Lactobacilli and other lacto-bacteria (e.g. Bifidobacteria) that perform lacto-fermentation and make lacto-fermented food, live also in the urinary, digestive and genital tracts of humans. They constitute so-called good flora and are believed to be in constant competition with bad flora. This picture is, however, rather simplified as some types of bacteria can be either friendly or harmful depending on circumstances.
All the microorganisms inhabiting the digestive tract are called gut flora. One person carries in their intestines 100 trillion of them, which is ten times more than the total number of cells in a human body*. Gut flora, in total, can weigh up to 2 kg and consists of around 1000 different species but only 150 to 170 predominate in an individual. The type of predominant flora determines a person’s enterotype.
Even though, people belong to three main enterotypes, the composition of gut flora is extremely individual. Age, gender and body weight influence neither individual gut flora composition nor enterotype, but the long-term diet does.
What happens with a baby during a delivery and in the first days of its life also has a huge impact. For example, babies born vaginally are exposed to different types of bacteria than those born by a caesarean section and breastfed babies are supplied a with different “starter set” of microflora than those fed on a formula. Moreover, kissing and touching babies by a healthy adult is believed to help with colonizing their bodies with proper types of bacteria.
The relationship between gut flora and humans is a mutualistic relationship. The host provides a protected, nutrient-rich environment for microorganisms and they in return perform a lot of useful functions. It is possible because the range of enzymes absent in human cells is produced in those microorganisms.
The main of those functions are:
a. Digesting polysaccharides (starches, fiber, etc.) in the stomach and small intestine;
b. Digesting and absorbing lactose, some alcohols, proteins and lipids
c. Supporting absorption of calcium, magnesium and iron
d. Producing and then facilitating absorption of vitamin K and some vitamins B
e. Increasing absorption of water
f. Controlling proliferation and differentiation of intestinal epithelial cells
g. Preventing injury to the gut mucosa
h. Preventing harmful bacteria and fungi from colonizing the gut through
– competitive exclusion
– producing bacteriocins (toxins killing other bacteria)
– lowering pH inside intestines with organic acids (mostly lactic acid) which are the products of fermentation
i. Stimulating the lymphoid tissue associated with the gut mucosa to produce antibodies to pathogens, thus increasing immunity
j. Reducing an overreactive immune response like those found in allergies and autoimmune disease by modulating T cells
k. Removing toxic heavy metals from the body
Taking care of gut flora means:
a. avoiding sugars, caffeine, alcohol, processed food, antibiotics, chlorinated water
b. eating prebiotics (fermentable fibre) and probiotics (live bacteria and yeast beneficial for health, i.e. good flora)
*25/04/2016 update: this fact became controversial recently; see an article in Nature and don’t skip the comment section, it’s the best part.
Read more about the gut flora:
Gut Microbiota Watch