[In vitro comparative study of piperacillin-aminoglycosides combinations against Enterococci and Enterobacteriaceae]

The hundred and ninety-two combinations were tested against 17 strains chosen from the results of MIC determination (disc method): 5 enterococci exhibiting low level resistance (r) or high level resistance (R) to streptomycin (S) and gentamicin (G): 2 strains Sr Gr, 2 strains SR Gr and 1 strain Sr GR; 12 enterobacteria chosen for their resistance phenotypes to beta-lactams and aminoglycosides and because they are the most frequent clinical isolates: 2 strains Amos Tics Ctns (group 1), 4 strains AmoR Tics CtnR (gr. II), 4 strains AmoR TicR Ctns (gr. III) and 2 strains AmoR TicR CtnR (gr. IV). MIC and MBC were assessed for the 17 strains (Mueller Hinton broth). Combinations were carried out by a checkerboard micromethod. FBC index was calculated for each combination. Against enterococci the 50 combinations were: piperacillin versus ampicillin + aminoglycosides (streptomycin, tobramycin, amikacin, gentamicin, netilmicin). Against enterobacteria piperacillin was combined with different aminoglycosides depending on their resistance phenotypes. These combinations were compared with ticarcillin or mezlocillin or cefotaxime + aminoglycosides (total number 142). The species studied produced different results: with the enterococci Gr synergistic effects (FBC = 0.62-0.75) were rare; additive and indifferent effects were predominant. With the GR strain some antagonistic effects were observed. With the enterobacteria, in groups I and II synergistic effects were frequent and almost equivalent regardless of the beta-lactam chosen. In groups III and IV (TicR) piperacillin MICs were greater than or equal to 128 mg/l and mezlocillin MICs greater than 512 mg/l; the synergistic effects were significant (FBC from 0.25 to 0.62). beta-lactam + amikacin or netilmicin, and especially piperacillin + amikacin, were found to have the most frequent synergistic effects upon the strains tested. Mezlocillin combinations cannot be used clinically; the use of piperacillin combinations requires further discussion. On the other hand, cefotaxime + aminoglycosides combinations are active against those TicR strains.