Contribution of the host to test results in assays of Staphylococcus epidermidis

From two human populations (one pediatric and one adult), clinically diagnosed with Staphylococcus epidermidis (S. epidermidis) sepsis of similar severity, bacteria were isolated from pre-antibiotic blood samples and evaluated for virulence. The LD50 of the bacteria in a mouse model was performed, with evaluation of animals dying acutely following intravenous S. epidermidis administration. More simple assays of virulence were also performed, including bacterial adherence to a fibrin clot and carbohydrate specific lectin binding. The eight pediatric-host S. epidermidis isolates required a significantly larger dose to produce lethality in dosed animals (LD50) when compared to the 20 adult-host S. epidermidis isolates. The fibrin clot assay, a test that has corroborated bacterial virulence in endocarditis models, did not differentiate the groups: all but one of the 28 isolates were well above the adherence seen with the ATCC control, suggesting endocarditis-producing potential. Glycocalyx (slime) from eight of the more virulent isolates showed reactivity with a glucose-specific biotinylated lectin which was lacking in other isolates. Necropsy of mice dying at 12 hr showed S. epidermidis strain differences in specific organ effects. Overall, this study demonstrates the utility of the LD50 to provide a highly sensitive quantification of bacterial virulence. Necropsy of test animals dying acutely has showed an apparent organ tropism of some of these isolates which are usually considered harmless commensals.