Impact of fluoroquinolones on the gastrointestinal flora.

A number of studies have evaluated the effect of fluoroquinolones on fecal flora. The results of these studies have consistently shown marked suppression or elimination of gram-negative facultative bacteria. The effect on gram-positive bacteria is more variable, with slight to moderate suppression reported most frequently. Total fecal anaerobes have generally been unchanged, although some anaerobic species have been moderately suppressed. The effect on facultative gram-negative bacteria can be explained by high fecal levels of quinolones and very low MICs for most of the organisms. The general lack of effect of anaerobes may be due to the higher MICs and an inoculum effect on the MBCs that is caused by the large number of organisms (10(9)-10(11) colony-forming units/g) characteristically found in feces.