Inactivation of A Putative Efflux Pump (lmrb) in Streptococcus Mutans Results in Altered Biofilm Structure and Increased Exopolysaccharide Synthesis: Implications for Biofilm Resistance

Efflux pumps are a mechanism associated with biofilm formation and resistance. There is limited information regarding efflux pumps in Streptococcus mutans, a major pathogen in dental caries. The aim of this study was to investigate potential roles of a putative efflux pump (LmrB) in S. mutans biofilm formation and susceptibility. Upon lmrB inactivation and antimicrobial exposure, the biofilm structure and expression of other efflux pumps were examined using confocal laser scanning microscopy (CLSM) and qRT-PCR. lmrB inactivation resulted in biofilm structural changes, increased EPS formation and EPS-related gene transcription (p < 0.05), but no improvement in susceptibility was observed. The expression of most efflux pump genes increased upon lmrB inactivation when exposed to antimicrobials (p < 0.05), suggesting a feedback mechanism that activated the transcription of other efflux pumps to compensate for the loss of lmrB. These observations imply that sole inactivation of lmrB is not an effective solution to control biofilms.