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

Abstract

Background and objectives: Ventilator-associated pneumonia (VAP) caused by carbapenem-resistant Gram-negative bacteria 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 carbapenem-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 for 62.7%. 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 prevalence 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 resistance. These findings highlight the urgent need for surveillance, stewardship, and new therapeutic options.

Keywords: Bacterial; Carbapenems; Drug resistance; Elizabethkingia meningoseptica; Raoultella terrigena; Ventilator-associated pneumonia.

Fig. 1.

Phenotypic and biochemical methods used for the characterization of carbapenem-resistant isolates A: Growth of bacterial colonies on a MacConkey agar plate, showcasing a distinct colony morphology with characteristics indicating lactose fermentation B: Antibiotic susceptibility testing using the Kirby-Bauer disk diffusion method. The zone of inhibition around antibiotic disks demonstrates the resistance of the bacteria to meropenam C. Modified carbapenem inactivation method (mCIM) and enhanced carbapenem inactivation method (eCIM) testing results. The image shows positive results for both mCIM and eCIM, indicating the presence of carbapenemase-producing bacteria (MBL-positive) as evidenced by growth within the antibiotic disk area. D: The API 20E strip showing multiple biochemical reactions, including tests for glucose, mannitol, indole, and others, used to identify and differentiate enteric bacteria