Distribution of MBL and serine-β-lactamase-producing pathogens in ventilator-associated pneumonia: insights into MDR and XDR strains

Background and Objectives: Ventilator-associated pneumonia (VAP) caused by carbapenem-resistant Gram-negative bac- teria is a serious ICU challenge. This study determined the prevalence, antimicrobial susceptibility profiles, and phenotypic carbapenemase resistance mechanisms of Gram-negative isolates from VAP patients in two tertiary hospitals in Karachi, Pakistan. Materials and Methods: We included 104 consecutive cases of VAP (July 2021–January 2023). A total of 67 carbapen- em-resistant Gram-negative isolates were identified and tested. Antibiotic susceptibility was assessed by disk diffusion and broth micro dilution, according to CLSI and EUCAST guidelines. Modified and enhanced carbapenem inactivation methods (mCIM/eCIM) were used to distinguish metallo-β-lactamase (MBL) and serine carbapenemase production. Results:The mean age was 44.6 ± 18.3 years; 52.2% were male. Early-onset VAP accounted for 37.3% and late-onset for62.7%. The most frequent pathogens were Acinetobacter baumannii (49.3%, 33/67) and Klebsiella pneumoniae (20.9%,14/67). Notably, 67% of isolates produced MBLs, and 33% produced serine carbapenemases (phenotypically). The preva-lence of multidrug-resistant (MDR), extensively drug-resistant (XDR), and pan drug-resistant (PDR) phenotypes was 42.6%,31.3%, and 19.4%, respectively. Conclusion: VAP in our ICUs was dominated by A. baumannii and K. pneumoniae with high levels of MBL-mediated resis- tance. These findings highlight the urgent need for surveillance, stewardship, and new therapeutic options.