The synergism between cetrimide and antibiotics against Pseudomonas aeruginosa.

Forty eight clinical isolates of P. aeruginosa were tested for susceptibility to seven different antimicrobial agents. When tested on Mueller-Hinton agar, the isolates were found to be resistant to ampicillin, sensitive to the antipseudomonal antibiotics, polymyxin B, gentamicin and carbenicillin. Polymyxin B inhibited all the isolates, whereas both carbenicillin and gentamicin inhibited 92.1% of the isolates. Neomycin, sulphamethoxypyridazine and chlortetracycline showed moderate activity and inhibited 50%, 28.9% and 15.8% of the isolates, respectively. However, on Mueller-Hinton agar supplemented with 0.03% cetrimide, the isolates succumbed readily to antimicrobial agents. In addition to polymyxin B, gentamicin and carbenicillin, all the strains were inhibited by neomycin and 94.7%, 92.1% and 63.6% of the isolates were inhibited by sulphamethoxypyridazine, chlortetracycline and ampicillin, respectively. Cetrimide, in concentrations ranging from 0.01 to 0.04% decreased the MIC of ampicillin against all the isolates, whereas 0.1% and 0.5% polysorbate 80 (tween 80) had no effect on the MIC. Growth inhibition studies have shown that the number of survivors was greatly reduced in presence of cetrimide and ampicillin. There was also an appreciable increase in the uptake of ampicillin by the bacterial cells in the presence of cetrimide.