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J.M. Reed, R.J. O'Callaghan, M.E. Marquart; Ocular virulence of capsule–deficient Streptococcus pneumoniae strain R6 . Invest. Ophthalmol. Vis. Sci. 2004;45(13):4638.
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Purpose: Determine the virulence of a Streptococcus pneumoniae unencapsulated strain in a rabbit keratitis model and compare its virulence to an encapsulated strain. Methods: The corneas of New Zealand white rabbits received intrastromal injections of 1 x 105 colony–forming units (CFU) S. pneumoniae strain R6 (unencapsulated) or TIGR4 (encapsulated). Eyes were scored by slit lamp examination (SLE) at 20, 24, and 48 hours post–infection. Rabbits were sacrificed and their corneas were homogenized and plated onto chocolate agar to determine the log CFU per cornea. Results: The mean SLE scores of rabbits infected with strain R6 were 3.81 ± 0.69, 3.50 ± 0.14, and 15.78 ± 1.33 at 20, 24, and 48 hours post–infection, respectively (n = 4). The mean SLE scores of rabbits infected with strain TIGR4 were 3.61 ± 0.44, 5.20 ± 0.44, and 11.05 ± 0.37 at 20, 24, and 48 hours post–infection, respectively (n = 7). The mean log CFU per cornea for strain R6 was 5.53 ± 0.37. The mean log CFU per cornea for strain TIGR4 was 5.88 ± 0.25. There were no significant differences between the two strains with respect to SLE at any of the timepoints examined (P ≥ 0.05), and there was no significant difference in the log CFU between the two strains (P = 0.78). Conclusion: Strain R6, an unencapsulated strain of S. pneumoniae often used as a host for genetic manipulations because of its presumed avirulence due to lack of capsule, is as virulent in the cornea as strain TIGR4, an encapsulated strain. These findings suggest that the S. pneumoniae capsule is not required for ocular virulence.
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