Escherichia coli O157 (n = 376) from 41 cattle farms were subtyped using pulsed field gel electrophoresis of endonuclease cleaved chromosomal DNA. Cleavage with XbaI resulted in 81 subtypes. Fifty-one isolates from subtypes found in more than one herd, or in herds on multiple sample collection dates were compared using the endonuclease NotI, resulting in 23 additional subtypes. Up to 11 XbaI subtypes were found per farm with up to 7 subtypes/farm identified from a single date. Indistinguishable subtypes (both XbaI and NotI) were found to persist on 4 farms for 6–24 months. Five subtypes were found on more than one farm separated by up to 640 km. Dairy farms where cattle had moved onto the farm had a similar number of subtypes as farms with no movement of cattle, and feedlots had more subtypes than dairy farms. These data indicate that there is a mechanism for multiple herd exposure to specific subtypes, there are multiple sources of exposure for cattle on farms, and on-farm reservoirs other than cattle may exist.
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