Bacteria belonging to the microbiota, and therefore considered as commensals, can also become pathogenic if their growth rate raises and if they outcompete other members of the intestinal flora. For bona fide pathogens, variability in the expression of virulence factors has also been observed.

How does commensal become pathogenic?

Particularly in the context of epithelial cells, a commensal bacterium can become pathogenic if there is a damage to the epithelial barrier. The pathogenic bacterium C. difficile can manipulate the epithelial tight junction barrier functions and can invade tissues.

How can a non pathogen become pathogenic?

Some nonpathogenic microorganisms are commensals on and inside the body of animals and are called microbiota. Some of these same nonpathogenic microorganisms have the potential of causing disease, or being pathogenic if they enter the body, multiply and cause symptoms of infection.