in those with diarrheal disease (Shulman et al. 2007). A preliminary communication
on this subject was made in 1885 at the Society for Morphology and Physiology in
Munich (Shulman et al. 2007). Although this was the first scientific evidence that
bacteria are part of the intestinal system, this situation was misunderstood and even
diabolized for a long time.
The number of bacteria in the gastrointestinal tract reaches 39 trillion with a total
mass of 200 g for a healthy young person. If we consider that human cells amount to
30 trillion, this finding refutes the traditional data of 10:1 (bacteria/human cells),
showing that bacteria are the same order as the number of human cells (Haahtela
2019; Salazar et al. 2017; Sender et al. 2016). Metagenomic studies have indicated
that the number of bacterial species in the gastrointestinal tract of a human body
varies between 1000 and 1500, with each human being harboring around 150 different species, possibly amounting to their being more genes in our symbionts than in
our own genome (Salazar et al. 2017).
The diversity and stability of these symbiotic microbiota are fundamental for
human health and for our adequate physiological development. Reduced contact
with the natural environment possibly is a factor that would result in a poor
symbiotic microbiota, which in turn might lead to immune imbalance (Haahtela
2019). This might contribute to the allergy epidemic that exists in modern societies
associated with global warming which prolongs and intensifies pollen seasons and
increases allergy crises (Haahtela 2019).
A good example of the importance of gut symbionts can be found in the serotonin
synthesis and role. Serotonin is a neurotransmitter of the central nervous system,
namely involved in immune cell activation, in intestinal inflammation process and
even in psychiatric disorders (Banskota et al. 2019). This molecule is synthesized in
two different areas of human body: 5% in serotonergic neurons of the central
nervous system and 95% in enterochromaffin cells of the gut (Banskota et al.
2019). We know that the precursor of this neurotransmitter is tryptophan, which is
involved in metabolic pathways associated with bacteria of the gastrointestinal tract,
and those microbes constitute a key factor of this molecular synthesis. However, it is
not clear why a neurotransmitter involved in the function of the central nervous
system is in great part synthesized in the gastrointestinal tract and has an important
evolutive influence on the gut–brain axis (GBA). Other microbial metabolites
produced in the gastrointestinal tract can also influence the GBA. Such is the case
of short chain fatty acids such as butyrate synthetized by gut microbiota that can
stimulate memory and synaptic plasticity and also influence the release of serotonin
from the intestinal enterochromaffin cells (Kaur et al. 2019). Furthermore, several
metabolites of bacterial tryptophan metabolism are important to modulate the health
of the host, namely in cases of mental depression. Various studies have also shown
the ability of certain bacteria such as Enterococcus and Pseudomonas to produce
serotonin, when those microbes are in tryptophan-rich media. Thusly, these other
microbes might also affect serotonergic neurotransmission, thereby in turn affecting
the functioning of human central and enteric nervous systems (Kaur et al. 2019).
27 The Importance of Being Symbiont and the Role of Symbiosis as a Driving Force. . .
605
on this subject was made in 1885 at the Society for Morphology and Physiology in
Munich (Shulman et al. 2007). Although this was the first scientific evidence that
bacteria are part of the intestinal system, this situation was misunderstood and even
diabolized for a long time.
The number of bacteria in the gastrointestinal tract reaches 39 trillion with a total
mass of 200 g for a healthy young person. If we consider that human cells amount to
30 trillion, this finding refutes the traditional data of 10:1 (bacteria/human cells),
showing that bacteria are the same order as the number of human cells (Haahtela
2019; Salazar et al. 2017; Sender et al. 2016). Metagenomic studies have indicated
that the number of bacterial species in the gastrointestinal tract of a human body
varies between 1000 and 1500, with each human being harboring around 150 different species, possibly amounting to their being more genes in our symbionts than in
our own genome (Salazar et al. 2017).
The diversity and stability of these symbiotic microbiota are fundamental for
human health and for our adequate physiological development. Reduced contact
with the natural environment possibly is a factor that would result in a poor
symbiotic microbiota, which in turn might lead to immune imbalance (Haahtela
2019). This might contribute to the allergy epidemic that exists in modern societies
associated with global warming which prolongs and intensifies pollen seasons and
increases allergy crises (Haahtela 2019).
A good example of the importance of gut symbionts can be found in the serotonin
synthesis and role. Serotonin is a neurotransmitter of the central nervous system,
namely involved in immune cell activation, in intestinal inflammation process and
even in psychiatric disorders (Banskota et al. 2019). This molecule is synthesized in
two different areas of human body: 5% in serotonergic neurons of the central
nervous system and 95% in enterochromaffin cells of the gut (Banskota et al.
2019). We know that the precursor of this neurotransmitter is tryptophan, which is
involved in metabolic pathways associated with bacteria of the gastrointestinal tract,
and those microbes constitute a key factor of this molecular synthesis. However, it is
not clear why a neurotransmitter involved in the function of the central nervous
system is in great part synthesized in the gastrointestinal tract and has an important
evolutive influence on the gut–brain axis (GBA). Other microbial metabolites
produced in the gastrointestinal tract can also influence the GBA. Such is the case
of short chain fatty acids such as butyrate synthetized by gut microbiota that can
stimulate memory and synaptic plasticity and also influence the release of serotonin
from the intestinal enterochromaffin cells (Kaur et al. 2019). Furthermore, several
metabolites of bacterial tryptophan metabolism are important to modulate the health
of the host, namely in cases of mental depression. Various studies have also shown
the ability of certain bacteria such as Enterococcus and Pseudomonas to produce
serotonin, when those microbes are in tryptophan-rich media. Thusly, these other
microbes might also affect serotonergic neurotransmission, thereby in turn affecting
the functioning of human central and enteric nervous systems (Kaur et al. 2019).
27 The Importance of Being Symbiont and the Role of Symbiosis as a Driving Force. . .
605