Purpose: To provide a historical reference for comparison for newly isolated EHEC strains, and to look at the genetics of O157:H7 in the time period of the first outbreaks, we sequenced a collection of EHEC isolates from years 1985 to 1993, isolated from food and animal fecal samples. This date range is important because it includes strains after the first identification of O157:H7 and after the first outbreak in 1993.
Methods: Sixty strains were sequenced by using next-generation sequencing (Illumina Miseq). Twelve of these strains were isolated from food, included milk, salami, meat and mayonnaise. Forty-eight of these strains were isolated from livestock fecal samples.
Results: A total of 54 of 60 samples were positive for O157:H7, 6 of 60 samples were not E. coli O157:H7, with 2 of these 6 being E.coli serotype O78:H10. 10 of the 54 O157:H7 strains isolated from food samples, while 44 of 54 the O157:H7 strains isolated from livestock fecal samples. Forty strains carried stx1, 46 strains carried stx2, and total of 35 strains carried both stx genes that these strains isolated from these dates are highly similar to more recent isolations.
Significance: These new historical reference sequences can be useful references library and valuable resources for further research of E. coli O157:H7. These strains have probably been causing disease even before the major outbreaks, but mostly unreported because we do not see evidence for a major evolutionary shift in the years between the recent expansion of whole genome sequencing and this reference library.