T Sakai

Expression of four virulence antigens of Shigella flexneri is positively regulated at the transcriptional level by the 30 kiloDalton virF protein.

On the virulence plasmid of Shigella flexneri the virG region required for cell‐to‐cell spread of the bacteriumencodes a 130 kiloDalton (kD) antigen and Region‐2essential for the bacterial invasion of epithelial cells encodes 57, 43 and 39 kD antigens. The expression of these four antigens is positively regulated by the 30 kD protein encoded by virF, whose nucleotide sequence had been determined and which was previously found to be essential for virulence. An approximately 3.8kilobase (kb) RNA transcript is found to be transcribed by the virG region and is positively regulated by the virF protein resulting in increased production of the 130 kD antigen. The virF sequence is conserved among all shigellae and enteroinvasive Escherichia coli.

Molecular Genetic Analysis of Virulence Factors of Shigella Flexneri

Labrec et al.7 established in 1964 that invasion into epithelial cells was the essential early step in the pathogenesis of bacillary dysentery. The virulent translucent strain of Shigella flexneri invaded the intestinal epithelium, while the avirulent opaque variant did not, in spite of the finding, that both had the same LD50 upon intraperitoneal injection. Ogawa et al.15 performed similar experiments and obtained essentially the same results. However, they made one important additional observation; within 24 hours of infection, the bacteria existed exclusively in the epithelial cells without direct penetration into the lamina propria and bacterial multiplication therein. Thus, virulent shigellae invade the colonic epithelium and spread successively to adjacent epithelial cells resulting in a severe inflammatory reaction, further spread of the bacteria to deeper tissue and, ultimately, to ulceration and bloody diarrhea.