Abstract

Escherichia coli O157:H7 is an important zoonotic pathogen, and one in which cattle are a major reservoir. E. coli O157:H7 type III secreted proteins are thought to contribute to rectal colonization and fecal shedding in cattle, but studies have not been conducted in slaughter-age animals, nor addressed the immune responses to these proteins in response to infection. This study addressed the significance of the translocated intimin receptor (Tir) in this regard. Adult cattle were orally challenged (C1) with a Tir⁺ or Tir^- E. coli O157:H7 strain and 42 days later rechallenged (C2) with the nalidixic acid (Nal) ^R parent organism. The Δtir, Tir complemented mutant, and Δtir vector control strains inadvertently did not express flagellin nor effectively colonize the intestine. Prior to C1 inoculation, cattle (n=30) had serum antibody titers to EspB and O157 lipopolysaccharide (LPS) that were higher (P<0.05) than those to Tir, intimin and EspA. After C1, those cattle inoculated with the wild type strain became colonized as evidenced by the magnitude and duration of fecal shedding, and developed significant increases in their serum antibody titers to the Tir, EspA, EspB, intimin and O157 LPS (P<0.05). However, as fecal shedding decreased, titers to all, except EspB, also decreased. In contrast, IgG and IgA antibodies to these same antigens were not detected in the feces, but were detected at low levels in the rectocolonic mucosal secretions. Hence, adult cattle respond serologically and mucosally to E. coli O157:H7 type III secreted proteins, intimin and O157 LPS during the course of infection and the response is correlated with the extent of fecal shedding. Due to the loss of flagellin expression by the Tir ^-stains, we were unable to determine whether Tir exposure had any effect on protection, and the cause of the loss of expression of flagellin was not addressed. However, we interpret these results as suggesting that either the flagellum or a factor that regulates both its production and that of some other effector has an important function in colonization. This information should aid in the understanding of factors involved in colonization and protection of cattle against E. coli O157:H7 infection.