Table 1.

Description of bacterial strains used in this study.

Strains	Description	Stx	Geno- typea	Origin	Source or reference
43894	Wild-type E. coli O157:H7 ATCC 43894	1&2	3b	Human isolate from outbreak associated with hamburger meat in Michigan, USA, 1982	ATCC
43894-Cu (277)	Isogenic mutant of 43894 with pO157 deletion	1&2		pO157 was cured using a mini plasmid incompatible with pO157d	(Sheng et al., 2006)
Sakai	Wild-type E. coli O157:H7	1&2	3	Human isolate from outbreak associated with white radish sprout in Osaka, Japan, 1996	(Tatsuno et al., 2001)
Sakai-Cu	Isogenic mutant of Sakai with pO157 deletion	1&2		pO157 was cured using a mini plasmid incompatible with pO157d	(Tatsuno et al., 2001)
WSU180	Wild-type E. coli O157:H7	1&2	6c	Bovine isolate from a dairy heifer in WA, USA, 2003	(Rice et al., 2003)

^aStx-encoding bacteriophage insertion site genotypes (Besser et al., 2007)

^bgenotype 3, a principle genotype of human clinical isolates

^cgenotype 6, a bovine-biased genotype

^dPlasmid deletion was confirmed by Southern-blot hybridization with a pO157-specific gene probe and chromosomal DNA integrity was confirmed by PFGE