Characterization of anaerobic gram-negative bacilli by using rapid slide tests for beta-lactamase production

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Abstract
Isolates (175) of anaerobic gram-negative bacilli were tested for .beta.-lactamase production by using a slide test modification of the chromogenic cephalosporin (Nitrocefin, Glaxo, Middlesex, England) assay and the iodometric slide test. Included isolates were Bacteroides melaninogenicus, B. fragilis, other Bacteroides isolates, Fusobacterium and other gram-negative bacilli. Both slide tests detected 25 B. melaninogenicus isolates that were .beta.-lactamase producers (minimal inhibitory concentration [MIC] of penicillin was > 0.78 .mu.g/ml). .beta.-lactamase produced by the other gram-negative anaerobes could only be detected by the Nitrocefin assay. This assay was positive in 70 of 77 B. fragilis against which the MIC of penicillin was > 0.78 .mu.g/ml. Of 11 other species of Bacteroides against which the MIC of penicillin was > 0.78 .mu.g/ml 10 were Nitrocefin test-positive. MIC of penicillin against all isolates of Fusobacterium and unidentified gram-negative bacilli were < 0.78 .mu.g/ml and were Nitrocefin assay-negative. .beta.-Lactamase-producing strains of B. melaninogenicus can be differentiated because the slide iodometric and Nitrocefin assays will be positive whereas .beta.-lactamase produced by other Bacteroides will only be detected by the Nitrocefin assay. Such penicillin-resistant isolates could be detected and reported to clinicians before final identification.