18. Multidrug-Resistant Organism Management

Quick Recap

Infectious Diseases System, new protocol (final ID System addition). Companion to Antimicrobial Stewardship Programs, Antimicrobial Dosing & TDM in Critical Illness, and Invasive Fungal Infections in Critical Illness (all Infectious Diseases System) — addresses the bacterial resistance counterpart, focusing on carbapenem-resistant Gram-negative organisms and the genuinely transformed treatment landscape novel beta-lactam/beta-lactamase inhibitor combinations have created.

1. Definition

Standardized resistance terminology: multidrug resistance (MDR) = non-susceptible to ≥1 agent in ≥3 antimicrobial categories; extensive drug resistance (XDR) = non-susceptible to ≥1 agent in all but ≤2 categories; pandrug resistance (PDR) = non-susceptible to all agents in all categories. Carbapenem-resistant Enterobacterales (CRE), carbapenem-resistant Pseudomonas aeruginosa (CRPA), and carbapenem-resistant Acinetobacter baumannii (CRAB) are the dominant clinical concern in the ICU — associated with mortality exceeding 40%, prolonged ICU stay, and substantial healthcare cost.

A genuinely alarming, real-world resistance trajectory: one recent ICU bacteremia surveillance study (India, 2022-2024) found carbapenem resistance reaching 79% in Klebsiella pneumoniae, 72% in Pseudomonas aeruginosa, and 100% in Acinetobacter baumannii and E. coli by 2024 — illustrating how severe this problem has already become in some settings, not a distant future concern. CDC's 2024 update found infections from these organisms increased approximately 20% from 2019 to 2022.

2. The Transformed Treatment Landscape — Novel Beta-Lactam/Beta-Lactamase Inhibitor Combinations

The introduction of novel BL/BLI combinations has significantly transformed treatment for carbapenem-resistant infections, previously limited largely to polymyxins, tigecycline, or aminoglycosides — agents with real toxicity and efficacy limitations. Each agent has a genuinely distinct activity spectrum, and none is universally effective, particularly against metallo-beta-lactamase (MBL) producers, which remain a persistent gap across most of these newer agents:

RESTORE-IMI 1 trial: imipenem-relebactam achieved similar clinical cure with LESS nephrotoxicity compared to colistin plus imipenem — concrete comparative evidence supporting the shift away from polymyxin-based regimens where a suitable BL/BLI alternative exists.

3. Colistin's Genuine Demotion

Current IDSA (2024) guidance explicitly discourages polymyxin (colistin) use when safer, effective beta-lactam options exist for CRE and CRPA, and prefers sulbactam-durlobactam over colistin-based regimens specifically for CRAB — reserving colistin mainly for salvage therapy, or when no better-activity alternative exists. This represents a genuine, guideline-level shift away from what was, for years, a default salvage option — colistin's real toxicity profile (nephrotoxicity in particular) is now weighed against genuinely better-tolerated alternatives wherever those alternatives retain activity.

4. Practical Selection Framework

1. Identify the specific resistance mechanism where possible (KPC, AmpC, MBL, OXA-48-like, or CRAB specifically) — agent selection is fundamentally mechanism-dependent, not a single universal "MDR cocktail"
2. For KPC-producing Enterobacterales or difficult-to-treat Pseudomonas: ceftazidime-avibactam, meropenem-vaborbactam, or imipenem-relebactam are current first-line options per IDSA 2024/ESCMID 2023 guidance
3. For MBL-producing organisms specifically: this remains the persistent gap — cefiderocol or aztreonam-avibactam are among the few options with genuine activity here, though real-world experience is still accumulating for the newest agent
4. For CRAB specifically: sulbactam-durlobactam is now preferred over colistin-based regimens where available
5. Reserve colistin for genuine salvage scenarios — not as a reflexive first-line choice for any carbapenem-resistant organism
6. Actively screen for emerging resistance to the novel agents themselves — though still uncommon, resistance to cefiderocol via siderophore receptor mutations or beta-lactamase upregulation after exposure has been documented, underscoring that these newer agents are not immune to the same evolutionary pressure that created the original problem

5. Practical Synthesis

The therapeutic landscape has genuinely improved — patients with carbapenem-resistant infections who a decade ago would have been managed with colistin/tigecycline/aminoglycoside-based regimens now often have better-tolerated, comparably or more effective options. However, none of these agents is universally effective, MBL-producing organisms remain a persistent challenge across most of the newer combinations, and the pace of novel-agent development continues to lag behind the pace of emerging resistance — particularly for CRAB and MBL-producing Enterobacterales, per the field's own acknowledgment. This protocol treats the current landscape as genuinely improved but not solved.

6. Consultation Matrix

7. Documentation & Medicolegal Checklist

• Resistance mechanism (where identified) and rationale for specific agent selection documented
• Rationale for colistin use specifically (salvage indication) documented if selected, given current guidance discouraging default use

8. Key Guidelines

• IDSA 2024 Guidance on the Treatment of Antimicrobial Resistant Gram-Negative Infections (updating the 2022 guidance)
• ESCMID 2023 guidance
• CDC 2024 Antibiotic Resistance Threats Report

9. Landmark Evidence

10. Controversies

• MBL-producing organisms remain a genuine, largely unresolved treatment gap across most novel BL/BLI combinations — this protocol does not present the current agent landscape as a complete solution, consistent with the field's own acknowledgment that development has "lagged behind the rapid emergence of resistance" particularly for this mechanism and for CRAB.
• Emerging resistance to the newest agents themselves (e.g., cefiderocol) is already being documented, though still uncommon — a reminder that novel-agent introduction doesn't permanently solve the underlying evolutionary pressure problem, reinforcing the importance of the stewardship framework (cross-reference Antimicrobial Stewardship Programs) alongside new-drug development.
• Real-world experience with the newest approvals (aztreonam-avibactam specifically) remains limited relative to the more established agents — promising in vitro/mechanistic rationale, but less mature real-world outcome data than ceftazidime-avibactam or meropenem-vaborbactam.

11. References

1. Precision Approaches to Carbapenem-Resistant Infections in the ICU: Integrating Diagnostics, Stewardship, and Novel Therapies. Diagnostics (Basel). 2026;16(7):1053.
2. Tamma PD, Aitken SL, Bonomo RA, Mathers AJ, van Duin D, Clancy CJ. Infectious Diseases Society of America 2024 Guidance on the Treatment of Antimicrobial Resistant Gram-Negative Infections.
3. Motsch J, et al. RESTORE-IMI 1: a multicenter, randomized, double-blind trial comparing efficacy and safety of imipenem/relebactam vs. colistin plus imipenem in patients with imipenem-nonsusceptible bacterial infections. Clin Infect Dis. 2020.
4. Filling the Gaps: A Review and Update on Beta-Lactamase Inhibitor Combinations. Curr Infect Dis Rep. 2026.
5. Management of CRE Infection with a Focus on New Antimicrobial Agents. 2025.
6. Activity of β-Lactamase Inhibitor Combinations Against Enterobacterales Isolated from Patients with Intra-Abdominal Infection from United States Medical Centres (2019-2023). 2025.
7. In Vitro Activity of Novel β-Lactam/β-Lactamase Inhibitors Against Carbapenem-Resistant Pseudomonas aeruginosa and Enterobacterales in Korea. 2025.
8. Bacteremia in the Intensive Care Unit: Pathogen Profiles and Antimicrobial Resistance Trends from January 2022 to June 2024 (India). 2025.
9. CDC 2024 Antibiotic Resistance Threats Report.

See also: Antimicrobial Stewardship Programs (Infectious Diseases System) for the program-level framework essential to preserving these newer agents' effectiveness; Antimicrobial Dosing & TDM in Critical Illness (Infectious Diseases System) for the pharmacokinetic dosing considerations relevant to these agents in critical illness; Invasive Fungal Infections in Critical Illness (Infectious Diseases System) for the fungal-resistance counterpart; Sepsis/Septic Shock (Infectious Diseases System) for the clinical syndrome these organisms most commonly complicate.

This completes the Infectious Diseases System additions (3 of 3: Antimicrobial Stewardship Programs, Invasive Fungal Infections, Multidrug-Resistant Organism Management).