Antimicrobial Chemotherapy and Resistance – Vocabulary Review

Learning Outcomes

• Grasp interplay between host immune defenses and microorganisms

  • Understand disease effects on host; significance of vaccination & immunotherapy

• Master core laboratory techniques

  • Isolation, culture & identification of pathogens

  • Problem‐solving & data interpretation in diagnostic microbiology

• Recognise importance of normal microbiota

  • Diversity of organisms in health & disease

  • Choice of diagnostic procedure informed by commensal presence

• Explain chemotherapy principles & resistance

  • Molecular, cellular & host perspectives

• Describe key virulence determinants & pathogenesis of named diseases

Overview of Topics Covered

• Types of antimicrobial agents (antibacterial, antifungal, antiparasitic, antiviral)

• Historical discovery & development of antibiotics

• Key antibiotic properties/definitions

• Bacterial cell‐target sites

• Antibiotic classes & mechanisms of action

• Antimicrobial-resistance (AMR) mechanisms & societal impact

Principles of Antimicrobial Chemotherapy

• Differential toxicity = drug harms pathogen > host

• Agents:

  • Antibacterial, antifungal, antiparasitic, antiviral

• Terminology

  • Antibiotic = natural microbial product that inhibits/kills bacteria at low [conc]

  • Chemotherapeutic drug = natural, semi-synthetic or synthetic compound used in vivo

Discovery & Development Milestones

• 1909 Salvarsan (arsenical vs. Treponema)

• 1928 Fleming discovers penicillin (Penicillium notatum)

• 1942 Mass production during WWII → dramatic fall in sepsis mortality

• Subsequent “Golden Era”: streptomycin, chloramphenicol, tetracycline, etc.

• Current pipeline: Teixobactin (first-in-class, cell-wall inhibitor expected ≤ 5 yrs)

• Socio-economic gains: ↑life expectancy, safer surgery, cancer chemo, obstetrics

Natural Producers of Antibiotics

• Gram-positive rods: Bacillus subtilis ⇒ Bacitracin; B. polymyxa ⇒ Polymyxin

• Fungi: Penicillium notatum ⇒ Penicillins; Cephalosporium spp. ⇒ Cephalosporins

• Actinomycetes (≈⅔ of all antibiotics):

  • Streptomyces venezuelae ⇒ Chloramphenicol

  • S. griseus ⇒ Streptomycin

  • S. nodosus ⇒ Amphotericin B

  • Micromonospora purpurea ⇒ Gentamicin

Ideal Antibiotic – Desirable Attributes

• Selective toxicity (high therapeutic index)

• Bactericidal (preferred) or reliable bacteriostatic

• Narrow spectrum when pathogen known; broad if empirical

• Low propensity to induce resistance

• Non-allergenic; minimal adverse effects

• Adequate tissue distribution & half-life; oral & parenteral forms

• Synergises with host immunity & other drugs

• Economical & chemically stable

Fundamental Definitions

• Spectrum of activity:

  • Narrow: active vs. limited taxa (e.g. penicillin G, isoniazid)

  • Broad: active vs. Gram ± , atypicals (e.g. tetracycline)

• \text{MIC} – minimum inhibitory concentration (prevents visible growth)

• \text{MBC} – minimum bactericidal concentration (\ge99.9 % kill)

• Bacteriostatic vs. bactericidal

• Time-dependent killing (efficacy ∝ T>MIC; e.g. β-lactams, vancomycin)

• Concentration-dependent killing (efficacy ∝ C\text{max}/MIC; e.g. aminoglycosides, quinolones)

• Prophylaxis = pre-emptive use; Treatment = curative/empirical/targeted use

Bacterial Cell Architecture & Why It Matters

• Cell wall (peptidoglycan) – maintains shape, prevents osmotic lysis

  • Gram + : thick PG, teichoic acids, no outer membrane

  • Gram − : thin PG, outer membrane with lipopolysaccharide (LPS), periplasm

• Cytoplasmic (inner) membrane – selective permeability, energy generation

• Ribosomes 70S (30S + 50S) – protein synthesis machinery

• Nucleoid – circular dsDNA; plasmids confer resistance/virulence

Major Antibacterial Target Sites

1. Cell wall synthesis

2. Cell-membrane integrity

3. Protein synthesis (ribosome)

4. Nucleic-acid synthesis (DNA/RNA)

5. Essential metabolite (folate) pathways

Detailed Mechanisms & Representative Classes

1 Inhibition of Cell-Wall Synthesis (Time-dependent, bactericidal)

• β-Lactams: penicillins, cephalosporins, cephamycins, carbapenems, monobactams

  • Bind PBPs → block transpeptidation → weakened PG → lysis

• Glycopeptides: vancomycin, teicoplanin (Gram + only)

  • Bind D-Ala–D-Ala termini → prevent cross-linking

2 Disruption of Cell-Membrane Function

• Polypeptides/AMPs: Polymyxin B, colistin (Gram −); miconazole (antifungal)

  • Cationic drug binds LPS/ergosterol ⇒ pore formation ⇒ leakage & lysis

3 Inhibition of Nucleic-Acid Synthesis

• Quinolones/Fluoroquinolones (ciprofloxacin, levofloxacin)

  • Inhibit DNA gyrase (Gram −) & topoisomerase IV (Gram +)

  • Bactericidal, concentration-dependent

• Rifamycins (rifampicin, rifabutin)

  • Bind β-subunit of RNA polymerase ⇒ block transcription (key for TB, leprosy)

4 Inhibition of Protein Synthesis (Ribosomal)

• 50S binders: Macrolides (erythro-, azithro-), Lincosamides (clindamycin), Chloramphenicol, Streptogramins, Oxazolidinones

• 30S binders: Aminoglycosides (gentamicin), Tetracyclines

• Effects: misreading, blocked initiation, inhibited peptidyl-transferase, or tRNA docking

5 Antimetabolites (Folate Pathway)

• Sulfonamides (PABA analogues) + Trimethoprim

  • Sequential blockade in folic-acid synthesis ⇒ synergistic bactericidal effect

Consolidated “Cheat Sheet” (Drug → Major Target)

• Cell wall: β-lactams, vancomycin, bacitracin

• Cell membrane: Polymyxins, daptomycin (lipopeptide)

• DNA gyrase/topoisomerase: Fluoroquinolones

• RNA polymerase: Rifampin

• 30S: Aminoglycosides, tetracyclines

• 50S: Macrolides, clindamycin, chloramphenicol, linezolid

• Folate: Sulfonamides, trimethoprim

Antiviral Chemotherapy Basics

• Viruses are obligate intracellular → fewer unique targets

• Key replicative steps targeted:

  1. Attachment/entry (e.g. enfuvirtide – HIV; docosanol – HSV)

  2. Uncoating (amantadine – influenza)

  3. Nucleic-acid synthesis (NRTIs/NNRTIs – HIV, HBV; acyclovir – HSV)

  4. Integration (INSTIs – HIV)

  5. Protein processing (Protease inhibitors – HIV/HCV)

  6. Release (neuraminidase inhibitors – influenza)

• HAART = ≥3 drugs acting at ≥2 distinct steps ⇒ sustained ↓viral load & resistance barrier

Safety & Toxicity Issues (selected examples)

• Aminoglycosides ⇒ nephro- & ototoxicity

• Chloramphenicol ⇒ aplastic anaemia (rare but fatal)

• Vancomycin ⇒ “Red-man” syndrome (histamine-mediated)

• β-Lactams ⇒ hypersensitivity (from rash → anaphylaxis)

Antimicrobial Resistance (AMR)

• Definition: ability of microbe to withstand drug conc. that kills/inhibits wild-type

• 2015 UK review projected 10\,\text{million} AMR deaths/year by 2050 ( > cancer)

• WHO Priority Pathogens:

  • Priority 1 Critical: carbapenem-resistant Acinetobacter, Pseudomonas, ESBL Enterobacteriaceae

  • Priority 2 High: VRE, MRSA/VRSA, fluoroquinolone-resistant Campylobacter/Salmonella, N. gonorrhoeae

  • Priority 3 Medium: penicillin-non-susceptible S. pneumoniae, ampicillin-resistant H. influenzae, Shigella

Types of Resistance

1. Inherent (intrinsic)

   • Example: outer-membrane impermeability of Gram − vs. vancomycin

2. Acquired

   • Mutation (vertical) or horizontal gene transfer (plasmid, transposon, phage)

3. Multi-drug resistance (MDR) – e.g. P. aeruginosa (β-lactams, quinolones, chloramphenicol)

4. Cross-resistance – within same class (all β-lactams → shared β-lactamase sensitivity)

Molecular Mechanisms

• Enzymatic inactivation (β-lactamases, aminoglycoside-modifying enzymes)

• Target modification (PBP2a from mecA; ribosomal methylation by erm genes; DNA-gyrase mutations)

• Reduced permeability (loss of porins in Gram −)

• Active efflux pumps (e.g. tetA, AcrAB-TolC)

• Metabolic bypass (D-Ala→D-Lac in VRE; altered folate enzymes)

Case Studies

• β-Lactamases: penicillinase, ESBL, AmpC, carbapenemases (KPC, NDM)

• MRSA: mecA encodes PBP2a ⇒ ↓β-lactam affinity

• Vancomycin resistance: vanA operon substitutes D-Ala–D-Lac ⇒ 1000× ↓binding

Drivers of AMR Epidemic

• Human overuse: viral URTIs treated with antibiotics; paediatric high consumption (e.g. Swedish children 0-6 yrs average 13 days/year)

• Veterinary/agricultural use: growth promoters & mass prophylaxis → resistant Campylobacter, Salmonella, Enterococci transferred to humans

Global & UK Action Plans (2013-18)

1. Stewardship – responsible prescribing & diagnostics

2. Infection prevention/control (IPC) in humans & animals

3. Public awareness campaigns

4. Research funding (new targets, rapid diagnostics)

5. Incentivise new drugs & vaccines

6. Strengthen surveillance systems

7. International collaboration & data sharing

Research Frontiers & Alternative Therapies

• Epidemiology: mapping transmission in healthcare & community

• Diagnostics: point-of-care PCR, CRISPR-based assays, MALDI-TOF

• New targets: essential genes (omics-driven), anti-virulence drugs

• Alternatives (pros / cons):

  • Bacteriophage therapy (specific, self-replicating / regulatory hurdles)

  • Anti-quorum sensing molecules (↓virulence / may not kill)

  • Immunomodulators & monoclonal antibodies (expensive)

  • Probiotics & microbiota transplant (limited spectrum)

  • CRISPR–Cas antimicrobials (precision / delivery challenges)

  • Metal nanoparticles, antimicrobial peptides, photodynamic therapy

High-Yield Diagrams to Review (slide refs)

• Gram-stain mechanism (peptidoglycan vs. LPS retention)

• β-Lactam vs. glycopeptide binding sites

• Ribosomal binding map (macrolide, tetracycline, streptomycin, chloramphenicol)

• Quinolone inhibition of DNA gyrase/topo IV

• Resistance flowchart: inactivation, efflux, reduced entry, target change, bypass

Key Numerical / Statistical Nuggets

• \text{AMR deaths (2015)} \approx 700\,000 → projected 10^{7}\text{/year} by 2050

• WHO: cancer deaths 8.2\,\text{million} vs. AMR future 10\,\text{million}

• Childhood antibiotic exposure (Sweden) ≈ 13\text{ days}/\text{yr} per child

Ethical & Societal Implications

• “Post-antibiotic era”: routine surgery, chemotherapy, neonatal & transplant medicine jeopardised

• Equity: LMICs bear disproportionate burden; counterfeit/sub-therapeutic drugs exacerbate AMR

• Stewardship balances individual benefit vs. collective risk of resistance emergence

Exam Tips & Common Pitfalls

• Always pair drug class with unique MOA & representative drug

• Distinguish time- vs. concentration-dependent killing for dosing rationales

• Remember intrinsic vs. acquired resistance examples (Pseudomonas, MRSA)

• Link drug target to likely toxicity (e.g. membrane-acting drugs → nephrotoxicity)

• For antivirals, map each drug class to replication step (attachment, RT, protease, integrase)

Quick Self-Check Questions

• Why are β-lactams ineffective against Mycoplasma?

• Explain how efflux pumps contribute to tetracycline resistance.

• Calculate \text{MBC/MIC} ratio & infer bactericidal vs. static activity (\<4 ≈ cidal).

• Which antibiotic is safest for treating MRSA pneumonia & why?

• Outline two advantages & two limitations of bacteriophage therapy.

Closing Quote