CRITERIA FOR TESTING THE SUSCEPTIBILITY OF STREPTOCOCCUS-PNEUMONIAE TO CEFOTAXIME AND ITS DESACETYL METABOLITE USING 1 MU-G OR 30 MU-G CEFOTAXIME DISKS

In vitro susceptibility tests were performed with 350 selected strains of Streptococcus pneumoniae to evaluate disk diffusion tests with 30 mu g and 1 mu g cefotaxime disks. Zones were compared to MICs of cefotaxime with and without its desacetyl metabolite. Cefotaxime was two to eight times more active than desacetyl cefotaxime, but the two compounds were additive when combined in vitro. For 30 mu g disks, zone size breakpoints were less than or equal to 27 mm, 28-30 mm and greater than or equal to 31 mm for resistant, intermediate and susceptible, respectively. For 1 mu g disks, those zone size criteria were reduced to less than or equal to 13 mm, 14-16 mm and greater than or equal to 17 mm. The 30 mu g disk that is currently available for testing other species can be used for testing pneumococci; however, the 1 mu g disk has some important advantages.