Plasmid-mediated resistance to nalidixic acid in Shigella dysenteriae type 1

In an epidemic of shigellosis in southern Bangladesh the causal organism, Shigella dysenteriae type 1, was resistant to nalidixic acid as well as to co-trimoxazole (trimethoprimsulphamethoxazole) and ampicillin. The genes coding for resistance to nalidixic acid, but not those coding for resistance to co-trimoxazole or ampicillin, are located on a conjugative 20 megadalton plasmid. This epidemic is of particular importance because of the resistance to nalidixic acid, an antibiotic to which shigellae are seldom resistant, and because plasmids were previously thought not to mediate resistance to nalidixic acid.