The antimicrobial activity in vitro of chlorhexidine, a mixture of isothiazolinones ('Kathon' CG) and cetyl trimethyl ammonium bromide (CTAB).

Chlorehexidine, two 4% chlorhexidine antiseptic handwashes ('Bioprep' and 'Hibiclens'), cetyl trimethyl ammonium bromide (CTAB) and isothiazolinones ('Kathon') were tested against Staphylococcus aureus, Micrococcus luteus, Escherichia coli, Serratia marcescens, Pseudomonas aeruginosa, Proteus vulgaris and Candida albicans. The activities measured were the minimum inhibitory concentration (MIC), minimum microbicidal concentration (MMC), rate of kill in water and broth, effect of organic soil, the development of microbial resistance on continuous exposure and agent bioavailability in media and formulation. 'Kathon' was the most active microbistatic agent showing maximal activity at low concentration, least inactivation by organic soil and media components and the lowest level of development of bacterial resistance. It was synergistic with chlorhexidine against S. marcescens and P. aeruginosa. Media, formulation components and organic soil affected the performance of chlorhexidine and CTAB. Chlorhexidine was more broadly active than CTAB but showed a greater reduction in activity in the presence of soil and engendered a greater level of bacterial resistance. It was more rapidly bactericidal to P. aeruginosa and S. marcescens than to S. aureus. Stable resistance to chlorhexidine and CTAB was developed by P. aeruginosa and S. marcescens, the latter showing the higher level of resistance. Chlorhexidine-resistant strains were also resistant to CTAB. The antiseptic formulations were more rapidly bactericidal than chlorhexidine alone but were otherwise of comparable activity. Mixtures of disinfectants, in particular a combination of chlorhexidine and a preservative level of 'Kathon', were more active than single disinfectants. The importance of standardization of media and test conditions and the use of chemically defined media for accurate and reproducible in-vitro testing of disinfectant activity is emphasized. Disinfection kinetics, expressed as time-kill curves, log reduction factors or decimal reduction times were shown to be valuable in differentiating microbistatic from microbicidal activity, showing the effects of dilution and soil on activity and indicating possible different mechanisms of action.