is among the top causes of bacterial endophthalmitis, an infectious disease of the intraocular fluids. The mechanisms by which grows and thrives in the intraocular cavity are not well understood. We used a bacterial genome-wide assessment tool (transposon insertion site sequencing) to determine genes essential for growth in vitreous humor. The results indicated that an ascorbic acid (AA) transport system subunit was important for growth. We created an isogenic gene deletion mutant of the AA transcriptional activator, , in 2 strains of . Growth curve analysis indicated that deletion caused attenuated growth in vitro for both strains. However, in vivo vitreous humor infection in rabbits with either strain determined that was necessary for growth in one strain but not the other. These results demonstrate that may be important for fitness during endophthalmitis depending on the background of the strain.