Influence of growth media on the sensitivity of Staphylococcus aureus and Pseudomonas aeruginosa to cationic biocides.
In this study, the influence of culturing Staphylococcus aureus and Pseudomonas aeruginosa under different growth conditions on their inactivation by the cationic active compds. benzalkonium chloride, chlorhexidine digluconate and octenidine dihydrochloride was investigated. Cells were grown in non-agitated tryptone soya broth as well as on tryptone soya agar according to national and international stds. for evaluating chem. disinfectants. In quant. suspension tests, cells of both test organisms grown on agar were significantly more sensitive to all three biocides than cells grown in broth. The differences in antimicrobial activity were greater in the case of S. aureus than in the case of P. aeruginosa. With S. aureus cultures, differences in the redn. factor of up to 5 log steps were found, with P. aeruginosa up to 2.5 log steps. The results of our uptake tests performed with S. aureus and octenidine dihydrochloride indicated that the growth conditions and the assocd. different stress factors either had an influence on the compn. of the cell surface of this test organism or induced the formation of an efflux system. Cells of S. aureus cultured in broth took up only one-fifth of the amt. of biocide mols. compared to cells from agar cultures. These data correlated with the results of the suspension tests. A low uptake of biocides apparently led to a reduced killing rate. In contrast to S. aureus, no significant differences in the uptake of octenidine dihydrochloride by cells of P. aeruginosa could be obsd. These cells took up the same amt. of the antimicrobial substance, whether on agar or in broth. In view of these results, possible consequences should be considered prior to changing test regulations.
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