Attenuated Virulence and Biofilm Formation in Staphylococcus aureus following Sublethal Exposure to Triclosan

Sub-effective exposure of Staphylococcus aureus to the biocide triclosan can reportedly induce a small colony variant (SCV) phenotype in Staphylococcus aureus. S. aureus SCVs are characterised by slow growth rates, reduced pigmentation and lowered antimicrobial susceptibility. Whilst they may exhibit enhanced intracellular survival, there are conflicting reports regarding their pathogenicity. The current study reports the characteristics of a SCV-like strain of S. aureus, created by repeated passage on sub-lethal triclosan concentrations. S. aureus ATCC 6538 (P0) was serially exposed ten times to concentration gradients of triclosan to generate strain P10. This strain was then further passaged ten times on triclosan-free medium (designated x10). The minimum inhibitory and bactericidal concentrations of triclosan for P0, P10 and x10 were determined and growth rates measured in biofilm and planktonic culture. Haemolysin, DNAse and coagulase activities were measured and virulence determined using a Galleria mellonella pathogenicity model. Strain P10 exhibited decreased susceptibility to triclosan and characteristics of a SCV phenotype, including considerably reduced growth rate and the formation of pinpoint colonies. However, this strain also had delayed coagulase production, impaired haemolysis (p<0.01), was defective in biofilm formation and DNAase activity, and displayed significantly attenuated virulence. Colony size, haemolysis, coagulase activity and virulence were only partially restored in strain x10, whereas planktonic growth rate was fully restored. However, x10 was at least as defective in biofilm formation and DNAse production as P10. These data suggest that although repeated exposure to triclosan may result in a SCV-like phenotype, this is not necessarily associated with increased virulence, and adapted bacteria may exhibit other functional deficiencies.