Phylum Spotlight: Actinobacteria

In this post we look at one of the final prominent phyla present in the human gut. This group of bacteria is called Actinobacteria and they occupy many habitats in and on the human body. Because of this ability to line in us, the pathogenic bacteria in this group are well known (Mycobacterium tuberculosis, Mycobacterium diphtheriae, and Corynebacterium leprae). However, there are many more examples of good Actinobacteria than the opposite. In essence, these bacteria are the gatekeepers of the microbiome, keeping the conditions in the body unfavorable for pathogens that may try to grow there.

The genera of Actinobacteria

There are five genera of Actinobacteria that interact with the human microbiome. We will now look at each one and see how exactly they interact with our bodies.

Corynebacterium

This genera of Actinobacteria lives mostly on the skin and in the mouth. In fact, all of the pathogens mentioned above fall under this class of bacteria. Besides the pathogens, the bacteria in this genera help to make biofilms in your mouth. This slimy layer of bacteria covers and protects your teeth and mouth from more harmful colonizers. Many forms of Corynebacterium live commensialistically with us; this means that they live on us without having much effect.

Propionibacterium

The genera Propionibacterium lives on the oily parts of your body such as the forehead, nose, and armpits. Specifically, these bacteria live inside the sebaceous follicles of our skin, using the oil we secrete as food. In return, these bacteria release propionate, an acid, to lower the pH of our skin and stop hostile bacteria from colonizing the skin. Again, this genera of bacteria lives exclusively on the surface of your body.

Rothia and Actinomyces

The two genera Rothia and Actinomyces are specialized to live in the upper GI tract. Rothia bacteria tend to live in the mouth and pharynx while Actinomyces tend to live exclusively in the oral cavity. The Rothia bacteria are critical in early microbiome colonization because of their metabolic activity. After landing in the mouth of an infant, these bacteria digest sugars and produce lactate as a byproduct. This lactate is a critical energy source for other beneficial colonists.

On the other hand, Actinomyces bacteria arrive on the scene later on. Since they only live in the oral cavity, these bacteria have evolved a special mechanism to stay in place. On the surface of Actinomyces, tiny hairs called pili grip their surroundings to keep the bacteria from being washed away by saliva. These hairs are specialized so that they only grab onto proteins in the saliva and the oral mucosal layer. After successfully anchoring themselves, these bacteria digest acids in the mouth for energy. This stops the pH of your mouth from dropping too much while providing Actinomyces with an energy source.

Bifidobacterium

This genera of Actinobacteria lives in the human gut. Generally, Bifidobacterium is considered a probiotic genera. These bacteria promote the development and maintenance of a healthy immune system in their host. When metabolizing food, Bifidobacterium produces acetate as a byproduct which is toxic to many pathogenic bacteria. This way, it is able to control the spread of pathogens in the gut.

The initial colonization by Bifidobacterium is supplemented by breast milk. Infants fed with breast milk have been observed to have much more diverse populations of this genera when compared to formula fed children. Just like Actinomyces does, Bifidobacterium utilizes pili to hold onto the gut mucosa.

Conclusion

The phylum of Actinobacteria plays a critical role in maintaining a stable microbiome. Without it, our gut bacterial populations would not be able to thrive and pathogenic organisms would plague us all. Only though the regulatory mechanisms of Actinobacteria is this symbiosis possible. Research into this phylum is currently limited but it is clear that without the help of Actinobacteria, the more prominent Bacteroidetes, Firmicutes, and Proteobacteria would have a much harder time thriving in our gut.