Endotoxin release from Escherichia coli after exposure to tobramycin: dose-dependency and reduction in cefuroxime-induced endotoxin release

Objective To study the release of free endotoxin from Escherichia coli expo sed to varying concentrations of the penicillin-binding protein (PBP) 3-spe cific beta-lactam antibiotic cefuroxime, the aminoglycoside tobramycin, and a combination of the two, and to test the relationship between bacterial k illing rate and endotoxin release. Methods A clinical isolate of Escherichia coli in logarithmic phase was exp osed to 0.1, 2, 10, and 50 x minimum inhibitory concentration (MIC) of cefu roxime, tobramycin, and a combination of the two. Samples for viable counts and endotoxin analysis were drawn immediately before and after the additio n of the antibiotics and at 1, 2, 4, 6, and 24 h. AU experiments were perfo rmed in triplicate. For the analysis of endotoxin, a chromogenic limulus am oebocyte lysate assay was used. Results Endotoxin liberation was found to bet proportional to the number of killed bacteria for each antibiotic regimen at each concentration level ju stifying the endotoxin-liberating potential to be expressed as release of e ndotoxin per killed bacterium, an expression that was independent of the in oculum size. At all concentration levels there was a statistically signific ant difference between the treatments, with the highest release of endotoxi n per killed bacterium for cefuroxime, lower for tobramycin and the lowest for the combination of the two drugs (P < 0.001). With increasing doses, th ere was a significant reduction (P < 0.001) in the propensity to release en dotoxin. When the bacterial killing rate was correlated to the propensity t o release endotoxin in bacteria exposed to tobramycin or the combination of tobramycin and cefuroxime, a significant negative correlation was found (P < 0.01). This reduction in endotoxin release nas nor caused by an unspecif ic endotoxin binding of tobramycin. Conclusions Addition of tobramycin reduced the cefuroxime-induced endotoxin release per killed bacterium to a level which was even lower than that of tobramycin alone in spite of an increased killing rare. Increasing concentr ations of tobramycin led to reduction in endotoxin release, which may be of benefit when dosing aminoglycosides once daily.