<P> The relationship between some gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship . Some human gut microorganisms benefit the host by fermentating dietary fiber into short - chain fatty acids (SCFAs), such as acetic acid and butyric acid, which are then absorbed by the host . Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols, and xenobiotics . The systemic importance of the SCFAs and other compounds they produce are like hormones and the gut flora itself appears to function like an endocrine organ, and dysregulation of the gut flora has been correlated with a host of inflammatory and autoimmune conditions . </P> <P> The composition of human gut flora changes over time, when the diet changes, and as overall health changes . A systematic review of 15 human randomized controlled trials from July 2016 found that certain commercially available strains of probiotic bacteria from the Bifidobacterium and Lactobacillus genera (B. longum, B. breve, B. infantis, L. helveticus, L. rhamnosus, L. plantarum, and L. casei), when taken by mouth in daily doses of 10--10 colony forming units (CFU) for 1--2 months, possess treatment efficacy (i.e., improved behavioral outcomes) in certain central nervous system disorders--including anxiety, depression, autism spectrum disorder, and obsessive--compulsive disorder--and improved certain aspects of memory . However, changes in the composition of gut microbiota has also been found to be correlated with harmful effects on health . In an article published by Musso et al., it was found that the gut microbiota of obese invidividuals had more Firmicutes and less Bacteroidetes than healthy individuals . Furthermore, a study done by Gordon et al., confirmed that it was the composition of the microbiota that causes obesity rather than the other way around . This was done by transplanting the gut microbiota from diet - induced obese (DIO) mice or lean control mice into lean germ - free mice that do not have a microbiome . They found that the mice transplanted with DIO mouse gut microbiota had significantly higher total body fat than the mice transplanted with lean mouse microbiota when fed with the same diet . </P> <P> Vaginal microbiota refers to those species and genera that colonize the lower reproductive tract of women . These organisms play an important role in protecting against infections and maintaining vaginal health . The most abundant vaginal microorganisms found in premenopausal women are from the genus Lactobacillus, which suppress pathogens by producing hydrogen peroxide and lactic acid . Bacterial species composition and ratios vary depending on the stage of the menstrual cycle . Ethnicity also influences vaginal flora . The occurrence of hydrogen peroxide - producing lactobacilli is lower in African American women and vaginal pH is higher . Other influential factors such as sexual intercourse and antibiotics have been linked to the loss of lactobacilli . Moreover, studies have found that sexual intercourse with a condom does appear to change lactobacilli levels, and does increase the level of Escherichia coli within the vaginal flora . Changes in the normal, healthy vaginal microbiota is an indication of infections, such as candidiasis or bacterial vaginosis . Candida albicans inhibits the growth of Lactobacillus species, while Lactobacillus species which produce hydrogen peroxide inhibit the growth and virulence of Candida albicans in both the vagina and the gut . </P> <P> Fungal genera that have been detected in the vagina include Candida, Pichia, Eurotium, Alternaria, Rhodotorula, and Cladosporium, among others . </P>

Where do most microbes reside in the human body