Ocular virulence of capsule-deficient streptococcus pneumoniae in a rabbit keratitis model

Abstract

Purpose: Determine the ocular virulence of noncapsular Streptococcus pneumoniae in a rabbit keratitis model.

Methods: Mice were infected intraperitoneally with 10(5) colony-forming units (CFUs) of Avery's strain (capsular type 2) or strain R6 (a noncapsular derivative of type 2), and mortality was monitored daily. In addition, 10(5) CFU of each strain was injected into rabbit corneas. Bacterial loads in rabbit corneas were determined at 20 or 48 hours after infection. Slit lamp examination (SLE) of rabbit eyes was performed at 24, 36, and 48 hours after infection. Controls included corneas inoculated with bacterial suspension medium and UV-killed bacteria.

Results: One hundred percent mortality was observed in mice infected intraperitoneally with the encapsulated strain at 2 days after infection, whereas all mice infected with the nonencapsulated strain survived for 21 days. The nonencapsulated strain caused the same pathologic effects in the rabbit cornea as the encapsulated strain at 24, 36, and 48 hours after infection (P > or = 0.080). Control corneas showed no pathologic effects and had significantly lower SLE scores than corneas infected with live bacteria (P < or = 0.001). Mean bacteria log CFU +/- SEM recovered at 20 hours after infection were 7.069 +/- 0.094 for the encapsulated and 6.533 +/- 0.116 for the nonencapsulated strain (P = 0.001). Bacteria recovered from the corneas at 48 hours after infection were 6.712 +/- 0.349 and 1.807 +/- 0.462 for the encapsulated and nonencapsulated strains, respectively (P < 0.001).

Conclusions: The S. pneumoniae noncapsular strain was as virulent in the rabbit cornea as was the encapsulated strain, but unlike the encapsulated strain, was avirulent in the mouse peritoneum.