Genetic subtyping of Escherichia coli O157 isolates from 41 Pacific Northwest USA cattle farms

Escherichia coli O157 (n = 376) from 41 cattle farms were subtyped using pu lsed field gel electrophoresis of endonuclease cleaved chromosomal DNA. Cle avage with XbaI resulted in 81 subtypes. Fifty-one isolates from subtypes f ound in more than one herd, or in herds on multiple sample collection dates were compared using the endonuclease NotI, resulting in 23 additional subt ypes. 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 N otI) were found to persist on 4 farms for 6-24 months. Five subtypes were f ound on more than one farm separated by up to 640 km. Dairy farms where cat tle 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. The se data indicate that there is a mechanism for multiple herd exposure to sp ecific subtypes, there are multiple sources of exposure for cattle on farms , and on-farm reservoirs other than cattle may exist.