Antibiotic-induced lipopolysaccharide (LPS) release from Salmonella typhi: delay between killing by ceftazidime and imipenem and release of LPS

It has been suggested that the antibiotic-induced release of lipopolysaccharide (LPS) is an important cause of the development of septic shock in patients treated for severe infections caused by gram-negative bacteria. Beta-lactam antibiotics change the integrity of the bacterial cell envelope by binding to penicillin-binding proteins (PBP) in the membrane and thus may affect the amount of LPS that is released and the kinetics of that release. In this respect, ceftazidime at intermediate concentrations binds with a high affinity to PBP 3 and PBP 1a and thus can induce filament formation in addition to killing, whereas imipenem preferentially binds to PBP 2 and PBP 1b, leading to spheroplast formation and rapid cell lysis. We investigated the effects of these antibiotics on the killing and the release of the radioactively labelled LPS of Salmonella typhi Ty 21A. A mathematical model was developed to calculate the delay between bacterial killing and LPS release, designated the lag time. At antibiotic concentrations inducing equal killing, the amount of LPS released was the same for both antibiotics. Only after 6 h of incubation at antibiotic concentrations above 0.5 microg/ml, the amount of 3H-LPS released was slightly higher (approximately 1.2-fold) in incubations with ceftazidime than in those with imipenem, and the maximum releases of the total label were 33.2% +/- 0.89% and 27.1% +/- 0.45%, respectively. Despite the clear concentration-dependent effect on the bacterial killing and subsequent LPS release, the lag time was independent of the antibiotic concentration. For ceftazidime as well as imipenem the lag time amounted to approximately 60 min. In conclusion, our findings imply that the mechanism of antibiotic-induced LPS release is independent of the PBP affinities for these beta-lactam antibiotics. Furthermore, once the organism is killed by either imipenem or ceftazidime, the rate of LPS release from S. typhi does not differ according to the antibiotic with which the organism is killed, and there is little difference in the relative amount of LPS released.