Comparative activity of several beta-lactam antibiotics against anaerobes determined by two methods.

The susceptibility of 120 strains of several species of anaerobes to a number of second and third generation beta-lactam antibiotics was determined by the National Committee for Clinical Laboratory Standards reference agar dilution and microdilution methods. The antibiotics tested were cefoperazone, cefotaxime, cefotetan, ceftizoxime, cefoxitin, and imipenem. The MIC50s ranged from 0.125 to 16 micrograms/ml. The MIC90s were lowest with imipenem at 0.5 micrograms/ml, followed by cefoxitin at 32 micrograms/ml; they were highest with cefotetan at 128 micrograms/ml and were 64 micrograms/ml with the others. In vitro drug activity varied with the antibiotic, the organism, the method used, and the breakpoint selected. Rates of resistance varied considerably between the taxonomic groups of organisms tested and also among species within a group. Overall, reproducibility with the agar dilution method ranged from 44% to 85%; testing with ceftizoxime was the least reproducible. Microdilution results agreed within +/- 1 dilution of the agar dilution mode 79% to 95% of the time, with some variation between drugs and organisms tested. Because there were distinct differences in the activity of some drugs against certain species, no antibiotic can substitute for others in in vitro testing.

Predicting the susceptibility of anaerobes to cefoperazone, cefotaxime, and cefoxitin with the thioglycolate broth disk procedure.

A variety of clinical anaerobic isolates were tested against cefoperazone (216 strains), cefoxitin (120 strains), and cefotaxime (120 strains) by the thioglycolate anaerobic broth disk method, and the results were compared with the National Committee for Clinical Laboratory Standards reference agar dilution method. The broth disk and reference breakpoint concentrations were as follows: cefoperazone, 60 and 64 or 30 and 32 micrograms/ml; cefotaxime, 30 and 32 micrograms/ml; cefoxitin, 18 and 16 micrograms/ml, respectively. Discrepant results were retested to obtain a mode. There was 99% agreement between the broth disk and reference methods for cefotaxime, 98% for cefoperazone with 60- and 64-micrograms/ml breakpoints and 91% with 30- and 32-micrograms/ml breakpoints, and 75% for cefoxitin. All but one of the strains that produced false susceptibility results by broth disk were members of the Bacteroides fragilis group, 1 with cefoperazone using the 60-micrograms/ml concentration, 14 with cefoperazone at the 30-micrograms/ml concentration, and 27 with cefoxitin. One strain of Clostridium difficile produced false susceptibility results to cefoperazone at the 30-micrograms/ml concentration. The lack of agreement between the broth disk and reference methods with cefoxitin may be a reflection of the number of isolates at the 16-micrograms/ml level and that the broth disk breakpoint was slightly higher than this concentration. Increased incubation time did not improve the results significantly.

Evaluation of the 10-micrograms clindamycin disk for susceptibility testing of anaerobes by the aerobically incubated thioglycolate broth disk method.

The reliability of the 10-micrograms clindamycin disk was evaluated for susceptibility testing of anaerobes by the aerobic thioglycolate broth disk method. A good correlation between the aerobic thioglycolate broth disk method and the reference agar dilution procedure of the National Committee for Clinical Laboratory Standards was obtained by using a 4-microgram/ml breakpoint. Improved correlation was obtained when the medium of the National Committee for Clinical Laboratory Standards was buffered.

The comparative in vitro susceptibility of cefazolin-resistant organisms to six cephalosporins, four penicillins, and three aminoglycosides.

One thousand thirty-seven cefazolin-resistant, gram-negative clinical isolates including members of the family Enterobacteriaceae, pseudomonads, and other nonfermenters were tested against a variety of newer antimicrobial agents by microdilution. Most of the Enterobacteriaceae were resistant to the second-generation cephalosporins, but highly susceptible to the third-generation agents and the broad-spectrum penicillins, 90% of the strains being inhibited at attainable serum concentrations. Cefoperazone and the penicillins had good activity against the Pseudomonas species, but the aminoglycosides remained the most active agents against all the gram-negative bacilli tested except Pseudomonas maltophilia.

Establishment of MICs of moxalactam for control and reference anaerobic organisms in agar dilution and microdilution techniques.

The MICs of moxalactam were determined for eight National Committee for Clinical Laboratory Standards control and reference strains and for Fusobacterium nucleatum ATCC 10953 by agar and microdilution techniques. The recommended MIC for the control strain Bacteroides fragilis in both agar and microdilution tests is 0.5 micrograms/ml. Recommended MICs for Bacteroides thetaiotaomicron ATCC 29741 and Clostridium perfringens ATCC 13124 by agar dilution are 8 and 0.063 micrograms/ml, respectively. These two strains gave inconsistent results in microdilution tests. Variation in results with microdilution procedures was seen, which illustrates problems in reading endpoints and with modifications of media. Recommended MICs for the reference strains are presented.

Establishment of minimum inhibitory concentrations of cefoperazone for control and reference anaerobic organisms.

The minimum inhibitory concentrations of cefoperazone were determined in a collaborative study for eight National Committee for Clinical Laboratory Standards control and reference strains of anaerobic bacteria by agar and microdilution techniques with several types and sources of media. Recommended minimum inhibitory concentrations for the control strains, Bacteroides fragilis ATCC 25285 and Bacteroides thetaiotaomicron ATCC 29741, are 32 to 64 micrograms/ml and 64 micrograms/ml, respectively. Clostridium perfringens ATCC 13124 gave inconsistent results, and no value is recommended. Recommended values for reference strains are presented. Modification of media did not significantly change the minimum inhibitory concentrations.

Differences in susceptibilities of species of the Bacteroides fragilis group to several beta-lactam antibiotics: indole production as an indicator of resistance.

Clinical isolates of the members of the Bacteroides fragilis group differ markedly in their susceptibilities to a variety of beta-lactam antibiotics, including cefoperazone, moxalactam, cefotaxime, cefoxitin, cefamandole, cephalothin, cefazolin, and carbenicillin, as determined by dilution techniques. The minimum concentrations required to inhibit at least 50% of the strains tested (MIC50) for the entire B. fragilis group were lowest with moxalactam and cefoxitin, 4 and 8 micrograms/ml, respectively, whereas the MIC90S of cefoperazone, cefotaxime, moxalactam, cefoxitin, and carbenicillin were equivalent (64 micrograms/ml); the MIC90S of cefamandole, cephalothin, and cefazolin were higher. Indole-positive members of the group (B. ovatus, B. thetaiotaomicron, and B. uniformis) were significantly more resistant to every antibiotic tested than were indole-negative members (B. fragilis, B. distasonis, and B. vulgatus). In a 6-month survey of clinical laboratory data, indole-positive strains comprised 40% of the B. fragilis group isolates and 22% of all Bacteroides isolates; B. fragilis was the most common species isolated (23%). The increased use of second-generation and the introduction of third-generation cephalosporins may dictate that clinical microbiology laboratories routinely identify members of the B. fragilis group as to species or, alternatively, test for indole production in addition to performing more extensive susceptibility testing.

Identification of members of the family Enterobacteriaceae by the R-B system.

The R-B system was evaluated in parallel with conventional bacteriological procedures for the identification of members of the family Enterobacteriaceae by using bacterial strains from a variety of clinical specimens and from stock cultures. The R-B tests found to be reliable were, in decreasing order, the reactions for phenylalanine deaminase, hydrogen sulfide, and indole, the production of gas from glucose, and the decarboxylation of ornithine. The reactions in the R-B system found to be unreliable were motility, the decarboxylation of lysine, and the fermentation of both glucose and lactose. In addition, the reactions of the R-B system were more difficult to read and interpret than those of the conventional system. On the basis of this evaluation, it was concluded that the R-B system is not an acceptable alternative to the conventional methods in the identification of the Enterobacteriaceae.