Influence of inoculum size on comparative susceptibilities of penicillinase-positive and -negative Neisseria gonorrhoeae to 31 antimicrobial agents.

The effects of two inoculum sizes (10(4) and 10(6) CFU) on the MICs of 20 beta-lactam antibiotics, 4 aminoglycosides, and 7 other antimicrobial agents were compared for 102 unselected strains of Neisseria gonorrhoeae (26 penicillinase positive and 76 penicillinase negative), with three replicates for each test. The method was agar plate dilution on Mueller-Hinton agar supplemented with 1% hemoglobin and 1% IsoVitaleX. For penicillinase-positive strains, a large inoculum (10(6) CFU) increased the MIC greater than or equal to 16-fold for benzylpenicillin, piperacillin, azlocillin, and mezlocillin and increased the MIC greater than or equal to 8-fold for ampicillin, cefoperazone, ceftazidime, cefonicid, and cefamandole. The inoculum size had little or no influence on MICs of any antimicrobial agent for penicillinase-negative strains. For penicillinase-positive strains, the most active antibiotics (MIC, less than or equal to 0.001 microgram/ml) were the new cephalosporins: cefotaxime, ceftazidime, ceftizoxime, ceftriaxone, and cefmenoxime. For penicillinase-negative strains, the most active antibiotics were piperacillin, ceftriaxone, ceftizoxime, and cefmenoxime (MIC, less than or equal to 0.001 microgram/ml), closely followed by ceftazidime, moxalactam, azlocillin, mezlocillin, and cefuroxime.

Correlation of moxalactam (LY127935) susceptibility tests by disk diffusion and agar plate dilution methods.

Susceptibility of pathogenic aerobic bacteria to moxalactam (LY127935) was compared by two methods, diffusion from 30-microgram disks and agar plate dilution. The two methods gave a satisfactory degree of correlation when compared by linear regression, but the slope of the linear regression was significantly steeper for gram-negative bacilli (Enterobacteriaceae and Pseudomonas aeruginosa) than for coccal organisms (streptococci and staphylococci). The largest zone diameters by disk diffusion were found with Citrobacter, Proteus, Escherichia coli, Klebsiella, and Enterobacter strains. The error rate-bounded method of Metzler and DeHaan (J. Infect. Dis. 130:588-594, 1974) gave useful breakpoints of disk zone sizes for moxalactam resistance and susceptibility.

Comparative activities of the oxa-beta-lactam LY127935, cefotaxime, cefoperazone, cefamandole, and ticarcillin against multiply resistant gram-negative bacilli.

A total of 91 multiply resistant bacterial strains, including Klebsiella pneumoniae (32 strains), Pseudomonas aeruginosa (16 strains), and Serratia marcescens (43 strains), were collected during hospital epidemics of nosocomial infection from 1975 to 1979. These strains were resistant to gentamicin, tobramycin, cephalothin, chloramphenicol, and ampicillin. Their susceptibility to three new broad-spectrum beta-lactams, LY127935 (a 1-oxa-beta-lactam), cefotaxime (HR 756), and cefoperazone (T 1551), was compared with the susceptibility of random strains of nine species of aerobic gram-negative bacilli collected in the same hospital in 1979. Susceptibility to cefamandole and ticarcillin was also determined. Strains of staphylococci and streptococci from that hospital and two nearby city-county hospitals were also compared for the three new cephalosporins and other effective antibiotics. The agar dilution method was used to measure the minimum inhibitory concentration for each antibiotic. The multiply resistant strains (minimum inhibitory concentration for gentamicin >/= 8 mug/ml) usually were as susceptible to the three new broad-spectrum beta-lactams as were non-multiply resistant strains. Both Klebsiella pneumoniae and Serratia marcescens, including multiply resistant and non-multiply resistant strains, were most susceptible to the 1-oxa-beta-lactam LY127935 and cefotaxime. P. aeruginosa (both multiply resistant and non-multiply resistant strains) were most susceptible to cefoperazone. All three new beta-lactams were active against non-multiply resistant strains of Escherichia coli, Enterobacter spp., Proteus spp., and Citrobacter spp. Providencia stuartii were most susceptible to cefotaxime and the 1-oxa-beta-lactam LY127935. The three new beta-lactams were all less active against staphylococci (especially methicillin-resistant Staphylococcus aureus) than cephalothin. Streptococcus pyogenes and S. pneumoniae were very susceptible to cefotaxime and cefoperazone, though less susceptible to LY127935. None of the three new beta-lactams was active against S. faecalis. All were very active against both penicillinase-positive and -negative strains of Neisseria gonorrhoeae.

Evaluation of a commercial microdilution system for quantitative susceptibility testing of aminoglycosides against multidrug-resistant, gram-negative bacilli.

Susceptibility of clinical isolates of Pseudomonas aeruginosa (29 isolates), Klebsiella species (54 isolates), Escherichia coli (28 isolates), Serratia marcescens (28 isolates), and Enterobacter species (29 isolates) to gentamicin, tobramycin, and amikacin was determined by the following three methods: commercial broth microdilution trays, standard agar dilution, and disk diffusion susceptibility. A total of 504 tests were performed by each method, and overall susceptibility or resistance determined by the broth microdilution method agreed with that determined by the agar dilution method in 92.7% of the tests, whereas results from the disk diffusion method agreed with those from the agar dilution method in 91.9% of the tests. The broth microdilution and disk diffusion methods agreed with each other 88.7% of the time. The broth microdilution system results varied from the agar dilution method results by more than one dilution in 121 of 504 determinations (24%); however, this altered susceptibility determinations in only 7.3% of the assays. E. coli isolates were found to be quantitatively more resistant to the aminoglycosides with the broth microdilution method than with the agar dilution method. In contrast, the broth microdilution method demonstrated P. aeruginosa to be quantitatively more susceptible to the aminoglycosides than when the results were obtained by the agar dilution method. The Micro-Media Systems method is economical, reliable, rapid, and simple to perform and yields quantitative minimum inhibitory concentrations.