Photodynamic inactivation diminishes quorum sensing-mediated virulence factor production and biofilm formation of Serratia marcescens.

Serratia marcescens is an opportunistic human pathogen causing nosocomial infections and displays expanded resistance towards the conventional antibiotics. In S. marcescens, quorum sensing (QS) mechanism coordinates the population-dependent behaviors and regulates the virulence factors production. Photodynamic inactivation (PDI) is a promising alternative for the treatment of infections caused by drug resistant bacteria. Although PDI should be applied at lethal doses, it is possible that during PDI treatment, pathogens encounter sub-lethal doses of PDI (sPDI). sPDI cannot kill microorganisms, but it can considerably influence the microbial virulence. So, in this study, the effect of methylene blue (MB)-mediated PDI on QS-mediated virulence factor production and biofilm formation of S. marcescens at lethal and sub-lethal doses was evaluated. The biofilm formation and virulence factor production of S. marcescens ATCC 13,880 and S. marcescens Sm2 were assessed before and after PDI treatment. Besides, the effect of lethal and sub-lethal PDI on expression of bsmA and bsmB (Biofilm maturation), fimA and fimC (Major fimbrial protein), flhD (Regulator of flagellar mediated swarming and swimming motility) and swrR (AHL-dependent regulator) genes were evaluated by quantitative real time polymerase chain reaction. Lethal and sub-lethal PDI resulted in a significant decrease in biofilm formation, swimming/swarming motility, and pigment and hemolysin production ability of S. marcescens strains. bsmA, bsmB, flhD and swrR genes were down-regulated after PDI treatments. In conclusion, QS-mediated virulence factor production and biofilm formation ability of the two studied S. marcescens strains decreased after both lethal and sub-lethal PDI.