Snyder, Robert

This study examined (i) the activity of gentamicin added to agar in ratios of 1:100 and 1:1,000 to produce a final concentration of 1 μg/ml, (ii) the uniformity of distribution of gentamicin in agar in relation to the amount of mixing, and (iii) the effect of agar depth on the activity of gentamicin against Pseudomonas aeruginosa . Although the ratio of antibiotic solution to agar had no significant effects on activity of the antibiotic, the amount of mixing did. Agar depth had no significant effect on the activity of gentamicin.

Ampicillin resistance (minimal inhibitory concentration ≥10 μg/ml) in the absence of beta-lactamase activity by Haemophilus influenzae was noted in tests performed with Mueller-Hinton agar containing one lot of supplement C. All strains, except five with known resistance due to beta-lactamase activity, were inhibited by 0.6 μg or less of ampicillin per ml of chocolatized blood agar.

Data derived from testing the bactericidal activity of combinations of penicillin with gentamicin or streptomycin and of clindamycin with gentamicin on nine isolates of Streptococcus mutans were analyzed by preparing isobolograms to determine the presence of additive, synergistic, or antagonistic effects. Synergy with penicillin-aminoglycoside combinations was found in two strains; additive effects occurred in seven instances with penicillin-gentamicin combinations; and antagonism occurred in eight instances with clindamycin-gentamicin combinations.

The penetrations of methicillin, oxacillin, and cephalothin into cortical bone and synovial tissues were studied 1 h after their intravenous administration in 105 patients having arthroplasty of the hip. Although the lowest serum levels were noted with cephalothin ( P < 0.01), more patients receiving cephalothin achieved osseous drug levels inhibitory to staphylococci ( P < 0.01). Differences in the penetration of the three agents into synovial tissues were not statistically significant.

Since the microbiological assay of the antibiotic content of serum generally requires 18 to 24 hr of incubation, results of such procedures may not become available in time to make appropriate adjustments in subsequent dosages of antibiotic. A 4-hr bioassay for determining concentrations of gentamicin in serum has been developed in which Staphylococcus aureus ATCC 6538P is used as the test organism. Poured plates have yielded satisfactory results after storage at 4 C for 5 days. Results of the 4-hr procedure agree closely with those of a conventional 18-hr disc-plate assay performed with the same test organism.