The fluoroquinolones are active against a wide range of gram-negative and a number of, gram-positive aerobes. They are highly effective against all intestinal bacterial pathogens, well as several intracellular pathogens, eg, Brucella spp. Quinolones also have sigmfica activity against Mycoplasma and Chlamydia spp. Obligate anaerobes tend to be resistant to most quinolones, as are most enterococcal group D Streptococcus spp (S faecalis and S faecium).

The older quinolones (eg, nalidixic acid and oxolinic acid) and the nonfluorinated quinolones (eg cinoxacin) tend to have only a moderately extended gram-negative spectrum. The newest third- and fourth-generation fluorinated quinolones may be characterized by an effective anaerobic spectrum.

A synergistic effect has been demonstrated in vitro between quinolones and p-lactams, aminoglycosides, clindamycin, and metronidazole.