1. The intestinal microbiota

We are 90% bacterial and 10% human.

The human digestive tract is a complex ecosystem, that is home to millions of microorganisms and hundreds of different types of bacteria. The human microbiome consists of ten times more microbial than human cells, i.e., 100 billion. Our gut flora contains more than three million microbial genes, or 150 times more genes than in the human genome. More than 1,000 different known bacterial species can be found in human gut microbiota but only 150 to 170 predominate in any given subject.

Most gut microbes are harmless or even beneficial to the host. They protect against stomach and intestinal pathogens (the bacteria that cause food-borne infections), help extract nutrients and energy from food and contribute to normal immune function. Some of these bacteria may be potentially dangerous. But they are usually kept in check by good bacteria to maintain the so-called microbiota balance.

Disruptions of this delicate balance are linked to obesity, malnutrition, unhealthy eating, illness, stress, etc.. A microbial imbalance can also trigger infections with opportunistic pathogens, causing various episodes of diarrhoea for example. Various factors can disrupt the microbial balance in the intestine, such as the use of medicines in case of illness, e.g., antibiotics. Treatments with antibiotics partly destroy microbial populations, the commensal microbiota is decreased, creating an environment that is susceptible to pathogenic and opportunistic intruders.

Dysbiosis of intestinal microbiota can also be caused by pathogenic (bacterial) infections such as listeria and salmonella.

Diarrhoea can be caused by viral infections such as the rotavirus and infections.

2. Why gut flora are crucial

The intestinal epithelium and the intestinal mucosal barrier are physical barriers, which regulate the exchange of water and nutrients between the environment outside the gut cavity and the body. This barrier function may be disrupted by an inflammatory process. This could give rise to a passive loss of water and dissolved substances from the body in the gut cavity, which can cause diarrhoea.

Viral infections can also undermine the epithelian/mucosal barriers. Various viruses, and the rotavirus especially, can infect the intestinal cells, causing loss of villi, impairing the exchange and absorption of water, electrolytes and nutrients. This situation can cause vomiting and diarrhoea, with significant loss of body fluids, which can give rise to dehydration. Various clinical studies have demonstrated the efficacy of specific  Lactobacillus rhamnosus GG for the prevention and treatment of diarrhoea caused by rotavirus in infants and children.¹⁰ It exerts protective effects because of its ability to reduce the viral replication and adhesion to the epithelian cells in the intestinal mucus. Certain studies also showed that the protective action of Lactobacillus rhamnosus GG against viral infections is enhanced when it is administered as a preventative measure, i.e., before the infection occurs. ¹¹

Sources:
¹⁰ Li YT, Xu H, Ye JZ, Wu WR, Shi D, Fang DQ, Liu Y, Li LJ. Efficacy of Lactobacillus rhamnosus GG in treatment of acute pediatric diarrhea: A systematic review with meta-analysis. World J Gastroenterol. 2019 Sep 7; 25(33): 4999–5016.
¹¹ Luoto R, Ruuskanen O, Waris M, Kalliomaki M, Salminen S, Isolauri E: Prebiotic and probiotic supplementation prevents rhinovirus infections in preterm infants: A randomized, placebo-controlled trial. J Allergy Clin Immunol 2013.