🦠 Chapter 104: Antibiotic Resistance & MDR Bacteria

MRSA · VRE · ESBL · Carbapenemase (KPC/NDM) · MDR/PDR · Acinetobacter · Pseudomonas · Antimicrobial stewardship · De-escalation · Infection prevention

🔍 Core Concepts: Antibiotic Resistance

📌 Definitions
MDR: non-susceptible to ≥1 agent in ≥3 categories. XDR: susceptible to ≤2 categories. PDR: resistant to all tested agents.
🦠 MRSA
mecA gene → altered PBP. HA-MRSA (multidrug resistant) vs CA-MRSA (PVL toxin, SSTI/necrotizing pneumonia). Tx: vancomycin, linezolid.
🧬 ESBL (E. coli/Klebsiella)
Hydrolyze penicillins, cephalosporins, aztreonam. Carbapenems = drug of choice. Avoid ceftriaxone even if susceptible in vitro.
⚙️ Carbapenemase (KPC, NDM, IMP)
Hydrolyze carbapenems. Tx: polymyxins (colistin), tigecycline (no bacteremia), aminoglycosides, ± combination.
🧫 VRE
VanA/vanB genes. Linezolid, daptomycin, tigecycline effective. Avoid ampicillin if resistant.
🛡️ MDR Acinetobacter/Pseudomonas
Polymyxins (colistin), tigecycline (Acinetobacter), high-dose sulbactam. Nebulized colistin for pneumonia.
📉 Antimicrobial Stewardship
De-escalation at 48h, shorten duration, optimize dosing, antibiotic cycling (limited evidence), infection prevention bundles.

🩺 Stepwise Approach: Managing MDR Infections

1
Identify risk factors for MDR
Prior antibiotics (90d), prolonged hospitalization (>5d), ICU stay, immunocompromise, chronic dialysis, prior MDR colonization.
2
Obtain appropriate cultures before antibiotics
Blood, urine, respiratory (BAL/ETA), wound. Perform susceptibility testing including ESBL, carbapenemase, VRE, MRSA screening.
3
Empiric therapy for suspected MDR
If MRSA risk: add vancomycin/linezolid. If ESBL risk: carbapenem (meropenem). If carbapenemase risk: polymyxin/tigecycline/aminoglycoside combination.
4
Implement infection control measures
Contact isolation (single room, gown, gloves). Chlorhexidine bathing. Cohort patients with same MDR organism. Strict hand hygiene.
5
De-escalate & optimize therapy based on susceptibility
Narrow to most effective, least toxic agent. Use therapeutic drug monitoring (vancomycin, aminoglycosides). Limit duration (7-14 days unless endocarditis, osteomyelitis).
6
Prevent spread & resistance
Antimicrobial stewardship program, audit antibiotic use, daily review, removal of unnecessary catheters, environmental cleaning with sporicidal agents.