Bacillus amyloliquefaciens-secreted cyclic dipeptide - cyclo(L-leucyl-L-prolyl) inhibits biofilm and virulence production in methicillin-resistant Staphylococcus aureus

The current study explores the inhibitory efficacy of cyclo(L-leucyl-L-prolyl) (CLP), a cyclic dipeptide from Bacillus amyloliquefaciens on the biofilm and virulence production of methicillin-resistant Staphylococcus aureus (MRSA). The minimal inhibitory concentration (MIC) and maximum bactericidal concentration (MBC) of CLP against three MRSA strains were found to be 256 and 512 mu g mL(-1) respectively. CLP at its sub-MICs (16, 32, 64 and 128 mu g mL(-1)) exhibited a phenomenal dose-dependent antibiofilm activity against MRSA strains with maximum inhibitions of 85-87%. Confocal and scanning electron microscopic examinations validated the antibiofilm efficacy of CLP. In addition, CLP was proficient enough to greatly modify the surface hydrophobicity and significantly reduced the slime synthesis of MRSA. Appreciable differences noticed in the EPS constituents of CLP treated MRSA signified that the possible antibiofilm mechanism could proceed by impeding the synthesis of EPS and thereby CLP prevents biofilm assemblage and the associated virulence cascade. Interestingly, CLP displayed a prominent disruption (52-54%) on a 48 h preformed biofilm of MRSA. Data from in vivo assays using Caenorhabditis elegans unveiled the non-toxic and anti-infective efficacy of CLP. Down-regulation of all studied virulence genes affirmed the results of the phenotypic and in vivo assays. Thus, the present study exemplifies the use of CLP as a plausible alternative to conventional antibiotics in controlling biofilm associated infections of MRSA.