Studies of the killing kinetics of benzylpenicillin, cefuroxime, azithromycin, and sparfloxacin on bacteria in the postantibiotic phase.

Most antibiotics are known to be incapable of killing nongrowing or slowly growing bacteria with few exceptions. Bacterial cell division is inhibited during the postantibiotic phase (PA phase) after short exposure to antibiotics. Only scarce and conflicting data are available concerning the ability of antibiotics to kill bacteria in the PA phase. The aim of the present study was to investigate the killing effect of four different antibiotics on bacteria in the PA phase. A postantibiotic effect (PAE) was induced by exposing Streptococcus pyogenes and Haemophilus influenzae to 10x MICs of benzylpenicillin, cefuroxime, sparfloxacin, and azithromycin. The bacteria were thereafter reexposed to a 10x MIC of the same antibiotic used for the induction of the PAE at the beginning of and after 2 and 4 h in the PA phase. Due to a very long PAE, the bacteria in PA phase induced by azithromycin were also exposed to 10x MICs after 6 and 8 h. A previously unexposed culture exposed to a 10x MIC was used as a control. The results seem to be dependent on both the antibiotic used and the bacterial species. The antibiotics exhibiting a fork bactericidal action gave significantly reduced killing of the bacteria in PA phase (cefuroxime with S. pyogenes, P < 0.01, and sparfloxacin with H. influenzae, P < 0.001), which was restored at 4 h for cefuroxime with S. pyogenes. There was a tendency to restoration of the bactericidal activity also with sparfloxacin and H. influenzae, but there was still a significant difference in killing between the control and the test bacteria in PA phase at 4 h. However, in the combinations with a lesser bactericidal effect (benzylpenicillin with S. pyogenes and sparfloxacin with S. pyogenes), there was no difference in killing between the control and the test bacteria in PA phase. Azithromycin induced long PAEs in both S. pyogenes and H. influenzae and exhibited a slower bactericidal action on both the control and the bacteria in PA phase especially at the end of the PAE, when the killing was almost bacteriostatic. Our findings in this study support the concept that a long interval (> 12 h) between doses of azithromycin, restoring full bactericidal action, may be beneficial to optimize efficacy of this drug but is not necessary for the other antibiotics evaluated, since the bactericidal effect seems to be restored already at 4 h.