Antimicrobial Chemotherapy & Resistance – Lecture Review

Learning Outcomes

• Describe interplay between immune defences & microorganisms, host-pathogen interactions, immunisation & immunotherapy.
• Perform laboratory isolation, diagnostic & interpretive procedures for pathogens.
• Recognise commensal flora’s role in health, micro-diversity in disease & choose appropriate diagnostics.
• Discuss antimicrobial chemotherapy principles & resistance development at molecular, cellular & host levels.
• Explain common pathogenic determinants/processes & the pathogenesis of specific named diseases.

Areas Covered in Lecture

• Types of antimicrobial agents.
• Discovery & development of antibiotics.
• Core antibiotic properties & definitions.
• Bacterial cellular target sites.
• Major antibiotic classes & their modes of action.
• Mechanisms leading to antimicrobial resistance.

Overview of Antimicrobial Chemotherapy

• Definition – use of drugs to combat infectious agents: antibacterial, antifungal, antiparasitic, antiviral.
• Differential toxicity – agent should be more toxic to the microbe than to the host.
• Antibiotic (strict sense) – natural substance produced by a microorganism that in small amounts inhibits or kills bacteria; clinical use now includes synthetic analogues.

Discovery of Antibiotics

• 1928 – accidental discovery of penicillin by Prof. Alexander Fleming (Penicillium fungus).
• Widespread production only from 1942 (WWII) → millions of lives saved.
• Pre-antibiotic era relied on cytotoxic metals (silver nitrate, arsenic, etc.).
• Current pipeline example: Teixobactin (expected availability ≈ 5 years from lecture date).
• Historical timeline highlights: Salvarsan (1900s), subsequent antibiotic classes through late 20ᵗʰ century.
• Socio-economic & healthcare advances mirrored antibiotic discovery.

Antibiotic-Producing Microorganisms

• Gram-positive rods:
– Bacillus subtilis → Bacitracin.
– Bacillus polymyxa → Polymyxin.
• Fungi:
– Penicillium notatum → Penicillin.
– Cephalosporium spp. → Cephalothin.
• Actinomycetes:
– Streptomyces venezuelae → Chloramphenicol.
– Streptomyces griseus → Streptomycin.
– Streptomyces nodosus → Amphotericin B.
– Micromonospora purpurea → Gentamicin.

What Makes an Ideal Antibiotic?

• Selective toxicity (high therapeutic index).
• Cidal rather than static where possible.
• Long plasma half-life → convenient dosing.
• Good tissue distribution (incl. CSF if required).
• Low plasma-protein binding.
• Oral & parenteral formulations.
• Minimal side-effects & interactions.
• Low propensity to induce resistance.
• Inexpensive to manufacture.

Core Definitions & Measurements

• Spectrum of Activity:
– Narrow spectrum – active against limited species (e.g. \text{Penicillin G} active chiefly on Gram +ve; Isoniazid only on Mycobacteria).
– Broad spectrum – active against wide range (e.g. Tetracycline on Gram +ve & –ve).
• Minimum Inhibitory Concentration (MIC) =\text{lowest drug concentration preventing visible growth}.
• Minimum Bactericidal Concentration (MBC) =\text{lowest concentration killing }\ge99.9\% \text{ of inoculum}.
• Bacteriostatic vs Bactericidal.
• Time-dependent vs Concentration-dependent killing.
• Prophylaxis – preventing infection. Treatment – curing existing/suspected infection.

Bacterial Cell Architecture – Relevance to Drug Targeting

• Cell wall (peptidoglycan) – maintains shape, prevents osmotic lysis; absent in host → prime selective target.
• Major wall constituents: alternating \text{NAG} & \text{NAM} sugars cross-linked by peptide chains.
• Gram +ve vs Gram –ve:
– Gram +ve: thick peptidoglycan, teichoic acids, traps crystal-violet stain.
– Gram –ve: outer lipid membrane with lipopolysaccharide (LPS), thin peptidoglycan, periplasmic space; crystal violet washed, counter-stained red.
• Internal structures: cytoplasmic membrane (selective permeability), nucleoid (circular dsDNA), 70S ribosomes \bigl(70S = 50S + 30S\bigr).

Antibacterial Target Sites & Modes of Action

1 – Inhibition of Cell Wall Synthesis

• β-lactams: penicillins, cephalosporins, cephamycins, carbapenems, monobactams.
– Bind Penicillin-Binding Proteins (PBPs) → block transpeptidation → weakened wall → lysis.
– Bactericidal, time-dependent.
• Glycopeptides (vancomycin, teicoplanin).
– Bind D-Ala-D-Ala termini of nascent peptides → prevent cross-linking.
– Active only on Gram +ve (molecules too large to penetrate Gram –ve outer membrane).

2 – Disruption of Cell Membrane Function

• Polypeptide antibiotics / Antimicrobial peptides (Polymyxin B, Colistin; antifungal Miconazole).
– Cationic; attracted to negatively charged LPS/LOS → insert, form pores → leakage → death.
– Bactericidal, time-dependent.

3 – Inhibition of Nucleic Acid Synthesis

• Quinolones / Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin).
– Inhibit DNA gyrase (Gram –ve) & Topoisomerase IV (Gram +ve).
– Bactericidal, concentration-dependent.
• Rifamycins (rifampicin, rifabutin).
– Bind bacterial RNA polymerase → block transcription.
– Key in Mycobacterium tuberculosis, leprosy; some Gram +ve.

4 – Inhibition of Protein Synthesis

• Ribosome-targeting; generally broad spectrum.
• Classes & specifics:
– Macrolides (erythromycin, azithromycin) – bind 50S; bacteriostatic; mainly Gram +ve, atypicals.
– Ketolides (telithromycin) – macrolide derivatives.
– Lincosamides (clindamycin) – 50S; Gram +ve, anaerobes.
– Streptogramins (quinupristin/dalfopristin).
– Tetracyclines (doxycycline) – 30S; prevent tRNA docking; broad spectrum.
– Aminoglycosides (gentamicin, amikacin) – 30S; misread mRNA; bactericidal, concentration-dependent, strong Gram –ve.
– Chloramphenicol – 50S; inhibits peptidyl transferase; broad spectrum but serious toxicity.
• Classic diagram: Streptomycin alters 30S decoding; Tetracyclines block tRNA entry; Chloramphenicol blocks peptide bond; Macrolides block exit tunnel.

5 – Antimetabolites

• Trimethoprim + Sulfamethoxazole (co-trimoxazole).
– Sequential blockade of folate pathway → no purine/AA synthesis.
– Bactericidal (synergy), concentration-dependent.

Master Summary Table (textual)

• Cell Wall – β-lactams, Vancomycin, Bacitracin.
• Cell Membrane – Polymyxins, Daptomycin.
• DNA/RNA – Quinolones (DNA gyrase), Rifampin (RNA polymerase).
• Protein 50S – Macrolides, Clindamycin, Linezolid, Chloramphenicol, Streptogramins.
• Protein 30S – Tetracyclines, Aminoglycosides.
• Folate – Sulfonamides, Trimethoprim.

Antiviral Chemotherapy

• Viruses are obligate intracellular; rely on host machinery → limited selective targets.
• Key replication steps exploitable:
1 Attachment & entry – blocked by Enfuvirtide (HIV), Docosanol (HSV), Palivizumab (RSV).
2 Uncoating – Amantadine/Rimantadine (influenza).
3 Nucleic acid synthesis – NRTIs/NNRTIs (HIV, HBV), Acyclovir (HSV), Foscarnet (CMV).
4 Integration – INSTIs (HIV).
5 Protein processing – Protease inhibitors (HIV, HCV).
6 Release – Neuraminidase inhibitors (influenza).
• HAART (Highly Active Antiretroviral Therapy): combination of ≥3 drugs targeting reverse transcriptase, protease, fusion/entry, integrase → reduces HIV viral load.

Safety Issues with Antimicrobial Use

• Toxicity (e.g. aminoglycoside nephro-/ototoxicity).
• Allergic reactions (β-lactam anaphylaxis).
• Drug–drug interactions (rifampicin CYP induction).
• Dysbiosis & superinfection (Clostridioides difficile after broad spectrum use).

Antimicrobial Resistance (AMR)

Scale of Crisis

• Current deaths ≈ 7\times10^{5} per year; projected 1\times10^{7} by 2050 (surpassing cancer).
• BBC & Guardian 2015–17 – described as “antibiotic apocalypse”.

WHO Priority Pathogens (2017)

• Priority 1 – Critical: Carbapenem-resistant Acinetobacter baumannii, Pseudomonas aeruginosa; ESBL-producing Enterobacteriaceae.
• Priority 2 – High: VRE Enterococcus, MRSA/VRSA, drug-resistant H. pylori, Campylobacter, Salmonella, N. gonorrhoeae.
• Priority 3 – Medium: Penicillin-non-susceptible S. pneumoniae, Ampicillin-R H. influenzae, Shigella.

Types of Resistance

• Inherent (intrinsic) – natural lack of target, impermeability, efflux; e.g. Pseudomonas outer membrane blocks many drugs.
• Acquired – mutation or acquisition of new genes via horizontal (plasmid, transposon, phage) or vertical transfer.
• Multi-resistance – e.g. P. aeruginosa resistant to β-lactams, quinolones, chloramphenicol.
• Cross-resistance – resistance to all drugs of same class (β-lactams).

Molecular Mechanisms

• Enzymatic degradation/modification – β-lactamases (penicillinases, ESBLs, carbapenemases); aminoglycoside-modifying enzymes.
• Target alteration – altered PBPs (MRSA mecA → PBP2a), VanA ligase replaces D-Ala with D-Lac (VRE/VRSA), mutations in gyrA (quinolones).
• Efflux pumps – tetracycline, macrolide, fluoroquinolone efflux.
• Reduced uptake – porin loss/mutation in Gram –ve.
• Bypass pathways – acquisition of alternate metabolic enzymes (e.g. sulfonamide-resistant dihydropteroate synthase).

Drivers of Resistance

• Overuse/misuse in human medicine – antibiotics prescribed for viral URTIs; Swedish stat: mean 13 treatment-days/child/year (0–6 yrs).
• Veterinary/agricultural use – growth promotion & mass prophylaxis led to Campylobacter/Salmonella resistance; practice banned in EU.

National/Global Action Plans (UK 2013–18 example)

1 Promote responsible prescribing & stewardship.
2 Strengthen infection prevention & control (human & animal sectors).
3 Public & professional awareness campaigns.
4 Research to understand resistance evolution & transmission.
5 Incentivise development of new drugs, vaccines, diagnostics.
6 Enhanced surveillance & data infrastructure.
7 International collaboration & capacity building.

Research Questions & Future Directions

• Epidemiology – mapping spread & prevalence of resistant strains.
• Rapid diagnostics – genomics, point-of-care tests for resistance genes/MIC.
• New antibiotic targets – essential, non-redundant bacterial proteins.
• Alternative therapies:
– Bacteriophage therapy.
– Anti-virulence factors (e.g. quorum sensing inhibitors).
– Monoclonal antibodies.
– Probiotics & microbiota modulation.
– CRISPR-based antimicrobials.
– Immunotherapy & vaccines.
– Nanoparticle drug delivery.
• Each alternative balances activity spectrum, host safety, resistance risk, cost & regulatory hurdles.

Key Take-Home Messages

• Antibiotics exploit structural/biochemical differences between microbes and host: cell wall, ribosome, nucleic acid machinery, metabolic pathways.
• Precise definitions (MIC, spectrum, bactericidal/static, time vs concentration dependence) guide clinical dosing.
• AMR threatens return to pre-antibiotic era; projections ≈ 10^{7} deaths/year by 2050.
• Resistance mechanisms are diverse; stewardship, surveillance, R&D, and alternative therapeutics are essential.
• Quote to remember – Dr Margaret Chan (ex-WHO): “Things as common as strep throat or a child’s scratched knee could once again kill…”.

Practice / Self-Check

• Explain how β-lactam antibiotics kill bacteria and how ESBLs defeat them.
• Distinguish intrinsic vs acquired resistance with examples.
• Pick one antiviral class and describe its molecular target.
• Calculate dosage adjustment if \text{AUC/MIC} target for fluoroquinolone is 125 and patient’s drug exposure is 500\,\text{mg·h/L} with MIC =4 mg/L (hint: \text{AUC/MIC}=125 achieved?).
• Describe two alternative non-antibiotic therapies under investigation.