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

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Abstract
Using the Rapid Inoculum Standardization System (RISS), the use of overnight agar cultures in the preparation of inocula for disc diffusion and microdilution susceptibility tests was investigated; results were compared with susceptibility results obtained with the standard methods. The RISS 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; 25 gram-positive cocci, 75 enteric and nonfermentive gram-negative bacilli and 40 more fastidious bacteria (Haemophilus, Neisseria and pneumococci) were evaluated. The geometric mean inoculum size for all organisms tested was 9.7 .times. 107 CFU [colony-forming units]/ml by the standard method and 1.1 .times. 108 CFU/ml for the RISS. The categories of susceptibility obtained by both methods in the disc diffusion tests were comparable, as were the minimal inhibitory concentrations. Thus, the use of overnight agar cultures and the RISS to prepare inocula for susceptibility tests are acceptable alternative procedures in the standard methods for susceptibility tests.