Inoculum standardization in antimicrobial susceptibility testing: evaluation of overnight agar cultures and the Rapid Inoculum Standardization System.

Using the Rapid Inoculum Standardization System [RISS; Minnesota Mining & Manufacturing Co. (3M Co.)], we investigated the use of overnight agar cultures in the preparation of inocula for disk diffusion and microdilution susceptibility tests and compared our results with susceptibility results obtained with the standard methods. The 3M system consists of an inoculation wand, with which a reproducible number of bacteria can be removed from an agar plate, and a diluent in which to suspend the organisms. We used 25 gram-positive cocci, 75 enteric and nonfermentive gram-negative bacilli, and 40 more fastidious bacteria (Haemophilus, Neisseria, and pneumococci) for the evaluation. The geometric mean inoculum size for all organisms tested was 9.7 X 10(7) CFU/ml by the standard method and 1.1 X 10(8) CFU/ml for the RISS. The categories of susceptibility obtained by both methods in the disk diffusion tests were comparable, as were the minimal inhibitory concentrations. We recommend that the use of overnight agar cultures and the use of the RISS to prepare inocula for susceptibility tests be acceptable alternative procedures in the standard methods for susceptibility tests.