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

Abstract

Objective. To study the release of free endotoxin from Escherichia coli exposed to varying concentrations of the penicillin-binding protein (PBP) 3-specific β-lactam antibiotic cefuroxime, the aminoglycoside tobramycin, and a combination of the two, and to test the relationship between bacterial killing rate and endotoxin release. Methods. A clinical isolate of Escherichia coli in logarithmic phase was exposed to 0.1, 2, 10, and 50 x minimum inhibitory concentration (MIC) of cefuroxime, tobramycin, and a combination of the two. Samples for viable counts and endotoxin analysis were drawn immediately before and after the addition of the antibiotics and at 1, 2, 4, 6, and 24 h. All experiments were performed in triplicate. For the analysis of endotoxin, a chromogenic limulus amoebocyte lysate assay was used. Results. Endotoxin liberation was found to be proportional to the number of killed bacteria for each antibiotic regimen at each concentration level justifying the endotoxin-liberating potential to be expressed as release of endotoxin per killed bacterium, an expression that was independent of the inoculum size. At all concentration levels there was a statistically significant difference between the treatments, with the highest release of endotoxin per killed bacterium for cefuroxime, lower for tobramycin and the lowest for the combination of the two drugs (P < 0.001). With increasing doses, there was a significant reduction (P < 0.001) in the propensity to release endotoxin. When the bacterial killing rate was correlated to the propensity to 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 was not caused by an unspecific 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 rate. Increasing concentrations of tobramycin led to reduction in endotoxin release, which may be of benefit when dosing aminoglycosides once daily.