Reference Agar Dilution Method Research Articles

Comparative antimicrobial activity of gatifloxacin tested against Campylobacter jejuni including fluoroquinolone-resistant clinical isolates

Campylobacter jejuni is an important pathogen that causes gastroenteritis, as well as other disease states such as meningitis and septic arthritis. In this study, the Etest (AB BIODISK, Solna, Sweden) results were compared to a reference agar dilution method using gatifloxacin, a new 8-methoxyfluoroquinolone. A total of 53 strains of C. jejuni initially isolated from patients in California and Mexico were tested. Results demonstrated a high correlation (r = 0.88) between the two utilized in vitro dilution methods. In addition, gatifloxacin activity was compared to that of ciprofloxacin, metronidazole, amoxicillin, erythromycin, chloramphenicol, gentamicin, tetracycline, and trimethoprim/sulfamethoxazole using the Etest. Gatifloxacin (MIC 90, 4μg/ml) was approximately eight- to 16-fold more potent than ciprofloxacin (Mic 90, > 32 μg/ml), a commonly used fluoroquinolone for Campylobacter infections. Eight strains highly resistant to ciprofloxacin (MIC 90, > 32 μg/ml) were tested for cross resistance against the newer fluoroquinolones (gatifloxacin, levofloxacin, trovafloxacin) and the rank order of potency was: gatifloxacin (MIC 50, 16 μg/ml) > trovafloxacin = levofloxacin (MIC 50, > 32 μg/mL). However, only 25% ciprofloxacin-resistant strains were inhibited by ≤ 1 μg/mL of gatifloxacin or trovafloxacin. These results for gatifloxacin against C. jejuni strains must be further assessed in the context of in vivo trials before the clinical role of this new fluoroquinolone can be determined. The Etest appears to be a simple and precise susceptibility test method for testing C. jejuni isolates against fluoroquinolones and other alternative therapeutic agents.

Antimicrobial activity of gatifloxacin tested against Neisseria gonorrhoeae using three methods and a collection of fluoroquinolone-resistant strains

Gatifloxacin, a new 8-methoxy fluoroquinolone, was tested against 131 strains of Neisseria gonorrhoeae by reference agar dilution, disk diffusion, and Etest (AB BIODISK, Solna, Sweden) methods on supplemented GC agar. Gatifloxacin activity was equal to ciprofloxacin (MIC 50, 0.008 μg/mL) against strains fully susceptible to fluoroquinolones, but was generally four-fold more active (MIC 90, 0.064–0.094 μg/mL) against strains with par C or gyr A mutations and resistance to ciprofloxacin. Etest results were comparable to those generated by the agar dilution test [correlation coefficient (r) = 0.97]. Gatifloxacin zone diameters using 5-μg disks also correlated well (r = 0.86–0.87) with the agar dilution and Etest MIC results. Breakpoints for laboratory testing of Neisseria gonorrhoeae strains await clinical trial outcome correlations, but susceptibility at ≤0.125 or ≤0.25 μg/mL (≥34 mm) seems appropriate. All three tests used in this study seem applicable for laboratory testing of isolates from patients with uncomplicated gonorrhoeae receiving therapy with gatifloxacin.

Survey of anaerobic susceptibility patterns: A French multicentre study

In 1996, the in vitro antibiotic susceptibility of 463 anaerobes was measured in five hospitals using the reference agar dilution method. None of the 209 B. fragilis group strains showed resistance to imipenem or ticarcillin-clavulanic acid. High resistance rates (29%) were observed for cefotetan and clindamycin. β-Lactamase production was detected respectively in 64% of the Prevotella and 7% of the Fusobacterium strains. Because the same standardized methods were used for many years, the authors were able to evaluate the evolution of antibiotic resistance. Clindamycin resistance had increased within the B. fragilis group (from 14% in 1992 to 29% in 1996) and also among strains of clostridia (32%), P. acnes (18%) and Peptostreptococcus (28%). In the B. fragilis group multidrug resistance was unlikely to occur.

In vitro comparison of disk diffusion and agar dilution antibiotic susceptibility test methods for Neisseria gonorrhoeae.

At present, most Neisseria gonorrhoeae testing is done with beta-lactamase and agar dilution tests with common therapeutic agents. Generally, in bacteriological diagnosis laboratories in Argentina, study of antibiotic susceptibility of N. gonorrhoeae is based on beta-lactamase determination and agar dilution method with common therapeutic agents. The National Committee for Clinical Laboratory Standards (NCCLS) has recently described a disk diffusion test that produces results comparable to the reference agar dilution method for antibiotic susceptibility of N. gonorrhoeae, using a dispersion diagram for analyzing the correlation between both techniques. We obtained 57 gonococcal isolates from patients attending a clinic for sexually transmitted diseases in Tucumán, Argentina. Antibiotic susceptibility tests using agar dilution and disk diffusion techniques were compared. The established NCCLS interpretive criteria for both susceptibility methods appeared to be applicable to domestic gonococcal strains. The correlation between the MIC's and the zones of inhibition was studied for penicillin, ampicillin, cefoxitin, spectinomycin, cefotaxime, cephaloridine, cephalexin, tetracycline, norfloxacin and kanamycin. Dispersion diagrams showed a high correlation between both methods.

Anti- Legionella activity of trovafloxacin compared with seven other antimicrobial agents including an intermethod evaluation

The activity of trovafloxacin, a new fluorinated naphtheridone, was tested against 61 Legionella spp. isolates and compared with that of 4 fluoroquinolones, 2 macrolides, and rifampin. Trovafloxacin MICs were determined by a reference agar dilution method and E-test (Solna, Sweden) strips on buffered charcoal yeast extract agar. Among the fluoroquinolone compounds, the rank order of activity (on the basis of MIC 90 results) determined with E-test strips was as follows: levofloxacin (MIC 90, 0.094 μg/ml) > trovafloxacin = sparfloxacin = ofloxacin (MIC 90, 0.19 μg/ml). Rifampin (MIC 90, 0.008 μg/ml) and clarithromycin (MIC 90, 0.032 μg/ml) were the most potent of all drugs tested, and erythromycin and ciprofloxacin were the least active. In this study, the E-test strips with trovafloxacin were validated (100% of results ± one log 2 dilution compared to the reference value) for susceptibility testing with Legionella isolates.

Broth Disk Elution Method for Anaerobic Bacteria: A Collaborative Study to Assess Its Reliability for Clinical Purposes

A collaborative study involving seven laboratories was undertaken to evaluate the reproducibility and the reliability of the broth disk elution test against anaerobic bacteria by comparing with the reference agar dilution method. A two breakpoint broth test was also evaluated. Assays were performed using the same testing conditions (i.e. medium, temperature, atmosphere and incubation time). One hundred Gram-negative and Gram-positive clinical isolates were initially studied. Overall agreement of 98.5% and 97.5%, were found for disk elution and the two breakpoint tests, respectively. In order to assess the reliability of the disk elution test, two different lots (LOT1 and LOT2) of disks of piperacillin and clindamycin were selected, to obtain two final concentrations after dilution (10 and 60 mg/mL; 1 and 4 mg/mL, respectively). Two hundred and eighty assays were performed against one strain of bothBacteroides fragilis(piperacillin MIC, 8.0 mg/mL; clindamycin MIC, <0.5 mg/mL) andBacteroides thetaiotaomicron(piperacillin MIC, 16.0 mg/mL; clindamycin MIC, <0.5 mg/mL). With LOT 1, considering both species and both antibiotics, the agreement among six laboratories ranged from 85% to 100% (P > 0.05) with the higher concentration. Overall agreement among all laboratories was 91%. No optimal agreement (>90%) for clindamycin-Bacteroides thetaiotaomicronusing the LOT1 (77%) was found. Since this finding was not observed with LOT2 (100% agreement), discrepancies were attributed to variation between lots. Overall agreement with LOT2 was 100% for all centres. The present study indicates that the broth disk elution method proved to be a reliable and suitable alternative for routine susceptibility testing for anaerobic bacteria, as a resistance screening method for clinical purposes.

Comparison of the Etest and a microbroth dilution system (Sceptor) to a reference agar dilution method for susceptibility testing of Bilophila wadsworthia.

OBJECTIVE: To compare the Etest and a microbroth dilution system (Sceptor) to a reference agar dilution method for susceptibility testing of Bilophila wadsworthia. METHODS: The susceptibility of 15 clinical isolates of Bilophila wadsworthia was determined by the National Committee for Clinical Laboratory Standards (NCCLS) agar dilution method using triphenyltetrazolium chloride for endpoint determination. The results were compared with the results obtained by the E test and a commercial microbroth dilution system (Sceptor). RESULTS: Comparison of the MICs obtained by the reference method and the Etest revealed few discrepancies, with piperacillin and metronidazole being the only exceptions. The overall agreement was 70% within one dilution step. The discrepancies did not result in major interpretative errors. The overall essential agreement using susceptibility categories was 98% for the E test and 99% for the microdilution system. CONCLUSIONS: Both methods may be considered as acceptable alternatives for testing individual isolates of B. wadsworthia.

Comparative evaluation of etest for susceptibility testing Neisseria meningitidis with eight antimicrobial agents An investigation using U.S. food and drug administration regulatory criteria

Invasive infections caused by Neisseria meningitidis continue to be a serious clinical problem for therapy, epidemiology, and potential prophylaxis. Multiple antimicrobial resistances have emerged among meningococcal strains including elevated MICs to penicillin, sulfonamides, tetracyclines, and rifampin. Thus, the need to perform accurate susceptibility testing of meningococci in clinical practice and for surveillance programs has renewed priority. In this study, for the first time Etest (AB Biodisk, Solna, Sweden) was compared to a reference agar dilution method using a large number (100) of clinical strains selected for a range of drug resistances. Etest quantitative accuracy (±one log 2 dilution agreement) ranged from 94% (penicillin) to 100% (three drugs) for the eight clinically useful antimicrobial agents tested. Intermethod categorical accuracy for all drugs ranged from 92% (penicillin, erythromycin) to 100% (five drugs), without false-susceptible or -resistant errors using Etest. Etest and reference methods were highly reproducible (99.6 to 100.0%, respectively). Quantitative discords between methods (≥two log 2 dilutions) were not reproducible and resolved following repeated testing. Etest method proved to be an accurate and reproducible quantitative method for testing N. meningitidis strains for the compared antimicrobial agents (eight) often utilized for therapy and prophylaxis of serious meningococcal disease.

Multidrug-resistant gram-positive pathogens. An update on current microbiological patterns.

Although resistance has developed among Gram-positive pathogens to penicillins, cephalosporins, aminoglycosides, quinolones and macrolides, the glycopeptides seem to remain largely unaffected. However, the recent emergence and range of glycopeptide resistance in enterococci, well documented in the USA but not in the rest of the world, have prompted this European surveillance study. The European Glycopeptide Resistance Survey was undertaken in 1995 in 9 countries and involved 70 microbiological centres. The primary aims of the survey were as follows: (i) to perform a microbiological quality assurance assessment to evaluate the ability of participating laboratories to correctly identify the strains and assess their glycopeptide susceptibility; and (ii) to accurately determine the level of glycopeptide resistance among staphylococci, streptococci and enterococci in European hospitals. The in vitro activity of several other antibiotics was assessed on strains isolated from the Italian centres. In total, 7078 Gram-positive isolates were collected in Europe, and national coordinators used the National Committee for Clinical Laboratory Standards (NCCLS) agar dilution reference method to successfully retest 96% of these. According to mode minimum inhibitory concentrations (MICs), teicoplanin activity was similar to that of vancomycin against Staphylococcus aureus. In general, the range of MICs for teicoplanin was wider than that for vancomycin against coagulase-negative staphylococci. Against Enterococcus spp. and Streptococcus spp., teicoplanin was 4 times more active than vancomycin. The greatest number of glycopeptide refractory organisms was evident among enterococci; resistance was observed to be approximately 10 times more frequent in Enterococcus faecium than in E. faecalis. The results from the Italian isolates were similar to those from the overall study. In particular, teicoplanin was 2- to 8-fold more active than vancomycin against the majority of the enterococci. The incidence of enterococcal resistance was lower in Italy (0.6% for teicoplanin and 0.9% for vancomycin) than in Europe (1.7% for teicoplanin and 2.3% for vancomycin). This extensive survey confirms that teicoplanin is more active than vancomycin against enterococci and streptococci, and that both display similar potency against staphylococci.

Evaluation of the standardized disk diffusion and agar dilution antibiotic susceptibility test methods by using strains of Neisseria gonorrhoeae from Tucumán, Argentina.

At present, most Neisseria gonorrhoeae testing is done with beta-lactamase and agar dilution tests using common therapeutic agents. Generally, in bacteriological diagnosis laboratories in Argentina, study of antibiotic susceptibility of N. gonorrhoeae is based on beta-lactamase determination and agar dilution method using common therapeutic agents. The National Committee for Clinical Laboratory Standards (NCCLS) recently described a disk diffusion test that produces results similar to the reference agar dilution method for antibiotic susceptibility of N. gonorrhoeae. We obtained 57 gonococcal isolates from patients attending a clinic for sexually transmitted diseases in Tucumán, Argentina. Antibiotic susceptibility tests using agar dilution and disk diffusion techniques were compared. The established NCCLS interpretive criteria for both susceptibility methods appeared to be applicable to domestic gonococcal strains. The correlation between the minimum inhibitory concentration (MIC's) and the zones of inhibition was studied for penicillin, ampicillin, cefoxitin, spectinomycin, cefotaxime, cephaloridine, cephalexin, tetracycline, norfloxacin and kanamycin. Dispersion diagrams showed a high correlation between both methods, with a sensitivity of 89% and specificity of 91%.

Critical appraisal of E test for the detection of fluoroquinolone resistance.

The ability of E test to accurately detect fluoroquinolone resistance was compared with an agar dilution reference method. Nearly 300 isolates belonging to 26 different species (62.5% with documented ciprofloxacin resistance) were tested with ciprofloxacin, fleroxacin, levofloxacin, norfloxacin, ofloxacin, and sparfloxacin. In contrast to earlier reports, E test MIC values for pneumococci and all rapid growing aerobes were routinely higher (approximately 0.5 log2 dilution step) than agar dilution results. The E test stable-gradient method also efficiently identified fluoroquinolone-resistant subpopulations which were not detected by the reference procedure with the standard inoculum. Categorical agreement for 1710 test comparisons was approximately 90% with no very major, false-susceptible errors. We found the E test to be a valid, reproducible method for fluoroquinolone susceptibility testing that provided quantitative results and produced a conservative (1.6% false-resistant results) assessment of susceptibility of bacteria including isolates of Streptococcus pneumoniae and Pseudomonas aeruginosa to compounds in this antimicrobial class.

In vitrosusceptibility of 124Xanthomonas maltophilia(Stenotrophomonas maltophilia) isolates

The in vitro susceptibility of 124 Xanthomonas maltophilia isolates was tested by four methods: Agar dilution (reference method), E-test, a disk diffusion and a tablet diffusion method. Trimethoprim-sulfamethoxazole had the highest activity against X. maltophilia, followed by a combination of aztreonam-clavulanic acid at different ratios, the ratio 1:1 being the most active with a susceptibility rate of 85% as compared to 2% for aztreonam alone. Addition of the beta-lactamase inhibitor tazobactam to piperacillin enhanced the rate of susceptible isolates from 31% to 53%, Relatively few isolates were susceptible to ciprofloxacin (27%) and gentamicin (9%). Generally, the disk diffusion method had a considerably higher frequency of "very major" discrepancies when compared with the agar dilution method than with the other methods. The susceptibility of X. maltophilia to trimethoprim-sulfamethoxazole and ciprofloxacin could reliably be determined by all the diffusion methods tested, but otherwise the agar dilution method is to be preferred. A standardized and reliable diffusion method for susceptibility testing of X. maltophilia remains to be found. Trimethoprim-sulfamethoxazole must be considered the drug of choice in the treatment of severe X. maltophilia infections. The combination aztreonam-clavulanic acid is promising, but must be proved in a clinical setting.

Evaluation of Susceptibility of Anaerobic Organisms by the Etest and the Reference Agar Dilution Method

Anaerobes continue to be recognized as clinically significant bacteria that play an important role in a variety of infections [1, 2]. Improvements in methodology-including techniques for sample collection, transportation, and culture-and availability of commercial identification kits have simplified the identification of anaerobes. The development of resistance to antimicrobial agents has made the reporting of susceptibilities of anaerobes less predictable [2, 3] and has increased the demand on laboratories to provide such susceptibility data. The Etest (AB BIODISK, Solna, Sweden) consists of an inert, nonporous plastic strip with a continuous exponential gradient of antibiotic immobilized on one side; the other side is marked with a scale for reading the MIC. Ninety-two gram-negative and 58 gram-positive clinical anaerobic isolates were tested, including Bacteroides fragilis (18 strains), Bacteroides distasonis (10 strains), Bacteroides ovatus (9 strains), Bacteroides thetaiotaolicron (9 strains), Bacteroides vulgatus (8 strains), Bacteroides uniformnis (5 strains), Porphyroillonas bivius (10 strains), Porphyromonas disiens (8 strains), Porplhvromonas intermnedius (6 strains), Porphyromonas mnelaninogenicus (5 strains), Porphyronmonas buccae (4 strains), Clostridiu,m difficile ( 12 strains), Clostridiuml perfringens (10 strains), other unidentified Clostridium7 species (14 strains), Peptostreptococcus species (7 strains), Propionibacteriuml species (10 strains), and Actinoii mvces species (5 strains). The following organisms were used for quality control: B. fragilis ATCC 25285, B. thetaiotaollicron ATCC 29741, B. vulgatus ATCC 29327, and C. perfringens ATCC 13124. If any ATCC strains tested outside their established MIC range, the entire batch of results was rejected. MICs were determined with use of Wilkens-Chalgren agar supplemented with 5% lysed horse blood, incorporating twofold dilutions of antibiotic as outlined by the National Committee for Clinical Laboratory Standards (NCCLS) [4]. The inoc-

Evaluation of the etest for antimicrobial spectrum and potency determinations of anaerobes associated with bacterial vaginosis and peritonitis

One hundred ninety-seven anaerobic organisms (24 Gardnerella vaginalis, 16 Mobiluncus spp., 19 Peptostreptococcus spp., 20 Lactobacillus spp., 20 Prevotella bivia/disiens, 81 Bacteroides fragilis group, 12 Clostridium spp., and five Fusobacterium spp.) were processed by the Etest (AB Biodisk, Solna, Sweden) and a reference ( Brucella blood agar) method against 10 antimicrobial agents. For the bacterial vaginosis-associated pathogens, the Etest was more reproducible and correlated acceptably with the reference agar test: within ±1 log 2 dilution for 74.4% of Mobiluncus spp. to 96.0% for Peptostreptococcus spp. (all organisms, 83.4%). The quantitative correlation ±2 log 2 dilution steps between test results was 94.3%. Results with B. fragilis group strains demonstrated 97.3% correlation (±2 log 2 dilution) with a trend toward slightly lower Etest minimum inhibitory concentrations for ampicillin-sulbactam, cefotaxime, imipenem, and clindamycin. The absolute qualitative interpretive agreement between Etest and the reference agar dilution method results was 94.4%, with only a 0.4% false-susceptible error rate. The Etest appears to be a very practical, quantitatively accurate, alternative procedure for clinical microbiology laboratories routinely testing the susceptibilities of anaerobes and, by these presented data, organisms associated with female tract infections.

Evaluations of the etest for antimicrobial susceptibility testing of Legionella pneumophila, including validation of the imipenem and sparfloxacin strips

Development of antimicrobial testing for Legionella spp. has been technically compromised by the fastidious growth requirements of this organism and by the most commonly used medium, buffered charcoal yeast extract agar (BCYE) that contains substances known to inhibit some antimicrobial agents. This study validated the potency of two newer antimicrobials (sparfloxacin and imipenem) and their Etest strip. The comparisons of antimicrobial minimum inhibitory concentration (MIC) values as determined by Etest on BCYE agar (98 Legionella pneumophila) demonstrated that sparfloxacin was the most potent drug (MIC 90, 0.19 μg/ml) among the fluoroquinolones, macrolides, tetracyclines, and β-lactams tested. Imipenem MICs (MIC 90, ⩽0.38 μg/ml) were also determined by a reference agar dilution method and validated Etest strips on buffered yeast extract agar, buffered charcoal yeast extract agar, and buffered charcoal yeast extract agar with defined supplements. The non- Legionella control strains used to demonstrate medium component influences on the imipenem MICs demonstrated the addition of supplements (particularly L-cysteine) markedly elevated the MICs. These data indicate that the Etest was a simple and accurate quantitative method for susceptibility tests with Legionella isolates. Sparfloxacin among the fluoroquinolones and imipenem among the β-lactams require further clinical studies for legionellosis therapy.

Susceptibility patterns and characterization of beta-lactamases in clinical isolates ofBacteroids fragilis

Strains of Bacteroides fragilis isolated from clinical specimens are known to be frequently resistant to penicillins or cephalosporins. The production of beta-lactamases is the major mechanism in this resistance. Up to now at least five types of beta-lactamase enzymes have been described in the Bacteroides fragilis group. The purpose of this study was to evaluate the susceptibility to different antibiotics and the prevalence of the beta-lactamase types among 100 Bacteroides fragilis strains isolated from clinical specimens in Italy. MICs of 11 antibiotics were determined by the reference agar dilution method and by the new E test (AB Biodisk, Sweden). Results showed an acceptable correlation between the MICs determined for most strains, although those obtained by the E test tended to be lower. In the beta-lactam resistant strains the enzyme activity and isoelectric points were determined on crude enzyme extract.

Evaluation of the E test for antimicrobial susceptibility testing of Porphyromonas gingivalis.

The antimicrobial susceptibilities of 81 recent clinical Porphyromonas gingivalis isolates and two reference strains were determined by the E test, a new method, and were compared with the minimal inhibitory concentrations for these strains obtained by the reference agar dilution method on supplemented Brucella blood agar. The following agreements were obtained: benzylpenicillin 100%, ampicillin 96%, cefaclor 82%, cefuroxime 91%, erythromycin 93%, clindamycin 99%, tetracycline 66%, doxycycline 89%, metronidazole 77% and ciprofloxacin 77%. Very major discrepancies were observed with ciprofloxacin. This study indicates that the E test is an acceptable method to determine the susceptibility of P. gingivalis for most antimicrobials.

Effect of CO2 on susceptibilities of anaerobes to erythromycin, azithromycin, clarithromycin, and roxithromycin

The Oxyrase agar dilution method (Oxyrase, Inc., Mansfield, Ohio), which provides an anaerobic environment without added CO2, was compared with the reference agar dilution method recommended by the National Committee for Clinical Laboratory Standards (anaerobic chamber with 10% CO2) to test the susceptibilities of 302 gram-negative and gram-positive anaerobes to erythromycin, azithromycin, clarithromycin, and roxithromycin. For erythromycin, the overall MIC for 50% of isolates tested (MIC50) was 0.5 micrograms/ml and the MIC90 was 8.0 micrograms/ml by the Oxyrase method, whereas they were 4.0 and 64.0 micrograms/ml, respectively, under standard anaerobic conditions with CO2. At a breakpoint of 4.0 micrograms/ml, 88% of strains were susceptible to erythromycin by the Oxyrase method, whereas 63% were susceptible in the chamber. The corresponding MIC50s and MIC90s of azithromycin, clarithromycin, and roxithromycin by the Oxyrase method were 0.5 and 8.0, 0.25 and 4.0, and 0.5 and 16.0 micrograms/ml, respectively, whereas in the chamber they were 4.0 and > 64.0, 2.0 and 64.0, and 2.0 and 64.0 micrograms/ml, respectively. At a breakpoint of 8.0 micrograms/ml for these three drugs, 89, 92, and 85% of the isolates, respectively, were susceptible by the Oxyrase method, whereas 67%, 72, and 68% of the isolates, respectively, were susceptible in the chamber. Most strains resistant to all four compounds by both methods were Bacteroides distasonis, Fusobacterium mortiferum, Fusobacterium varium and non-Clostridium perfringens Clostridium species. Results of the study may lead to a reappraisal of the role played by macrolides and azalides in the treatment of anaerobic infections.

Comparison of susceptibility test methods to detect penicillin-resistant Streptococcus pneumoniae

The detection of penicillin-resistant Streptococcus pneumoniae was assessed by six different methods: agar dilution, oxacillin screen by disk diffusion, E-test, and three overnight microdilution test methods that included commercial panels from MicroScan and Micro Media and in-house-made conventional panels using a commercial Haemophilus test medium (HTM) broth. Of the 52 pneumococcal isolates tested, 12 were resistant, 16 were relatively resistant, and 24 were susceptible to penicillin as defined by the reference agar dilution method. The oxacillin screen detected as resistant all 28 resistant and relatively resistant strains. The percentage of penicillin-resistant isolates detected by each minimum inhibitory concentration (MIC) test method was as follows: E-test (100%), Micro Media (75%), MicroScan (0%), and HTM (0%). With the relatively resistant isolates, the detection percentage was as follows: E-test (88%), Micro Media (94%), MicroScan (69%), and HTM (69%). In conclusion, the E-test and Micro Media MIC tests are acceptable confirmatory tests for detecting penicillin resistance among S. pneumoniae isolates.

Cefotetan Susceptibility Testing Against AnaerobicBacteria From Obstetrical and Gynecologic Sources:Comparison of Five Different Methods

Five different antibiotic susceptibility methods were utilized to test the effectiveness of cefotetan against 200 anaerobic bacteria recovered from patients with obstetrical or gynecological infections. The object of this study was to determine if a more economical and rapid method for anaerobic susceptibility testing was as acceptable as the reference agar dilution method. The five methods were: 1) broth disk elution, 2) microbroth technique, 3) a commercially available microbroth technique, 4) a commercially available spiral gradient technique, and 5) reference agar dilution. The minimal inhibitory concentrations (MICs) calculated from the spiral gradient technique were equal to or within one doubling dilution of the reference system in 99.5% of cases, while the percentage for the commercially available microbroth system was 96.8%, very similar to the microbroth technique used in our laboratory that yielded a percentage of 96.3. The disk elution method correlated to the reference agar dilution method in 95.3% cases. While the overall agreement between these techniques is good, especially for the spiral gradient system, clustering of certain organisms near the breakpoint of the antibiotic tested results in variability in the labeling of these organisms as susceptible or resistant. This problem appears to be particularly significant for the disk elution method. Therefore, further refinements in these methods of suscleptibility testing are needed in order to provide a more clinically useful assessment of the susceptibility or resistance of certain bacterial isolates.