Influence of growth medium on the in vitro activities of second- and third-generation cephalosporins against Streptococcus faecalis

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Abstract
The influence of culture medium on the minimum inhibitory concentration (MIC) of 8 cephalosporins for 45 strains of S. faecalis was investigated. The MIC of cephalothin, cefamandole and cefoperazone were not substantially influenced by the type of culture medium used. MIC of cefuroxime, ceftizoxime, cefotaxime, cefenoxime and ceftriaxone varied markedly with both the commercial brand and the blood content of the broth used. The use of Mueller-Hinton broths (from Oxoid Ltd., GIBCO Diagnostics, and Difco Laboratories) supplemented with 5% lysed sheep blood frequently resulted in MIC that were .gtoreq. 16 times lower than the MIC obtained with the same broths without blood. Similar, but less marked, patterns were observed when supplemented and unsupplemented brain heart infusion and Sceptor broths were used. The influence of broth on MIC suggests a complex interaction between some cephalosporins, medium components and organisms. The cephalosporins that were affected by media share an identical moiety at the 7-acyl position (cefuroxime is slightly different), but this structure is not shared by those cephalosporins that were not affected. This commonality in structure at the 7-acyl position may be partially responsible for the observed results.