Medium-dependent zone size discrepancies associated with susceptibility testing of group D streptococci against various cephalosporins

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Abstract
Mueller-Hinton (MH) agar media from various commercial sources, supplemented or not supplemented with 5% sheep blood, were studied to determine their effect on disc diffusion susceptibility testing results obtained with 90 strains of group D streptococci and 4 cephalosporins. The cephalosporins investigated were cephalothin, cefamandole, moxalactam and cefotaxime. A number of Streptococcus faecalis and S. faecium strains were susceptible to cephalothin, cefamandole and cefotaxime; the number varied with the commercial source and blood content of the MH medium used. Regardless of the MH medium used, none of the S. faecalis or S. faecium strains were susceptible to moxalactam. The apparently medium-associated variations in the number of strains susceptible to cephalothin, cefamandole and cefotaxime were largely due to minor discrepancies (one result being intermediate) among the various types of MH media used. Major discrepancies (one result being resistant and the other susceptible or vice versa) were observed when S. faecalis strains were tested against cefotaxime. These major discrepancies were associated with the commercial source of the MH medium and the blood content of the medium.