Our Microbial Frenemies

So, what do we know about the microbiota; specifically, our microbiota? Firstly, we know that the microorganisms occupying our gut do not cause disease, under normal circumstances. Secondly, we know that they can provide us with a multitude of benefits, such as helping us digest complex organic molecules, and preventing invasion of foreign microbes by directly competing for resources and keeping the immune system stimulated. These are just a few of the advantages our microbial allies provide us. However, that is not to say that they pose no danger to ourselves either. Typically, these microorganisms are categorised into being in a beneficial, pathogenic or commensal relationship with its host.

Beneficial microbes, or probiotics, are as the name suggests: these microbes typically provide some form of health benefit to the host and are usually non-pathogenic. Many of the bacterial species found in our gut lumen, for example, have the capability to digest cellulose. As such, without these microbes, digesting vegetables would be a much harder and less rewarding task. Most of the probiotics found in our microflora are of lactic acid bacteria origin and are most common in diets that incorporate fermented dairy products (12). Pathogenic microbes, on the other hand, mostly describe microbes of foreign origin. These microorganisms will infect and exploit the host’s cells, ultimately causing disease. Commensal microorganisms walk an interesting line, in comparison to beneficial and pathogenic microbes. This group of microbes encompasses all of the characteristics described above, depending on circumstance. This ranges from benefiting both the host and microbe, the microbe itself, or even causing disease within its host when given the opportunity. An example of a commensal microorganism is Escherichia coli, or E. coli. It is a bacterium that colonises our gastrointestinal tract as soon as we are born, where it fends off more than 500 competing bacteria species, thanks to its versatility and adaptations to our gut environment (13). Furthermore, the presence of E. coli along our gut epithelium helps to stimulate mucin production, inhibiting any foreign microbes from invading the epithelium (14). However, as is typical of a commensal organism, when given the chance, E. coli is capable of causing intestinal or extraintestinal disease in our bodies. Urinary tract infections due to E. coli are among the most common causes of a microflora-associated infection and often occur when the bacterium is allowed to enter the urinary tract via cross contamination with the anus, where E. coli is typically shed as part of the faeces (15).
​
Typically, these beneficial and commensal bacteria are found all over our body. They can be found in our hair, on our skin, and as we have discussed, in our gut. Malassezia, for example, is a fungus that colonises our scalp, and is what causes dandruff in most people. While dandruff may be a nuisance to those who experience it, do the disadvantages necessarily outweigh the benefits? The presence of Malassezia on our scalps means that other, possibly dangerous, microorganisms will have to compete with Malassezia in order to invade. Additionally, the stimulation of our body’s defenses due to Malassezia aids in repelling foreign invaders (16). Staphylococcus aureus is another example of a commensal microbe, and an even better example of an opportunistic pathogen that can be found living harmoniously on our skin and nasal passages, helping us fend off other competing microbes just as Malassezia does on our scalp. However, when the skin is pierced, whether by means of injury or even medically through surgeries or treatments, the Staphylococcus bacteria will opportunistically attempt to invade and infect its host (17). As such, Staph infections and outbreaks are among some of the most common forms of hospital-related infections (18).