When bacteria and parasites attack the gut microbiota
Viruses are not the only causes of diarrhea: there are other enteric pathogens (microorganisms infecting the gastrointestinal tract). Bacteria such as Salmonellae and Escherichia coli, or unicellular organisms (protozoans) such as Giardia lamblia, are able to colonize the GI tract, to disrupt the gut microbiota, and to cause short- and long-term consequences13. There are other factors: some drugs, such as antibiotics that disrupt the microbiota, impair its functioning and promote colonization by pathogens, of which Clostridium difficile is the most frequent.
Why are some people particularly sensitive when others are more resistant? “It depends on the microbiota”, answer the researchers, whose studies underline the impact of interactions between invasive pathogens and microorganism living in our intestines.
An uphill battle
To understand the infectious process of enteropathogenic bacteria, researchers reviewed the mechanisms employed by the body to fight the colonization of the digestive tract by Salmonella typhimurium14, a bacterium causing food poisoning with diarrhea, that is sometimes severe although short-lived. The first mechanism comes into play in the stomach where the acid environment destroys between 95 and 99% of ingested bacteria (through food). For bacteria that are able to reach the intestines, it is too early to claim victory: they can only grow if the level of resistance to colonization allows it. But the latter depends on the composition of the gut microbiota, specific to each person, that has an arsenal of tools to fight this colonization: secretion of components blocking the invader’s growth and virulence, competition for the same binding sites, creation of an oxygen-poor environment that is unfavorable to its growth…
A fierce fight
And our defenses have not said their last word: bacteria must be present in sufficient amounts to trigger diarrhea, and it only happens between 12 to 36 hours (sometimes 72 hours, depending on the number of ingested bacteria) after they cross the intestiWhen bacteria and parasites attack the gut microbiota nal barrier. Based on animal models, S. typhimurium accomplishes this by secreting toxic substances, thus allowing it to reach the mucosa and then the submucosa. The body reacts by expelling infected intestinal cells thus dividing the number of pathogenic bacteria in the tissues by 100– and sets off a massive inflammatory response which affects the enemy in two ways: by reducing the bacterial load in the body, while serving as fuel to the remaining bacteria.
There is another enemy: Giardia lamblia. This protozoan infects humans through the consumption of contaminated water or food. Although the frequency of giardiasis does not exceed 7% in developed countries, it can reach 30% in developing countries. In most cases, the infection takes a few weeks to resolve but it can sometimes last several months and become chronic. There is currently no vaccine and available treatments have different levels of effectiveness. Giardiasis is often asymptomatic, but it can cause diarrhea, cramps and nausea. In newborns, disease severity hinges on its long-term consequences: at the age of two, they show a significant growth deficit. And some people may develop post-infectious syndromes several years after the elimination of the parasite, such as irritable bowel syndrome or chronic fatigue. According to several papers, Giardia lamblia could act by decreasing the immune response and causing dysbiosis.
13 Fink MY et al. The Intersection of Immune Responses, Microbiota, and Pathogenesis in Giardiasis. Trends Parasitol. 2017 Nov
14 Wotzka SY et al. Salmonella Typhimurium Diarrhea Reveals Basic Principles of Enteropathogen Infection and Disease-Promoted DNA Exchange. Cell Host Microbe. 2017 Apr 1