We analyzed the impact of the biocides chlorhexidine digluconate (CHG) and benzalkonium chloride (BAC), in combination with minimal glucose medium on the biofilm formation ability of Escherichia coli strains producing ESBL enzymes (ESBL-EC). ESBL-EC strains were isolated between 2018 and 2022 from lower respiratory tract samples of patients hospitalized in in the Central Slovenian region and identified by the Institute of Microbiology and Immunology in Ljubljana. The strains were classified into two groups based on their affiliation with sequence type ST131. Minimum inhibitory concentrations for both biocides were determined using agar dilution and microdilution methods. For the biocide CHG, both methods yielded comparable MIC values (most frequently 4 mg/L), while for the biocide BAC, greater discrepancies were observed (64 mg/L with agar dilution, 32 mg/L with microdilution). Biofilm formation of ESBL-EC strains was tested in LB medium and minimal MG medium supplemented with 0,2 g/L glucose (0,02 % w/v), in the presence or abscence of very low concentrations of biocides (0,04 mg/L of CHG and 0,32 mg/L of BAC). In LB medium, biofilm formation was generally weak across most ESBL-EC strains, which changed slightly after the addition of biocide CHG. A greater effect was observed after the addition of the biocide BAC, increasing the proportion of moderately biofilm-forming strains by 11,2 % (from 19,8 % to 31,0 %) and strongly biofilm-forming strains by 4,3 % (from 18,1 % to 22,4 %). We observed minor differences between the ST131 and NON-ST131 groups. In MG medium 72,1 % of strains, belonging to the ST131 group, exhibited strong biofilm formation even without biocide exposure, increasing to 80,3 % after addition of BAC. This difference was statistically significant (p value < 0,001), as only 41,8 % of NON-ST131 strains were strongly biofilm-forming under the same conditions.
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