ROUTINE DETECTION OF INDUCIBLE BETA-LACTAMASE IN GRAM-NEGATIVE BACILLI BY MEANS OF DISK SUSCEPTIBILITY TESTING

A modified disk approximation test using a disk dispenser was employed to detect inducible beta-lactamase in gram-negative bacilli. In 186 (63.1%) of 295 clinical isolates beta-lactamase activity was detected by demonstration of antagonism between cefoxitin and third generation cephalosporins including ceftazidim, ceftriaxone, cefotaxim, or latamoxef. The rates of inducible beta-lactamase were: for Pseudomonas spp., 92.4% (134 of 145 isolates); for Enterobacter spp., 88.2% (30 of 34); for Morganella morganii, 88.9% (8 of 9); for Citrobacter freundii, 85.7% (6 of 7); and for Serratia spp., 63.6% (7 of 11). Resistance to cefoxitin was associated with inducible beta-lactamase in 172 out of 190 (90.5%) isolates, intermediate sensitivity in 10 out of 16 (62.5%), and sensitivity in 4 out of 89 (4.5%). Subculture studies of isolates with inducible beta-lactamase on media containing antibiotics showed persistent resistance in 5.4%. The disk approximation test using a dispenser can be performed easily in a routine laboratory and provides rapid and potentially important information for the clinician caring for patients treated with beta-lactam antibiotics.