Antibiotic synergy and antagonism against clinical isolates of Klebsiella species.

Minimal inhibitory concentrations of kanamycin, gentamicin, amikacin, cephalothin, and chloramphenicol were determined in Trypticase soy broth for 70 clinical isolates of Klebsiella species. Gentamicin and amikacin were the most active on a weight basis. Chloramphenicol was more active than kanamycin, and cephalothin was the least active of all. Studies using a microtiter modification of the checkerboard technique were performed to evaluate the comparative activity of the three aminoglycosides in combination with either chloramphenicol or cephalothin. The cephalothin-aminoglycoside combinations demonstrated synergy in >80% of the isolates tested. No antagonism was noted. The chloramphenicol-aminoglycoside combinations showed antagonism in 35 to 45% of the isolates tested. The data suggest that the chloramphenicol-aminoglycoside combinations be used with caution when treating serious infections where Klebsiella is a potential pathogen.