Genetic Diversity and Virulence Potential of Shiga-toxigenic Escherichia coli Strains of O113:H21 Serotype Isolated from the Environment and Foods from Various Countries

Introduction:  Shiga toxin-producing Escherichia coli (STEC) that cause severe diseases most often also produce the intimin binding protein that enables them to attach to intestinal cells.  However, O113:H21 strains do not produce intimin, but have caused hemolytic uremic syndrome (HUS) in Australia and elsewhere.  O113:H21 strains have been isolated from the environment in many countries and also from foods and fresh produce. But it’s uncertain if these strains are closely related or are pathogenic.

Purpose: Sixty-five clinical, environmental and food isolates of O113:H21 from Argentina, Brazil, France, Germany and the U.S. were compared to the HUS-causing pathogenic strains from Australia to determine if they may be pathogenic. The strains were also examined for genetic diversity.   

Methods: A Microarray was used to test for 41 STEC traits. The specific stx subtype they carried was determined by PCR.  Multilocus Sequence Typing was used to examine clonal relations and genetic diversity was determined by Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) sequence polymorphisms.

Results: All the environmental strains were O113:H21 and indistinguishable from the known pathogens. Most strains carried Stx2a or Stx2c, but a few also had Stx2d or combinations of these subtypes. Stx1a was also found in 14 strains, mainly bovine isolates from Brazil. Majority of the strains had sequence type (ST) 223 or had ST that is closely related to 223.  Five strains, 3 from Brazil and 2 from Germany, had ST846 and belonged in a different clonal group. The Australian strains had ST820 that differed from ST223 by a single mutation but are within the STEC-2 clonal group.  Although in the same clonal group, CRISPR analysis showed considerable genetic sequence diversity among the O113:H21 strains.

Significance: Environmental O113:H21 strains from the various countries are indistinguishable from the pathogenic strains suggesting that they may also be pathogenic. Most belong in the same clonal group, but are genetically diverse.