Carbapenem resistance genes and biofilm-virulence uncoupling in Acinetobacter baumannii

Acinetobacter baumannii, a World Health Organization (WHO) critical-priority pathogen, causes difficult-to-treat nosocomial infections. The link between resistance genes, biofilm, and virulence carriage remains poorly understood in clinical isolates of high-burden South Asian settings. This study intended to thoroughly describe and explain linkages among clinical isolates collected in Haryana, India.

A total of 200 non-duplicate A. baumannii isolates underwent antimicrobial susceptibility testing by broth microdilution against 17 antibiotics, interpreted according to CLSI M100 guidelines (32nd edition). Biofilm forming capacity was quantified using the standardized microtiter plate crystal violet assay. Polymerase chain reaction (PCR) was used to detect six carbapenemase genes (blaOXA-23, blaOXA-24, blaOXA-58, blaNDM-1, blaVIM, and blaIMP), beta-lactamase genes (blaPER-1, AmpC, and blaTEM), and virulence genes (bap, ompA, and csuE). Concordance was assessed by Cohen's κ; gene-resistance and gene-biofilm links were tested using regression and non-parametric analyses. A total of 200 pure, non-duplicate A. baumannii isolates were included and contaminated cultures were excluded. Sample size adequacy was assessed using Buderer's formula based on expected sensitivity, specificity, carbapenem-resistance prevalence, 95% confidence level, and desired precision.

Among 200 isolates, 54% were MDR, 34% XDR, and 61% carbapenem-resistant. The blaOXA-23 gene, present in 61%, showed near-perfect concordance with carbapenem resistance (κ = 0.81). Composite carbapenemase carriage improved concordance (κ = 0.88). Biofilm capacity was unrelated to overall resistance, but strong biofilm formers had higher colistin MICs (p = 0.001). Virulence genes (bap, ompA, and csuE) were linked to strong biofilm formation (p < 0.001) without resistance escalation.

blaOXA-23 emerged as the dominant genetic driver of carbapenem resistance, underscoring its value in rapid diagnostics. Colistin tolerance was linked to biofilm-associated phenotypes rather than cumulative resistance.