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

Mueller-Hinton (MH) agar media from various commercial sources, either supplemented or not supplemented with 5% sheep blood, were studied to determine their effect on disk diffusion susceptibility testing results obtained with 90 strains of group D streptococci and four cephalosporins. The cephalosporins investigated included cephalothin, cefamandole, moxalactam, and cefotaxime. Results showed that a number of Streptococcus faecalis and Streptococcus faecium strains were susceptible to cephalothin, cefamandole, and cefotaxime, but the number varied with both 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 found to be 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. However, 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 both the commercial source of the MH medium and the blood content of the medium.