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At least 19 recordsLinked to original sources
PubMed1976

Short-term storage of six penicillins and cephalothin in microdilution trays for antimicrobial susceptibility tests.

To perform microdilution antimicrobial susceptibility tests efficiently, broth dilutions of the antimicrobial drugs should be prepared in large batches and stored frozen until needed. Studies were carried out to document the stability of ampicillin, benzylpenicillin, carbenicillin, methicillin, oxacillin, nafcillin, and cephalothin in microdilution trays during storage at -20 and -60 C. No significant deterioration was noted after 21 days of storage, as detected by bioassay and by replicate minimal inhibitory concentration determinations with control organisms. No significant differences were noted moreover between trays stored for three weeks at -20 C and those stored at -60 C. The microdilution technique was found to be a highly reproducible method for quantitative determination of antimicrobial susceptibility.

Biological Assay↗
PubMed1979

Rapid antimicrobial susceptibility test using tetrazolium reduction.

A rapid method of antimicrobial susceptibility testing has been developed, which uses a modified microdilution procedure and an inoculum of 10(7) bacteria per ml. Results are determined within 4 h with an indicator consisting of 2(p-iodophenyl)-3(p-nitrophenyl)-5-phenyltetrazolium chloride. The precipitation of a red formazan by bacteria uninhibited by antimicrobials is accelerated by the addition of phenazine methosulfate. Isolates are classified as resistant, indeterminate, or susceptible, based on growth in up to two antimicrobial concentrations which conform closely to concentrations which correlate with the millimeter breakpoints used in the Bauer-Kirby method. Results of testing 10 antimicrobial agents against 1,126 isolates were compared with results obtained when the Bauer-Kirby method and the agar dilution procedure were used as reference methods. Enterococci were excluded because of false resistance. Discrepancies were classified as very major (false susceptibility), major (false resistance), and minor (combinations of susceptibility or resistance with indeterminate results). The rapid method versus the agar dilution method yielded 2.3% very major, 0.7% major, and 2.9% minor discrepancies, for a total of 6.0%. Of 58 organism-antimicrobial agent combinations tested, 23 displayed 1% very major discrepancies between the rapid method and the agar dilution method. Six were not therapeutically important. The remainder involved Staphylococcus aureus, Staphylococcus epidermidis, Acinetobacter sp., and most organisms tested with chloramphenicol. It is suggested that adjustments in antibiotic concentrations and/or inoculum size may eliminate these discrepancies. The rapid method appeared economical when compared with Autobac 1 and the Bauer-Kirby procedure.

Evaluation Studies as Topic↗
PubMed1976

Antimicrobial susceptibility testing.

Considerable advances have been made in antimicrobial susceptibility testing. This review emphasizes the continued efforts toward standardization of methods for dilution and diffusion testing, particularly in the area of variation in medium performance, methods for detection of ampicillin resistant Haemophilus influenzae, and attempts to develop rapid automated systems for susceptibility testing. Susceptibility testing of anaerobes continues to be controversial from the standpoint of both the selection of methods and the application of susceptibility results to the clinical situation. It is expected that these and other problems in susceptibility testing will be solved by continued application of the investigative approaches that have brought us where we are today.

Ampicillin↗
PubMed1978

Evaluation of microdilution trays (sensititre) for antimicrobial susceptibility testing of Bacteroides fragilis.

Microdilution trays (Sensititre) for antimicrobial susceptibility testing of Bacteroides fragilis has been compared for accuracy with the standard agar dilution method. The microtrays were found to be reliable for all antimicrobials tested (benzylpenicillin, clindamycin, doxycycline, chloramphenicol, fusidic acid, cefoxitin, and cephalotin) but not for metronidazole.

Anti-Bacterial Agents↗
PubMed1979

Antimicrobial susceptibility testing of pneumococci: determination of Kirby-Bauer breakpoints for penicillin G, erythromycin, clindamycin, tetracycline, chloramphenicol, and rifampin.

Antimicrobial susceptibility testing of pneumococci is now essential to monitor for the presence of resistance to agents such as the penicillins, macrolides, lincomycins, chloramphenicol, and tetracycline. In this study, clinical isolates of a selection of resistant South African strains were tested for antimicrobial susceptibility by minimal inhibitory concentration (MIC) determination and by a modified Kirby-Bauer disk diffusion technique, using Mueller-Hinton medium supplemented with 5% horse blood. Disk diffusion breakpoints were determined for penicillin G, erythromycin, clindamycin, tetracycline, chloramphenicol, and rifampin. Reliable results were obtained on disk diffusion for all these agents except for penicillin G. With 6-mug penicillin G disks, zones of strains with intermediate penicillin susceptibility overlapped those of sensitive and resistant strains. With 5-mug methicillin disks, clearer separation of strains based on susceptibility to penicillin G occurred. Strains with zones of <35 mm around penicillin G disks and <25 mm around methicillin disks should have penicillin G MICs determined to confirm their resistance to penicillin G. In view of the potential for pneumococci to be resistant to the agents used in this study, antimicrobial susceptibility of all clinically significant isolates should be determined.

Chloramphenicol↗
PubMed1976

Sisomicin versus netilmicin: in vitro susceptibility testing.

Antimicrobial susceptibility to sisomicin and netilmicin (Sch 20569) was determined on 164 clinical isolates using a broth microdilution method. Sisomicin was active against 86.1%, and netilmicin against 96.4%, of the isolates. In addition, netilmicin was active against 93.7% of the strains that were resistant to gentamicin, kanamycin, tobramycin, and sisomicin.

Aminoglycosides↗
PubMed1979

Antimicrobial susceptibility testing of Mycobacterium fortuitum complex.

A total of 24 strains of the Mycobacterium fortuitum complex were tested for susceptibility to antimicrobial agents by the disk diffusion and agar dilution techniques. By comparing zones of inhibition obtained with the disk diffusion technique with results of minimal inhibitory concentration determinations, it was shown that disk diffusion results could predict in vitro susceptibility to selected antimicrobial agents. All of 17 strains of M. fortuitum were susceptible to </=1 mug of amikacin per ml. The corresponding average zone of inhibition around a 10-mug amikacin disk was 37 mm. Seven M. chelonei strains were more resistant to amikacin, with minimal inhibitory concentrations ranging from 1 to 32 mug/ml, and the corresponding average zone size was 21 mm. Susceptibility of both M. fortuitum and M. chelonei to tetracycline was variable and none of the M. chelonei strains was inhibited by polymyxin B, whereas M. fortuitum strains consistently had zones of inhibition around the polymyxin disk. It appears that identification to species of the M. fortuitum complex may be of importance with regard to antibiotic susceptibility. Separation of M. fortuitum and M. chelonei was readily accomplished in the present study by the nitrate reduction and 3-day arylsulfatase tests.

Aminoglycosides↗
PubMed1979

A rapid (4--6-hour) urine-culture system for direct identification and direct antimicrobial susceptibility testing.

This study evaluates a new direct rapid system for urine cultures, including detection and quantitation of positive specimens by Gram stain, direct identification by 4--6-hour incubation of sediment with reagent strips, and antibiotic susceptibility testing by direct (3--4-hour) disk-elution methods. Of 987 routine urine specimens, 121 had significant (less than or equal to 10(5) colony-forming units/ml) gram-negative bacilluria, of which 89% were detected by the Gram stain. Direct rapid identification was correct in 94%. Results of direct disk-elution antimicrobial tests showed overall agreement with results of standard disk diffusion of 93% of tests, and major discrepancies in 4%. For urine specimens with gram-negative bacilluria, this system permitted detection, quantitation, identification, and antimicrobial susceptibility testing in four to six hours with reasonable, though not complete, accuracy.

Anti-Bacterial Agents↗
PubMed1977

Chemically defined antimicrobial susceptibility test medium for Pseudomonas aeruginosa.

A chemically defined growth medium containing physiological concentrations of magnesium and calcium ions was utilized in a microdilution procedure for antimicrobial drug susceptibility testing of Pseudomonas aeruginosa. Determinations of growth end points were simplified by use of sodium citrate as a sole carbon source and bromothymol blue as a pH indicator. Growth of the test organisms was detectable by a change in the indicator color from green to blue after alkalinization of the medium due to citrate utilization. Minimal inhibitory concentrations of amikacin, carbenicillin, gentamicin, and tobramycin were determined on 100 recent clinical isolates of Pseudomonas. Parallel determinations using the microdilution procedure and a conventional tube-broth dilution technique incorporating Mueller-Hinton broth with identical magnesium and calcium content generally agreed within one twofold dilution. Modal minimal inhibitory concentrations for susceptible strains using the microdilution method were: amikacin, 6 mug/ml; carbenicillin, 50 mug/ml; gentamicin, 1.5 mug/ml; tobramycin, 1.5 mug/ml. This modified microdilution technique allowed rapid, definitive minimal inhibitory concentration determinations, using growth end points defined by a color indicator change.

Amikacin↗
PubMed1976

Antimicrobial susceptibility testing of yeasts: a turbidimetric technique independent of inoculum size.

The current development of potent antifungal drugs necessitates reliable methods of in vitro susceptibility testing of clinical isolates. Other laboratories have noted with some antimicrobials that conventional tube dilution visual end points for determining minimal inhibitory concentrations (MIC) vary markedly with the inoculum. With seven isolates (four species) we compared the conventional method with one developed with a spectrophotometer. Yeasts were diluted to inocula of 10(2) to 10(5) cells/ml and incubated at 37 degrees C with 5-fluorocytosine or miconazole or without drug in a synthetic defined medium. At frequent intervals MICs were determined by the lowest drug concentration without visible turbidity. Concurrently, the percent transmission of each tube was measured and an inhibitory concentration (IC((1/2))) was calculated from the turbidimetric measurements. MICs of both drugs for all strains varied with the inoculum. Intrastrain differences averaged 128-fold for miconazole and greater than 8-fold with 5-fluorocytosine. This variability in MIC occurred when readings were made after 12 to 48 h of incubation. In contrast, IC((1/2)) values of all isolates varied less than two-fold with the same range of inocula. Most isolates showed this independence when measured at 48 h. Fast-growing yeasts showed greater IC((1/2)) variability unless the turbidimetric measurements were taken earlier while the drug-free growth was still in a rapid growth phase. This method is reproducible, inoculum independent, rapid, free from subjectivity and observer variability, and suitable for adaptation to a clinical setting.

Methods↗
PubMed1976

Comparison of direct and standardized antimicrobial susceptibility testing of positive blood cultures.

In an effort to determine the feasibility and accuracy of performing direct susceptibility tests from positive blood cultures, minimal inhibitory concentrations (MICs), determined by the agar dilution method, of direct and standardized tests with seven antibiotics were compared. Results were analyzed as to the number of very major (change in MIC from susceptible in preliminary direct testing to resistant in final standardized testing), major (change in MIC from resistant to susceptible), and minor (change in MIC without change in interpretation) discrepancies. The results for gram-positive cocci and for gram-negative bacilli were 5, 6, and 82 of 162 strains tested and 3, 12, and 79 of 90 strains tested, respectively. Of the total number of susceptibility tests compared, major and very major discrepancies occurred in only 1 and 2.4% of instances with gram-positive and -negative isolates, respectively. The majority of discrepancies were noted with Staphylococcus epidermidis (four very major, five major), Klebsiella (two very major, four major), and Alcaligenes (five major). The antibiotics most often exhibiting discrepancies were penicillin, ampicillin, and cephalothin. The results indicate that preliminary susceptibility testing directly from positive blood culture bottles is generally both feasible and accurate.

Anti-Bacterial Agents↗
PubMed1975

Microdilution technique for antimicrobial susceptibility testing of anaerobic bacteria.

A microdilution technique using commercially available media and materials was developed and used to determine the minimal inhibitory concentrations (MICs) of clindamycin, chloramphenicol, tetracycline, minocycline, ampicillin, carbenicillin, cephalothin, and gentamicin for 101 anaerobic isolates. Representative strains of Bacteroides, Clostridium, Fusobacterium, Peptococcus, and Peptostreptococcus were tested. The use of Schaedler broth at pH 7.2, an inoculum of 10(5) to 10(7) colony-forming units per ml, and incubation at 35 C in an anaerobic glove box with an atmosphere of 80% nitrogen, 10% hydrogen, and 10% carbon dioxide resulted in good growth and easily interpretable results. After 48 h of incubation, 97% of strains tested were inhibited by 3.1 mug or less of clindamycin per ml and 98% were inhibited by 12.5 mug or less of chloramphenicol per ml. Tetracycline and minocycline inhibited 81 and 88% of strains tested in concentrations of 1.6 mug or less per ml and 1.6 mug or less per ml, respectively. Ampicillin inhibited all strains other than B. fragilis in concentrations of 3.1 mug or less per ml. Excluding certain strains of Bacteroides and Clostridium, carbenicillin in concentrations of 12.5 mug or less per ml and cephalothin in concentrations of 6.2 mug or less per ml inhibited all strains tested. Gentamicin was inactive although some strains of anaerobic cocci and Bacteroides were inhibited by 3.1 mug or less per ml. After 18 to 24 h of incubation, eight of the 101 strains had not grown sufficiently for MICs to be determined; for the 93 strains which had grown sufficiently, 93% of 744 MICs were the same or one concentration lower than the 48-h MICs.

Anaerobiosis↗
PubMed1975

Simplified method for antimicrobial susceptibility testing of anaerobic bacteria.

A simple, abbreviated broth dilution test (tube test) utilizing a commercially available medium and inexpensive disposable materials, and which could be performed entirely in room air, was developed and used to test the susceptibility of 100 strains of anaerobic bacteria to clindamycin, chloramphenicol, ampicillin, and tetracycline. Results are reported in categories of susceptibility: susceptible to concentrations surpassed in vivo with usual dosage, susceptible to concentrations surpassed in vivo with high dosage, and resistant to concentrations achievable in vivo. Results are compared to minimal inhibitory concentrations which were determined simultaneously by using a microdilution method in an anaerobic glove box. Twenty strains of Bacteroides fragilis, 10 strains of Fusobacterium, 20 strains of Clostridium, 10 strains of gram-positive non-sporeforming bacilli, and 30 strains of cocci grew to visible turbidity within 1 day of incubation. Of the 360 antibiotic-organism combinations tested, 98% were in a susceptibility category that corresponded (within one concentration) to the actual minimal inhibitory concentration as determined by the microdilution method. After 2 days of incubation, growth was more abundant, but results often indicated inappropriate degrees of resistance. Variation in inoculum size had little effect on results. Ten strains of B. melaninogenicus did not grow enough for susceptibility to be categorized accurately. The tube test could be used in any clinical microbiology laboratory for a limited number of susceptibility tests on anaerobic bacteria other than B. melaninogenicus without preparation of special media or purchase of special equipment.

Ampicillin↗