When might commensal bacteria cause pathology and why? (Hint: Location, location, location…)

Commensal bacteria live on our bodies (e.g. on skin) and in our bodies (e.g. mucosal surfaces). These bacteria typically do not cause pathology. When might they cause pathology and why? (Hint: Location, location, location…)

The first line of immune defense is innate immunity. The forefront of innate immune protection comes from barriers; the skin and mucous membranes that are exposed to the environment, the acidity of those areas, and secondary barriers such as the blood-brain barrier all contribute to innate immunity. Often, bacteria or other microbes that don’t cross these barriers are tolerated (e.g. bacteria in your gut). Breakdown of the barriers provides a route of entry for the “commensal” that can then become a “pathogen”. However, immediate response from the innate immune system occurs immediately through pathogen (aka danger) pattern recognition.