Jean Barnishan

In vitro susceptibilities of four species of coagulase-negative staphylococci.

The in vitro susceptibilities of 260 strains of coagulase-negative staphylococci to penicillin G, oxacillin, nafcillin, methicillin, cephalothin, and seven non-beta-lactam antimicrobial agents were determined and compared with the susceptibilities of 54 strains of Staphylococcus aureus with known patterns of susceptibility. Penicillin G susceptibility for S. aureus, Staphylococcus epidermidis, Staphylococcus haemolyticus, and Staphylococcus hominis was readily determined by using beta-lactamase tests with induced cells and with a standardized microdilution test. MIC criteria for susceptibility used for S. aureus were applicable to the coagulase-negative species. Percentages of organisms susceptible were as follows: S. epidermidis, 7%; S. haemolyticus, 5%; and S. hominis, 47%. Oxacillin susceptibility for these four species was readily determined by using a modification of the microdilution test. MIC criteria for susceptibility used for S. aureus were applicable to S. haemolyticus and S. hominis, but alternate criteria were necessary for S. epidermidis. Percentages of organisms susceptible were as follows: S. epidermidis, 29%; S. haemolyticus, 36%; and S. hominis, 97%. Staphylococcus saprophyticus differed from the other staphylococcal species; all strains were beta-lactamase negative and were penicillin susceptible but had higher penicillin G MICs than did susceptible strains of the other species. There was total cross resistance among the penicillinase-resistant penicillins and cephalothin for the coagulase-negative staphylococci as well as for S. aureus; oxacillin MICs were more reliable than MICs of the other drugs or a standardized disk diffusion test for distinguishing resistant from susceptible strains. Vancomycin, rifampin, and ciprofloxacin were consistently active against all staphylococci. Erythromycin, clindamycin, gentamicin, and trimethoprim-sulfamethoxazole were more active against oxacillin-susceptible staphylococci than against oxacillin-resistant staphylococci.

Penicillinase production and in vitro susceptibilities of Staphylococcus lugdunensis.

Of 59 clinical isolates of Staphylococcus lugdunensis, 76% were beta-lactamase negative, with penicillin G MICs of less than or equal to 0.13 microgram/ml, and 24% were beta-lactamase positive, with penicillin MICs of greater than or equal to 0.5 microgram/ml. Bimodal distributions were observed also with ampicillin, ampicillin-sulbactam, and amoxicillin-clavulanate. All strains were susceptible to oxacillin, cephalothin, gentamicin, rifampin, and vancomycin; 98% were erythromycin susceptible.

Effect of Divalent Cation Concentrations on the Antibiotic Susceptibilities of Nonfermenters Other than Pseudomonas aeruginosa

The effects of supplementing Mueller-Hinton broth with calcium and magnesium on the minimal inhibitory concentrations (MICs) of eight aminoglycosides, colistin, tetracycline, and carbenicillin for 11 nonfermenters other than Pseudomonas aeruginosa were studied and compared with the effects for Escherichia coli and P. aeruginosa. MICs were simultaneously performed in unsupplemented Mueller-Hinton broth and Mueller-Hinton broth supplemented to contain 5 mg of calcium and 2.5 mg of magnesium per dl. Changes in MICs were expressed as the increases in the number of log2 concentrations caused by supplementation. The usual increases in MICs of aminoglycosides caused by supplementation were: zero concentrations for E. coli, one to six concentrations for P. aeruginosa, and one to two concentrations for most other nonfermenters. The largest increases (five to six concentrations) were observed with gentamicin and P. aeruginosa. The usual increases in MICs of colistin were: zero concentrations for E. coli, two concentrations for P. aeruginosa, and one to two concentrations for other nonfermenters. Increases in MICs of tetracycline were: one to five concentrations for all organisms tested. The usual increases in MICs of carbenicillin were: zero concentrations for E. coli and P. aeruginosa and zero to two concentrations for other nonfermenters. These observations indicated that supplementation of Mueller-Hinton broth to contain recommended concentrations of calcium and magnesium had little effect on MICs of aminoglycosides and colistin for E. coli but increased MICs for most nonfermenters, increased MICs of tetracycline for E. coli and all nonfermenters, and had little effect on MICs of carbenicillin for E. coli and P. aeruginosa but increased the MICs for several nonfermenters other than P. aeruginosa.

The utility of non-β-lactam antimicrobial MICs as markers to distinguish oxacillin-resistant from oxacillin-susceptible strains of Staphylococcus epidermidis

Among 6,068 strains of Staphylococcus epidermidis, 75.5% were oxacillin-resistant. Oxacillin-susceptible strains were more frequently susceptible to erythromycin, clindamycin, ciprofloxacin, trimethoprim/sulfamethoxazole, gentamicin, and tetracycline than oxacillin-resistant strains. With the exception of erythromycin, non-beta-lactam MICs were less discriminatory for identifying oxacillin-resistant strains with oxacillin MICs < or = 2 micrograms/ml than for those with oxacillin MICs > or = 4 micrograms/ml.