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

Efficacy of the E-test in evaluating the antimicrobial susceptibility of anaerobic bacteria of veterinary interest

A study was made of the sensitivity of 39 clinical isolates of anaerobic bacteria to 10 antimicrobial agents. Minimal inhibitory concentrations (MICs) were calculated using a new method--the E-test (AB Biodisk, Solna, Sweden)--and compared with those obtained using the conventional agar dilution method. Agreement between the MICs obtained by the two methods with a variation of +/- 2 dilutions was 78.7%. The E-test, though less sensitive than the conventional agar dilution method, may be of value in clinical veterinary practice when rapid selection of treatment for a given infectious process is required.

Review of methods for susceptibility testing of anaerobes

In the USA, the National Committee for Clinical Laboratory Standards has studied and published a reference agar dilution method for the susceptibility testing of anaerobic bacteria. While numerous investigators both in Europe and the USA have evaluated a variety of methods with a variety of modifications, only the broth microdilution method appears to be appropriate for routine use. The problems of the choice of breakpoint, inoculum size, media, media additives, endpoint recognition and other parameters affecting test performance and interpretation, while troublesome for anaerobes, are not unique to this group of organisms. The increasing resistance of anaerobes and the ever existing need to provide therapeutic guidance, surveillance for resistance and susceptibility data on new drugs make the need for an accurate and reliable susceptibility test for anaerobes critical. The newer methods, while showing promise, need further evaluation with all agents that have a therapeutic indication for anaerobic infections.

Comparison of the E test (PDM epsilometer) and broth microdilution susceptibility tests for members of the Bacteroides fragilis group

The E test, a quantitative disk diffusion susceptibility test, was compared with the National Committee for Clinical Laboratory Standards (NCCLS) standardized broth microdilution (BMD) test. A total of 101 isolates in the Bacteroides fragilis group were tested against five antibiotics: cefoxitin, cefotaxime, clindamycin, imipenem, and piperacillin. An agreement of 85%-87% was observed between the E and BMD tests for cefotaxime and clindamycin, 74%-75% for piperacillin and imipenem, and 65% for cefoxitin. Four major (false-resistant) interpretive discrepancies (two each with cefotaxime and piperacillin) and no very major (false-susceptible) discrepancies were observed at 24 hr of incubation. The number of major discrepancies increased to nine when the E test strips were incubated for 48 hr. Although the number of major and very major discrepancies were low for the individual antibiotics, the E test is currently unacceptable with the high proportion of minor discepancies.

Susceptibility testing of anaerobic bacteria: myth, magic, or method?

The demand for susceptibility testing of anaerobes has increased, yet consensus as to procedure and interpretation in this area has not been achieved. While routine testing of anaerobic isolates is not needed, certain isolates in specific clinical settings should be tested. Also, laboratories may monitor their local antibiograms by doing periodic surveillance batch testing. The National Committee for Clinical Laboratory Standards has published a protocol of methods approved for susceptibility testing of anaerobic bacteria. Both agar and broth microdilution are included; however, the broth disk elution method is no longer approved by the National Committee for Clinical Laboratory Standards because of method-related interpretive errors. A number of newer methods are undergoing evaluation and seem promising. Clinicians and microbiologists reviewing susceptibility reports should be aware of sources of variability in the test results. Variables in susceptibility testing of anaerobes include the media and methods used, organisms chosen for testing, breakpoints chosen for interpretation, antibiotic, and determination of endpoint. Clustering of MICs around the breakpoint may lead to significant variability in test results. Adherence of testing laboratories to approved methods and careful descriptions of the method and the breakpoints used for interpretation would facilitate interlaboratory comparisons and allow problems of emerging resistance to be noted. A variety of resistance mechanisms occurs in anaerobic bacteria, including the production of beta-lactamase and other drug-inactivating enzymes, alteration of target proteins, and inability of the drug to penetrate the bacterial wall. Antimicrobial resistance patterns in the United States and abroad are described.

Evaluation of the E test for susceptibility testing of anaerobic bacteria

The susceptibilities of 105 clinical isolates of anaerobic bacteria were determined by a new method, the E test (AB Biodisk, Solna, Sweden), and were compared with the MICs for these organisms obtained by the reference agar dilution method by using supplemented brucella and Wilkins-Chalgren agars. The E test is a plastic strip with a predefined antibiotic gradient immobilized on one side and a MIC interpretive scale printed on the other side. Strips with cefoxitin, cefotaxime, imipenem, penicillin, metronidazole, and clindamycin were used in this study. A suspension of the test strain equal to the visual turbidity of a no. 0.5 McFarland standard was prepared and swabbed onto a 150-mm-diameter plate. The strips were applied in a radial fashion, and the plates were incubated under anaerobic conditions. After growth had occurred, an ellipse of inhibition was seen around each strip. At the point of intersection of the ellipse with the strip, the MIC was read from the interpretive scale. For most antibiotic-organism combinations, the ellipse was clear and the endpoint was sharp. The E-test MICs were interpreted after overnight and 48-h incubation for 58 of the strains. After overnight incubation, 87% of the E-test MICs were within 1 dilution of the agar dilution MICs, and 98% were within 2 dilutions. After 48 h of incubation, agreement was 86 and 97% respectively. E-test MICs obtained for the Bacteroides fragilis group after overnight incubation were more comparable than those obtained after 48 h of incubation to agar dilution MICs determined at 48 h for all drugs except clindamycin. On brucella agar, there was a 2% categorical discrepancy rate between the E-test MICs and agar dilution MICs, which occurred mostly with cefoxitin. The E test is easy to perform and read, is suitable for all anaerobes, can be used to test single patient isolates as needed, and offers the laboratory a reliable method for susceptibility testing of anaerobic bacteria.

Cefotaxime. An update of its pharmacology and therapeutic use

Cefotaxime was the first 'third generation' cephalosporin to be marketed and is administered intramuscularly or intravenously. Similar to other agents of this class, it has a broad spectrum of in vitro activity, particularly against Enterobacteriaceae, including beta-lactamase-producing strains. Cefotaxime forms a metabolite, desacetylcefotaxime, which is antibacterially effective against many bacteria per se and acts additively or synergistically with cefotaxime against many strains. Since the first review of cefotaxime in the Journal, further studies have confirmed its value in the treatment of various infections: complicated urinary tract infections, lower respiratory tract infections, bacteraemia, meningitis, uncomplicated gonorrhoea, infections of skin and soft tissue and of bone and joints, and obstetric and gynaecological infections. Cefotaxime is effective as an empirical treatment of suspected infection due to susceptible organisms in immunocompromised patients and is of proven efficacy in serious, life-threatening infections in general. Cefotaxime reduces the incidence of postsurgical infection but the role of third generation cephalosporins in prophylaxis remains to be determined. The indications for which cefotaxime and other 'third generation' cephalosporins would be considered the most appropriate therapy remain largely dependent upon such factors as varied as cost, local medical custom, decisions of regulatory agencies and geographical patterns of bacterial resistance. Cefotaxime nevertheless represents a valuable 'third generation' cephalosporin of great clinical value in certain infectious conditions, in particular those which are serious and life-threatening and where resistance to therapies is creating a clinical problem.

Discordant results between the broth disk elution and broth microdilution susceptibility tests with Bacteroides fragilis group isolates

Susceptibility testing of 161 clinical isolates of the Bacteroides fragilis group was performed to compare interpretive results generated by the broth disk elution and broth microdilution methods recommended by the National Committee for Clinical Laboratory Standards. Among the cephalosporin-cephamycin compounds tested, correlation was poorest for ceftizoxime (71%), ceftriaxone (57%), and cefotaxime (47%); when the tests did not correlate, false resistance was seen 92, 95, and 93% of the time, respectively. Cefotetan and cefoperazone showed lack of correlation in 19 and 20% of the tests, respectively. For cefotetan, false resistance was more frequent, while with cefoperazone, false susceptibility occurred more often. Cefoxitin produced the fewest discrepancies; 10% of the disk elution tests produced either false-resistance or false-susceptibility results. Mezlocillin and piperacillin showed lack of correlation in 8 and 14% of the tests, respectively, and discrepancies were due primarily to false-resistance results. Overall with the beta-lactams, 84% of the discordant interpretive results were false resistance by the broth disk elution test. Clindamycin had a discrepancy rate of 10%, with the majority of discrepancies being false susceptibility disk elution results. Because of the high number of discrepancies noted with ceftizoxime, ceftriaxone, and cefotaxime, we recommend that these drugs not be tested by the disk elution method and that they be tested by a quantitative MIC method such as the broth microdilution test. Furthermore, caution should be exercised when interpreting broth disk elution results with all the beta-lactams included in this study except imipenem. These data indicate the lack of correlation of results between these two tests for many beta-lactams and suggest the need for a reexamination of the disk elution method to provide a more accurately standardized test.

Determination of susceptibility of anaerobic bacteria to beta-lactam antibiotics by a tablet diffusion test

A standardized tablet diffusion test and a reference agar dilution test was evaluated for susceptibility testing of anaerobic bacteria to beta-lactam antibiotics. 74 freshly isolated anaerobic bacteria and three control strains (Cl. perfringens ATCC 13124 B. fragilis ATCC 25285, B. thetaiotaomicron ATCC 29741) were tested. The in vitro activities of 7 beta-lactam antibiotics were compared with metronidazole and clindamycin. Most active were metronidazole and clindamycin. Cefoxitin had the best activity of the beta-lactam antibiotics, whereas piperacillin and carbenicillin had good activities. High resistance rates were found for penicillin, ampicillin, cefuroxime and cefotaxime. MIC on control strains fell well within range set by the National Committee for Clinical Laboratory Standards (NCCLS). Correlation between MIC and inhibition zone diameters was generally good. Tablet diffusion can be used to divide anaerobic bacteria into three susceptibility categories. In addition all bacterial strains were tested for production of beta-lactamase by a nitrocefin tube test. Beta-lactamase production by the nitrocefin test indicated reduced sensitivity to beta-lactam antibiotics.

Evaluation of in vitro methods for testing ceftriaxone against anaerobic bacteria, including quality control guidelines

Tests with 100 anaerobic bacterial isolates demonstrated comparability between ceftriaxone MICs obtained with the reference agar dilution procedure and those obtained with a broth microdilution susceptibility testing procedure. The aerobically incubated thioglycolate disk elution test was also evaluated. Six 30-micrograms ceftriaxone disks in 5 ml of thioglycolate separated strains for which MICs were less than or equal to 32 micrograms/ml from those for which MICs were greater than or equal to 64 micrograms/ml (6% overall discrepancies). Quality control limits for ceftriaxone agar dilution tests were determined to be as follows: Bacteroides fragilis ATCC 25285, 32 to 128 micrograms/ml; and Bacteroides thetaiotaomicron ATCC 29741, 64 to 256 micrograms/ml.