Characterization of enteroinvasive Escherichia coli serotype 0164 by means of plasmid analysis and virulence assay.

Escherichia coli 0164 is the most prevalent serotype among the enteroinvasive E. coli in Bulgaria and is annually isolated in sporadic, as well in epidemic cases. We studied 243 strains of this serotype, isolated in different regions of the country over the period 1954-1988. The plasmid analysis performed revealed that the presence of large plasmid DNA with molecular size of 140 MDa was related to the virulence of the strains, assessed by the test of SERENY. The absence of this plasmid or its deletion led to the loss of virulence. In 92% of the strains two small plasmids were demonstrated with molecular size of 6.0 MDa and 4.8 MDa, which appeared to be serotype-specific. On the basis of the plasmid content of the strains, 7 plasmid profiles were differentiated. The stability of the plasmid profile within a given epidemic focus was confirmed.