Who Controls Who?

Scientists have long since been aware of the microbiota in the human gut and its influences on the human digestive system. However, the importance and extent of influence of the human microbiome in other parts of the body has only recently surfaced. It is intriguing to know that a multitude of bacteria is living in the human body affecting its physiological processes and functions. It raises the question whether humans, as hosts, control the millions of microorganisms inhabiting them or is it the other way around? Koenig et al.’s study of the developing infant gut microbiome attempts to answer this question.

In their study, the Succession of microbial consortia in the developing infant gut microbiome, over 60 fecal samples, taken over two and a half years, from a single infant were analyzed. The gut microbial composition of each sample was obtained using 454-pyrosequencing of rRNA. From this approximately 318,620 16S rRNA gene sequences were generated and plotted onto a timeline coinciding with life events such as introduction of solid food, onset of ear infection and weaning. On basis of the patterns observed on this timeline, more than 500,000 sequences from 12 samples were studied in greater detail with respect to certain life events. This metagenomic analysis formed a foundation on which further results were acquired (Koenig et al., 2010).

Major results noted specific variations in the microbiome concerning time, composition and function. For instance, diversity of bacterial communities decreased before the occurrence of a fever and after the ingestion of antibiotics. Also, succession of bacterial communities positively correlated with age and succession of life stages. Furthermore, bacterial communities occupied specific niches and some communities were never present together (Koenig et al, 2010). These two findings show the dynamic nature of our microbiome and the effect of human life stages over them. It is essential to note that in this case the change in the microbiome occurred after the life event or stage.

The last lot of results focused on the link between gene function and occurrence of life events. Meta Genome Rapid Annotation using Subsystem Technology showed that certain gene sequences were enriched according to the need of their function. For example, genes that help digest plant-based glycans were enriched before the introduction of solid food in the infant’s diet. It wasn’t just gene sequences that were being enriched; entire phyla as well became more abundant corresponding to certain life events. For example, after the onset of fever, phyla Actinobacteria and Proteobacteria had a relatively greater abundance in the gut. (Koenig et al, 2010). Thus, it is evident that the bacterial diversity depends on the function required from the microbiome.

Regarding the digestion of plant-based glycans, the microbiome changed before the life event occurred, unlike the the earlier result. This raises some rather interesting questions. Do microbiome changes trigger transitions through life stages? Without these microbial changes would an individual not proceed to the next life stage? Do microbiomes truly control human physiology? Is their control substantial?

It is certain that their control is much more than hitherto known. Several studies focus on finding out more on this curious case of microbial control on the human physiology. One such study directly linked the gut microbiota and its production of metabolites to physiological control. It concluded that gut microbiomes produced certain metabolites which can affect the production of the neurotransmitter, serotonin, and in turn affect physiological functions of the body (Ridaura, 2015). Another study established that microbial secretions consisted of mood-controlling neurochemicals such as dopamine and GABA (Smith, 2015). Another research shows that certain bacteria are predominantly found in people who suffer from depression (Smith, 2015).

All of these studies provide an insight into the workings of the human gut microbiome. The extent of its influences into the domain of human moods and emotions is indeed astonishing. However, scientists have barely touched the surface of the subject of microbial control on humans. The aforementioned studies covered only the microbiome in the human gut. Bacteria inhabit several other parts of the body giving scientists a variety of microbiomes to study. Also, the Koenig et al study analyzed just a single infant. A possible future study could be devoted to a meta-analysis of several infant gut microbiomes which could give further insights into the workings of the human microbiomes.

The findings from Koenig et al’s research and the other discussed studies have undoubtedly enhanced the ecological understanding of the human microbiome. The question of “Who controls who?” does remain unanswered. However, it has given rise to several other questions worth finding answers to.

References:

          1. Koenig JE, Spor A, Scalfone N, Fricker AD, Stombaugh J, Knight R, Angenent LT, Ley RE. Succession of microbial consortia in the developing infant gut microbiome. PNAS. June 24, 2010. http://www.pnas.org/content/108/Supplement_1/4578.full.pdf

2. Vanessa Ridaura, Yasmine Belkaid. Gut Microbiota: The link to your Second Brain. Science Direct. 9^th April, 2015; Volume161 (Issue 2).  http://www.sciencedirect.com/science/article/pii/S0092867415003530

3.  Peter Andrey Smith. Can the Bacteria in Your Gut Explain Your mood? The New York Times Magazine. June 23, 2015. http://www.nytimes.com/2015/06/28/magazine/can-the-bacteria-in-your-gut-explain-your-mood.html?_r=1

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