A hemolysin-encoding plasmid contributes to bacterial virulence in experimental Enterococcus faecalis endophthalmitis

Abstract

Beta-hemolysin production is a variable trait of the Lancefield group D streptococcus, Enterococcus faecalis. The E. faecalis hemolysin is encoded by large transmissible plasmids. The variable nature of this putative virulence factor provided an ideal system for testing its contribution in experimental endophthalmitis. In this study, isogenic E. faecalis strains were compared to determine whether the presence of the hemolysin-encoding plasmid affected the severity of disease in a rabbit endophthalmitis model. Experimental infections (n = 6) with 10(1)-10(4) E. faecalis organisms harboring the hemolysin-encoding plasmid resulted in a 98% loss of retinal function (by electroretinography [ERG]) and white reflex by postoperative day 3. By contrast, infections of similar numbers of plasmid-free E. faecalis organisms (n = 5) resulted in retention of some retinal function (23% per ERG) with a red reflex demonstrated on postoperative day 3. Results of light microscopy, slit-lamp examination, ERG, and indirect ophthalmoscopy indicated that infections with hemolysin-encoding plasmid-containing E. faecalis resulted in a more aggressive endophthalmitis compared with the endophthalmitis caused by plasmid-free E. faecalis. This is the first endophthalmitis model to the authors' knowledge that specifically evaluates bacterial virulence using isogenic strains.