As I mentioned earlier, the best known (and best studied) microbes are the pathogens – those that cause illness in the host organisms they invade. But let us not forget about our unsung heroes, the commensals. These microbes are (usually) more friendly to their host, using a microscopic region of the host’s body as their habitat. And make no mistake – we are covered in them. You have about 10 times as many commensals living on you as you do body cells. Commensals are also the first line of defense against pathogenic microbes. If a few pathogens try to find a parking space on your body (and they will!) fear not – because your commensals have already taken up residence, filling up most of the available niches and habitats. Pathogens usually don’t last long because they have trouble wedging their way into the competition. Now sometimes pathogens can get past this first line of defense and start finding a means to flourish and reproduce within your system. That’s when you get sick and your immune system becomes active, with specialized cells such as T-lymphocytes and macrophages leading the charge.
Commensals and their hosts can usually live together in harmony – a type of symbiosis where both host and microbial agent benefit. But this level of mutualism is a delicate balance. How does a relationship between a commensal and its host become adversarial? This question will be explored in an upcoming post – stay tuned.
