[Inactivation of chlorhexidine for in vitro testing of disinfectants]

An insufficient neutralization of chlorhexidine (4%), tested in vitro against Enterococcus faecium ATCC 6057, was suspected in the quantitative suspension test although the combination Tween 80, cysteine, histidine and saponine was assessed as sufficient for neutralising chlorhexidine. Bactericidal activity of each neutralising compound and their combinations was excluded. Sufficient neutralization was observed when all dilution steps and the corresponding agar plates were supplemented with the neutralising compounds. Successful neutralization in broth is not equivalent with successful neutralization in tryptic saline. We suggest to supplement at least the agar plates of the first dilution step with neutralising compounds when testing chlorhexidine.