The antibacterial activity of tobramycin, gentamicin, erythromycin, cloxacillin, kanamycin, cephalexin, penicillin, carbenicillin and polymyxin were compared against 303 clinical bacterial isolates from a pediatric hospital patient population. Standard disk diffusion and agar-dilution methods were employed. Significant activity was demonstrated for tobramycin against Pseudomonas, Klebsiella, Escherichia coli and both Staphylococcus aureus and albus. Tobramycin was significantly more active against Pseudomonas than gentamicin or the other antibiotics tested. Comparable activity to gentamicin was present for the other types of bacteria. Cross-resistance was not encountered between tobramycin and gentamicin. 30 isolates of E. coli were tested against the combination of tobramycin and ampicillin by the growth-curve method. Synergism was demonstrated in 4 isolates, antagonism in 1 and an additive effect in 25. A bactericidal effect was present at 24 h against 17 isolates with tobramycin alone and against 25 isolates when combined with ampicillin. These results provide in vitro rationale for the consideration of tobramycin for clinical use in patients with Pseudomonas infections and for the combination of ampicillin and tobramycin for the treatment of selected E. coli infections.

The activities of trimethoprim (TMP) and sulfamethoxazole (SMZ), alone and in combination (SMZ-TMP), and of the following antibiotics were tested against 115 clinical isolates of-nontyphoid Salmonella species: tobramycin, gentamicin, ampicillin, amoxicillin, neomycin, kanamycin, chloramphenicol, and tetracycline. The methods of disk diffusion, microtiter broth dilution, and agar dilution were employed for all single antimicrobial agents as well as for SMZ-TMP studies. Growth curves were performed in broth. SMZ-TMP, TMP, gentamicin, tobramycin, and neomycin were the most active drugs in vitro. All strains were inhibited by < 1 Ag of TMP per ml, but > 100 qg of SMZ per ml was required for at least 10% of strains. SMZ and TMP in a ratio of 10:0.5, respectively, inhibited all isolates and were synergistic for 105 strains. All strains inhibited by the combination of 10:0.5 SMZ-TMP had a zone diameter of >22 mm by using a combination disk containing 1.25 pg of TMP and 23.75 jig of SMZ. Seven isolates were resistant to > 100 ug/ml of ampicillin or amoxicillin; all isolates were sensitive to chloramphenicol at <6.3 ug/ml. SMZ-TMP appears to be active against nontyphoid salmonellae in vitro; this is usually due to a synergistic effect.

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Hospital nursery A has used chloramphenicol and nursery B has used the combination of penicillin G sodium and kanamycin sulfate routinely in the treatment of neonatal sepsis and other bacterial infections. A hypothesis was formulated that these different antibiotic pressures would select out a substantial number of populations of resistant bacteria in each of the two nurseries. This was tested by periodic sampling of the skin, mouth, and rectal flora of babies and the permanent personnel in these nurseries. These bacteria were studied for susceptibility to a number of antibiotics. The population of resistant strains selected out was correlated with the antibiotics used in each nursery. There is a need for continuing surveillance of hospital nursery strains of bacteria for in vitro susceptibilities to commonly prescribed antimicrobials.