US BioTek GI Advanced Profile: Results Explained

Acetate is the most abundant SCFA in the colon and makes up more than half of the total SCFA detected in feces.

Opportunistic intestinal bacteria

may represent 3–5% of the microbial composition in the healthy human intestinal tract

part of the penicillin class

Intestinal parasitic roundworm

works by inhibiting bacterial cell wall synthesis

Gram-negative species of the Bacteroidetes phylum.

The Bacteroidetes Phylum marker refers to a group of bacteria commonly found in the human gut, playing a crucial role in the digestive process. This phylum includes various species that are essential for breaking down complex molecules like carbohydrates and fibers, helping in the absorption of nutrients and the production of short-chain fatty acids which are vital for gut health. The presence and balance of Bacteroidetes can be an indicator of gut flora health; for instance, a healthy balance between Bacteroidetes and another major gut bacteria group, Firmicutes, is often associated with good digestive health. On a GI panel, the levels of Bacteroidetes are assessed to help diagnose potential issues such as dysbiosis, where there is an imbalance in gut microbial communities. This can be linked to various health conditions, including obesity, diabetes, and inflammatory bowel diseases. By monitoring Bacteroidetes levels, healthcare providers can gain insights into a patient’s gut health, which can inform dietary recommendations and treatment plans aimed at restoring gut flora balance and improving overall health.

Bifidobacterium adolescentis is a beneficial bacterium commonly found in the human gastrointestinal (GI) tract, particularly in the intestines. The presence and quantity of Bifidobacterium adolescentis are significant because it plays a crucial role in maintaining a healthy digestive system. This bacterium helps in breaking down complex carbohydrates, producing essential vitamins like B12, and supporting the immune system by inhibiting harmful pathogens. An optimal level of Bifidobacterium adolescentis indicates a balanced gut microbiome, which is vital for overall digestive health, nutrient absorption, and immune function. Conversely, an imbalance, either too high or too low, can be a sign of underlying gastrointestinal issues such as dysbiosis, which might lead to symptoms like bloating, irregular bowel movements, and inflammation. Therefore, monitoring Bifidobacterium adolescentis on the GI – Advanced Profile panel provides valuable insights into the state of one's gut health and helps in devising appropriate dietary or probiotic interventions to restore balance and promote a healthy digestive system.

Bifidobacterium bifidum is a significant bacterium to evaluate gut health. This specific bacterium is a member of the Bifidobacterium genus, known for its beneficial effects on the digestive system. It naturally inhabits the intestines and plays a crucial role in maintaining a healthy balance of gut flora. Bifidobacterium bifidum helps in the digestion of dietary fiber, producing short-chain fatty acids like acetate, propionate, and butyrate, which are essential for colon health. Moreover, it enhances the immune response by inhibiting harmful pathogens and reducing inflammation. This bacterium is also involved in the production of vitamins, such as B-complex vitamins, and assists in the absorption of minerals like calcium and magnesium. In a GI – Advanced Profile panel, the presence and levels of Bifidobacterium bifidum are analyzed to assess the overall condition of the gastrointestinal tract. An optimal level indicates a well-functioning digestive system, while deviations can suggest issues such as dysbiosis, which may be linked to conditions like irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), or even systemic impacts like allergies and metabolic disorders. Therefore, monitoring Bifidobacterium bifidum provides valuable insights into gut health and helps guide dietary and therapeutic interventions to restore and maintain a balanced microbiome.

a beneficial probiotic bacterium

Bifidobacterium longum is a specific species of microscopic non-pathogenic bacteria found naturally in the gastrointestinal tracts of humans as well as in most other animals. It is added to a variety of foods or dietary supplements as a probiotic due to its many health benefits. It is a gram-positive, branched rod-shaped bacterium that produces lactic acid from the fermentation of sugar in the gastrointestinal tract.

Amongst the first bacterial colonizers of the intestine.

is found throughout the world in both people with and without symptoms

a short-chain fatty acid (SCFA)

a calcium-binding protein with antimicrobial properties.

Commensal fungi that can be pathogenic to immunocompromised patients.

Gram-negative bacteria in the Proteobacteria phylum.

Opportunistic gut bacteria

Clostridium species, when identified in a gastrointestinal (GI) test, can be of significant clinical interest due to their diverse roles in human health and disease. Members of the Clostridium genus are a part of the normal gut flora in humans and animals, but certain species can become pathogenic under specific conditions. Clostridium difficile (C. difficile) is particularly noteworthy; it's known for causing infections in the GI tract, especially after the use of broad-spectrum antibiotics that disrupt the normal gut microbiota. C. difficile infections (CDI) are a major concern in healthcare settings, as they can lead to conditions ranging from mild diarrhea to life-threatening colitis. Other Clostridium species, such as Clostridium perfringens, can also cause GI infections, often linked to food poisoning. These bacteria produce toxins that can lead to abdominal cramps, diarrhea, and in severe cases, more serious complications like necrotic enteritis. The presence of Clostridium in a GI test might not always signify a pathogenic infection; it could also reflect the normal bacterial composition of the gut. However, in the right clinical context, such as a patient with recent antibiotic use or hospitalization, the detection of Clostridium species, especially C. difficile, can be crucial in diagnosing and managing infections. Symptoms of Clostridium infections can vary depending on the species and the individual's overall health but typically include diarrhea, abdominal pain, and fever. In severe cases, there may be signs of systemic illness, such as dehydration, elevated white blood cell count, and in the case of C. difficile, pseudomembranous colitis. Treatment approaches for Clostridium infections depend on the severity and specific species involved. For mild to moderate C. difficile infections, specific antibiotics such as metronidazole or vancomycin are commonly used, while more severe cases might require fidaxomicin or even fecal microbiota transplantation to restore a healthy gut flora. For infections caused by other Clostridium species, the treatment may involve different antibiotics, supportive care, and addressing any underlying conditions that might have predisposed the individual to the infection.

Coptis, often referred to in the context of traditional Chinese medicine, is a genus of plants known for their potent medicinal properties, particularly their natural inhibitory effects against various pathogens and health conditions. The most well-known species within this genus is Coptis chinensis, commonly called Chinese goldthread. The primary active component of Coptis is berberine, an alkaloid that imparts the distinct yellow color to the roots of these plants and is responsible for many of their therapeutic effects. Berberine, extracted from Coptis, has been studied for its broad-spectrum antimicrobial properties. It has shown inhibitory effects against bacteria, viruses, fungi, and parasites. This makes it a potential natural alternative or complement to conventional antibiotics, especially in the face of increasing antibiotic resistance. Berberine's mechanism of action is believed to involve the disruption of microbial cell function, although the exact processes vary depending on the type of pathogen. Apart from its antimicrobial properties, Coptis and berberine have been found to have natural inhibitory effects on several other health conditions. For example, berberine has demonstrated potential benefits in managing metabolic disorders like diabetes and hyperlipidemia by improving insulin sensitivity and reducing cholesterol levels. It may also have anti-inflammatory and antioxidative properties, making it beneficial in conditions like arthritis and certain cardiovascular diseases. Furthermore, some studies suggest that Coptis and berberine may have anticancer properties, potentially inhibiting the growth of various cancer cells. This is thought to be due to its ability to induce apoptosis (programmed cell death) in cancer cells and inhibit their proliferation. However, it's important to approach the use of Coptis and berberine with caution. While they offer potential health benefits, their interactions with conventional medicines and possible side effects, particularly at higher doses, warrant careful consideration and professional guidance, especially in individuals with pre-existing health conditions or those taking other medications. The efficacy and safety of Coptis as a natural inhibitor in various applications continue to be an area of active research, and its use should be informed by scientific evidence and clinical judgment.

commonly found in the human gut microbiome

Intestinal protozoan organism

Enterobacter cloacae complex is part of the Enterobacteriaceae family. E cloacae complex is a group of six closely related species with similar resistance patterns: E. cloacae, E. asburiae, E. hormaechei, E. kobei, E. ludwigii, and E. nimipressuralis. This gram-negative bacterium is considered dysbiotic at levels of 3+ or greater. E. cloacae complex is considered an opportunistic pathogen associated with diarrhea in children. A Shiga-like toxin-producing E. cloacae was isolated from the feces of an infant with hemolytic-uremic syndrome. However, E. cloacae complex is most often involved in extraintestinal infections including the urinary tract, respiratory tract, and cutaneous wounds. Widely distributed in the environment, Enterobacter spp. is commonly isolated from both human and animal feces. Environmental strains of Enterobacter spp. are capable of growth in foods at refrigeration temperature. E. cloacae complex is known to possess inducible ß-lactamases. Isolates may become resistant to all cephalosporins after initiation of therapy. Avoid ß-lactam-inhibitor drugs such as amoxicillin/ clavulanate, ampicillin/sulbactam, and piperacillin/tazobactam. Antibiotics may be indicated in systemic infections if symptoms are prolonged.

Opportunistic Bacteriology

Indicator of gut health, balance, and potential dysbiosis in GI tests.

Gram-positive species in the Firmicutes phylum.

can provide important insights into the health and balance of the gut microbiome

Enterococcus species, commonly detected in gastrointestinal (GI) tests, represent a significant group of bacteria naturally inhabiting the human gut. These species, particularly Enterococcus faecalis and Enterococcus faecium, are noteworthy for their dual nature: while being normal constituents of the intestinal microbiota, they can also act as opportunistic pathogens. Their presence in GI tests can sometimes be indicative of a healthy, balanced gut flora, but their overgrowth or presence in certain clinical contexts can signal an imbalance or an infection. Enterococci are known for their resilience, having the ability to survive harsh conditions, which also makes them a concern in healthcare settings due to their resistance to many antibiotics. This resistance complicates treatment of infections caused by these bacteria, such as urinary tract infections, bacteremia, and endocarditis. Furthermore, Enterococcus species are often used as indicators of fecal contamination in water and food, reflecting their ubiquity in the environment. In GI testing, the detection and quantification of Enterococcus can provide crucial information for diagnosing infections or assessing the microbial balance in the gut. However, interpreting these results requires careful consideration of the patient’s clinical history and symptoms, as the mere presence of Enterococcus species does not unequivocally indicate disease. Advances in molecular biology and microbiology have enhanced the detection and characterization of these bacteria, contributing to a better understanding of their role in health and disease.

Euryarchaeota in gut panels indicate metabolic health and digestive system balance.

Anti-inflammatory effect (potentially useful in Crohn disease relief)

The marker "Firmicutes Phylum" refers to a significant group of bacteria commonly found in the human gut. Firmicutes are one of the most abundant types of bacteria in the human digestive tract, playing a crucial role in the body's ability to digest and absorb nutrients. They are also involved in the synthesis of certain vitamins and the regulation of our immune system. A GI panel often measures the levels of Firmicutes to assess the balance of microbial populations in the gut. An imbalance in these bacteria can be linked to various health conditions, including digestive disorders, obesity, and metabolic issues. When analyzing GI panel results, high or low levels of Firmicutes can provide insights into the individual's gut health, pointing towards potential dysbiosis (microbial imbalance) and guiding appropriate dietary or medical interventions to restore balance and support overall health.

provides insights into the composition of gut microbiota

The marker Fusobacterium species refers to a group of bacteria that are part of the normal flora in the human gut but can also act as pathogens under certain conditions. Fusobacterium species are anaerobic bacteria, meaning they thrive in environments with little to no oxygen, such as the deeper layers of the mucous membrane lining the gut. In a healthy individual, these bacteria coexist peacefully with other microorganisms and contribute to the gut's ecological balance. However, if the balance is disrupted, Fusobacterium can become opportunistic pathogens, leading to infections. The presence of these bacteria in elevated levels on a GI panel can indicate an infection or dysbiosis (an imbalance in the microbial community), which may be associated with various gastrointestinal disorders, including inflammatory bowel disease and colon cancer. Therefore, the detection of Fusobacterium species on a GI panel is crucial for diagnosing potential infections or imbalances, guiding effective treatment plans to restore gut health.

The Geotrichum species marker on a gastrointestinal (GI) panel is indicative of the presence and relative abundance of Geotrichum, a genus of fungi, in the gut microbiome. Geotrichum species are commonly found in soil, water, air, and various food products; they are also a part of the normal flora of the skin, respiratory system, and gastrointestinal tract in humans. In a balanced gut microbiome, the presence of Geotrichum species is typically harmless and may even play a role in the complex ecosystem of the gut. However, an overgrowth or significant presence indicated by a GI panel can have clinical implications. In immunocompromised individuals or those with disrupted gut flora due to factors such as antibiotic use or gastrointestinal disorders, an overgrowth of Geotrichum can lead to geotrichosis, a rare fungal infection. Symptoms of geotrichosis might include gastrointestinal discomfort, diarrhea, and in severe cases, systemic infection. The diagnosis is often confirmed through the isolation of the fungus in stool cultures and identification in a GI panel. Treatment for an overgrowth of Geotrichum species typically involves antifungal therapy. The choice of antifungal medication depends on the severity of the symptoms and the patient's overall health status. In mild cases, especially when symptoms are not prominent, monitoring and adjustments in diet or probiotic supplementation may be sufficient to restore the balance of the gut microbiota. Moreover, addressing underlying conditions that may have contributed to the overgrowth is crucial. This includes careful use of antibiotics, treatment of any existing gastrointestinal diseases, and measures to improve the immune system function in immunocompromised patients. In addition to medical treatment, dietary modifications to support a healthy gut environment can be beneficial. These might include the incorporation of fermented foods, fiber-rich diet, and possibly prebiotics and probiotics to encourage a diverse and balanced gut microbiome.

Intestinal protozoan parasite

Golden seal, or Hydrastis canadensis, is a North American herb known for its natural inhibitory properties, mainly due to alkaloids like berberine and hydrastine. It's traditionally used for its antimicrobial activity against bacteria, fungi, and parasites, making it a popular remedy for gastrointestinal and respiratory infections. Goldenseal also has anti-inflammatory properties, beneficial in treating conditions like gastritis and ulcerative colitis, and is thought to support the immune system, often used for colds and flu. Additionally, it aids in digestion and has antioxidant activity. Topically, it's applied for its antiseptic properties in treating skin and eye infections. However, caution is advised due to overharvesting concerns, potential side effects and interactions, especially during pregnancy and breastfeeding, and the need for more clinical evidence to fully validate its effectiveness and safety. In essence, goldenseal is a historically valued herb for various health conditions, but responsible use and further scientific research are necessary to confirm its therapeutic benefits.

H. pylori virulence marker

Helicobacter pylori (H. pylori) is a spiral-shaped bacterium that lives in the lining of the stomach and, in some cases, the upper small intestine. It is one of the most common bacteria found in humans worldwide. While many people carry H. pylori without symptoms, in some individuals it can cause inflammation of the stomach lining (gastritis), ulcers, and, over time, digestive discomfort or nutrient absorption issues. H. pylori is typically acquired during childhood and can persist throughout life if untreated. It spreads mainly through saliva, food, or water contaminated with the bacteria.

gram-negative bacilli belonging to the Enterobacteriaceae family

non-motile, Gramnegative rods that belong to the Enterobacteriaceae family

Lactobacillus acidophilus, measured in CFU/g (Colony Forming Units per gram), is a key biomarker included in the GI Standard Test Panel by US BioTek. This gram-positive, rod-shaped bacterium is a vital member of the gut microbiome, primarily residing in the small intestine. It is one of the most well-studied probiotic species and plays a critical role in maintaining gut and overall health. The Role of Lactobacillus acidophilus: Lactic Acid Production: L. acidophilus produces lactic acid, which helps maintain an acidic environment in the intestines. This pH regulation is essential for inhibiting the growth of harmful bacteria and yeast. Gut Barrier Support: It promotes the integrity of the intestinal lining, reducing the risk of "leaky gut" and supporting nutrient absorption. Immune Modulation: L. acidophilus interacts with the immune system to enhance defense mechanisms and modulate inflammation. Digestive Health: It assists in breaking down lactose, aiding individuals with lactose intolerance, and contributes to carbohydrate fermentation. Pathogen Resistance: By competing for attachment sites and nutrients, L. acidophilus helps prevent colonization by pathogenic microbes. By monitoring Lactobacillus acidophilus on the GI Standard Test Panel, healthcare providers can develop personalized treatment plans to optimize gut health and overall well-being.

Lactobacillus delbrueckii is a member of the Lactobacillus genus, known for its beneficial role in the gut microbiome. It is commonly found in fermented dairy products like yogurt and plays a crucial role in maintaining a balanced gut environment. On the GI – Advanced Profile panel, the presence and levels of Lactobacillus delbrueckii are important because this bacterium helps in the digestion process by producing lactic acid, which in turn lowers the pH of the gut. This acidic environment is essential for inhibiting the growth of harmful bacteria, supporting the integrity of the gut lining, and enhancing nutrient absorption. A healthy level of Lactobacillus delbrueckii is indicative of a well-functioning digestive system, while low levels might suggest a disruption in the gut microbiome, potentially leading to issues like poor digestion, inflammation, or an overgrowth of pathogenic bacteria. By monitoring this marker, healthcare providers can gain insights into the overall health of the GI tract and make informed decisions about dietary adjustments, probiotic supplementation, or other interventions to restore and maintain optimal gut health.

a species of probiotic bacteria

one of the most studied probiotics

Lactobacillus salivarius is a type of probiotic bacteria that plays a significant role in maintaining a healthy gastrointestinal (GI) tract. Lactobacillus salivarius is used to assess the levels of this beneficial bacterium in the digestive system. Lactobacillus salivarius is known for its ability to produce lactic acid, which helps to lower the pH in the gut and creates an environment that is less favorable for harmful bacteria and pathogens. This probiotic also contributes to the overall balance of the gut microbiome, which is crucial for proper digestion, nutrient absorption, and immune function. Additionally, Lactobacillus salivarius has been associated with anti-inflammatory properties and may help in alleviating symptoms of certain gastrointestinal disorders such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD). Monitoring the levels of Lactobacillus salivarius through a GI – Advanced Profile panel can provide valuable insights into an individual's gut health and help in tailoring probiotic and dietary interventions to support a healthy digestive system. Regular assessments of this marker can aid in maintaining optimal GI health and preventing potential imbalances that could lead to digestive issues.

A type of bacteria.

a prominent archaeon (=a microorganism) in the human gut microbiome

Mucous production may indicate the presence of an infection and/or inflammation. The GI – Advanced Profile by US BioTek features a comprehensive marker known as "Mucuous," which plays a pivotal role in assessing gastrointestinal health. This marker is crucial for understanding the mucosal integrity within the GI tract. A balanced mucous level is essential for proper digestion, nutrient absorption, and barrier function against pathogens. When discussing the GI – Advanced Profile, highlighting the Mucuous marker is key, as it provides invaluable insights into gut health, potentially indicating issues such as inflammation or infections. The mucous membrane of the large intestine helps stool to pass. It is normal for a small amount of mucus to be present in stool. A typical bowel movement will not produce much mucus. It may be tawny or clear in color, and a person is unlikely to notice it. When stool has visible mucus, it can be a sign of other health conditions, such as an infection, irritable bowel syndrome (IBS), ulcerative colitis (UC), or Crohn’s disease. Other signs to look for: - increased amounts of mucus - blood or pus in the stool - stomach pain, cramping, or bloating - sudden changes in stool frequency, consistency, or color A person should contact a doctor if they notice mucus in their stool.

The presence of blood in the stool may be the result of several causes besides colorectal bleeding, including hemorrhoids or gastrointestinal infection.

Can reduce the incidence of kidney stones.

Pancreatic insufficiency test

Intestinal protozoan organism

short-chain fatty acid

Proteobacteria are a diverse phylum, comprised with several subclasses: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Epsilonproteobacteria, and Zetaproteobacteria. The Proteobacteria are commonly occurring in healthy mammalian GI microbiomes and include common human pathogens. Proteobacteria contribute to homeostasis of the anaerobic environment of the gastrointestinal tract. It has a role in protein and sugar degradation and maintaining oxygen homeostasis within the gut.

Opportunistic Bacteria associated with Autoimmunity.

Gram-negative bacteria in the Proteobacteria phylum.

belonging to the bacterial family Pseudomonadaceae

A marker of gut secretory immunity and barrier function.

Staphylococcus species, commonly found in the gastrointestinal tract, are a diverse group of bacteria. Some species are harmless and part of the normal gut flora, while others, like Staphylococcus aureus, can be pathogenic. The presence of Staphylococcus in the gut microbiome is a natural occurrence, but an overgrowth may indicate an imbalance or potential infection. Scientifically, their role in the GI tract is complex, as they can interact with the immune system and impact gut health. Elevated levels of pathogenic Staphylococcus species in the GI tract require careful clinical evaluation, as they might be linked to various health conditions, including food poisoning and intestinal infections. Treatment approaches depend on the specific Staphylococcus species identified and the overall health context.

Steatocrit is a diagnostic test specifically designed to measure the fat content in feces, offering a non-invasive method to evaluate fat malabsorption. This condition, where fats are not properly absorbed by the intestines, can lead to various gastrointestinal disorders, including steatorrhea (the excretion of abnormal quantities of fat with the feces), which is indicative of issues such as pancreatic insufficiency, celiac disease, Crohn's disease, or other conditions affecting the bile ducts or small intestine. The steatocrit test involves analyzing a small sample of stool to determine the percentage of fat, providing a quantitative measure of the extent of fat malabsorption. Elevated steatocrit levels signify higher than normal fat content in the stool, pointing to malabsorption issues that warrant further investigation to identify the underlying cause. Understanding steatocrit levels helps clinicians tailor treatment strategies, which may include dietary adjustments, enzyme supplements, or other interventions aimed at managing the malabsorption and alleviating associated symptoms. As a part of a comprehensive GI panel, steatocrit results contribute valuable insights into a patient's digestive health, guiding diagnostic and therapeutic decisions to improve gastrointestinal function and overall well-being.

Sample form is categorised using the Bristol stool chart.

Higher abundance of S. salivarius and S. thermophilus (Firmicutes phylum) have been associated with a moderate to severe disease course in newly diagnosed ulcerative colitis (UC) patients. These findings are in accordance with a study that showed that UC patients have significantly increased Streptococcus spp. and depletion of Bifidobacterium spp. Higher levels of Streptococcus spp. were also observed in patients with colorectal cancer compared to healthy controls. Administration of S. salivarius together with Bifidobacterium bifidum was shown to reduce the incidence of acute diarrhea and rotavirus shedding in infants. S. salivarius and S. thermophilus are also widely used in dairy products like yogurt and cheese.

a common bacterium often carried in the intestines or lower genital tract.

refers to a genus of roundworms

probiotic bacteria

Total Lactobacilli measured in x10^3 CFU/g (Colony Forming Units per gram) is a critical biomarker indicating the presence and abundance of beneficial Lactobacillus bacteria in the gastrointestinal tract. These gram-positive bacteria are key contributors to the gut microbiome, supporting both digestive and overall health. Clinicians use Total Lactobacilli levels to assess the state of beneficial bacteria in the gut ecosystem. Healthy levels are associated with improved digestive function and immune health, while low levels may signal an imbalance in the gut microbiome, also known as dysbiosis. Benefits of Lactobacilli: Produce lactic acid to maintain a healthy intestinal pH. Compete with harmful bacteria for resources and attachment sites. Strengthen the intestinal barrier. Synthesize certain B vitamins. Aid in the breakdown of lactose and other carbohydrates. Modulate immune system function. Low levels of Total Lactobacilli may result from: Antibiotic use Poor diet Stress Gastrointestinal conditions Interpretation: This biomarker is interpreted alongside other gut health indicators to provide a comprehensive view of gastrointestinal function. Clinicians can use this information to guide targeted therapeutic interventions, such as dietary changes, probiotic supplementation, or stress management strategies.

Short Chain Fatty Acid

The marker "Verrucomicrobia Phylum" refers to a group of bacteria belonging to the phylum Verrucomicrobia, which is often evaluated during stool tests to understand gut health. Verrucomicrobia are a less common but interesting group of bacteria that reside in various environments, including soil, freshwater, and marine ecosystems, but they are also present in the human gut, particularly in the large intestine. In the context of a GI Panel, the presence and levels of Verrucomicrobia can provide insights into the microbial diversity of the gut, which is crucial for maintaining digestive health and immune function. These bacteria are particularly noted for their role in breaking down complex polysaccharides (large sugar molecules), which is an important process in the human digestive system. An imbalance in the levels of Verrucomicrobia, either too high or too low, can indicate disturbances in the gut microbiome, which might be associated with various health conditions such as inflammatory bowel disease or obesity. Therefore, monitoring this phylum can help healthcare providers assess gut health and contribute to the diagnosis and management of gastrointestinal disorders.

Opportunistic gut bacteria

a protein that opens intercellular tight junctions in the gut lining