Penicillin Resistance and Penicillinase Production in Clinical Isolates of Bacteroides melaninogenicus

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Abstract
The minimum inhibitory concentrations (MIC) of penicillin and six other antimicrobials were determined for 50 clinical isolates of Bacteroides melaninogenicus . Agar dilution susceptibilities were performed using supplemented brucella blood agar and the proposed National Committee for Clinical Laboratory Standards standard method for anaerobes; results with the two methods were comparable. A penicillin concentration ≥0.8 μg/ml was needed to inhibit 56% of the isolates, whereas 100% were susceptible to 0.1 μg of clindamycin per ml. All isolates with penicillin MIC values ≥0.8 μg/ml produced β-lactamase using a slide method. A micro-iodometric assay was used to quantitate β-lactamase production in six isolates. The β-lactamase activity of B. melaninogenicus was comparable to that of a Staphylococcus aureus isolate but was not inducible, and the specific amount produced correlated only partially with penicillin MIC values. A clinical review of patients from whom the β-lactamase-producing strains of B. melaninogenicus were isolated did not suggest any increased virulence in these strains or an unexpectedly poor clinical response to appropriate therapy.