---

Antibiotic Resistance — Comprehensive Notes for MD Examinations (15 Marks)

---

1. DEFINITION

---

2. MAGNITUDE OF THE PROBLEM

• 2.8 million antibiotic-resistant infections occur annually in the USA; >35,000 deaths/year (CDC 2019 Antibiotic Resistance Threat Report).

• WHO declared antimicrobial resistance (AMR) one of the top 10 global public health threats to humanity.
• Emergence of carbapenem-resistant Enterobacteriaceae (CRE) is considered a harbinger of a potential "post-antibiotic era" (Harrison's, 21st ed., p. 4261).

---

3. CLASSIFICATION OF RESISTANCE

---

4. MECHANISMS OF ANTIBIOTIC RESISTANCE

A. Drug Inactivation / Enzymatic Destruction (Most Common)

NDM-1 (New Delhi Metallo-β-lactamase) — confers resistance to virtually all β-lactams including carbapenems; encoded on transmissible plasmids.

---

B. Alteration of Drug Target

VRE mechanism (Harrison's, p. 4508): Vancomycin resistance in E. faecium is mediated by VanA/VanB operons — the D-Ala-D-Ala terminal of peptidoglycan precursors is replaced by D-Ala-D-Lac (VanA, VanB) or D-Ala-D-Ser (VanC), reducing vancomycin binding affinity >1000-fold.

---

C. Reduced Drug Accumulation

1. Decreased permeability / Porin loss
   • Pseudomonas aeruginosa: loss of OprD porin → carbapenem resistance
   • Klebsiella: OmpK35/36 porin loss
2. Active efflux pumps (Major mechanism)
   • Pump drug out of bacterial cell before it can act
   • Families: MFS, RND, SMR, MATE, ABC
   • Examples:
     • MexAB-OprM in Pseudomonas → resistance to β-lactams, fluoroquinolones
     • AcrAB-TolC in E. coli → broad resistance
     • NorA in S. aureus → fluoroquinolone resistance
     • Tet(A) in Enterobacteriaceae → tetracycline efflux

---

D. Bypass / Alternative Metabolic Pathway

• Sulfonamide resistance: bacteria acquire ability to use exogenous folate, bypassing the need for PABA-dependent folate synthesis
• Trimethoprim resistance: overproduction or altered DHFR (dihydrofolate reductase) enzyme

---

E. Biofilm Formation

• Bacteria encased in polysaccharide matrix — antibiotics cannot penetrate
• Minimum biofilm eradication concentration (MBEC) >> MIC
• Key pathogens: Pseudomonas, coagulase-negative Staphylococci, Candida
• Clinical relevance: catheter-associated infections, prosthetic valve endocarditis

---

5. GENETIC BASIS — HOW RESISTANCE IS TRANSFERRED

Vertical Transmission       Horizontal Gene Transfer (HGT)
(Mutation during           ┌────────────────────────────────┐
 replication)              │ Conjugation  → Plasmids (most  │
                           │               common & dangerous│
                           │ Transformation → Naked DNA uptake│
                           │ Transduction  → Bacteriophage  │
                           │ Transposons   → "Jumping genes" │
                           └────────────────────────────────┘

• Plasmids (R-factors): carry multiple resistance genes simultaneously → multi-drug resistance (MDR)
• Integrons: gene-capture elements that can incorporate resistance gene cassettes
• Transposons: mobile genetic elements that move resistance genes between chromosomes and plasmids

---

6. IMPORTANT RESISTANT ORGANISMS (ESKAPE Pathogens)

CDC 2019 Threat Levels (Harrison's, p. 4261)

---

7. SPECIFIC HIGH-YIELD RESISTANCE PATTERNS

MRSA (Methicillin-Resistant S. aureus)

• Gene: mecA → encodes PBP2a (low affinity for all β-lactams)
• Both hospital-acquired (HA-MRSA) and community-acquired (CA-MRSA) types
• Treatment: Vancomycin, Linezolid, Daptomycin, Teicoplanin, Ceftaroline (5th-gen cephalosporin active against MRSA)

ESBL-Producing Organisms

• Enzymes hydrolyze oxyimino-cephalosporins (ceftriaxone, cefotaxime) + monobactams
• Common in E. coli, Klebsiella pneumoniae
• Inhibited by clavulanate (key for detection — double-disc synergy test)
• Treatment: Carbapenems (drug of choice)

Carbapenem-Resistant Enterobacteriaceae (CRE)

• Mechanisms: Carbapenemases (KPC, NDM-1, OXA-48, VIM, IMP)
• NDM-1 (New Delhi Metallo-β-lactamase) — discovered in 2009; widespread in South Asia
• Treatment options (all last resort): Ceftazidime-avibactam, Polymyxins (colistin), Fosfomycin, Tigecycline, Aztreonam-avibactam (for NDM)

VRE (Vancomycin-Resistant Enterococci)

• VanA: resistant to vancomycin + teicoplanin
• VanB: resistant to vancomycin only (teicoplanin sensitive)
• Treatment: Linezolid, Daptomycin, Quinupristin-dalfopristin (not E. faecalis)

---

8. DETECTION OF ANTIBIOTIC RESISTANCE

---

9. FACTORS PROMOTING ANTIBIOTIC RESISTANCE

1. Overuse/misuse of antibiotics in humans — self-medication, incomplete courses
2. Agricultural use — antibiotics as growth promoters in livestock
3. Subtherapeutic dosing — selects for partially resistant mutants
4. Inadequate infection control — nosocomial spread
5. Global travel and trade — worldwide dissemination of resistant clones
6. Poor diagnostics — empirical broad-spectrum therapy
7. Limited new antibiotic development — pipeline gap (pharma economics)

---

10. PREVENTION AND CONTROL

A. Antimicrobial Stewardship Programs (ASP)

• Right drug, right dose, right duration, right route
• De-escalation based on culture/sensitivity
• Formulary restrictions, prior authorization for broad-spectrum agents
• Reduces resistance rates and C. difficile infections

B. Infection Prevention and Control (IPC)

• Hand hygiene (WHO 5 moments) — single most effective intervention
• Contact precautions for MDR organisms
• Active surveillance cultures
• Environmental decontamination

C. Surveillance

• National programmes: IDSP (India), ESKAPE surveillance, WHO GLASS (Global Antimicrobial Resistance and Use Surveillance System)

D. New Strategies Under Research

---

11. SUMMARY DIAGRAM — MECHANISMS AT A GLANCE

ANTIBIOTIC ENTERS BACTERIA
         │
    ─────┼──────────────────────────────────────────
         │
    ┌────▼─────────────────────────────────────────┐
    │  RESISTANCE MECHANISMS                        │
    │                                               │
    │  1. PUMP IT OUT → Efflux pumps               │
    │     (MexAB, AcrAB-TolC, NorA)                │
    │                                               │
    │  2. KEEP IT OUT → Porin loss                 │
    │     (OprD loss in Pseudomonas)                │
    │                                               │
    │  3. DESTROY IT → Enzymatic inactivation      │
    │     (β-lactamases, ESBLs, NDM-1)             │
    │                                               │
    │  4. CHANGE THE TARGET → Altered PBPs/rRNA    │
    │     (MRSA: PBP2a; VRE: D-Ala-D-Lac)         │
    │                                               │
    │  5. BYPASS IT → Alternative pathway          │
    │     (Sulfonamide resistance)                  │
    │                                               │
    │  6. HIDE IN BIOFILM → Impenetrable matrix    │
    └───────────────────────────────────────────────┘

---

12. QUICK REVISION TABLE — HIGH-YIELD FACTS

---