Comparative susceptibility of resident and transient hand bacteria to para-chloro-meta-xylenol and triclosan

Aims: To determine the susceptibility of planktonic and biofilm-grown strains of resident and transient skin bacteria to the liquid hand soap biocides para-chloro-meta-xylenol (PCMX) and triclosan. Methods and Results: Freshly isolated hand bacteria were identified by partial 16S rRNA gene sequencing. Two resident and three transient strains, as well as four exogenous potential transient strains, were selected for biocide susceptibility testing. The minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) of planktonic cells were determined. Resident and transient strains showed a range of susceptibilities to both biocides (PCMX, MIC 12.5-200 mg l(-1) , MBC 100-400 mg l(-1) ; triclosan, MIC 0.6- > 40 mg l(-1) , MBC 1.3- > 40 mg l(-1) ). Strains were attached to polystyrene plates for 65 h in 96-well microtitre plates and challenged with biocide to determine the biofilm inhibitory concentration and biofilm eradicating concentration. For all strains tested, biofilms were two- to eightfold less susceptible than planktonic cells to PCMX. Conclusions: Very few transients were detected on the hand. Transients were not more sensitive than residents to the biocides and susceptibility to PCMX and triclosan was strain dependent. Biofilm-grown strains were less susceptible to PCMX than planktonic cells. Significance and Impact of the Study: The study provides increased knowledge about the susceptibility of skin bacteria to biocides present in typical liquid antibacterial hand soaps and suggests that the concentration of biocide employed in such products is in excess of that required to kill the low numbers of transient bacteria typically found on skin.