BETA-LACTAMASE DETECTION IN BACTEROIDES-FRAGILIS GROUP

Beta-lactamase activity among Bacteroides species is generally considered as due to cephalosporinase. We have tested 133 strains of the Bacteroides fragilis group isolated from clinical specimens for their ability to produce a beta-lactamase. The detection method was a microbiological assay, testing for the ability of Bacteroides filtrates to inactivate amoxycillin, cefaloridin, cefuroxime, cefotaxime and cefoxitin. The enzyme was extracted by sonication and centrifugation. The enzyme production was detected by the level of inactivation for each Bacteroides filtrate, using a Clostridium perfringens as test-strain. Isoelectric points of the detected enzymes were determined by isoelectric focusing. We found 128 of the 133 strains producing beta-lactamase. The major beta-lactamase activity was found to be broad spectrum activity = penicillinase and cephalosporinase (101 strains). A second group (20 strains) showed only weak cephalosporinase activity. A third group (4 strains) showed a wide beta-lactamase activity inactivating all beta-lactam antibiotics tested, included cefoxitin and imipenem for 3 strains. Five strains failed to show any enzymatic activity. The isoelectric points were found at pH : 4,9 and occasionally pH = 5,3; pH = 5,6 and pH = 8. No correlation was found between the pHi and the enzymatic activity.