In vitro antimicrobial effect against Streptococcus pneumoniae of adding rifampin to penicillin, ceftriaxone, or 1-ofloxacin.

Adding rifampin to penicillin or l-ofloxacin diminished the rate at which these antibiotics killed 21 clinical isolates isolates of Streptococcus pneumoniae in vitro. A less pronounced inhibitory effect was observed when rifampin was added to ceftriaxone. Synergy was not observed for any bacterial isolate. The in vitro demonstration of indifference or antagonism using these antibiotic combinations argues against the empirical addition of rifampin to beta-lactams or fluoroquinolones in treating serious pneumococcal infections.