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Abstract Background We evaluated the association between mortality and colistin resistance in Acinetobacter baumannii infections and the interaction with antibiotic therapy. Methods This is a secondary analysis of a randomized controlled trial of patients with carbapenem-resistant gram-negative bacterial infections treated with colistin or colistin-meropenem combination. We evaluated patients with infection caused by carbapenem-resistant A. baumannii (CRAB) identified as colistin susceptible (CoS) at the time of treatment and compared patients in which the isolate was confirmed as CoS with those whose isolates were retrospectively identified as colistin resistant (CoR) when tested by broth microdilution (BMD). The primary outcome was 28-day mortality. Results Data were available for 266 patients (214 CoS and 52 CoR isolates). Patients with CoR isolates had higher baseline functional capacity and lower rates of mechanical ventilation than patients with CoS isolates. All-cause 28-day mortality was 42.3% (22/52) among patients with CoR strains and 52.8% (113/214) among patients with CoS isolates (P = .174). After adjusting for variables associated with mortality, the mortality rate was lower among patients with CoR isolates (odds ratio [OR], 0.285 [95% confidence interval {CI}, .118–.686]). This difference was associated with treatment arm: Mortality rates among patients with CoR isolates were higher in those randomized to colistin-meropenem combination therapy compared to colistin monotherapy (OR, 3.065 [95% CI, 1.021–9.202]). Conclusions Colistin resistance determined by BMD was associated with lower mortality among patients with severe CRAB infections. Among patients with CoR isolates, colistin monotherapy was associated with a better outcome compared to colistin-meropenem combination therapy. Clinical Trials Registration NCT01732250 In a retrospective analysis of a randomized controlled trial, infection with carbapenem-resistant, colistin-resistant Acinetobacter baumannii was associated with lower mortality than carbapenem-resistant, colistin-susceptible Acinetobacter. Colistin-carbapenem combination therapy was associated with higher mortality than colistin monotherapy in patients with colistin-resistant isolates.

Background Colistin-carbapenem combination therapy is frequently used for carbapenem-resistant Gram-negative infections, but its impact on subsequent acquisition of carbapenem-resistant Enterobacterales (CRE) requires further investigation. We evaluated the incidence of CRE acquisition and performed molecular characterization of recovered isolates following treatment with colistin–meropenem versus colistin monotherapy. Methods This analysis addressed a pre-specified secondary aim of the AIDA multicenter randomized controlled trial, which compared colistin monotherapy to colistin–meropenem combination therapy for carbapenem-resistant Gram-negative infections at six hospitals in Israel, Greece, and Italy. Rectal swabs were obtained at enrollment and weekly until day 28 or discharge. Swabs were processed centrally by plating onto MacConkey agar supplemented with imipenem to selectively isolate CRE. Recovered colonies were identified using MALDI-TOF mass spectrometry, and meropenem minimum inhibitory concentrations (MICs) were determined by broth microdilution. Clinical cultures were obtained as indicated and processed locally, and CRE isolates were sent to the central laboratory for confirmation and characterization. Whole-genome sequencing was used to determine sequence types and resistance genes. Patients were excluded if they had CRE detected at baseline, either by rectal culture or as the index clinical isolate, or if no follow-up rectal cultures were available. Results Among 197 eligible patients (99 colistin; 98 colistin–meropenem), CRE acquisition occurred in 6 (3.0%): 1/99 (1.0%, 95% CI 0.03–5.5%) in the monotherapy arm and 5/98 (5.1%, 95% CI 1.7–11.5%) in the combination arm ( p  = 0.12). Two patients in the combination arm developed clinical infections caused by CRE (bacteremia and pneumonia); none occurred in the monotherapy arm. Carbapenemase genes were detected in four of the six acquired CRE isolates: one in the monotherapy arm ( bla VIM ) and three in the combination arm (all bla KPC ). Identified species included Klebsiella pneumoniae and Escherichia coli belonging to established and emerging high-risk, multidrug-resistant clones. Conclusions Patients treated with colistin-meropenem had a higher, though not statistically significant, rate of CRE acquisition. Early detection of high-risk CRE clones highlights the need to weigh potential unintended consequences when selecting combination regimens for multidrug-resistant infections. Trial Registration AIDA trial was registered with ClinicalTrials.gov, number NCT01732250 (submitted 19-11-2012).