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To assess the mycobactericidal efficacy of an agent relatively new to disinfection, ortho-phthalaldehyde (OPA) and to compare the resistances of three Mycobacterium species. Mycobacterium bovis (strain BCG) was compared with Mycobacterium chelonae and Mycobacterium terrae to investigate the feasibility of using either of the latter two species in tuberculocidal testing. M chelonae (a rapid grower) and M terrae (an intermediate grower) both grow faster and are less virulent than M bovis (a slow grower).
The quantitative suspension protocol specified by the Environmental Protection Agency (EPA), the Tuberculocidal Activity Test Method (EPA test), was used throughout this study. Standard suspensions of all three species were prepared in a similar manner. Two suspensions of M bovis, created in different laboratories, were used. These were tested against two concentrations of alkaline glutaraldehyde to provide reference data. Two concentrations of OPA were evaluated against all mycobacterial test suspensions. Four replicates of each organism-disinfectant combination were performed.
Results were assessed by analysis of variance. M terrae was significantly more resistant to 0.05% OPA than either M bovis or M chelonae. At 0.21% OPA, M terrae was slightly more susceptible than one test suspension of M bovis, but not significantly different from the other. M chelonae was significantly less resistant than the other species at both OPA concentrations. At their respective minimum effective concentration, OPA achieved a 6-log10 reduction of M bovis in nearly one sixth the time required by glutaraldehyde (5.5 minutes vs 32 minutes).
These data, along with other recent studies, lend support to the idea that M terrae may be a suitable test organism for use in the tuberculocidal efficacy testing of disinfectants. They also confirm the relatively rapid tuberculocidal activity of OPA.
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