In vitro activity of cefoxitin, alone and in combination with aminoglycoside or other beta-lactam antibiotics against common gram-negative pathogens.

We examined 160 strains of Enterobacteriaceae and Pseudomonas aeruginosa for susceptibility to cefoxitin, aminoglycosides, and other beta-lactam antibiotics, alone and in combination. The aminoglycosides were the most effective agents tested, but, except for some strains, cefoxitin inhibited all organisms at a 90% minimal inhibitory concentration of 25 micrograms or less per ml. We observed a relatively high incidence of antagonism or nonsynergy when cefoxitin was tested in combination with other agents against 33 gentamicin-susceptible gram-negative bacilli. No synergy was observed when various combinations with cefoxitin were tested against three strains of cephalothin-resistant, carbenicillin-resistant Enterobacteriaceae of varying gentamicin susceptibility.