Human Gut Flora Genes

Thousands of Species, Millions of Genes

A research paper in this week's Nature shows a major advance in the study of the role of the human gut flora in disease and health.  Metagenomics of the Human Intestinal Tract (MetaHIT) Consortium examined the role of gut flora in health by a massive project (ref. below) to determine the DNA base sequence of the majority of the millions of genes in the thousands of species of bacteria that abide in human feces.  Gut flora have been linked to many human diseases, as well as the normal function of the human immune system.

Major Findings:

• 576.7 gigabases sequenced (150 times larger than human genome)
• Identified 3.3 million gut flora genes
• Represents more than 1,000 bacterial species
• Each individual harbors approx. 160 different species
• Most prevalent species are identified and shared by everyone

Feces from 124 Europeans

Feces samples (124 individuals total) from a Danish project to determine the role of gut flora in obesity and from a Spanish project to determine the role of gut flora in Crohn’s disease and ulcerative colitis, were extracted for total DNA.

Illumina Multiplex Sequencing of PCR Amplified Fragments

DNA samples from each individual were fragments (<800 bp) and amplified to include indexing information and sequencing primers.  The sequences of a dozen of the fragments at a time was determined using a dozen different dyes (Illumina multiplexing).  Sample preparation and analysis was fully automated to permit identification of overlapping contiguous sequences and assembly of bacterial genomes (1,000-1,150 most common).  The sequence data in this study represents most of the bacterial species of the gut.

Comparison to 89 Existing Human Gut Bacteria Genome Sequences

The new study represented more than 200 times the sequence information than in all previous studies combined.  Existing genome sequences could be identified among the bacterial genomes assembled from the new data.  The identified and sequenced bacterial species represent approximately a tenth of the common bacteria in the gut.

Polysaccharide/sugar Metabolism Common among Gut Flora

Genes coding for enzymes needed to metabolize pectin, sorbitol, mannose, fructose, cellulose and sucrose, were common among the gut flora.  Bacteriophages were also significantly (5%) represented in the total gut metagenome.  Most of the genes were assigned recognizable bacterial functions, but many genes, presumably involved in interactions among the bacterial community or in modification of gut function have not been characterized.

Obese, Crohn’s Disease, Ulcerative Colitis, Compared to Healthy

The bacterial gene compostion of individuals diagnosed with Crohn’s Disease and ulcerative colitis were different and distinct from healthy individuals.  Individuals with Crohn’s disease had 25% fewer species of gut flora than comparable healthy controls.

This study demonstrates the feasibility of using current techniques to examine in detail the interactions between gut flora and tissues of the gut that are involved in health and disease.  This also suggests the risks of antibiotics in altering critical functions of the gut flora, as well as the alteration of gut flora to support health and cure disease.

reference:
Junjie Qin, et al.  2010. A human gut microbial gene catalogue
established by metagenomic sequencing. Nature 464, 59-65.

Biofilm Transformation, Helicobacter, Klebsiella

Helicobacter pylori causes stomach cancer, but it feeds on hydrogen gas produced by Klebsiella pneumoniae in gut biofilms. DNA released by biofilm bacteria not only transfers antibiotic resistance, but it also provides protection against host antibacterial peptides, such a cathelicidins and defensins.

Exploding Labs

When I was working on host/pathogen interactions and plant disease resistance, I also became familiar with research on the formation of the plant equivalent of cancer, crown galls, and symbiotic bacterial nitrogen fixation. I mention this, because this also exposed me to the free-living bacterial nitrogen fixing system in Klebsiella and to the memorable urban legion of exploding labs. As the story goes, as bacteria convert atmospheric nitrogen gas into ammonia, nitrogen fixation, they use high energy electrons, e.g. from ferrodoxin, and lots of ATP, but they also produce hydrogen gas. In labs where they are researching nitrogen fixation, the excess hydrogen gas would accumulate on the ceiling until... boom! Now those labs are properly vented.

Helicobacter Uses Hydrogen as an Energy Source

Helicobacter pylori is considered the most common bacterial pathogen of humans and is the primary cause of ulcers and stomach cancer. H. pylori lives in the stomach by neutralizing stomach acid with ammonia. Another interesting ability of this bacterium is its ability to use hydrogen dissolved in circulating blood as an energy source. The high energy electrons from molecular hydrogen are transported to its electron transport chain, and the energy is used in membrane transport and ATP production. The circulating hydrogen is produced by gut bacteria.

Klebsiella Is not just a Soil Bacterium, Gut Gases

Klebsiella pneumonia is a lung pathogen and it also forms gut biofilms. Presence in the gut and the ability to produce hydrogen gas has some implications for hydrogen utilizing bacteria like H. pylori. Clearly, the stomach of someone with an abundant source of hydrogen fuel in their blood is a better target for H. pylori colonization. This explains why even at age 50, individuals who were exclusively breastfed have a lower incidence of H. pylori and stomach cancer, since even a single bottle of formula can shift an infant to adult, i.e. Klebsiella gut flora.

Klebsiella Needs Carbs to Produce Hydrogen

K. pneumoniae has been associated with Crohn’s Disease and Ankylosing Spondylitis. It grows in gut biofilms and produces pullulanase, an enzyme that can utilize the branched glucosides left over from the action of amylase on plant starch. So K.p. has an untapped food source and it needs lots of ATP to produce hydrogen gas. The nitrogenase needed for nitrogen fixation and hydrogen production is very sensitive to oxygen, so this means that K.p. needs a partially anaerobic environment and must get its energy from fermentation. Fermentation yields much less ATP than respiration using oxygen, which means that K.p. can only produce hydrogen with lots of glucose from starch.

Low Carb Diet Cures Crohn’s Disease

It turns out that the antigen causing Crohn’s disease is the pullulanse (with collagen mimetics.) As you should expect, it has a basic triplet. Eating a low carb diet reduces the flareups of Crohn’s disease, presumably by starving out the K.p.. It is interesting that nitrogenase is the antigen involved in Ankylosing Spondylitis.

Biofilms Promote Transformation and Antibiotic Resistance

Just as a footnote to the benefit of K.p. as a citizen of a biofilm community, H.p. should also live in those biofilms, since that is the source of the hydrogen it uses. Biofilms also stimulate the exchange of DNA, because the quorum sensing chemical signals trigger the release of DNA. The DNA is a component in the matrix that binds bacteria in the biofilm and can work in conjunction with bacterial acidic polysaccharides and host heparan sulfate. These acidic polymers tend to bind the basic antimicrobial peptides, e.g. defensins and cathecidins produced as a major non-adaptive defense against bacteria. Thus, the release of DNA triggered by quorum sensing, builds matrix, facilitates DNA transformation that is the foundation for the spread of antibiotic resistance in gut biofilms and provides resistance against antimicrobial peptides.

Vagus Nerve Controls Gut Inflammation II

Inflammatory Mast Cells Silenced

In a previous article, I outlined the role of the vagus nerve in responding to infection/damage signals by producing signals that inhibit inflammation. In a recent article (ref. below), the role of the vagus nerve in gut inflammation was examined using real-time biophotonic labeling. Basically that means that a video camera sensitive to infrared can be used to detect infrared dyes produced when NFkB is activated -- the camera is able to visualize regions of inflammation in living mice. Using this technique, researchers were able to demonstrate that cutting the vagus nerve produced heightened inflammation in gut treated with an irritant. The vagus nerve appears to stimulate regulatory T cells that lower the activity of inflammatory cells.

Inflammation/NFkB Activation Visualized in Live Mice

The studies were performed in a mouse line constructed to express an infrared fluorescent protein in cells in which the inflammation transcription factor, NFkB, is activated. Mice of this strain were prepared with and without the vagus nerve intact leading to the intestines. The mice were then exposed to sodium dextran sulfate (DSS) to simulate inflammatory bowel disease symptoms.

Cutting the Vagus Nerve Permits Inflammation

Mice with intact vagus nerves exhibited much less inflammation in their gut than those without vagus innervation. The cut vagus experiments demonstrated that the vagus nerve was responsible for suppressing inflammation. Further experiments were performed to determine if the inflammatory and anti-inflammatory reactions could be transferred to other mice by transferring cells from the treated mice.

Regulatory T Cells (CD4+, CD25+) Block Inflammation

Transfer experiments showed that inflammatory T cells (CD4+, CD25-) from cut vagus, DSS mice would cause bowel inflammation in other mice, but that did not happen with the same type of cells from mice with intact vagus nerves. Further tests showed that either cutting the vagus or adding inflammatory T cells from a mouse with a cut vagus, reduced the population of regulatory T cells (CD4+, CD25+) in control mice treated with DSS. So, without the vagus stimulation, the regulatory T cell population declined in the presence of inflammatory signals.

Absence of Regulatory T Cells Can Explain Many Inflammatory Diseases

In many inflammatory diseases, e.g. celiac, Crohn’s disease, rosacea, there appears to be a deficiency of regulatory T cells. In the absence regulatory T cells, signals from vagus nerves will no longer produce anti-inflammatory suppression. In fact the same nerve signals may become inflammatory. This would explain why rosaceans will become inflamed by hot or cold stimulation that would normally lead to anti-inflammatory stimulation of regulatory T cells. Similarly, capsaicin, castor oil and menthol, which normally produce an anti-inflammatory response, produce inflammation in rosaceans.

[Vagal stimulation exercise links:  here and here.]

reference:
O'Mahony C, van der Kleij HP, Bienenstock J, Shanahan F, O'Mahony L. 2009. Loss of vagal anti-inflammatory effect - in vivo visualization and adoptive transfer. Am J Physiol Regul Integr Comp Physiol. Aug 12. [Epub ahead of print]

Crohn’s Disease and Cryptidins

The intestines produce enzymes to digest food, antimicrobial peptides to kill pathogens and have lots of surface area to absorb nutrient molecules released from the food macromolecules (protein, polysaccharides, fats). The epithelial cells that line the intestines, enterocytes, must communicate with bacteria in the gut, the gut flora, to maintain bacteria helpful in food digestion, i.e. probiotic bacteria, and trigger an immune response to eliminate pathogens. Probiotic bacteria are tolerated and pathogens are identified and attacked.

Enterocytes are produced by division of stem cells at the bottom of the crypts that are in the valleys between the villi that project into the lumen where the digesting food is. New enterocytes are added at the base of the villi and old enterocytes are sloughed off at the top of the villi. As the new enterocytes move up the villi, they differentiate to produce the dramatic surface of microvilli, the furry brush border that further expands their surface area. The mature enterocytes produce transport proteins on their microvilli to take up sugars, amino acids and fats. The small nutrient molecules pass through the base of the enterocytes and bath the cells below, the lamina propia. The nutrients enter the capillaries of the villi and travel to the liver. Fats are transported through the lymphatic system.

Bacteria that slip through the enterocyte layer encounter macrophages and other types of white blood cells of the lamina propia. Among these cells are the Paneth cells. Fragments of the cell walls of bacteria bind to the NOD proteins of the Paneth cells and trigger the secretion of antimicrobial peptides, the cryptidins. Cryptidins are antimicrobial because of their array of basic amino acids surrounded by hydrophobic amino acids. These short proteins are able to disrupt the membrane function of most bacteria. I think they work on bacteria the same way that amyloid proteins, e.g. amyloid plaque proteins of Alzheimer’s disease, kill human cells. In fact, amyloid fibers bind to heparin and so do antibiotic peptides.

Here is an example of an antibiotic peptide, cryptidin 4,

GLLCYCRKGHCKRGERVRGTCGIRFLYCCPRR

Note the pairs of basic amino acids (blue). These amino acids are necessary for toxicity to bacteria. Heparin binding domains from proteins are produced naturally as proteins are digested to peptides in the stomach by pepsin. Pepsin hydrolyzes proteins next to the basic amino acids and leaves antimicrobial peptides that sterilize incoming food. I have illustrated the cryptidin protein to show how the basic amino acids (blue) are displayed on its surface.

With each meal, the fat content normally stimulates the production of a hormone, cholecystokinin, that binds to a receptor and causes an anti-inflammatory release of cytokines from the vagus nerves that reach the villi. Thus, food normally makes the intestines more tolerant of food antigens.

If the intestines become chronically inflamed, then exposure to normal probiotic bacteria can lead to cycles of inflammation that damage the integrity of the intestines. The intestines lose the ability to discriminate between probiotic and pathogen.

Crohn’s disease is an inflammatory, autoimmune disease of the bowel. The chronic inflammation of the lamina propia eliminate the ability of the Paneth cells to produce cryptidins and bacteria set up residence in the crypts and cause continual inflammation. This disease is typically treated by suppressing inflammation and treating with antibiotics.

Other treatment approaches that have been found effective are omega-3 oils to stimulate production of anti-inflammatory prostaglandins, pre- and probiotics, heparin and helminth eggs, e.g. wireworm.

Crohn’s disease would seem to benefit from the standard recommendation of an anti-inflammatory diet and lifestyle.