Activity of Newer Aminoglycosides and Carbenicillin, Alone and in Combination, Against Gentamicin-Resistant Pseudomonas aeruginosa

The in vitro activity of the aminoglycoside antibiotics tobramycin, sisomicin, amikacin, gentamicin, and netilmicin (SCH 20569) were compared against 26 gentamicin-resistant isolates of Pseudomonas aeruginosa cultured from hospitalized children. Tobramycin had the greatest activity on a weight basis, followed by sisomicin, gentamicin, amikacin, and netilmicin. All isolates were resistant to achievable concentrations of netilmicin and gentamicin, but 23% were inhibited by achievable concentrations of tobramycin, 8% by amikacin, and 4% by sisomicin. The combinations carbenicillin/tobramycin, carbenicillin/sisomicin, and carbenicillin/amikacin were synergistic for 92% of strains; antagonism was not encountered. These in vitro results suggest that tobramycin, sisomicin, or amikacin in combination with carbenicillin would be the safest initial regimen in the therapy of gentamicin-resistant Pseudomonas infections pending susceptibility studies.