In vitro synergistic activity of some chinolinic compounds combined with beta-lactam antibiotics against gram-positive and gram-negative clinical isolates

The antimicrobial activities of nalidixic acid-cephalexin (ratio 1:1) and cinoxacin-cefadroxil (ratio 1:2) combinations have been evaluated against 396 clinical isolates; many of them were nalidixic acid- or cinoxacin-resistant organisms (MIC greater than or equal to 100 micrograms/ml). We have also tested the nalidixic acid-amoxicillin combination (ratio 1:1) against 225 amoxicillin-resistant bacterial strains (MIC greater than or equal to 800 micrograms/ml). Synergy was found for 62-70% of the Enterobacteriaceae and nonfermenter bacilli tested and for 85-92% of the gram-positive bacterial strains. The 225 clinical isolates resistant to amoxicillin (MIC greater than or equal to 800 micrograms/ml) were synergistically inhibited by the nalidixic acid-amoxicillin combination.