In-vitro interactions of FCE 22101 with aminoglycosides against gram-negative rods

Agar dilution MICs of FCE 22101 were measured for 894 consecutively-isolated Gram-negative rods from clinical specimens (413 Escherichia coli, 104 Klebsiella spp., 54 Enterobacter spp., 19 Citrobacter spp., 131 Proteus spp., 43 Acinetobacter spp., 9 Serratia spp., 3 Providencia spp., 4 Yersinia spp., 3 Hafnia spp. and 111 Pseudomonas spp.). Excluding Pseudomonas spp., 98% of these isolates were susceptible to 8 mg/l FCE 22101. Resistance (MIC greater than 8 mg/l) varied from 11% in Serratia spp. to 0% in Citrobacter spp. Interactions between FCE 22101 and gentamicin, tobramycin and amikacin were tested by the chequerboard method for 150 of the isolates. The geometric mean sigma FICs were 0.80 for FCE + gentamicin, 0.78 for FCE + tobramycin and 0.79 for FCE + amikacin. Synergy, defined at FIC index (sigma FIC) less than 0.5, for at least one combination, was found in only 16/150 (11%) of the isolates. Most other isolates showed an additive response (sigma FIC = greater than 0.5-1). No antagonism was detected. No significant difference was observed between the various species tested, except that sigma FIC values were lower (geometric mean sigma FIC = 0.61-0.68) for non-fermentative species than for fermentative species (geometric mean sigma FIC = 0.83-0.86). Likewise, geometric sigma FIC values were lower (0.67-0.69) for isolates resistant to FCE and/or aminoglycosides than for those susceptible to both components (sigma FIC 0.85-0.88).