that these include some that can adapt quickly to the new conditions (de Mazancourt
et al. 2008). Perhaps the same is true of the gut microbiota?
Another possibility is that biodiversity inhibits potentially dangerous biofilm
formation. Biofilms are communities of one or more species that adhere to each
other and to surfaces. The “decision” to switch from planktonic to biofilm growth is
regulated by environmental factors and quorum sensing. The normal healthy symbiotic gut microbiota forms physiological biofilms associated with the mucus layers
of the large bowel. But the crucial point is that the physiology of an organism
changes when it switches to biofilm mode, and with that switch some organisms
become pathogenic as well as resistant to antimicrobials and resistant to the immune
system. For example, most pathology caused by Candida albicans follows the
switch from yeast to hyphal forms that occurs when it makes biofilm (Tsui et al.
2016). In patients with IBD (inflammatory bowel disease), gut microbiota can
bypass the mucus barrier and abnormal biofilm is found adherent to the epithelial
surface. Organisms from such biofilm can translocate across human intestinal
epithelial cell monolayers in vitro, whereas bacteria from the microbiota of healthy
donors do not (Buret et al. 2019). It is possible that high gut microbiota biodiversity
affects the quorum sensing signals, and stops potential pathogens from switching to
biofilm.
18.4.11 Modulation of the Gut–Brain Axis
Being deprived of contact with green space during childhood increases the risk of
mental illness in later life (Maas et al. 2009; Engemann et al. 2019). Is this an effect
of exposure to environmental microbiota, or is it due to other factors such as
relaxation, sun, and exercise? There is abundant experimental evidence that the
gut microbiota has potent effects on brain function (Mayer et al. 2014; Rook et al.
2018). For example, exposing pregnant rats to a nonabsorbable antibiotic leads to
behavioural abnormalities in the offspring (Degroote et al. 2016), and even the adult
mouse brain can be modified by antibiotics. Administering a broad spectrum antibiotic mixture to adult mice reduced hippocampal neurogenesis, and memory
retention (Möhle et al. 2016). These defects could be treated by reconstituting a
normal microbiota, particularly when supplemented with a probiotic mixture
(VSL#3). Similarly, germ-free animals, although of uncertain relevance to humans,
have abnormal stress responses that can be corrected by early restoration of the
microbiota, but cannot be corrected by normalization of the microbiota in adulthood
(Sudo et al. 2004; Diaz Heijtz et al. 2011). Interestingly, transfer of microbiota from
depressed humans to germ-free or antibiotic-treated rodents induces depression-like
behavior in the recipient animals (Kelly et al. 2016; Zheng et al. 2016). So these
experiments establish the potential relevance of the microbiota, which is inevitably
influenced by environmental inputs to the microbiota itself, and to the immune
system that controls it. Many microbial components that modulate brain function
have been identified (Caspani and Swann 2019).
18 Darwinian Medicine: We Evolved to Require Continuing Contact with the. . .
349
et al. 2008). Perhaps the same is true of the gut microbiota?
Another possibility is that biodiversity inhibits potentially dangerous biofilm
formation. Biofilms are communities of one or more species that adhere to each
other and to surfaces. The “decision” to switch from planktonic to biofilm growth is
regulated by environmental factors and quorum sensing. The normal healthy symbiotic gut microbiota forms physiological biofilms associated with the mucus layers
of the large bowel. But the crucial point is that the physiology of an organism
changes when it switches to biofilm mode, and with that switch some organisms
become pathogenic as well as resistant to antimicrobials and resistant to the immune
system. For example, most pathology caused by Candida albicans follows the
switch from yeast to hyphal forms that occurs when it makes biofilm (Tsui et al.
2016). In patients with IBD (inflammatory bowel disease), gut microbiota can
bypass the mucus barrier and abnormal biofilm is found adherent to the epithelial
surface. Organisms from such biofilm can translocate across human intestinal
epithelial cell monolayers in vitro, whereas bacteria from the microbiota of healthy
donors do not (Buret et al. 2019). It is possible that high gut microbiota biodiversity
affects the quorum sensing signals, and stops potential pathogens from switching to
biofilm.
18.4.11 Modulation of the Gut–Brain Axis
Being deprived of contact with green space during childhood increases the risk of
mental illness in later life (Maas et al. 2009; Engemann et al. 2019). Is this an effect
of exposure to environmental microbiota, or is it due to other factors such as
relaxation, sun, and exercise? There is abundant experimental evidence that the
gut microbiota has potent effects on brain function (Mayer et al. 2014; Rook et al.
2018). For example, exposing pregnant rats to a nonabsorbable antibiotic leads to
behavioural abnormalities in the offspring (Degroote et al. 2016), and even the adult
mouse brain can be modified by antibiotics. Administering a broad spectrum antibiotic mixture to adult mice reduced hippocampal neurogenesis, and memory
retention (Möhle et al. 2016). These defects could be treated by reconstituting a
normal microbiota, particularly when supplemented with a probiotic mixture
(VSL#3). Similarly, germ-free animals, although of uncertain relevance to humans,
have abnormal stress responses that can be corrected by early restoration of the
microbiota, but cannot be corrected by normalization of the microbiota in adulthood
(Sudo et al. 2004; Diaz Heijtz et al. 2011). Interestingly, transfer of microbiota from
depressed humans to germ-free or antibiotic-treated rodents induces depression-like
behavior in the recipient animals (Kelly et al. 2016; Zheng et al. 2016). So these
experiments establish the potential relevance of the microbiota, which is inevitably
influenced by environmental inputs to the microbiota itself, and to the immune
system that controls it. Many microbial components that modulate brain function
have been identified (Caspani and Swann 2019).
18 Darwinian Medicine: We Evolved to Require Continuing Contact with the. . .
349