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Mutation in Shigella flexneri 2a Resulting in Increased Electrophoretic Mobility

An avirulent mutant of Shigella flexneri 2a, when grown in broth containing calcium ions, was found to have a three- to fourfold increase in electrophoretic mobility toward the anode when compared to its virulent parent. Under these same growth conditions, it was found that the avirulent strain was made more electronegative in the presence of ethylenediaminetetraacetic acid (EDTA). EDTA had no effect on the electrophoretic mobility of the virulent strain. In the absence of added calcium in the growth medium, no differences between the strains were observed. The alteration which has resulted in an increased negative charge has also stabilized the avirulent strain against loss of lipopolysaccharide from EDTA treatment.