Role of Escherichia coli O157:H7 Virulence Factors in Colonization at the Bovine Terminal Rectal Mucosa

Abstract

The human pathogen Escherichia coli O157:H7 causes hemorrhagic colitis and life-threatening sequelae and transiently colonizes healthy cattle at the terminal rectal mucosa. This study analyzed virulence factors important for the clinical manifestations of human E. coli O157:H7 infection for their contribution to the persistence of E. coli in cattle. The colonizing ability of E. coli O157:H7 was compared with those of nonpathogenic E. coli K-12 and isogenic deletion mutants missing Shiga toxin (Stx), the adhesin intimin, its receptor Tir, hemolysin, or the ϳ92-kb pO157. Fully ruminant steers received a single rectal application of one E. coli strain so that effects of mucosal attachment and survival at the terminal rectum could be measured without the impact of bacterial passage through the entire gastrointestinal tract. Colonization was monitored by sensitive recto-anal junction mucosal swab culture. Nonpathogenic E. coli K-12 did not colonize as well as E. coli O157:H7 at the bovine terminal rectal mucosa. The E. coli O157:H7 best able to persist had intimin, Tir, and the pO157. Strains missing even one of these factors were recovered in lower numbers and were cleared faster than the wild type. In contrast, E. coli O157:H7 strains that were missing Stx or hemolysin colonized like the wild type. For these three strains, the number of bacteria increased between days 1 and 4 postapplication and then decreased slowly. In contrast, the numbers of noncolonizing strains (K-12, ⌬tir, and ⌬eae) decreased from the day of application. These patterns consistently predicted long-term colonization or clearance of the bacteria from the bovine terminal rectal mucosa.Enterohemorrhagic Escherichia coli (EHEC) strains are a subset of Shiga toxin-producing E. coli (STEC) that can cause human disease and are threats to public health worldwide (46,49). Human illnesses caused by EHEC range from self-limiting watery diarrhea or hemorrhagic colitis to life-threatening sequelae, the hemolytic-uremic syndrome or thrombotic thrombocytopenic purpura. The predominant EHEC serotype associated with the most severe disease in North America, the United Kingdom, and Japan is O157:H7 (23,42,44,46,59).Cattle are considered the primary reservoir for E. coli O157:H7 and the most common source for food-borne and direct animal contact infections (5, 25, 69). Healthy cattle carry E. coli O157:H7 transiently without suffering pathological symptoms (2,4,26). Individual animals can passively shed E. coli O157:H7 in their feces for a short time (a few days) without establishing a colonized state or can pass fecal E. coli O157:H7 for a longer time (a month or more) if the bacteria colonize and persist (22). The conditions that lead to these different host-bacterium interactions are not understood.It is well accepted that reducing the carriage or prevalence of E. coli O157:H7 in cattle would reduce the risk of human exposure to this pathogen (61). Recently, the recto-anal junction (RAJ) mucosa was identified as the primary site of E. coli O157:H7 colonizatio...

Escherichia coli O157:H7-secreted cytotoxins are toxic to enterocytes and increase Escherichia coli O157:H7 colonization of jejunum and descending colon in cattle

“…In contrast, Shiga toxins do not affect either EHEC O157:H7 colonization of the terminal rectum in single strain rectal challenged cattle (Sheng et al 2006) or secretory responses in bovine ileal intestinal loops from older calves (Stevens et al 2002). The terminal rectum studies (Naylor et al 2003;Sheng et al 2004Sheng et al , 2006Low et al 2005;Lim et al 2007) have focused on the role of lymphoid tissue in the EHEC O157:H7 infection process in cattle without comparisons with absorptive epithelium in other intestinal sites despite the reported association of EHEC O157:H7 with small and large intestinal sites in challenged cattle (Cray and Moon 1995;Grauke et al 2002;Van Baale et al 2004). In a previous study, we reported that there were no differences in the dose-dependent colonization of small and large intestinal sites compared with the terminal rectum (Baines et al 2008, manuscript in preparation) and in this study, we add to this by showing that the small and large intestinal tissues also respond to EHEC O157:H7 secreted cytotoxins in a similar manner to that reported for a model system purported to better represent the response of ruminant systems to EHEC O157:H7 colonization (Robinson et al 2006).…”

“…Other factors affect EHEC O157:H7 colonization in cattle including age (Cray and Moon 1995) and exposure dose (Besser et al 2001;Sheng et al 2004). Two EHEC effector molecules, Intimin and Tir, are important EHEC O157:H7 virulence factors, enhancing the magnitude and duration of shedding in calves and cattle (Dean-Nystrom et al 1998;Stevens et al 2002;Sheng et al 2006;Sinclair et al 2006) with the benefit derived from their involvement in the intimate attachment of bacteria to mucosal tissues. In contrast, Shiga toxins do not affect either EHEC O157:H7 colonization of the terminal rectum in single strain rectal challenged cattle (Sheng et al 2006) or secretory responses in bovine ileal intestinal loops from older calves (Stevens et al 2002).…”

“…Two EHEC effector molecules, Intimin and Tir, are important EHEC O157:H7 virulence factors, enhancing the magnitude and duration of shedding in calves and cattle (Dean-Nystrom et al 1998;Stevens et al 2002;Sheng et al 2006;Sinclair et al 2006) with the benefit derived from their involvement in the intimate attachment of bacteria to mucosal tissues. In contrast, Shiga toxins do not affect either EHEC O157:H7 colonization of the terminal rectum in single strain rectal challenged cattle (Sheng et al 2006) or secretory responses in bovine ileal intestinal loops from older calves (Stevens et al 2002). The terminal rectum studies (Naylor et al 2003;Sheng et al 2004Sheng et al , 2006Low et al 2005;Lim et al 2007) have focused on the role of lymphoid tissue in the EHEC O157:H7 infection process in cattle without comparisons with absorptive epithelium in other intestinal sites despite the reported association of EHEC O157:H7 with small and large intestinal sites in challenged cattle (Cray and Moon 1995;Grauke et al 2002;Van Baale et al 2004).…”

“…Bacterial toxins can have both lethal (Keenen et al 1986;Paton et al 1995;Bauer and Welch 1996;Ferens and Hovde 2000;Schuller et al 2004;Figueiredo et al 2007) and sublethal (Menge et al 2004;Ferens and Hovde 2007) effects on cells that benefit pathogen colonization through a reduction in host defenses or by providing a foothold on the mucosa for replication. Persistent pathogen colonization like that observed in challenged cattle (Cray and Moon 1995;Besser et al 2001;Grauke et al 2002;McAllister et al 2006;Sheng et al 2006) also support a potential role for bacterial toxins in subverting the immune defense system (Galan 2005). We recently identified EHEC O157:H7 colonies associated with focal mucosal hemorrhages in the absorptive epithelium from the jejunum and colon of persistent shedding cattle (Baines et al 2008, manuscript in preparation).…”

“…EHEC O157:H7 has been isolated from the small and large intestine of older calves and mature cattle but not necessarily associated with A/E lesions (Cray and Moon 1995;Brown et al 1997;Grauke et al 2002;Naylor et al 2003;van Baale et al 2004). The current model for EHEC O157:H7 colonization in cattle suggests that the lymphoid-follicle dense mucosa in the terminal rectum is the primary colonization site and it is this colonization that is responsible for persistent shedding (Naylor et al 2003Sheng et al 2004Sheng et al , 2006Lim et al 2007). The basic assumption of this model is that cattle are not adversely affected by EHEC O157:H7 virulence factors, and although a reduction in the expression of some EHEC O157:H7 virulence factors has been demonstrated (Rashid et al 2006), it is not true for all virulence factors.…”