AbstractThe human microbiome is the home of trillions and trillions of bacterial microorganisms, also known as microbiota. In humans, the gut is the central location of the microbiome. Gut microbiota has lasting effects in different parts of the body. In this paper, gut microbiota in humans will be discussed, as well as its effects on intestinal permeability, autoimmune diseases, the brain, and cancer. The purpose of the research presented is to understand the human microbiome and its significance to health and disease in humans. This is an ongoing topic in health and disease and further research is still advancing in order to continue to gain knowledge about human microbiota and its significant effects on the human body.Introduction Human microbiota is defined as a place where different forms of microorganisms reside or live. The human microbiome consists of bacteria, viruses, archaea, and eukaryotic microbes that live on and within the body (Shreiner 2015). These microorganisms not only live within the body, they have a huge impact on human health and disease. Gut microbiota help protect the body from harmful pathogens, affect the immune system, and ultimately affect the physiologic functions and everyday activities of humans (Shreiner 2015). Human gut microbiota is the center of health and disease. The effects and alterations of gut microbiota on health and disease within the body are very significant because of their specific effects on intestinal or “gut” permeability, autoimmune diseases, effects on the brain, and its significant effects on cancer. The purpose of the intestinal barrier is to protect the body from incoming pathogens that may be harmful. However, the intestinal barrier must be semi-permeable to allow nutrients and important fluids to enter. The gut microbiota, and alterations of it, have significant effects on intestinal permeability, which lead to health problems and disease when unwanted pathogens are let in (Bischoff 2014). Gut microbiota in humans have many effects on autoimmune diseases such as celiac disease. Although autoimmune diseases have a lot of genetic and environmental factors, more and more research is advancing in the area of gut microbiota and its correlation with celiac disease and effects on other autoimmune diseases. New and emanate research has shown evidence that human behavior and Central Nervous System (CNS) function is influenced by microbiota in the gut. Gastrointestinal research has exhibited the importance of the “gut-brain” axis and its relationship with different disorders and psychiatric illnesses (Neufeld 2009). Certain intestinal bacteria can cause dysbiosis, which is linked to oncogenesis and tumor progression. Antibiotic use and different abdominal infections have an impact on dysbiosis. Ultimately, gut microbiota and the alteration of it, influences these diseases (Zitvogel 2015). Intestinal Permeability. The intestinal barrier in the gut has a mucosal surface that is the home of trillions of bacteria that ultimately come into contact with the body’s immune system. The barrier protects the human body by blocking harmful pathogens and toxins from entering. The intestinal barrier also has to allow essential nutrients and fluids to be able to pass through (Bischoff 2014). This barrier in the intestine has the power to do both of these extremely important jobs because it is semi-permeable. A semi-permeable membrane is a membrane that allows certain substances to move through but not others that are potentially pathogenic. This membrane regulated by tight junctions. These are larger structures in the lining that link intestinal epithelial cells together and ultimately help seal the lining in the intestines. This structure helps protect the gastrointestinal tract. Tight junctions are made up of multiprotein complexes that control epithelial permeability and diffusion (Dokladny 2016). These tight junctions are the doorways that allow essential nutrients to pass through. The regulation of intestinal permeability has shown to be affected by different bacteria in the gut, as well as diet. Different factors can disrupt the intestinal barrier such as drugs, infections, genetics, and a wide range of environmental factors. Some of these environmental factors include diet, sleep and stress. Increased intestinal permeability caused by these disruptors can disturb the healthy gut microbiota. This disruption can cause many health problems in humans. Some of these health problems are linked to diseases such as celiac disease, irritable bowel syndrome, different metabolic diseases, and obesity (Bischoff 2014). Overall, intestinal permeability is extremely important to the health of humans and the homeostasis of the human microbiome. The human microbiome, which consists of trillions of microorganisms, has a huge effect on intestinal permeability and the way it functions. Although other factors influence the way the intestinal barrier functions, the gut microbiota and the alteration of it, ultimately have the biggest influence on human health and a wide range of diseases (Bischoff 2014). Autoimmune Diseases. Autoimmune diseases occur when the immune system in the body attacks its own cells. Human gut microbiota come into contact with the immune system on an ongoing basis. Autoimmune diseases such as celiac disease, derive from the interaction of the immune system and the gut. The interaction between gluten (the main protein in wheat, barley and rye), genetic and environmental factors all play a role in the development of celiac disease. This autoimmune disease is exasperated by the ingestion of gluten. Gluten is very poorly digested in the gastrointestinal tract and cannot be broken down by different brush border membranes. It is then passed through the intestinal barrier during increased intestinal permeability and ultimately creates an inflammatory reaction in the small and large intestine. The response of this inflammatory reaction is mediated by the adaptive and innate immune systems because of the formation of cytotoxic cells (Peter 2007). In addition to this research, other studies have shown that zonulin, the only human protein discovered to date that is known to regulate intercellular tight junctions in the intestinal barrier, is known to be secreted at the same time that intestinal permeability is increased (Fassano 2015). The tight junctions open when the body produces too much zonulin, allowing the uninhibited passage of antigens, including gluten proteins, from the intestinal lumen within the gastrointestinal tract. When this happens in the body, it can be extremely harmful for those who have a genetic susceptibility to the dangerous antigens that are let through. This can trigger an onset of immune-related diseases like celiac disease (Fassano 2015). Human gut microbiota are affected by this immune response to gluten. There is a distinctive connection between gut microbiota, the intestinal barrier, and celiac disease. The Brain. Other types of inflammatory reactions are produced by an immune response in connection to human gut microbiota. Another example of an inflammatory reaction is Irritable Bowel Syndrome (IBS). IBS is a disorder that consists of chronic abdominal pain and bowel changes (Lee 2011). The pathogenic mechanism associated with IBS is known to be an adjusted “gut-brain” axis. This adjusted gut-brain axis is linked to issues with the gastrointestinal nervous system. Aberrations in gut microbiota motility result from these alterations which cause the symptoms of IBS. As such, when the gut microbiota is altered IBS becomes more prevalent. Alterations in gut microbiota also result from the use of antibiotics and intestinal infections (Lee 2011). Psychiatric illnesses show another way the brain is affected by the human microbiome. Research has shown that IBS and many other gastrointestinal disorders have a very high correlation with psychiatric illness. Mood disorders have shown to be the most common disorder associated with functional bowel disorders (Neufeld 2009). This is significant because in a study, about 50% of patients with a functional bowel disorder, like IBS, fell under the same criteria for mood disorders (Neufeld 2009). This is an emerging topic because different preclinical work has brought out the importance of microbiota. These findings, combined with investigating the impact of antibiotics and probiotics on the Central Nervous System (CNS) and its development and function, will help gain more knowledge on brain function and the correlation with psychiatric illness (Neufeld 2009). Cancer. Alterations of the human gut microbiota and immune system can result in many different health issues, diseases, and cancer. Some studies have linked the accumulation of incidences of colorectal cancer with the use of antibiotic treatments and abdominal injections (Zitvogel 2015). Dysbiosis of the gut microbiota has shown to lead to oncogenesis and tumor progression. The overuse of antibiotics results in the formation of different determinants through inflammatory and metabolic parts by alteration of the gut microbiota. This results in a rise in neoplasms, which are abnormal growths of tissue, and are usually cancerous (Zitvogel 2015). However, the activity of anticancer therapy may be improved by altering the gut microbiota. Studies have brought attention to the fact that some antibiotics, probiotics, prebiotics and postbiotics are used in correlation with anticancer therapy (Zitvogel 2015). Recent findings show that alterations of the gut microbiota to an extent can be beneficial. Conclusions/Further Directions The human microbiome is an ongoing topic of discussion and more and more new research is coming out everyday about how the gut microbiota affects the human body and how it influences health and disease in humans. The most significant part of this research is that human gut microbiota is the center of conversation in the body when it comes to health and disease. The human microbiome influences human daily activities and is the home of the biggest immune system in the body. Therefore, it influences different diseases and illnesses that come into contact with it. Human gut microbiota is the most important aspect of the body and it is extremely important to be knowledgeable about the different ways the human microbiome influences our daily lives and its role in health and disease. This topic is an ongoing discussion and there are many unanswered questions about the gut microbiota in humans and how important it is for our bodies. Further research is still progressing everyday on this topic, especially in the areas of autoimmune diseases and cancer because these diseases are becoming more and more common in humans every day. After researching about these four topics and their correlation with gut microbiota, it is clear that the human microbiome is one of the most essential parts to human life.