Protection from Escherichia coli O157:H7 infection by Bifidobacterium
Bifidobacteria: an important member of the gut microbiota
The human gastrointestinal tract is inhabited along all its length by a diverse microbial ecosystem formed by around 1 x 1014 colony forming units of bacteria from more than 1000 different species which belong to about 190 genera. The predominant genera of those 190 are Bacteroides, Bifidobacterium, Escherichia, Enterobacter, Enterococcus, Lactobacillus, Eubacterium, Fusobacterium, Ruminococcus, Peptococcus, Peptostreptococcus and Clostridium; and all of these genera are set in 5 core phyla: Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, and Verrucomicrobia. This stable microbiota participates in many physiological functions, such as the immune system stimulation or the digestion and absorption of diverse nutrients (Russell et al. 2011; Leahy et al. 2005).
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Bifidobacteria can be defined as Gram-positive polymorphic rods, non-motile, non-spore-forming and non-filamentous bacteria belonging to Actinobacteria phyla; with morphology ranges from uniform to branched bifurcated Y and V forms which are spatula or club shaped. Although it exist a few strains which can tolerate the oxygen, they are predominantly anaerobes. They are saccharolytic; they are able to ferment glucose, galactose and fructose via the fructose-6-phosphate to obtain acetic and lactic acid (Leahy et al. 2005).
Ten species of Bifidobacteria have been isolated from the human intestine, being the most representatives B. longum, B. breve, B. bifidum, B. adolescentis and B. pseudocatenulatum. Moreover than in the human gastrointestinal tract, where they constitute about 95% of the total gut microbiota in breast-fed newborns and around 3% in adults, Bifidobacteria can be found as well in several niches such as the human vagina (B. adolescentis), food, other animal gut (B. pseudolongum, B. thermophilus and B. animalis) or sewage (B. angulatum) (Russell et al. 2011) .
Probiotic Bifidobacteria and diarrhoea illness
According to their features, several species of Bifidobacteria are considered probiotics microorganisms, since they are non-pathogenic and viable bacteria able to survive the stomach acid and bile, reaching the intestines in a sufficient amount to confer health benefits to the host (Ohland & MacNaughton 2010). Among these benefits, Bifidobacteria strains have been demonstrated to have inhibitory and preventive effects against acute diarrhoeal disease caused by the pathogens Escherichia coli, Salmonella, Campylobacter and Shigella, or by other multiple reasons as, for instance, the intake of antibiotics or rotavirus infection (Leahy et al. 2005; Gagnon et al. 2004). Enterohemorrhagic E. coli (EHEC) serotype O157:H7, also known as Shiga toxin-producing E. coli (STEC) is one of the most relevant causes of traveller’s diarrhoea disease, which in some cases can produce a hemorrhagic colitis and haemolytic uremic syndrome, both of which may lead to death. Currently, probiotic Bifidobacteria are being studied as an alternative therapy against STEC infection, due to their capability to protect animals and humans from E.coli O157:H7 infection by different mechanisms (Carey et al. 2008)
Mechanisms of prevention
Probiotic Bifidobacteria strains can ferment different carbohydrates, such as fructose or mannose, to short-chain fatty acids (SCFA), mainly acetate and formate (dissociated forms of the acetic acid and formic acid), thanks to having genes which encodes ABC-type carbohydrate transporters. The quantity of acetate generated by Bifidobacteria differs from one strain to another. At the same time, those probiotic Bifidobacteria strains which present more transporters can produce more acetate, having more protective capacity (Fukuda et al. 2012).
Although the acetic acid has a high bactericidal activity, the concentration produced by Bifidobacteria is not sufficient to inhibit the growth of E.coli O157:H7. Nevertheless, high concentrations of these organic acid and it inherent decreased in the pH reduce the expression of the EHEC Shiga-toxin genes, inhibiting the Shiga-toxin production and making STEC less virulent (Carey et al. 2008; Asahara et al. 2004). Moreover, high concentrations of acetate can also block the translocation of Shiga toxin through the epithelium to blood, preventing the diffusion of the toxin throughout the body by protecting the epithelial monolayer from STEC-induced cell death (Fukuda et al. 2012), as it is shown in figure 1. Fukuda et al. (2012) determined than endogenous acetate production by probiotic Bifidobacteria in mice is enough to prevent an EHEC infection, but acetyled starch taken as prebiotic increase dramatically the acetate production in the mice gut and enhance significantly the protection against EHEC infection.
Antibody synthesis stimulation
The major part of the human immune system is located in the gut. The Peyer’s patch and the mesenteric lymph nodes are the immune tissues located along the epithelium of the gastrointestinal tract. They are formed principally by B cells and lymphocytes which synthetize Immunoglobulin A (IgA), the most predominant immunoglobulin type of the mucosa. These IgAs opsonize pathogens as E.coli O157:H7 in order to other components of the immune system can process them. Park et al. (2002) found that Bifidobacterium bifidum is not only able to stimulate the production of IgA by mesenteric lymph nodes and Peyer’s patch cells but also they make them more reactive to cytokines such as TGF-β1 and IL-5 for further secretion of IgAs. Furthermore, B.bifidum and other Bifidobacteria strains, as B. thermacidophilum RBL 71, also increase other both serum IgM and IgG antibody types. Thus, Bifidobacterium is, like LPS, a potent B cell polyclonal activator which enhances the humoral immunity without causing immune reactivity in the host as they are a natural component of the gut microbiota (Gagnon et al. 2006; Park et al. 2002).
Inhibition of the EHEC adhesion to the intestinal epithelium
One of the most important virulence factors which allow E.coli pathogenic strains colonize the human gut is their capability to adhere to the intestinal epithelium. However, probiotic bifidobacteria also show this ability, hence they can compete with EHEC strains and reduce their adhesion to the epithelial cells (Gagnon et al. 2004). According to that, Fujiwara et al. (2001) found a protein factor named BIF secreted by Bifidobacterium longum SBT2928 which inhibits the Escherichia coli Pb176 binding to the human intestinal epithelium by interfering in the interaction between the CFA/II antigen of the bacteria and the GA1 molecule on the epithelial cell surface.
Competition for nutrients or the hydrogen peroxide and bacteriocins production are other mechanisms that Bifidobacteria show against E.coli O157:H7 infection (Leahy et al. 2005).
Probiotic Bifidobacteria strains are a representative part of the human gut microbiota. They also can be administered within food products or drugs to humans and reach the intestines conferring multiple benefits to the host. Among these benefits, it has been highlighted their ability to protect the host against E.coli O157:H7 infection by several mechanisms, such as the production of acetate as an anti-toxin factor, the inhibition of the E.coli pathogenic strains adhesion to the epithelium, the stimulation of the immune system or the competition for nutrients.
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