Rough mutant of Shigella flexneri 2a that penetrates tissue culture cells but does not evoke keratoconjunctivitis in guinea pigs.

A rough mutant, designated 5503-01, has been isolated from a virulent strain of Shigella flexneri 2a 5503. Strain 5503-01 produced smooth opaque colonies, whereas its parent strain produced characteristic green-gold translucent ones. Characterization of 5503-01 by agglutination tests, rhamnose content, and sensitivity spectra to "rough-specific" phages revealed that it had lost the specific somatic antigens. When 5503-01 was used to infect HeLa or L cells, it penetrated the cells and multiplied within the cytoplasm. On the other hand, it could not evoke keratoconjunctivitis in guinea pigs. The properties of this strain were remarkably stable against serial passages and preservation for a long period. The presence of a rough mutant with the ability to penetrate tissue culture cells suggest that specific O antigen is not of significance in the early step of the invasive process of shigellosis.