The streptococcus pneumoniae capsule is required for full virulence in pneumococcal endophthalmitis

Purpose. To determine whether Streptococcus pneumoniae capsule was necessary for pathogenesis of pneumococcal endophthalmitis. Methods. An isogenic capsule-deficient strain was created using homologous recombination. New Zealand White rabbits were injected intravitreously with 10² colony-forming units (CFU) of the parent strain or the capsule mutant. Slit lamp examination (SLE), electroretinography, and myeloperoxidase activity were performed 24 and 48 hours post infection (PI). Serial dilutions of vitreous were plated to quantitate CFU, eyes were extracted for histology, and host cytokine mRNA expression was determined. Results. Eyes infected with the parent strain had significantly higher SLE scores than eyes infected with the capsule-deficient strain 24 and 48 hours PI (P < 0.001). CFU recovered from eyes infected with the capsule mutant were significantly fewer than CFU recovered from eyes infected with the parent strain 24 and 48 hours PI (P < 0.001). The parent strain caused a significantly greater decrease in retinal function and more retinal destruction than the mutant strain 48 hours PI (P = 0.026). Vitreal IL-1β, IL-6, and TNF-α were up regulated by both the parent and mutant strain 12 hours PI. By 48 hours PI, there was significantly more neutrophil infiltration in the vitreous infected with the parent strain. Conclusions. Endophthalmitis caused by the encapsulated strain is more damaging to retinal function and structural integrity. These findings indicate that capsule is an important virulence factor of S. pneumoniae endophthalmitis, in contrast to keratitis, suggesting that the anatomic host site in pneumococcal ocular infections is important.