COMPARISON OF THE E-TEST AND CONVENTIONAL AGAR DILUTION METHODS FOR SUSCEPTIBILITY TESTING OF GRAM-NEGATIVE ANAEROBIC RODS

The E test was used to determine the susceptibility of 209 anaerobic G ram-negative rods to ampicillin, amoxicillin-clavulanate, ticarcillin, ticarcillin-clavulanate, piperacillin, piperacillin-tazobactam, clind amycin, chloramphenicol, and cefoxitin. Minimum inhibitory (MICs) were read where growth intersected the strips or where the upper indentati on intersected the strips for beta-lactamase inhibitor-containing stri ps. Reference MICs were performed by the National Committee for Clinic al Laboratory Standards agar dilution method. Organ isms tested consis ted of 117 Bacteroides fragilis group, 24 fusobacteria, and 68 other s pecies (mainly Prevotella species). Agreement of MICs by both methods occurred in 33.4% of cases; 80.8% were within one, 99.0% were within t wo, and 99.95% were within three doubling dilutions apart. E-test resu lts, expressed as MIC(50)/MIC(90) values (mu g/ml), were as follows: a mpicillin: 16/>256; amoxicillin-clavulanate (2:1), 0.5/4; ticarcillin, 16/>256; ticarcillin-clavulanate (2 mu g/ml), 0.5/4; piperacillin, 16 />256; piperacillin-tazobactam (4 mu g/ ml), 1/8; piperacillin-tazobac tam (8:1), 2/8; cefoxitin, 8/64; clindamycin, 0.25/4; and chlorampheni col, 2/8. Conventional MIC methodology yielded MIC(50)s and MIC(90)s e ither identical to, or within one doubling dilution of, E-test results . Percentages of strains susceptible varied by <3% between the two met hods, except for cefoxitin (E test 5.2% lower). Based on these results , the E test is an accurate and practical method for use with the agen ts and organisms evaluated.