Escherichia coli O157 : H7 forms attaching and effacing lesions at the terminal rectum of cattle and colonization requires the LEE4 operon

1 Animal Health Group, Scottish Agricultural College, West Mains Road, Edinburgh EH9 3JG, UK 2 Zoonotic and Animal Pathogens Research Laboratory, Division of Veterinary Biomedical Sciences, Teviot Place, Edinburgh University, Edinburgh EH8 9AG, UK 3 Functional Genomics Unit, Moredun Research Institute, Pentland Science Park, Mid Lothian EH26 0PZ, UK Correspondence David L. Gally dgally{at}ed.ac.uk Enterohaemorrhagic Escherichia coli O157 : H7 is a human pathogen that causes no apparent disease in cattle, its primary reservoir host. Recent research has demonstrated that E. coli O157 : H7 predominately colonizes the distal few centimetres of the bovine rectum, and in this study, the LEE4 operon encoding a type III secretion system translocon and associated proteins was shown to be essential for colonization. A deletion mutant of LEE4 failed to colonize cattle, in contrast to a co-inoculated strain containing a chromosomal complement of the operon, therefore fulfilling ‘molecular’ Koch's postulates for this virulence determinant. In addition, attaching and effacing (A/E) lesions were detectable in E. coli O157 : H7 microcolonies from the terminal rectum of both naturally and experimentally colonized cattle when examined by transmission electron microscopy. This study proves that type III secretion is required for colonization of cattle by E. coli O157 : H7, and that A/E lesion formation occurs at the bovine terminal rectum within E. coli O157 : H7 microcolonies. The research confirms the value of using type III secreted proteins as vaccine candidates in cattle. Abbreviations: A/E, attaching and effacing; EHEC, enterohaemorrhagic Escherichia coli ; EPEC, enteropathogenic Escherichia coli ; Esp, EPEC secreted protein; FAE, follicle associated epithelium; LEE, locus of enterocyte effacement; MPN, most probable number; TEM, transmission electron microscopy; Tir, translocated intimin receptor; TTSS, type III secretion system