Phenotypic changes in biofilm formation of Escherichia coli under the influence of antibacterial drugs and their combinations

This study examined the effects of subinhibitory concentrations of the fluoroquinolones enrofloxacin and levofloxacin, as well as their combination, on the formation of bacterial biofilms in Escherichia coli upon induction of the SOS response. The study involved 46 clinical isolates of E. coli, isolated from various farm animal habitats, as well as the reference strain E. coli JW5503 (ΔtolC), carrying the TurboRFP fluorescent reporter system. Biofilm formation was quantified using gentian violet staining followed by optical density measurement, and SOS response activation was recorded using a transilluminator and fluorescence microscopy. It was found that the optical density of the reference strain increased from 0.26 ± 0.05 to 0.50 ± 0.04 (an increase of 92.3%; p < 0.05) under the influence of enrofloxacin, and to 0.462 ± 0.06 (an increase of 76.9%; p < 0.05) under the influence of levofloxacin, which corresponded to the transition from a low to a moderate degree of biofilm formation.  Upon exposure of the studied clinical isolates to enrofloxacin (0.125 µg/mL), the maximum increase in biofilm biomass was observed, amounting to 40.2% (p < 0.01), whereas levofloxacin induced a dose-dependent decrease in this parameter by 39.8% at a concentration  of 0.25 µg/mL (p < 0.01). Combined administration of enrofloxacin and levofloxacin to the clinical isolates resulted in the most pronounced enhancement of biofilm formation, reaching 61.2% relative to control values (p <  0.01). 

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