[In vitro activity of mezlocillin, azlocillin and carbenicillin against bacteroidaceae with particular reference to bacteroides fragilis (author's transl)].

The in vitro susceptibility of approximately 90 Bacteroidaceae strains to the the new broad-spectrum penicillins mezlocillin and azlocillin was determined by tube dilution tests and compared to susceptibility to carbenicillin. Bacteroides fragilis and Bacteroides thetaiotaomicron predominated among the strains tested. These strains are particularly important as pathogens and are known to produce beta-lactamase. At concentrations of 16 microng/ml or less, mezlocilin inhibited 81% of the Bacteroides cultures, and at 32 microng/ml 96% of the cultures. The amount of Bacteroides strains inhibited by 16 and 32 microng/ml of azlocillin was 67% and 93% respectively. Similar results were obtained with carbenicillin, concentrations of 32, 64 and 256 microng/ml inhibiting 55%, 87% and 94% of the strains respectively. Mezlocillin and azlocillin did not appear to be any more effective than carbenicillin against Sphaerophorus strains. Statistical analysis of the relationship between minimal inhibiting concentration and inhibition zone diameter showed that the susceptibility of Bacteroidaceae to mezlocillin, azlocillin and carbenicillin cannot be determined with sufficient accuracy by a standardized agar diffusion test.