It seems to be so: the microbes in our gastrointestinal tract (GIT) influence our choice of food. No wonder: microbes, primarily bacteria, are present in significant amounts in GIT, more than 10 bacterial cells for each of our cells, a total of 1014 (The human body has about 1013 cells). This amounts to about 1-1.5 kg. And these bacteria have lived with us always, since all mammals have them. So, they have evolved with our ancestors and therefore they are well suited to our internal environment. Being our bodies their habitat, much the better if they can control what reaches the intestine. And how can they do? Then giving orders to the brain to eat such a thing or that other, appropriate for them, the microbes.
Well, gone seriously, there is some previous work in this direction. It seems there is a relationship between preferences for a particular diet and microbial composition of GIT (Norris et al 2013). In fact, it is a two-way interaction, one of the many aspects of symbiotic mutualism between us and our microbiota (Dethlefsen et al 2007).
There is much evidence that diet influences the microbiota. One of the most striking examples is that African children fed almost exclusively in sorghum have more cellulolytic microbes than other children (De Filippo et al 2010).
The brain can also indirectly influence the gut microbiota by changes in intestinal motility, secretion and permeability, or directly releasing specific molecules to the gut digestive lumen from the sub epithelial cells (neurons or from the immune system) (Rhee et al 2009).
The GIT is a complex ecosystem where different species of bacteria and other microorganisms must compete and cooperate among themselves and with the host cells. The food ingested by the host (human or other mammal) is an important factor in the continuous selection of these microbes and the nature of food is often determined by the preferences of the host. Those bacteria that are able to manipulate these preferences will have advantages over those that are not (Norris et al 2013).
Recently Alcock et al (2014) have reviewed the evidences of all this. Microbes can manipulate the feeding behaviour of the host in their own benefit through various possible strategies. We’ll see some examples in relation to the scheme of Figure 2.
Figure 2. As if microbes were puppeteers and we humans were the puppets, microbes can control what we eat by a number of marked mechanisms. Adapted from Alcock et al 2014.
People who have “desires” of chocolate have different microbial metabolites in urine from people indifferent to chocolate, despite having the same diet.
Dysphoria, id est, human discomfort until we eat food which improve microbial “welfare”, may be due to the expression of bacterial virulence genes and perception of pain by the host. This is because the production of toxins is often triggered by a low concentration of nutrients limiting growth. The detection of sugars and other nutrients regulates virulence and growth of various microbes. These directly injure the intestinal epithelium when nutrients are absent. According to this hypothesis, it has been shown that bacterial virulence proteins activate pain receptors. It has been shown that fasting in mice increases the perception of pain by a mechanism of vagal nerve.
Microbes can also alter food preferences of guests changing the expression of taste receptors on the host. In this sense, for instance germ-free mice prefer more sweet food and have a greater number of sweet receptors on the tongue and intestine that mice with a normal microbiota.
The feeding behaviour of the host can also be manipulated by microbes through the nervous system, through the vagus nerve, which connects the 100 million neurons of the enteric nervous system from the gut to the brain via the medulla. Enteric nerves have receptors that react to the presence of certain bacteria and bacterial metabolites such as short chain fatty acids. The vagus nerve regulates eating behaviour and body weight. It has been seen that the activity of the vagus nerve of rats stimulated with norepinephrine causes that they keep eating despite being satiated. This suggests that GIT microbes produce neurotransmitters that can contribute to overeating.
Neurotransmitters produced by microbes are analogue compounds to mammalian hormones related to mood and behaviour. More than 50% of dopamine and most of serotonin in the body have an intestinal origin. Many persistent and transient inhabitants of the gut, including E. coli, several Bacillus, Staphylococcus and Proteus secrete dopamine. In Table 1 we can see the various neurotransmitters produced by GIT microbes. At the same time, it is known that host enzymes such as amine oxidase can degrade neurotransmitters produced by microorganisms, which demonstrates the evolutionary interactions between microbes and hosts.
Table 1. Diversity of neurotransmitters isolated from several microbial species (Roschchina 2010)
|GABA (gamma-amino-butyric acid)||Lactobacillus, Bifidobacterium|
|Norepinephrine||Escherichia, Bacillus, Saccharomyces|
|Serotonin||Candida, Streptococcus, Escherichia, Enterococcus|
Some bacteria induce hosts to provide their favourite nutrients. For example, Bacteroides thetaiotaomicron inhabits the intestinal mucus, where it feeds on oligosaccharides secreted by goblet cells of the intestine, and this bacterium induces its host mammal to increase the secretion of these oligosaccharides. Instead, Faecalibacterium prausnitzii, a not degrading mucus, which is associated with B. thetaiotaomicron, inhibits the mucus production. Therefore, this is an ecosystem with multiple agents that interact with each other and with the host.
As microbiota is easily manipulated by prebiotics, probiotics, antibiotics, faecal transplants, and changes in diet, controlling and altering our microbiota provides a viable method to the otherwise insoluble problems of obesity and poor diet.
Alcock J, Maley CC, Aktipis CA (2014) Is eating behavior manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanisms. BioEssays 36, DOI: 10.1002/bies.201400071
De Filippo C, Cavalieri D, Di Paola M, Ramazzotti M, et al (2010) Impact of diet in shaping gut microbiota revealed by a comparative study in children from Europe and rural Africa. Proc Natl Acad Sci USA 107:14691–6
Dethlefsen L, McFall-Ngai M, Relman DA (2007) An ecological and evolutionary perspective on human-microbe mutualism and disease. Nature 449:811-818
Lyte M (2011) Probiotics function mechanistically as delivery for neuroactive compounds: Microbial endocrinology in teh design and use of probiotics. BioEssays 33:574-581
Norris V, Molina F, Gewirtz AT (2013) Hypothesis: bacteria control host appetites. J Bacteriol 195:411–416
Rhee SH, Pothoulakis C, Mayer EA (2009) Principles and clinical implications of the brain–gut–enteric microbiota axis. Nature Reviews Gastroenterology and Hepatology 6:306-314
Roschchina VV (2010) Evolutionary considerations of neurotransmitters in microbial, plant, and animal cells. In Lyte M, Freestone PPE, eds; Microbial Endocrinology: Interkingdom Signaling in Infectious Disease and Health. New York: Springer. pp. 17–52