Antibacterial activity of tobramycin against gram-negative bacteria and the combination of ampicillin/tobramycin against E. coli.

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 testedmcomparable 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 24h 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 Psuedomonas infections for the combination of ampicillin and tobramycin for the treatment of selected E.coli infections.