Susceptibilities of coagulase-negative staphylococci to aminoglycosides antibiotics as compared by early bactericidy assays.

Twenty four strains of coagulase-negative staphylococci from clinically significant blood cultures were separated in four groups on the basis of oxacillin susceptibility and phenotype of resistance to aminoglycoside antibiotics. Susceptibility testing included disk diffusion testing, M.I.C. and M.B.C. determinations and finally, measurements, at sequential times (from 0 to 6 hours), of the bactericidal effect of aminoglycosides alone or combined with oxacillin or vancomycin. Aminoglycosides clinical use (gentamicin, netilmicin, tobramycin and amikacin) exhibited an efficient early bactericidy over the sensitive strains to any tested aminoglycosides and to oxacillin (group 1, 13 strains). When strains having a phenotype of resistance to kanamycin and neomycin (group 2, 4 strains) or kanamycin and tobramycin (group 3, 2 strains) were considered, amikacin inefficiency was assessed by time-killing assay even though disk diffusion testing as well as M.I.C. and M.B.C. determinations failed to demonstrate it. In the same way, only bactericidy measurements indicated the lack of activity of amikacin or netilmicin over strains exhibiting a phenotype of resistance to gentamicin plus kanamycin and tobramycin (group 4, 5 strains). Netilmicin appeared as the most active aminoglycoside over the strains studied. Crude results of disk diffusion testing might appear as an oversimplification, especially when coagulase-negative staphylococci are considered; the early-bactericidy assay findings were stressing for the need of interpreting the results of antibiotic testing on the ground of resistance mechanisms.