Monday, 13 July 2015

Why does gut dysbiosis always involve Enterobacteriaceae?

Several studies by Maes et al. have implicated Enterobacteriaceae in CFS. Specifically there are elevated antibody responses to the LPS of commensal Enterobacteriaceae which correlates immune markers and abdominal symptoms 1,2. This suggests Enterobacteriaceae or their components (LPS) have translocated from the gut into the body (i.e. leaky gut) and stimulated an immune response. This post compiles some factors found to influence Enterobacteriaceae growth and translocation in other diseases, which may also be of some relevance in ME/CFS.

Enterobacteriaceae and disease
Enterobacteriaceae are a large family of gram-negative facultative bacteria, which belong to the class Gammaproteobacteria and phylum Proteobacteria. The Enterobacteriaceae family contains gut symbionts but also many familiar pathogens (e.g. Klebsiella, E. coli, Salmonella, Citrobacter, Enterobacter, etc). Proteobacteria and Enterobacteriaceae are normally present in the gut at relatively low levels, and exist in close proximity to the mucosa, since as facultative bacteria they can tolerate oxygen diffusing from the epithelium 3. However they are amongst the most frequently overgrown gut bacteria in many conditions, including gut infections, IBD, IBS, constipation, celiac disease, AIDS, SIRS, obesity, Parkinson’s disease and major depression.

Enterobacteriaceae promote disease via immune activation; largely because they are a major source of potent inflammatory PAMPs such as lipopolysaccharide (LPS) 4. For instance in the gut Enterobacteriaceae/LPS can increase inflammatory tone 5, slow intestinal motility 6, exacerbate NSAID-induced intestinal injury 7, increase intestinal permeability in celiac disease 8, promote intestinal hypersensitivity in IBS 9 and exacerbate inflammation in IBD, amongst other things. Translocation of LPS into blood is associated with systemic immune activation, neuroinflammation 10, insulin resistance 11, etc.

So elevated levels of Enterobacteriaceae is bad! But how does it occur in the first place? Below are some mechanisms which could be important.

Dietary factors
Diets high in sugar, fat and protein, but low in plants and indigestible carbohydrate (e.g. western or weight-loss diets), favour the growth of Proteobacteria and Enterobacteriaceae 12,13. This could be for several reasons. Diets high in protein promote a putrefactive microbial metabolism which generates harmful metabolites 14, while diets high in indigestible carbohydrate (resistant starch and fibre) promote a saccharolytic metabolism which generates beneficial short-chain fatty acids (SCFAs) 12. SCFAs acidify the colon and inhibit Enterobacteriaceae 12. Diets high in fat, saturated fat and omega-6 promote Enterobacteriaceae growth and LPS translocation, while omega-3 does the opposite 15–18. The beneficial effects of omega-3 on the gut microbiota are due to regulation of intestinal alkaline phosphatase (IAP) 18.

Low stomach acid
Suppression of gastric acid secretion by proton pump inhibitor (PPI) administration was found to induce jejunum dysbiosis, consisting of an overgrowth of aerobic bacteria and Enterobacteriaceae, and a decrease in Bifidobacteria 7. Many other studies have found an association between PPI use and small intestinal bacterial overgrowth (SIBO) in humans 19 (note that Enterobacteriaceae can be hydrogen-producers 20). This may involve several mechanisms: gastric acid can inhibit the growth of many bacteria, promote protein digestion and trigger other intestinal secretions/processes.

The gut barrier regulates levels of mucosal bacteria by releasing antimicrobial peptides and IgA 21,22. Innate immune functions are impaired in inflammatory bowel disease (IBD), especially Crohn’s disease, which allows for increased growth of bacteria such as invasive E. coli  23,24. Also genetic variations which impair function of the NOD2 gene (encodes an intracellular immune receptor) is associated with increased abundance of Enterobacteriaceae in IBD 25. In both HIV/AIDS and ICL there is major disruption of the intestinal immune system, resulting in barrier disruption and translocation of LPS 26,27.

Inflammation & oxidative stress
Gut inflammation has been shown to induce blooms in Proteobacteria and Enterobacteriaceae. This is due to the increased formation of oxidation products (e.g. nitrate) which can serve as electron acceptors in the anaerobic respiration of some facultative bacteria 28,29. In fact nitrate reductase activity is most prevalent in the genomes of Enterobacteriaceae 30. Moreover some Enterobacteriaceae pathogens (e.g. Salmonella) may actually induce inflammation as part of an evolutionary survival strategy 29. Notably antibiotic treatment can induce low-grade gut inflammation which enhances the growth of Enterobacteriaceae 28–30. Inflammation can also increase intestinal permeability and may therefore allow bacterial translocation, perhaps especially in the ileum 31.

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