One explanation for the variability of the bacterial suspension test

Disinfection kinetic studies of sodium dodecyl sulphate, benzalkonium chlor ide and sodium hypochlorite against Staphylococcus aureus revealed that whe n a higher inoculum level of Staph. aureus than normal was used (approximat ely 1 log higher), the efficacy of disinfection was severely attenuated. Ki netic analysis using the Hom model for experiments carried out on tests usi ng 3 x 10(8) organisms ml(-1) were unable to account for the large increase in disinfection power observed when smaller inoculum levels were used. Sin ce the inoculum was the same in every way except for the numbers used, the large variations in the log reduction/time curves could not be explained by a variation in the resistance of the population to the biocide, as identic al log reduction-time curves should have resulted. The level of disinfectio n achieved for a given concentration of biocide was found to be approximate ly linearly related to the cell number ml(-1) of test solution and not to t he log number. The variation observed is believed to occur due to intrinsic self-quenching of the biocide by the microbes during the course of the dis infection test. As the level of free biocide decreases, the rate of reactio n decreases, giving the tails of the log reduction/time curves. Such intrin sic self-quenching could explain the large variations known to occur in the legally required disinfection suspension tests.