In-vitro antimicrobial susceptibility pattern and clinical outcome of Carbapenem-resistant Acinetobacter baumannii causing ventilator-associated pneumonia: Is colistin the last resort?

INTRODUCTION

Acinetobacter baumannii (A. baumannii) specifically carbapenem-resistant A. baumannii (CRAB) is one of the most common causative agents of ventilator-associated pneumonia (VAP) worldwide. This research was designed to evaluate the prevalence of CRAB in VAP cases within a North Indian tertiary ICU and to correlate in-vitro susceptibility with clinical survival, particularly assessing the "last-resort" status of colistin in the current era of antibiotic resistance.

METHODS

This prospective observational study included 400 A. baumannii isolates, all obtained from clinical specimens from patients of VAP. Bacterial identification and antimicrobial susceptibility testing were performed using the VITEK-2 system, while colistin minimum inhibitory concentrations were determined by broth microdilution. Patient outcomes were monitored for 28 days to determine the impact of targeted therapy on survival rates. The association between colistin therapy and survival was analyzed using the Chi-square test, and odds ratios (OR) were calculated.

RESULTS

During the study period, the prevalence of CRAB-associated VAP was 52.6%. All isolates were carbapenem-resistant and multidrug-resistant, with 42% classified as extensively drug-resistant and 13.5% as colistin-intermediate susceptible-only. All isolates were categorized as "Intermediate" to colistin. The overall survival rate was 74.5%. Survival was significantly higher in the colistin-treated group (81.7%) compared to the non-colistin group (64.7%) (χ2 = 14.47, p < 0.001; OR ≈ 2.45).

CONCLUSION

The study demonstrated an alarmingly high prevalence (52.6%) of carbapenem-resistant A. baumannii among VAP isolates in the study setting. Carbapenem resistance data (100% resistance) from the current study clearly indicates towards the lost-superiority of carbapenems among the treatment regimens for A. baumannii-mediated VAP. Although colistin therapy was associated with higher survival rates, this finding should be considered exploratory due to the observational design and the fact that the study was not powered primarily for outcome assessment.