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 f-lactams, LY127935 (a 1-oxa-p-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 2 8 ,ug/ml) usually were as susceptible to the three new broad-spectrum ,B-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-/ilactam LY127935 and cefotaxime. P. aeruginosa (both multiply resistant and non-multiply resistant strains) were most susceptible to cefoperazone. All three new B-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-,-lactam LY127935. The three new ,6-lactams were all less active against staphylococci (especially methicillinresistant 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 ,B-lactams was active against S. faecalis. All were very active against both penicillinase-positive and -negative strains of Neisseria gonorrhoeae.

The purpose of the present study was to compare the results for the MIC determinations obtained for three aminoglycosides by the MMS method with the standard agar dilution MIC and the standard disk diffusion susceptibility test when performed on a large number of multidrug-resistant, gram-negative bacilli. MATERIALS AND METHODSTest strains. A total of 168 gram-negative bacilli which had been saved by storage at -76°C (6) from patients with multidrug-resistant, gram-negative rod infections were tested (7). The organisms tested were a combination of selected isolates which had been previously found to be either susceptible to aminoglycosides or resistant to gentamicin and tobramycin. There were 29 isolates of Pseudomonas aeruginosa, 54 of Klebsiella (K. pneumoniae and K. oxytoca), 28 of Escherichia coli, 28 of Serratia marcescens, and 29 of Enterobacter species. Susceptibility of each of these strains to gentamicin, tobramycin, and amikacin was investigated by the MMS method, the standard agar dilution method, and the standard disk diffusion method. Tests were also performed on E. coli ATCC 25922 and P. aeruginosa ATCC 27853, which were used as reference bacteria.Microdilution susceptibility tests Sets of commercially available MMS trays were purchased from the nearest distribution center. Susceptibility tests were performed according to the manufacturer's instructions. The inoculum is prepared by picking four to five isolated colonies from an isolation plate and placing them into a tube containing 0.5 ml of brain heart infusion broth. This broth is then incubated for 4 to 6 h to achieve a stationary growth phase. The inoculum is then diluted by placing 0.05 ml of the brain heart infusion broth culture into a tube contain-

The effects of two inoculum sizes (104 and 106 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 Muelier-Hinton agar supplemented with 1% hemoglobin and 1% IsoVitaleX. For penicillinase-positive strains, a large inoculum (106 CFU) increased the MIC 216-fold for benzylpenicillin, piperacillin, azlocillin, and mezlocillin and increased the MIC >8-fold for ampicillin, cefoperazone, ceftazidime, cefonicid, and cefamandole.

Susceptibility of pathogenic aerobic bacteria to moxalactam (LY127935) was compared by two methods, diffusion from 30-pg 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 organisns (streptococci and staphylococci). The Assay plates contained Muelier-Hinton agar (4-mm depth). In the case of fastidious organisms the agar was supplemented as described above. Several colonies of bacterial isolate were suspended in tryptic soy broth until the turbidity matched that of a 0.