Agar disk diffusion susceptibility characteristics of azlocillin, carbenicillin, mezlocillin, piperacillin, and ticarcillin

Abstract

The agar disk diffusion susceptibility of Enterobacteriaceae to mezlocillin and piperacillin was correlated with agar minimal inhibitory concentrations and compared with the susceptibility to carbenicilhin. The agar disk susceptibility of Pseudomonas aeruginosa to azlocillin, mezlocillin, and piperacilhin was correlated with agar minimal inhibitory concentrations and compared with the susceptibility to carbenicillin and ticarcillin. Criteria are offered for the zones of inhibition to provide information about resistant and susceptible isolates that correlate with known serum levels.A number of penicillins with extended antibacterial spectra have been developed in the past few years. A 100-,ug carbenicillin disk has been used to determine the susceptibility of Enterobacteriaceae and Pseudomonas species to carbenicillin and ticarcillin, the antibiotics currently available for general use in the United States. A 75-,tg ticarcillin disk has been evaluated, and zone sizes for resistance have been correlated with the miniimal inhibitory concentration (MIC)(9). Two ureido penicillins, mezlocillin and azlocillin, and a piperazine derivative of ampicillin have been shown by several groups to be more active in vitro than carbenicillin against many bacteria (2,3,5,12 Susceptibility tests. MICs were determined by the agar dilution method, utilizing a replicating device which delivered a drop to agar plates (11). More detailed descriptions of the method have been published (9). The standard inoculum used was 105 colonyforming units. The MICs and the disk diffusion tests were performed at the same time. Organisms were grown overnight in Mueller-Hinton broth and diluted to the proper inoculum in broth. The inoculum for the disk diffusion was adjusted to a 0.5 MacFarland opacity standard. Both agar dilution plates and disk diffusion plates (Muelier-Hinton agar, BBL) were incubated for 18 h at 35°C.The breakpoints used for determination of strains susceptible to carbenicillin were based on the NCCLS recommendations (6). P. aeruginosa were considered susceptible if the inhibition zone size was .-17 mm and resistant if the zone size was c14 mm. When testing Enterobacteriaceae the zone size for susceptibility was >23 mm and that for resistance was c17 mm. The carbenicillin MICs corresponding to these zone sizes were used as criteria to determine susceptibility and resistance with azlocillin, mezlocillin, piperacillin, and ticarcillin.The zone sizes were reported to the nearest 0.5 mm.

Mezlocillin: tentative interpretive standards for disk diffusion susceptibility testing

“…The pharmacokinetics of mezlocillin appear to be well established (2,3,8,13). We report herein attempts to confirm a recent report on the interpretive zone standards for this semisynthetic penicillin with commercially prepared disks and the disk diffusion susceptibility testing technique (11,15 (1,6). Disk diffusion tests on the same isolates were performed, utilizing the latest method recommended by the National Committee for Clinical Laboratory Standards (NCCLS) (11).…”

Importance of methodology in determining bactericidal and bacteriostatic activities of azlocillin and ticarcillin against Pseudomonas aeruginosa

“…Strains that gave zones of inhibition of 13 mm or less were regarded as resistant. This is consistent with the criteria laid down by Brown and Blowers (1978) for gentamicin, Phaneuf and Neu (1979) for ticarcillin and Fass (1982) for azlocillin. Disks were controlled with a standard sensitive strain, P .…”

Quality control and interpretive criteria for the azlocillin disk diffusion susceptibility test

“…When P. mirabilis and A. hydrophila were included, the correlation coefficient was -0.87. In a previous study (7), in which regression line analyses of inhibitory zone diameters and MICs performed by the agar dilution method were calculated, the correlation coefficient for P. aeruginosa was -0.60 and there was no zone (6 mm) for MICs of >115 ,ug/ ml; Enterobacteriaceae were not studied. In the present study, the correlation coefficient was -0.92 for P. aeruginosa and the calculated nozone MIC was >258 ,ug/ml.…”

“…Azlocillin is a new ureidopenicilin which is similar to mezlocillin but is more active against Pseudomonas aeruginosa and less active against Enterobacteriaceae (2,7). In this study, values were established for monitoring quality control when performing the disk diffusion susceptibility test with 75-,Lg azlocillin disks.…”