However, different structures, such as the teeth or cheeks, host unique communities of both aerobic and anaerobic microbes. Some factors appear to work against making the mouth hospitable to certain microbes.
For example, chewing allows microbes to mix better with saliva so they can be swallowed or spit out more easily. Saliva also contains enzymes, including lysozyme , which can damage microbial cells.
Additionally, fluids containing immunoglobulins and phagocytic cells are produced in the gingival spaces. Despite all of these chemical and mechanical activities, the mouth supports a large microbial community. As food leaves the oral cavity, it travels through the pharynx, or the back of the throat, and moves into the esophagus , which carries the food from the pharynx to the stomach without adding any additional digestive enzymes.
The stomach produces mucus to protect its lining, as well as digestive enzymes and acid to break down food. Partially digested food then leaves the stomach through the pyloric sphincter , reaching the first part of the small intestine called the duodenum. Pancreatic juice, which includes enzymes and bicarbonate ions, is released into the small intestine to neutralize the acidic material from the stomach and to assist in digestion.
Bile, produced by the liver but stored in the gallbladder , is also released into the small intestine to emulsify fats so that they can travel in the watery environment of the small intestine.
Digestion continues in the small intestine, where the majority of nutrients contained in the food are absorbed. Simple columnar epithelial cells called enterocytes line the lumen surface of the small intestinal folds called villi. Each enterocyte has smaller microvilli cytoplasmic membrane extensions on the cellular apical surface that increase the surface area to allow more absorption of nutrients to occur.
Figure 4. Microvilli are cytoplasmic extensions on individual cells that increase the surface area for absorption. Digested food leaves the small intestine and moves into the large intestine , or colon , where there is a more diverse microbiota.
Near this junction, there is a small pouch in the large intestine called the cecum , which attaches to the appendix. Further digestion occurs throughout the colon and water is reabsorbed, then waste is excreted through the rectum , the last section of the colon, and out of the body through the anus. The environment of most of the GI tract is harsh, which serves two purposes: digestion and immunity.
The stomach is an extremely acidic environment pH 1. The environment in the small intestine is less harsh and is able to support microbial communities. Microorganisms present in the small intestine can include lactobacilli , diptherioids and the fungus Candida. On the other hand, the large intestine colon contains a diverse and abundant microbiota that is important for normal function.
These microbes include Bacteriodetes especially the genera Bacteroides and Prevotella and Firmicutes especially members of the genus Clostridium. Methanogenic archaea and some fungi are also present, among many other species of bacteria. These microbes all aid in digestion and contribute to the production of feces, the waste excreted from the digestive tract, and flatus, the gas produced from microbial fermentation of undigested food.
They can also produce valuable nutrients. At a high level, we can group bacteria into approximately thirty— although hypothesized to be closer to one hundred — formally described phyla, a term used to group together related organisms.
Focusing in on more closely related organisms within each phylum at the genus level, several bacterial groups have been recognized for their probiotic, or beneficial, effects. These include species of bacteria in the genus Lactobacilli , Bifidobacteria , and Roseburia , along with specific species such as Faecalibacterium prausnitzii , and Akkermansia mucinophila.
Those with IBD have a decreased number of these beneficial bacteria and overall less bacterial diversity, with increases in organisms such as Escherichia coli , members of the phylum Proteobacteria. Fibre is one example of a complex carbohydrate that humans consume. Increased fibre intake has been associated with decreased cholesterol, as well as decreased risk of developing colorectal cancer.
In humans, diets rich in dietary fibre have been associated with increases in beneficial bacteria such as Bifidobacteria and Lactobacilli. Consumption of concentrated supplements containing indigestible food ingredients that promote growth of beneficial bacteria prebiotics such as inulin might also increase populations of Bifidobacteria and F. Prebiotics such as inulin are in foods such as green bananas, legumes, leeks, asparagus, and seaweed.
On the other hand, lack of dietary fibre, in animal models, can lead to increases in intestinal bacteria that degrade and feed off of intestinal mucus, leading to a thinner mucus layer and potentially disrupted barrier. Protein recommendations in the new food guide encourage consumption of more plant-based proteins rather than animal-based protein sources.
Reasons for this include an association between intake of red and processed meats and increased risk for colorectal cancer, in addition to soy protein being linked to lower cholesterol levels. In contrast, plant-based diets, rich in legumes, grains, vegetables, and fruits, had increased presence of Roseburia and F. Interestingly, the intestinal bacteria of the individuals consuming the animal-based diet returned to their original pre-diet bacterial structure in two days after discontinuation of the diet.
Fat comes in many forms, and there is evidence to suggest that its heavy consumption in the saturated form those that are typically solid at room temperature can lead to increased risk of cardiovascular disease. If this disease is untreated, death eventually occurs, with the victim dying extremely malnourished. However, tropical sprue can be fatal in extremely young or old victims, even with treatment.
Tropical sprue is mostly prevalent in tropical areas, however, not all of these areas have cases of tropical sprue. This may be due to dietary differences of the inhabitants in these regions. There has been a study linking the amount of long-chain unsaturated fatty acids consumed to the occurrence of tropical sprue in the population [6]. Small intestine bacterial overgrowth SIBO , also known as small bowel bacterial overgrowth SBBO , is a condition of the small intestine, defined as an increase in the number of bacteria in the upper gastrointestinal tract [9].
It can be caused by any condition that interferes with the muscular activity in the small intestine that can allow bacteria to multiply [8]. SIBO can result in villous atrophy and mucosal inflammation, altering the absorptive functions of the small intestine [9].
The main cause of SIBO is thought to be mainly bacterial. The production of enterotoxins by facultative anaerobes injures the intestinal surface while aerobic bacteria produce enzymes and metabolic products that cause epithelial injury. Symptoms of SIBO result mainly from malabsorption, and these symptoms include combinations of cramping, diarrhea, dyspepsia, and weight loss.
In addition, anemia can result from malabsorption caused by occult blood loss, vitamin B12 deficiency, or both [9]. The diagnosis of SIBO usually starts with a clinical exam.
The presence of 10 5 or more colony units of non-pharyngeal bacteria, usually coliforms, suggest SIBO [9]. Screening tests, which include urine ladicans, serum D-lactic acid, and the glucose breath hydrogen test, are also used. The most common of these, the glucose breath hydrogen test, works as an increase in the hydrogen production in the breath after glucose consumption usually means that there is a significant small intestine bacterial fermentation of carbohydrates [9].
Treatment is usually given when the symptoms become too severe, and they usually depend on the species of bacteria involved and the severity of the symptoms. Most treatments include the use of oral antibiotics, which reduce the number of bacteria in the intestinal tract. Surgery may be needed for anatomical anomalies such as diverticula, which are pouches in the colon that provide bacteria with a safe area in which to multiply. The most popular antibiotic used is metronidazole [9].
Trimethoprim-sulfamethoxazole, aminoglycosides, spectrum penicillins, and cephlasporins are usually used against facultative anaerobes [9]. If a patient has increased symptomatic activity, increased dosage may be administered. Sulfasalazine or corticosteroids may be used if there is an increased inflammatory response, however, dosage is based only on bacterial response [9].
A new and upcoming treatment used to combat SIBO is the use of bacterial substitution, which utilizes probiotic bacteria to replace existing bacteria, since the probiotic bacteria can eliminate pathogenic bacterial infections [9]. These probiotic bacteria have a beneficial effect on the small intestine. The probiotic bacteria commonly used are able to control the growth of pathogenic bacteria such as Salmonella typhimurium , Shigella sp , Clostridium difficile , Ampylobacter jejuni , and Escherichia coli , and in addition, are able to offer protection against Gardnerella vaginalitis , Bacteroides bivius , Candida albicans , and Chlamydia trachomatis.
Lactic acid bacteria such as Lactobacillus GG and Actobacillus plantarum V are able to inhibit many gram-negative bacteria by removing toxic substances in the small intestine and stimulating the immune system [9]. Crohn's Disease [2]. Salmonellosis [3]. Irritable Bowel Syndrome [4]. Prior studies have shown that the small intestines have undergone morphological changes, such as the crypt depth and villous height, after inoculating germ-free pigs with different types of bacteria.
Two gnotobiotic experiments were performed where 16 piglets were allocated into 4 types of treatment groups: Germ-Free, monoassociation with Lactobacillus fermentum , Escherichia coli , or sow feces. The piglets were reared for 14 days of age where the intestinal tissue and enterocytes were collected each day for histology, gene expression, and protein analysis. Quantitative PCR was used to measure proliferating nuclear cell antigen and it was concluded that the Escherichia coli and not the Lactobacillus fermentum helped stimulate an increase apoptosis and cell proliferation.
Thus, only by the death of the receptors and commensal bacteria were the enterocyte able to have a significant turnover [3]. Researchers from Harvard Medical School have discovered that the bacteria in the small intestine may help promote the invasion of typhoid. This triggers epithelial cell trafficking of a protein, therefore serving as a receptor for the pathogenic bacteria. As a result, the cells become more susceptible to infection of Salmonella enterica serovar Typhi [4].
Bacterial flora in the ilea and ceca of chickens are analyzed by studying 1, partial 16S rRNA gene sequences. Bacterial DNA was isolated using density gradient centrifugation. Studies show that the microbes that live in the ileum consist of: The large amount and variety of bacteria in the intestine make it difficult to distinguish the function of each one; however, overall that intestinal bacterial affect: nutrition, immune responses, pathogenesis of intestinal disease, and degradation of mucus.
The bacterial community in birds is affected by diet, age, and antibiotics. Overall, results showed that as the bacterial community started out as a stable community in the ileum and cecum. However, as the chickens matured, the bacterial community becomes more complex and increasingly differs between the ileum and cecum, which shows that each region develops its own unique set of microbes [16].
Using three-dimensional imaging, researchers at the Washington University School of Medicine studied the impact of intestinal angiogenesis on the microbes in the small intestine of mice. Angiogenesis, the growth of new blood vessels, takes place in the complex network of blood vessels in small intestinal villi.
However, the villi is also the site where gut microorganisms, or microbiota, live. This experiment studies the differences between the villi in germ-free mice and mice with Bacteroides thetaiotaomicron colonies during or after postnatal gut development — approximately a ten day period. Studies have shown that Bacteroides thetaiotaomicron is of the major microbes that reside in both mouse and human gut.
This experiment focuses on Paneth cells, which defend the host against microbes by secreting antibacterial peptides that affect the microbes in the lumen, or interior of the blood vessels. The results show that microbes function in breaking down carbohydrate polymers by facilitating lumen breakdown of dietary macromolecules so that the host does not have to cleave the various linkages in their food themselves.
Symbiosis is observed since the host is able to gain more nutrients, while microbes are provided a safe niche with a plentiful carbon source [17]. Symbiosis between microbes and their human host can be observed by microbes preventing intestinal inflammatory disease. One of the main microbes this experiment focuses on is Bacteroides fragilis , which protects its human host against colitis, a chronic inflammation of the membrane lining the gastrointestinal tract, caused by Helicobacter hepaticus , a commensal bacterium.
Although colitis is mainly affects the large intestine, in some rare cases it affects the ileum as well since it precedes the large intestine.
In order for Bacteroides fragilis to be beneficial to its host, it must express polysaccharide A, or PSA, which affects the interleukinproducing CD4 T cells; otherwise Helicobacter hepaticus will continue to grow and cause inflammatory disease.
It is shown that purified PSA can prevent gut pathology. This experiment gives purified PSA to mice and measures the changes of the inflammatory disease. Mice without Helicobacter hepaticus had a very mild colitis, while mice with Helicobacter hepaticus developed severe colitis. When purified PSA was given to the diseased mice, they were almost completely protected against the Helicobacter hepaticus , and the disease level was reduced to match those who did not develop colitis.
Thus, Bacteroides fragilis is just one example of a symbiosis relationship where a single bacterial organism promotes human health; and suggests that there are others yet to be discovered [18].
PCR with denaturing gradient gel electrophoresis DGGE studies how different fat sources in the diets of broiler chickens affected the microbial community in the ileum of the chickens at different ages. This experiment differs from previous experiments, which only focused on the cecum, by examining the parts of the small intestine — the jejunum and ileum, which are largely responsible for nutrient absorption. The experiment focuses only on the ileum, but notes that the same micro flora exist in both sites.
Written By John Easton. Although the vast majority of research on the gut microbiome has focused on bacteria in the large intestine, a new study — one of a few to concentrate on microbes in the upper gastrointestinal tract — shows how the typical calorie-dense western diet can induce expansion of microbes that promote the digestion and absorption of high-fat foods.
Several studies have shown that these bacteria can multiply within 24 to 48 hours in the small bowel in response to consumption of high-fat foods. The findings from this work suggest that these microbes facilitate production and secretion of digestive enzymes into the small bowel. Those digestive enzymes break down dietary fat, enabling the rapid absorption of calorie-dense foods. Concurrently, the microbes release bioactive compounds. These compounds stimulate the absorptive cells in the intestine to package and transport fat for absorption.
Over time, the steady presence of these microbes can lead to over-nutrition and obesity. The goals of the study, published April 11, in the journal Cell Host and Microbe , were to find out if microbes were required for digestion and absorption of fats, to begin to learn which microbes were involved, and to assess the role of diet-induced microbes on the digestion and uptake of fats.
The germ-free mice, even when fed a high-fat diet, were unable to digest or absorb fatty foods. They did not gain weight.
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