This review surveys the literature on carriage and transmission of enterohemorrhagic Escherichia coli (EHEC) O157:H7 in the context of virulence factors and sampling=culture technique. EHEC of the O157:H7 serotype are worldwide zoonotic pathogens responsible for the majority of severe cases of human EHEC disease. EHEC O157:H7 strains are carried primarily by healthy cattle and other ruminants, but most of the bovine strains are not transmitted to people, and do not exhibit virulence factors associated with human disease. Prevalence of EHEC O157:H7 is probably underestimated. Carriage of EHEC O157:H7 by individual animals is typically shortlived, but pen and farm prevalence of specific isolates may extend for months or years and some carriers, designated as supershedders, may harbor high intestinal numbers of the pathogen for extended periods. The prevalence of EHEC O157:H7 in cattle peaks in the summer and is higher in postweaned calves and heifers than in younger and older animals. Virulent strains of EHEC O157:H7 are rarely harbored by pigs or chickens, but are found in turkeys. The bacteria rarely occur in wildlife with the exception of deer and are only sporadically carried by domestic animals and synanthropic rodents and birds. EHEC O157:H7 occur in amphibian, fish, and invertebrate carriers, and can colonize plant surfaces and tissues via attachment mechanisms different from those mediating intestinal attachment. Strains of EHEC O157:H7 exhibit high genetic variability but typically a small number of genetic types predominate in groups of cattle and a farm environment. Transmission to people occurs primarily via ingestion of inadequately processed contaminated food or water and less frequently through contact with manure, animals, or infected people.

Human infections with Shiga toxin (Stx)-producing Escherichia coli (STEC) cause hemorrhagic colitis. The Stxs belong to a large family of ribosome-inactivating proteins (RIPs) that are found in a variety of higher plants and some bacteria. Many RIPs have potent antiviral activity for the plants that synthesize them. STEC strains, both virulent and nonvirulent to humans, are frequently isolated from healthy cattle. Interestingly, despite intensive investigations, it is not known why cattle carry STEC. We tested the hypothesis that Stx has antiviral properties for bovine viruses by assessing the impact of Stx type 1 (Stx1) on bovine peripheral blood mononuclear cells (PBMC) from cows infected with bovine leukemia virus (BLV). PBMC from BLV-positive animals invariably displayed spontaneous lymphocyte proliferation (SLP) in vitro. Stx1 or the toxin A subunit (Stx1A) strongly inhibited SLP. Toxin only weakly reduced the pokeweed mitogen-or interleukin-2-induced proliferation of PBMC from normal (BLV-negative) cows and had no effect on concanavalin A-induced proliferation. The toxin activity in PBMC from BLV-positive cattle was selective for viral SLP and did not abrogate cell response to pokeweed mitogen-or interleukin-2-induced proliferation. Antibody to virus or Stx1A was most effective at inhibiting SLP if administered at the start of cell culture, indicating that both reagents likely interfere with BLV-dependent initiation of SLP. Stx1A inhibited expression of BLV p24 protein by PBMC. A well-defined mutant Stx1A (E167D) that has decreased catalytic activity was not effective at inhibiting SLP, suggesting the inhibition of protein synthesis is likely the mechanism of toxin antiviral activity. Our data suggest that Stx has potent antiviral activity and may serve an important role in BLV-infected cattle by inhibiting BLV replication and thus slowing the progression of infection to its malignant end stage.

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Ruminants often carry gastrointestinal Shiga toxin (Stx)-producing Escherichia coli (STEC).

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Staphylococcus aureus is a major mastitis-causing pathogen in cattle. The chronic nature of bovine staphylococcal mastitis suggests that some products or components of S. aureus may interfere with the development of protective immunity. One class of molecules that could be involved are superantigens (SAgs). Although a significant number of mastitis isolates produce SAgs, the effect of these molecules on the bovine immune system is unresolved. To determine if immunosuppression caused by SAgs could play a role in pathogenesis, we monitored bovine lymphocytes exposed to staphylococcal enterotoxin C1 (SEC1). Activation of bovine lymphocytes by either SEC1 or concanavalin A (ConA) was influenced by the γδ/αβ T-cell ratio in the culture. Compared to ConA-induced stimulation, cultures stimulated with SEC1 generated small numbers of CD4+ αβ T cells expressing high levels of interleukin-2 receptor α chain (IL-2Rα) and major histocompatibility complex class II (MHCII), suggesting that SAg exposure does not lead to full activation of these cells. This state of partial activation was most pronounced in cultures with a high γδ/αβ ratio. In contrast, significant numbers of CD8+ αβ T cells expressed high levels of IL-2Rα and MHCII, regardless of the γδ/αβ ratio and the stimulant used. CD8+ blasts in cultures stimulated with SEC1 also expressed another activation marker, ACT3, previously detected predominantly on thymocytes and CD4+ T cells. Although γδ CD2− and CD2+ T cells expressed MHCII and IL-2Rα following stimulation with SEC1, only a few cells increased to blast size, suggesting that they were only partially activated. The results suggest ways in which SAgs might facilitate immunosuppression that promotes the persistence of bacteria in cattle and contributes to chronic intramammary infection.

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This study expanded our earlier finding that Shiga toxin type 1 (Stx1) has activity against bovine leukemia virus (BLV) (W. A. Ferens and C. J. Hovde, Infect. Immun. 68:4462-4469, 2000). The Stx molecular motifs required for antiviral activity were identified, and a mechanism of Stx action on virally infected cells is suggested. Using inhibition of BLV-dependent spontaneous lymphocyte proliferation as a measure of antiviral activity, we showed that Stx2 had antiviral activity similar to that of Stx1. Enzymatic and antiviral activities of three StxA1 chain mutants deficient in enzymatic activity or aspects of receptor-mediated cytotoxicity were compared. Using protein synthesis inhibition to measure enzymatic activity, the mutant E167D was 300-fold less catalytically active than wild-type StxA1, was minimally active in antiviral assays, and did not inhibit synthesis of viral proteins. Two StxA1 mutants, A231D-G234E and StxA 1 1 (enzymatically active but unable to kill cells via the classical receptor-mediated route), had undiminished antiviral activity. Although binding of radiolabeled StxA1 to bovine blood cells or to free virus was not detected, flow cytometric analysis showed that the number of BLV-expressing cells were specifically reduced in cultures treated with Stx. These unique and rare lymphocytes were highly permeable to 40-and 70-kDa fluorescent dextrans, indicating that direct absorption of toxins by virus-expressing cells is a potential mechanism of target cell intoxication. These results support the hypothesis that Stx-producing Escherichia coli colonization of the gastrointestinal tract may benefit ruminant hosts by the ability of Stxs to exert antiviral activity.Ruminant animals are a reservoir for Shiga toxin (Stx)-producing Escherichia coli (STEC) that can cause hemorrhagic colitis and life-threatening sequelae in humans (15,23,25). STEC are part of the normal ruminant gastrointestinal microbiota and are frequently isolated from cattle, sheep, and deer (8,38,41,45). Surveys of healthy domesticated cattle routinely show a high prevalence of STEC in animals around the world (7,8,10,12,(34)(35)(36). The reasons for the wide distribution of STEC in ruminants are not known. The possible benefits arising from gastrointestinal tract colonization by STEC include postulated enhancement of gastrointestinal mucosal architecture