The microbiome is defined as “the collective genomes of the microbes (composed of bacteria, bacteriophages, fungi, protozoa and viruses) that live inside, and on the human body” . As a relatively new concept, the microbiome offers a great deal of speculation, with building expectations as to what lies within this black box of science; with the linking of the microbiome to characteristics of our very personalities.
The question: “What makes us who we are?” has been discussed for many years, with emphasis on both free will and determinism as contributing factors. Microbial populations, particularly within the gut, have been linked with many behavioural characteristics, acting via the ‘brain-gut-microbiota axis’, providing a whole new level for such a discussion. Deviations from a ‘standard’ microbiome are believed to affect not only immunity and bowel function, but also many psychiatric conditions. Research involving ‘germ-free’ mice and a variety of different behavioural tests has demonstrated how such mice display greater than average anxiety and emotional traits. Such abnormalities have also been shown in mice that have been treated with antibiotics, raising great interest into the applicability of these studies to humans .
“We may one day be able to medically treat criminality”
Speculation of the microbiome’s ability to change our personalities poses huge questions not only within the field of medicine, but also that of the law. It allows for the possibility in the future, not only for the blaming of the human genome for criminal offences in court, but for blaming our microbiome also. This speculation could go as far as to say that we may one day be able to medically treat criminality and certain behavioural defects via interference with the microbiome. However, more research is needed within this field in order to determine how the host microbial populations are able to control behaviour in such a way.
Figure 1. : Can personality really be regarded as a ‘human’ trait? With the human microbiome being linked to psychiatric disabilities such as autism and schizophrenia; and recorded occasions where a person’s likes and dislikes were altered after a transplant; it is becoming an increasingly grey area as to whether our very personalities may be regarded as what makes us human. 
The composition of the gut microbiota has been theorized to be involved with several psychiatric comorbidities – autism being one such example . In fact, research carried out on DNA extracted from stool samples has identified clusters of Desulfovibrio or Clostridium over-represented in children with both an autistic spectrum disorder (ASD) and a gastrointestinal (GI) disorder; in comparison to children displaying a similar GI complaint without an ASD . This suggests that a link that is often seen to be present between ASDs and many GI complaints, ranging from irritable bowel syndrome to being ‘fussy eaters’ and preferring to snack rather than eating regular and substantial meals; may be down to the microbiome. One possible theory aiming to explain the abnormalities in the GI microbiota of children with ASDs implicates the mothers’ breast-feeding habits. Early-life diet plays an invaluable role in the acquisition and development of the human microbiome, with breast-fed infants showing considerable differences in their microbial make-up to those who are formula fed. Moreover, some evidence supports that the different microbial make-up due to variations in breast-feeding habits may possibly be linked to ASDs in a dose-response relationship .
“In the past, people have…suggested a particular diet will “cure” children of autism”
Complex animal studies on the gut microbiome have discovered the importance of the ‘brain-gut-microbiota axis’ showing concordance between the gut microbiota and brain function – further illustrating the possible link with ASDs. However, although this leads to further suggestions into possible causations and treatments for ASDs, attributing the microbiome to the causation of autism has proven very controversial. In the past, people have jumped on a rather large bandwagon under this line of research, suggesting a particular diet will “cure” children of autism; hence the need for much more careful research before any real conclusions may be drawn, as well as careful communication to the public.
Hence, with bacteria being able to mediate our very own distinct personalities, it could well be time to reconsider what it is that makes us truly human.
1. Bercik, P., Denou, E., Collins, J., Jackson, W., Lu, J., Jury, J. et al (2011). The Intestinal Microbiota Affect Central Levels of Brain-Derived Neurotropic Factor and Behaviour in Mice. Gastroenterology 141 (2); 599-609
2. Cooney, E. (June 13 2012). HMP Maps the Healthy Human Microbiome. SciTech Daily. Available: http://www.scitechdaily.com/hmp-maps-the-healthy-human-microbiome/ (last accessed 21/1/14)
3. Cryan, J. & O’Mahon, S. (2011). The microbiome-gut-brain axis: from bowel to behavior. Neurogastroenterology & Motility 23 (3) 187-192
4. Mulle, J., Sharp, W., & Cubells, J. (2013). The Gut Microbiome: A New Frontier in Autism Research. Current Psychiatry Reports 15 (337)
5. Yang, J (July 16 2012). National Human Genome Research Institute. Genome.gov. Available: http://www.genome.gov/27549400