Abstract
Purpose:
Endophthalmitis due to viridans group streptococci (VGS) has been shown to result in poor visual prognosis despite antibiotic therapy. We hypothesized that VGS possess toxins that are involved in ocular pathogenesis. The purpose of this study was to identify the virulence characteristics of endophthalmitis strains of VGS.
Methods:
Concentrated extracellular milieu from 20 endophthalmitis strains of VGS, and concentrated culture media (controls), were subjected to 3 assays: lysis of sheep erythrocytes, retinal pigmented epithelial (RPE) cell toxicity, and Western blot for detection of pneumolysin (the major toxin of S. pneumoniae). Each strain was assigned a profile based on assay results. Four strains with different profiles were chosen for intravitreous injection of 100 colony-forming units (CFU) of each strain into the left eyes of each of 3 rabbits, followed by clinical examination and bacterial recovery from the vitreous. The concentrated extracellular milieu from each of the 4 strains was also tested for protease activity by gelatin zymography.
Results:
Eight of 20 strains (40%) lysed sheep erythrocytes and 15 (75%) were cytotoxic at a level of ≥2 on a scale from 0 (no activity) to 4 (full activity). Pneumolysin was detected in 6 (30%). Two of the 4 strains tested in vivo caused anterior chamber inflammation in addition to vitreous haze, with severity increasing with time up to 72 hours after infection. The most severe eyes, which were infected with strain E664, had clinical scores with a mean of 25.33 ± 2.08 (scale of 0 to 32) and bacterial recovery with a mean of 6.95 ± 0.53 log10 CFU/mL at 72 hours. Interestingly, E664 was not hemolytic nor did it produce pneumolysin, but was highly toxic to RPE cells. The second-most virulent strain in vivo, 144065, was negative for all of the in vitro assays. The least virulent strain in vivo, E618, was positive for all of the in vitro assays. Zymography of the 4 strains tested in vivo showed protease activity in E664 and 144065, but not in the other 2 strains.
Conclusions:
VGS from endophthalmitis are diverse in toxin activity, and toxin activity cannot necessarily predict virulence in the rabbit eye. Strains with protease activity induce more damage in the rabbit eye than those without detectable protease. Determination of whether protease activity contributes to pathogenesis will aid in the first steps of characterizing the virulence of VGS in the eye.