Postantibiotic effect, and postantibiotic effect of subinhibitory concentrations, of sparfloxacin on gram-negative bacteria.

The postantibiotic effect (PAE), the postantibiotic sub-MIC effect (PA SME), and the sub-MIC effect (SME) of a new oral quinolone, sparfloxacin, were determined for different strains of gram-negative bacteria. A PAE was induced by 10 x MIC of sparfloxacin for 2 h and the antibiotic was then eliminated by washing and dilution. The PA SME was studied by adding different sub-MICs during the postantibiotic phase, and the SME by exposing the controls to the sub-MICs alone. Growth curves were followed for 20 h by continuous monitoring of optical density in an automatic incubator. Sparfloxacin induced a PAE of 0.9-2.4 h for all strains except two clinical strains of Enterobacter cloacae. In accordance with earlier experiments with sparfloxacin and gram-positive bacteria, this study showed a PA SME that was nearly equal to the PAE plus the effect of the subinhibitory concentration alone (SME).