Colistin resistance and biofilm formation in carbapenem-resistant Acinetobacter baumannii isolates.

This study aimed to evaluate colistin susceptibility and biofilm formation in carbapenem-resistant Acinetobacter baumannii (CRAB) clinical isolates and to compare planktonic minimum inhibitory concentrations (MICs) with biofilm-associated minimum biofilm inhibitory concentrations (MBICs). A total of 105 non-duplicate CRAB isolates were analyzed. Colistin MICs were determined using broth microdilution, biofilm formation was quantified by crystal violet staining, and MBIC values were assessed using a resazurin-based biofilm susceptibility assay. Nine of 105 isolates (8.57%) were colistin-resistant under planktonic conditions, while all isolates were classified as weak biofilm producers, with mean OD₅₇₀ values of 0.1639 ± 0.021. MBIC values were significantly higher than corresponding MIC values (p < 0.0001), ranging from 16 to 128 µg/mL, with the majority of isolates (65.7%) exhibiting MBIC values of 64 µg/mL. Biofilm growth was associated with a marked increase in colistin concentrations required for inhibition in both colistin-susceptible and colistin-resistant isolates. Although MIC and MBIC values represent different growth conditions and are not directly comparable, the findings indicate that even weak biofilm formation is associated with substantially reduced colistin activity. These results suggest that reliance on planktonic MIC values alone may underestimate colistin response in biofilm-associated CRAB, and incorporation of biofilm-related parameters may improve the interpretation of antimicrobial susceptibility in these infections. These findings highlight the need for cautious interpretation of in vitro susceptibility data and support further investigation into biofilm-targeted therapeutic strategies.