Clostridioides difficile

Clostridioides difficile, formerly called Clostridium difficile, is a bacterium. It is a bacillus, meaning it is rod-shaped, and it is gram-positive, meaning it stains purple with the Gram stain. It is also an obligate anaerobe, meaning it grows only where there is no oxygen, and it forms spores, a dormant form of the bacterium itself. It is the leading cause of antibiotic-associated diarrhea.

The curious part is that it can live in the gut of healthy people without making them sick. Think of the gut microbiome, the community of bacteria that live in the intestine, as a garden full of plants. If an antibiotic pulls up plants, the weeds that were already hiding among them grow. That is what happens here: the antibiotic can disrupt the balance of the microbiome, and that allows C. difficile to multiply and cause infection.

Then there are its toxins, A and B. Once they get inside colonocytes, the cells of the colon, they shut down cell pathways that depend on Rho proteins. The colonocytes are damaged, the tight junctions, the seals that hold each cell to its neighbor, are disrupted, and the colon becomes inflamed: that is colitis.