Comparison and Evaluation of Carbenicillin Disks in Diffusion Susceptibility Testing

Abstract

A broad variety of bacterial strains, including 79 Pseudomonas aeruginosa, were studied in an in vitro evaluation of carbenicillin disk susceptibility testing.Regression analysis with both 50-,gg and 100-ag carbenicillin disks was carried out. Organisms having minimal inhibitory concentration values of 100 to 200 ,sg/ml demonstrated zones of less than 11 mm with the 50-,ug disk, resulting in very little opportunity for appropriate discrimination of results. The line of regression for the 50-,gg disk intersected the ordinate at a point just above the minimal inhibitory concentration value considered to be the limit of intermediate susceptibility for Pseudomonas. These considerations, together with evidence of greater disk content variation in the 50-,ag than in the 100-ag disks assayed, considerable manufacturer-to-manufacturer variability with the 50-,ug disk, and the more appropriate performance of the 100-,gg disk, lead us to conclude that the 100-,ug disk better serves the clinical test requirements for this agent than does the 50-Ag disk, which is currently the only disk available for laboratory testing.Carbenicillin (disodium-alpha-carboxybenzyl penicillin), although a relatively new antimicrobial agent, has become very much a part of the antibiotic armamentarium of the physician treating infections caused by Pseudomonas aeruginosa, Proteus species, Providencia, and, to a lesser extent, strains of Enterobacter and Escherichia coli resistant to other antibiotics (3,12,16).Susceptibility testing is now carried out in most diagnostic microbiology laboratories on a routine basis with carbenicillin. Currently, only the 50-,ug disk is approved for carbenicillin testing by the Food and Drug Administration (6, 9). Previously work was done with disks of varied antibiotic content (10-12, 17, 20, 21). However, subsequent to the advent of the Food and Drug Administration decision, a few in vitro studies were published which contain 50-,ug disk data (16,19,22,23

Comparison and Evaluation of Ticarcillin and Carbenicillin Using Disk Diffusion Methods

“…This is within Food and Drug Administration specifications for allowable error in disk content (3). Matsen (6) and Washington (11) found that 50-,g carbenicillin disks were not suitable. Both disk content variation and ability to detect susceptibility at higher MIC values was a problem.…”

“…Original work with disks of varying carbenicillin content (5,9,10) led to the approval of a 50-pug carbenicillin disk and the adoption of criteria for zone size interpretation (3). Recent work by Matsen et al (6) and by Washington et al (11) has suggested that the 100-,ug carbenicillin disk is subject to less variation in content of antibiotic and produces zone sizes that correlate better with carbenicillin minimal inhibitory concentration (MIC) values determined by agar dilution methods.…”

Normalized Resistance Interpretation as a Tool for Establishing Epidemiological MIC Susceptibility Breakpoints

“…This would require a species-related aspect which was not yet accepted, although some reports on this issue had been published (2,12,14). Earlier, O'Brien had shown that an adjustment of interpretive breakpoints in relation to the susceptible peak in zone diameter histograms made results from different laboratories comparable and thereby foresaw later developments (17).…”

Comparison of the In Vitro Activity of BL-P1654 with Gentamicin and Carbenicillin Against Pseudomonas aeruginosa

“…3. When an MIC of 32 ytg/ml or less and a zone of inhibition of 12 mm or more were used as criteria indicating susceptibility, 10.6% of the isolates would be falsely considered resistant, whereas 4.2% of the isolates would be falsely susceptible by the disk susceptibility test.…”

“…Perhaps disks containing different concentrations of this antibiotic should be tested in order to find an optimal concentration for the disk test. The same situation existed in the case of carbenicillin and three different disk concentrations (100, 50, and 25 gg) were evaluated (4,10,12,14,16,18). The Food and Drug Administration has recently approved the 100-Ag carbenicillin disk for use in the clinical laboratory.…”