[Effect of multiresistance on the minimal inhibitory concentration of N-formimidoyl thienamycin and 6 comparing substances against P. aeruginosa and 4 Enterobacteriaceae species].

The minimal inhibitory concentrations (MICs) of multiresistant strains of P. aeruginosa and four species of Enterobacteriaceae were compared with those of more sensitive ones in N-f-thienamycin and six other antibiotics. The greatest difference between these two differently resistant bacterial populations was found in cefoperazone and the beta-lactamase-instable piperacillin. The latter was used for comparison purposes. The activity of cefsulodin was distinctly reduced in multiresistant strains of P. aeruginosa. Cefotaxime and lamoxactam displayed slightly elevated MICs in multiresistant strains. In N-f-thienamycin a significant difference between the multiresistant and the more sensitive population could not be verified (p greater than 0.05). In this regard it resembles fosfomycin. beta-lactam antibiotics could be ranked by means of correlation analysis of their MICs. Using piperacillin as a reference antibiotic the following order of succession could be arranged according to correlation coefficients in P. aeruginosa: cefoperazone, cefsulodin, lamoxactam, cefotaxime and N-f-thienamycin. For the Enterobacteriaceae species some antibiotics could not be evaluated in this test. Moreover, it was recognized that there was an obvious correlation between the activities of lamoxactam and cefotaxime in P. aeruginosa (r = 0.81) and E. coli (r = 0.80). A correlation was also observed between those of cefsulodin and cefoperazone in P. aeruginosa (r = 0.75). Lower correlation coefficients were seen in N-f-thienamycin with cefoperazone, piperacillin and cefsulodin in P. aeruginosa and with cefoperazone in Klebsiella spec.. Notwithstanding, strains resistant to cefsulodin and cefoperazone were inhibited by concentrations less than or equal to 8 micrograms/ml of N-f-thienamycin. These results showed that in N-f-thienamycin, cefotaxime and lamoxactam multiresistance has only a minimal influence on the MICs of the species of Enterobacteriaceae examined. For therapeutic considerations concerning multiresistance the small differences recognized among these three antibiotics might be without significance. However, N-f-thienamycin was distinctly more active in multiresistant strains of P. aeruginosa than the other beta-lactam antibiotics tested.