Antibiotic Susceptibility Testing of Gram-Negative Nonfermentative Bacilli

Abstract

A study was undertaken to determine if current methods of antibiotic susceptibility testing could be successfully applied to the gram-negative nonfermentative bacilli. Using clinical isolates and reference strains, experiments were conducted on the inherent reliability of the Bauer-Kirby method, as well as the .effect of certain modifications on the method such as elimination of the 2-to 5-h incubation in broth and use of different agar media. Results obtained using these modifications were compared to the results obtained by the standard method. It was shown that the two modifications investigated had a significant effect on the interpretation of zone diameters. It was further shown that the standard Bauer-Kirby method with some exceptions correlates with minimal inhibitory concentrations as determined by broth dilution methods. Results suggest that the Bauer-Kirby method may be a reliable technique for testing the antibiotic susceptibility of the nonfermentative bacilli.

Stakelama marina sp. nov., isolated from seawater of the Tangyin hydrothermal field in the Okinawa Trough

“…Antibiotic susceptibility tests were carried out using the disc diffusion assay (Kirby-Bauer method) according to Ruddell and Anselmo [22] on R2A agar after growth at 28 °C for 3 days. Strain LXI357 T was found to be sensitive (µg per disc) to kanamycin (30), vancomycin (30), chloramphenicol (100), ampicillin (30), carbenicillin (100), cefalexin (30), cefoperazone (75), cephradine (30), cefalexin (30), ceftriaxone (30), cefazolin (30), ceftazidime (30), cefuroxim (30), neomycin (30), doxycycline (30), erythromycin (15), penicillin (10), piperacillin (100), aboren (30), ofloxacin (5), ciprofloxacin (5), paediatric compound sulfamethoxazole tablets (23.75/1.25), gentamicin (10) and norfloxacin (10).…”

Emergence in a Burn Center of Populations of Bacteria Resistant to Gentamicin, Tobramycin, and Amikacin: Evidence for the Need for Changes in Zone Diameter Interpretative Standards

“…At that time, however, few if any gentamicin-resistant strains had emerged, and the discriminatory capacity of the breakpoint could not be fully evaluated. Subsequently, it was reported (25,27) that some strains of P. aeruginosa showing gentamicin zones in the resistant range had MICs of less than 4 ,ug/ml; conversely, others reported encountering strains with agar dilution MICs of over 4 p,g/ ml that showed zones of up to 15 to 16 mm (22,23,26). These disparities are explainable, at least in part, by the demonstrated effect of divalent cation content on the susceptibility of Pseudomonas to gentamicin.…”

“…Many of the earlier studies designed to determine criteria for susceptibility by the diffusion test were performed before the difficulty with the cationic content of media was recognized. More recent studies have brought the validity of the present laboratory definition of resistance to gentamicin into question (15,22,23,26).…”