Effect of inoculum size on in vitro activity of norfloxacin against fecal anaerobic bacteria. Rationale for selective decontamination of the digestive tract.

Previous studies have shown that anaerobic bacteria are susceptible to norfloxacin at the levels attained in the feces. Conversely, studies in laboratory animals and neutropenic humans using norfloxacin for selective decontamination of the digestive tract have shown that norfloxacin markedly reduces the aerobic enteric flora without reducing fecal anaerobic flora. In an effort to resolve this paradox, the effect of a 10(9) colony-forming units (cfu)/ml inoculum, which is more reflective of actual fecal counts than the standard 10(5) cfu/ml inoculum, on the activity of norfloxacin against two fecal Escherichia coli isolates and 16 fecal anaerobic isolates was studied. The results showed a marked inoculum effect at 10(9) cfu/ml for most anaerobic isolates but not for the E. coli strains tested. At 256 micrograms/ml, all E. coli were killed while the anaerobic bacteria maintained colony counts greater than or equal to 10(9) cfu/ml. Hence, the lack of anaerobic activity (minimal inhibitory concentration greater than or equal to 512 micrograms/ml) at higher fecal inocula might explain the utility of norfloxacin in selective decontamination of the bowel.