Comparative in vitro activity of ciprofloxacin and other unrelated antimicrobials against bacterial respiratory tract pathogens.

Ciprofloxacin is a new fluorinated 4-quinolone with a broad spectrum of antimicrobial activity which includes both Gram-negative and Gram-positive bacteria. In this study the in vitro activity of ciprofloxacin has been determined against bacteria associated with respiratory tract infections and compared with that of other antimicrobial agents used in the therapy of such infections. Ciprofloxacin (MIC90 0.008 mg/l) was highly active against Haemophilus influenzae, including isolates producing beta-lactamase which were resistant to amoxycillin. Ciprofloxacin (MIC90 0.06 mg/l) was also highly active against Branhamella catarrhalis, again including those isolates resistant to amoxycillin as a result of beta-lactamase production. Isolates of Streptococcus pneumoniae were less susceptible to ciprofloxacin (MIC90 2 mg/l) but were highly susceptible to amoxycillin (MIC90 less than 0.12 mg/l) and erythromycin (MIC90 0.25 mg/l). Isolates of Klebsiella aerogenes were highly susceptible to ciprofloxacin (MIC90 0.06 mg/l) but much less so to amoxycillin, sulfamethoxazole, trimethoprim, oxytetracycline and erythromycin. Ciprofloxacin (MIC90 0.5 mg/l) was very active against Staphylococcus aureus, including those isolates resistant to amoxycillin and flucloxacillin, and against Mycoplasma pneumoniae. Together with rifampicin and erythromycin, ciprofloxacin was highly active against Legionella pneumophila (MIC90 0.015 mg/l). These results suggest that clinical evaluation of ciprofloxacin in the treatment of respiratory tract infections is justified.