we describe a strain of shigella dysenteriae serotype 7 which had novel biochemical and genetic characters . unlike other s.  dysenteriae , it produced gas , fermented mannitol , was a late - lactose fermenter and harboured the set 1a and set 1b genes . 
 the significance of such atypical strains is that they are difficult to identify . 
 if such strains are missed , they could prove to be a serious public health problem because the infectious dose is very low and they may harbour integrons contributing to drug resistance .