The post-antimicrobial suppressive effect of quinolone agents.

A post-antimicrobial effect (PAE) was seen with all of the new 4-quinolones studied: ciprofloxacin, ofloxacin, Cl-934, Ro 23-6240, A-56619 and S-25930. The post-antimicrobial suppression of growth was for 4-8 h in the case of Escherichia coli, Klebsiella pneumoniae, Serratia marcescens and Pseudomonas aeruginosa. PEA was demonstrated both in Mueller-Hinton broth at pH 7.4 and in urine at pH 5.5 with a Mg2+ concentration of 9.5 mM. Resistant organisms, with MICs greater than 4 micrograms/ml, showed a PAE after a 2-h exposure to 200 micrograms/ml of 4-quinolones. This demonstrates that PAE is seen for 4-quinolones even under conditions in which they are less active.