Combinations of antimicrobial agents. I. The in vitro sensitivity of 100 strains of Pseudomonas aeruginosa to polymyxin B, colistin, carbenicillin, gentamicin and doxycycline and to various combinations of these antibiotics.

The MICs for 100 strains of Pseudomonas aeruginosa were determined for polymyxin B, colistin, carbenicillin, gentamicin and doxycycline and various combinations of these antibiotics. This study was carried out by means of a tube dilution method. Additive actions were observed in the combinations of polymyxin B plus carbenicillin, polymyxin B plus gentamicin, polymyxin B plus doxycycline, carbenicillin plus doxycycline and gentamicin plus doxycycline. A synergistic action was observed for the combinations of colistin plus carbenicillin, colistin plus gentamicin, colistin plus doxycycline and carbenicillin plus gentamicin. In none of the combinations was any antagonistic action encountered. Numerous reasons for using combined antibiotic treatment are listed. The results were obtained exclusively in in vitro investigations.