Invasive strains of Escherichia coli belonging to serotype O121:NM

Ten strains of Escherichia coli isolated from 10 travellers with sporadic diarrhea who were returning to Tokyo, Japan, from abroad were found to be of serotype O121:NM and were positive in the Serény test for invasiveness; this suggests that this serotype can cause a shigellosis-like illness in humans. The E. coli O121:NM strains were positive in the cell invasion test in HeLa cells. Analysis of the plasmid content of these strains showed that they contained a high-molecular-mass plasmid of 120 to 140 MDa which has been associated with invasiveness and were positive in an enzyme-linked immunosorbent assay for detection of the virulence plasmid-encoded proteins of Shigella spp. and enteroinvasive E. coli.