic Resistance

Learning Outcomes

• Understand the mechanisms of action of antibiotics.

• Determine appropriate antibiotic use based on infection and microbial characteristics.

• Explain resistance mechanisms and their relationship to specific antibiotics.

• Discuss challenges in developing antiviral, antifungal, and new antibiotic drugs.

• Describe virulence factors of Staphylococcus aureus and factors influencing its pathogenicity.

Introduction to Antimicrobials

Definition and Importance

• Antimicrobials are agents that kill or inhibit the growth of microorganisms, including bacteria, viruses, fungi, and parasites.

• They are essential for treating infectious diseases and reducing morbidity and mortality.

• Antibiotics are a subset of antimicrobials that specifically target bacteria.

Why We Need Antimicrobials

• To control and treat infections that would otherwise be life-threatening.

• To prevent the spread of infectious diseases in communities and healthcare settings.

General Features of a Good Antimicrobial Drug

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

• Broad or narrow spectrum: Effective against a wide range or specific group of microbes.

• Low potential for resistance development.

• Good pharmacokinetics: Reaches effective concentrations at the site of infection.

• Minimal side effects and toxicity.

Consequences of Antibiotic Use

Impact on Microbiota and Resistance

• Broad-spectrum antibiotics can disrupt normal microbiota, leading to superinfections by drug-resistant pathogens.

• Overuse and misuse of antibiotics accelerate the selection of resistant strains.

Example: Clostridioides difficile infection following broad-spectrum antibiotic therapy.

Antibiotic Administration

Routes and Pharmacokinetics

• Common routes: oral, intramuscular (IM), intravenous (IV).

• IV administration achieves rapid and high plasma concentrations; oral and IM routes have slower absorption.

Key Concept: The route of administration affects the onset, intensity, and duration of drug action.

Social Aspects of Antibiotic Use

Patient Pressure and Public Perception

• Patients often pressure healthcare providers for antibiotics, even when not medically necessary.

• Misconceptions exist about antibiotics treating viral infections.

Spectrum of Antimicrobials

Classification by Target Organism

• Broad-spectrum antibiotics: Active against a wide range of bacteria (e.g., tetracyclines).

• Narrow-spectrum antibiotics: Target specific bacteria (e.g., penicillin for Gram-positive bacteria).

• Antifungals, antivirals, and antiparasitics target eukaryotic pathogens and viruses.

Potential Targets of Antimicrobials

• Cell wall synthesis (e.g., beta-lactams, vancomycin)

• Protein synthesis (e.g., tetracyclines, macrolides)

• DNA gyrase (e.g., quinolones)

• RNA polymerase (e.g., rifampin)

• Metabolic pathways (e.g., sulfa drugs)

• Cell membrane structure and function

Cell Wall Synthesis Inhibitors

Beta-lactams

• Include penicillins and cephalosporins.

• Inhibit transpeptidation, preventing cross-linking of peptidoglycan in bacterial cell walls.

• Structure: Beta-lactam ring essential for activity.

Example: Penicillin G is effective against Gram-positive bacteria but is acid-sensitive and susceptible to beta-lactamases.

Cephalosporins

• Similar mechanism to penicillins but more resistant to beta-lactamases.

• Used for patients allergic to penicillins or for resistant infections.

Non-beta-lactam Inhibitors

• Examples: Vancomycin, bacitracin, daptomycin.

• Inhibit cell wall synthesis by different mechanisms (e.g., blocking transglycosylation or lipid carrier recycling).

Growth Factor Analogs

• Structurally similar to essential growth factors but disrupt metabolic pathways.

• Example: Sulfa drugs mimic para-aminobenzoic acid (PABA) and inhibit folic acid synthesis.

Resistance Mechanism: Some bacteria bypass inhibition by taking up folic acid from the environment.

Combination Therapy: Sulfa drugs + trimethoprim target sequential steps in folic acid synthesis for synergistic effect.

Antibiotics Targeting Protein Synthesis

• Bind to bacterial ribosomal subunits (30S or 50S) and inhibit translation.

• Major classes: Chloramphenicol, macrolides, lincosamides (50S); aminoglycosides, tetracyclines (30S).

Example: Tetracyclines block tRNA binding to the ribosome, inhibiting protein synthesis.

Antibiotics Targeting DNA Gyrase

• Quinolones (e.g., ciprofloxacin, moxifloxacin) inhibit DNA gyrase, preventing DNA replication and transcription.

Antibiotics Targeting Nucleic Acid Synthesis

• Rifampin binds bacterial RNA polymerase, blocking transcription initiation.

• Actinomycin binds DNA, interfering with RNA polymerase elongation.

Note: Bacterial and eukaryotic RNA polymerases are structurally similar, but eukaryotic enzymes are more complex.

Antibiotic Resistance

Key Concepts

• Resistance is pre-existing in microbial populations and is selected for by antibiotic use.

• Most resistance genes are carried on plasmids or other mobile genetic elements, facilitating horizontal gene transfer.

Mechanisms of Resistance

• Efflux pumps: Actively expel antibiotics from the cell.

• Blocked penetration: Altered porins prevent antibiotic entry.

• Inactivation of enzymes: Enzymes degrade or modify antibiotics (e.g., beta-lactamases).

• Target modification: Alteration of antibiotic targets reduces binding (e.g., altered ribosomal proteins).

Additional info: Other mechanisms include target overproduction, target mimicry, and enzymatic bypass.

Antiviral Drugs

Challenges and Targets

• Viruses use host cell machinery, making selective toxicity difficult.

• Targets include viral entry, nucleic acid synthesis, protease activity, and specific viral enzymes.

Antifungal Drugs

Challenges in Treatment

• Fungi are eukaryotes, sharing many cellular features with humans, making selective toxicity difficult.

• Antifungal drugs often target ergosterol synthesis or cell wall components unique to fungi.

Example: Azoles inhibit ergosterol synthesis; echinocandins inhibit cell wall glucan synthesis.

Staphylococcus aureus and Antibiotic Resistance

• S. aureus is a common cause of hospital- and community-acquired infections.

• Development of resistance (e.g., MRSA, VISA, VRSA) has followed the introduction of new antibiotics.

Virulence Factors: Include protein A, toxins, and enzymes that help evade the immune system.

Discovery and Development of New Antimicrobials

Current Challenges

• Only about 1% of isolated antibiotics are clinically useful due to toxicity, poor pharmacokinetics, or rapid resistance development.

• FDA approvals of new antimicrobials have declined in recent decades.

Strategies for New Drug Discovery

• Computer modeling and structural biology (e.g., saquinavir for HIV).

• Screening environmental microbes for novel compounds (e.g., platensimycin).

• Combination therapy to enhance efficacy and reduce resistance (e.g., beta-lactam + beta-lactamase inhibitor, ART for HIV).