Healthy ruminants are the main reservoir of Shiga toxin-producing
Escherichia coli (STEC). During their transit through the ruminant gastrointestinal tract, STEC encounters a number of acidic environments. As all STEC strains are not equally resistant to acidic conditions, the purpose of this study was to investigate whether acid resistance confers an ecological advantage to STEC strains in ruminant digestive contents and whether acid resistance mechanisms are induced in the rumen compartment. We found that acid-resistant STEC survived at higher rates during prolonged incubation in rumen fluid than acid-sensitive STEC and that they resisted the highly acidic conditions of the abomasum fluid, whereas acid-sensitive strains were killed. However, transit through the rumen contents allowed acid-sensitive strains to survive in the abomasum fluid at levels similar to those of acid-resistant STEC. The acid resistance status of the strains had little influence on STEC growth in jejunal and cecal contents. Supplementation with the probiotic
Saccharomyces cerevisiae CNCM I-1077 or
Lactobacillus acidophilus BT-1386 led to killing of all of the strains tested during prolonged incubation in the rumen contents, but it did not have any influence in the other digestive compartments. In addition,
S. cerevisiae did not limit the induction of acid resistance in the rumen fluid. Our results indicate that the rumen compartment could be a relevant target for intervention strategies that could both limit STEC survival and eliminate induction of acid resistance mechanisms in order to decrease the number of viable STEC cells reaching the hindgut and thus STEC shedding and food contamination.Shiga toxin-producing
Escherichia coli (STEC) strains are food-borne pathogens that cause human diseases ranging from uncomplicated diarrhea to hemorrhagic colitis (HC), as well as life-threatening complications, such as hemolytic-uremic syndrome (HUS). Most outbreaks and sporadic cases of HC and HUS have been attributed to O157:H7 STEC (
http://www.cdc.gov/ecoli/outbreaks.html;
http://www.euro.who.int). However, in some geographic areas, non-O157:H7 STEC infections are considered to be at least as important as
E. coli O157:H7 infections, but they are often underdiagnosed (
21,
46). In spite of diverse virulence characteristics, one common trait of pathogenic STEC strains could be resistance to the gastric acidity in humans. Indeed, it has been suggested that acid resistance of
E. coli O157:H7 is negatively correlated with the infectious dose required for this organism to cause disease in humans (
17).Healthy cattle and other ruminants appear to be the main reservoir of STEC strains. However, colonization of the cattle gastrointestinal tract (GIT) by STEC seems to be a transient event, with a mean duration of 14 days to 1 month (
4,
8,
38). The site of STEC persistence and proliferation in the GIT depends on the STEC strain and seems to vary from one individual to another. Some previous studies identified the rumen as the primary site of colonization (
8), whereas other studies referred to the cecum, the colon, or the rectum (
10,
18,
23,
32,
42). Although STEC strains adhere
in vitro to bovine colonic mucosa, forming the characteristic attaching and effacing lesions (
35), they are very rarely associated with tissues in animal carriers and are generally isolated from the digesta (
8). STEC does not, therefore, seem to colonize the gut mucosa, except for the anorectal mucosa, which has been described as the preferred colonization site for O157:H7 strains but not for non-O157:H7 strains (
24,
32). During their transit through the ruminant GIT, STEC strains encounter various acidic conditions. Volatile fatty acid (VFA) concentrations are high in the rumen of grain-fed animals, and the pH may vary from 5.0 to 6.5. In these conditions, VFAs are in the undissociated form and can freely enter the bacterial cells, dissociate, and acidify the cytosol. In hay-fed animals, less fermentation occurs in the rumen, and the pH remains between 6.5 and 7. In the abomasum, STEC encounters strongly acidic conditions, regardless of the diet, due to the presence of mineral acids, resulting in a pH below 3. Then the pH increases from the proximal part to the distal part of the small intestine, and in the cecum and the colon STEC encounters more neutral pH conditions.All STEC strains are not equally resistant to acidic conditions (
2,
9,
30,
45). Therefore, it could be hypothesized that acid-resistant (AR) STEC survives and persists better in the GIT of ruminants than acid-sensitive (AS) STEC. Acid resistance mechanisms can be induced during exposure to a moderately acidic environment (
12,
26,
41). The rumen contents of a grain-fed animal could be such an environment favorable for the induction of acid resistance in STEC. While the diet does not seem to affect the acid resistance of an
E. coli O157:H7 strain (
19), grain feeding increases the number of acid-resistant generic coliforms (
15,
19), either by inducing acid resistance mechanisms in the rumen or by selecting acid-resistant
E. coli strains during passage through the abomasum. Hence, generic coliforms behave differently than
E. coli O157:H7 in ruminants (
19), and the potential ecological advantage conferred by acid resistance to non-O157:H7 STEC strains for persistence in the ruminant GIT has never been investigated.Inhibition of STEC proliferation in the ruminant gut may be mediated through probiotic supplementation. Several studies have demonstrated the capacity of certain lactic acid bacteria or yeast to reduce
E. coli O157:H7 counts
in vitro (
1,
34) or
in vivo (
5,
40). The mechanisms of action of probiotics are not well characterized but could involve competition for nutrients and adhesion sites in the GIT, an increase in the VFA concentration and a decrease in the pH, production of antimicrobial molecules, or interference with quorum-sensing signaling (
27-
29). However, the impact of probiotics on non-O157:H7 STEC has been poorly investigated (
36). Although not all non-O157:H7 STEC strains are pathogenic, limiting their carriage by ruminants should decrease the risk of food-borne illness. The impact of probiotics and of the physicochemical conditions of the rumen digesta on the survival of non-O157:H7 STEC strains or on induction of acid resistance mechanisms could have significant implications for farm management practices and food safety.The purpose of this work was to investigate whether the level of acid resistance, determined using an
in vitro assay, confers an ecological advantage to STEC strains in ruminant digestive contents and whether acid resistance mechanisms are induced in the rumen compartment. Moreover, we evaluated the potential of probiotics to limit STEC survival and induction of acid resistance in the ruminant GIT.
相似文献