Antibiotics and Antibiotic Resistance: Mechanisms, Challenges, and New Frontiers

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.

Features of Good Antimicrobial Drugs

Desirable Properties

• Selective toxicity: Target microbial structures or functions not found in the host.

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

• Low potential for resistance development.

• Minimal side effects and toxicity to the host.

• Good pharmacokinetics: Adequate absorption, distribution, metabolism, and excretion.

Consequences of Antibiotic Use

Impact on Microbiota and Resistance

• Normal microbiota suppress opportunistic pathogens.

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

• 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 and lower peaks.

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

Societal Factors in Antibiotic Use

Patient Pressure and Public Perception

• Patients often pressure physicians for antibiotics, even when not indicated.

• Misconceptions exist about antibiotics treating viral infections.

Perception	General Public 2010	General Public 2019	Parents (0-6 yrs) 2010	Parents (0-6 yrs) 2019
Only on bacteria	40%	50%	46%	51%
Both viruses and bacteria	14%	22%	30%	30%
Only on viruses	14%	11%	8%	7%
You don't know	17%	32%	25%	11%

Additional info: Data shows persistent misconceptions about antibiotic specificity.

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 groups (e.g., penicillin for Gram-positive bacteria).

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

Type	Examples
Bacteria	Penicillins, Cephalosporins, Quinolones
Fungi	Azoles, Polyenes, Echinocandins
Viruses	Reverse transcriptase inhibitors, Protease inhibitors, Fusion inhibitors

Potential Targets of Antimicrobials

Major Cellular Processes Targeted

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

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

• DNA replication (e.g., quinolones)

• RNA synthesis (e.g., rifampin, actinomycin)

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

• Cell membrane integrity (e.g., polymyxins, daptomycin)

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 beta-lactamase and acid sensitive.

$\text{Beta-lactam structure:}~\ce{C8H9NO4}$

Penicillin Type	Properties
Methicillin	Acid-stable, beta-lactamase-resistant
Oxacillin	Acid-stable, beta-lactamase-resistant
Ampicillin	Broad spectrum, acid-stable, beta-lactamase-sensitive
Carbenicillin	Broad spectrum, acid-labile, beta-lactamase-sensitive

Cephalosporins

• Similar mechanism to beta-lactams but with increased resistance to beta-lactamases.

• Example: Ceftriaxone.

Non-beta-lactam Cell Wall Inhibitors

• Include vancomycin, bacitracin, daptomycin.

• Inhibit different steps in peptidoglycan synthesis or cell membrane function.

Growth Factor Analogs

Mechanism and Examples

• Structurally similar to essential growth factors but disrupt metabolism when incorporated.

• Sulfa drugs mimic para-aminobenzoic acid (PABA), inhibiting folic acid synthesis.

• Trimethoprim inhibits a later step in folic acid synthesis.

Resistance: Some bacteria bypass inhibition by importing folic acid from the environment.

Combination therapy: Sulfa drugs + trimethoprim for synergistic effect.

Antibiotics Targeting Protein Synthesis

Major Classes and Mechanisms

• Bind to bacterial ribosomal subunits (30S or 50S), inhibiting translation.

• Chloramphenicol, macrolides, lincosamides: Bind 50S subunit, inhibit peptide bond formation.

• Aminoglycosides: Bind 30S subunit, cause misreading of mRNA.

• Tetracyclines: Bind 30S subunit, block tRNA attachment.

Antibiotics Targeting DNA Gyrase

Quinolones

• Inhibit DNA gyrase, preventing DNA replication and transcription.

• Examples: Ciprofloxacin, moxifloxacin.

$\text{DNA gyrase inhibition leads to double-stranded DNA breaks and cell death.}$

Antibiotics Targeting Nucleic Acid Synthesis

RNA Synthesis Inhibitors

• Rifampin: Binds bacterial RNA polymerase, blocks transcription initiation.

• Actinomycin: Binds DNA, prevents 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: Expel antibiotics from the cell.

• Blocked penetration: Altered porins prevent drug entry.

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

• Target modification: Alteration of antibiotic targets (e.g., ribosomal mutations).

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.

Target	Example Drug/Class
Fusion/Entry	Fusion inhibitors
DNA/RNA synthesis	Nucleoside analogs, reverse transcriptase inhibitors
Protease activity	Protease inhibitors
Viral enzymes	Neuraminidase inhibitors (e.g., Tamiflu)

Example: Tamiflu inhibits neuraminidase, blocking influenza virus release from host cells.

Antifungal Drugs

Challenges in Treatment

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

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

• Immunocompromised individuals are at higher risk for fungal infections.

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

Staphylococcus aureus and Antibiotic Resistance

Virulence and Resistance Trends

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

• Has developed resistance to multiple antibiotics, including penicillin (MRSA), methicillin (MRSA), and vancomycin (VRSA).

• Virulence factors include toxins, enzymes, and immune evasion mechanisms.

Graph: Shows increasing burden of resistance over time, with emergence of MRSA and VRSA.

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 for new antimicrobials have declined in recent decades.

Years	New Antimicrobials Approved
1983-1987	16
1988-1992	14
1993-1997	8
1998-2002	7
2003-2007	4
2008-2012	2

Strategies for New Antimicrobial Discovery

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

• Screening for antibiotic activity in uncultured bacteria and new environments (e.g., platensimycin).

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