Evaluation of carriers used in the test methods of the Association of Official Analytical Chemists

Revision of the official test method for the determination of the tuberculocidal activity of disinfectants is being undertaken. The current procedure lacks precision and accuracy and is not quantitative. Variability associated with carriers and the lack of temperature control were evaluated in this paper. The use of porcelain versus stainless steel carriers was also evaluated. When carriers of either type were contaminated with Mycobacterium bovis BCG, the number of organisms on the carriers varied by as much as 1.0 on the log10 scale. The average number of organisms attached to each porcelain carried was 1.10 x 10(5) CFU (range, 2.7 x 10(4) to 2.7 x 10(5) CFU), whereas the average number of organisms attached to each stainless steel carrier was 1.38 x 10(5) CFU (range, 2.9 x 10(4) to 4.0 x 10(5) CFU). The average number of cells attached to the carrier was directly proportional to the number of cells in the contaminating cell suspension. Variations in drying time did not alter the number of cells attached to the carrier. When porcelain carriers were placed in a test solution, the average number of organisms washed from the carriers was 55% of the total, with a range of 19 to 80%, whereas for stainless steel carriers, the average number was 82% of the total, with a range of 52 to 96%. Data for B. subtilis spores were similar to those for M. bovis BCG, suggesting that there may be similar problems with the Association of Official Analytical Chemists sporicidal test, which uses carriers. It was also found that the lack of an exacting temperature control could influence the outcome of the test. Changes in temperature as little as 1 degree C could influence the rate of killing of M. bovis BCG.