Rabbit Anti-S. flexneri Polyclonal antibody for sELISA. Shigella flexneri is a species of Gram-negative bacteria in the genus Shigella that can cause diarrhea in humans. There are several different serogroups of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become resistant. Less severe cases are not usually treated because they become more resistant in the future.