Pneumococcal Endophthalmitis: Prevalence of Serotype 6 in Human Endophthalmitis Isolates and Comparison to a Non-Type 6 Isolate in a Rabbit Endophthalmitis Model

Abstract

Purpose:: Streptococcus pneumoniae is a common cause of bacterial endophthalmitis. The purpose of this study was to determine whether a specific serotype was predominant in human pneumococcal endophthalmitis isolates and to test whether the predominant serotype was more virulent than another serotype in a rabbit endophthalmitis model.

Methods:: Multiplex PCR was used to determine the capsule types of 9 pneumococcal strains isolated from human cases of endophthalmitis. Rabbits were maintained according to the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. Rabbit vitreous was injected with 10² colony-forming units (CFU) of the type 6 strain (n = 6) or the non-type 6 strain (n = 6). Severity of endophthalmitis infection was graded by two observers using slit lamp examination (SLE). Eight different parameters were each assigned a grade ranging from 0 (normal) to 4 (most severe), resulting in a total score of 0 to 32. Vitreous was removed from rabbits infected with both strains, and serial dilutions of vitreous were plated on blood agar for bacterial CFU quantitation.

Results:: Of the 9 strains, 4 (44%) were type 6, 1 (11%) was type 11, 1 (11%) was type 14, 2 (22%) were type 19f, and 1 (11%) was of undetermined type. One of the type 6 strains was compared to the strain of undetermined type in a New Zealand white rabbit model of endophthalmitis. At 24 hours post-infection, the average SLE score for the type 6 pneumococcal strain was 25.21 ± 2.1493 whereas the average SLE score for the non-type 6 strain was 8.33 ± 2.7586 (P = 0.0007). Rabbits infected with type 6 were sacrificed immediately following SLE due to the severity of infection. The CFU present in eyes infected with the type 6 strain were not significantly different than the CFU in eye infected with the non-type 6 strain (8.7205 ± 0.1418 and 8.5554 ± 0.3369, respectively; P= 0.6612).

Conclusions:: These data show that the endophthalmitis isolates in this study are predominantly type 6 and that one of these type 6 isolates has increased virulence in the vitreous compared to a non-type 6 strain.

Keywords: endophthalmitis • bacterial disease