Role of Efflux Pump-Mediated Antibiotic Resistance in Quorum Sensing-Regulated Biofilm Formation by Salmonella Typhimurium

This study was designed to assess the influence of efflux pump activity on the biofilm formation in Salmonella Typhimurium. Salmonella enterica subsp. enterica serovar Typhimurium ATCC 19585 (ST(WT)) and clinically isolated S. Typhimurium CCARM 8009 (ST(CI)) were treated with ceftriaxone (CEF), chloramphenicol (CHL), ciprofloxacin (CIP), erythromycin (ERY), norfloxacin (NOR), and tetracycline (TET) in autoinducer-containing media in the absence and presence of phenylalanine-arginine β-naphthylamide (PAβN) to compare efflux pump activity with biofilm-forming ability. The susceptibilities of ST(WT) and ST(CI) were increased in the presence of PAβN. ERY+PAβN showed the highest decrease in the minimum inhibitory concentration (MIC) of ERY from 256 to 2 μg/mL against ST(WT) and ST(CI). The antimicrobial activity of NOR against planktonic cells was significantly increased in the presence of PAβN, showing the lowest numbers of ST(WT) (3.2 log CFU/cm(2)), and the TET+PAβN effectively inhibited the growth of ST(CI) (5.2 log CFU/cm(2)). The lowest biofilm-forming abilities were observed at NOR+PAβN against ST(WT) (biofilm-forming index, BFI < 0.41) and CEF+PAβN against ST(CI) (BFI = 0.32). The bacteria swimming motility and relative fitness varied depending on the antibiotic and PAβN treatments. The motility diameters of ST(WT) were significantly decreased by NOR+PAβN (6 mm) and TET+PAβN (15 mm), while the lowest motility of ST(CI) was observed at CIP+PAβN (8 mm). The significant decrease in the relative fitness levels of ST(WT) and ST(CI) was observed at CIP+PAβN and NOR+PAβN. The PAβN as an efflux pump inhibitor (EPI) can improve the antimicrobial and anti-biofilm efficacy of antibiotics against S. Typhimurium. This study provides useful information for understanding the role of efflux pump activity in quorum sensing-regulated biofilm formation and also emphasizes the necessity of the discovery of novel EPIs for controlling biofilm formation by antibiotic-resistant pathogens.