All of us carry Clostridium bacteria in our gut as they are a normal component of a healthy digestive microbiome.

Clostridia seem particularly sensitive to diet, and although they can be abundant in the guts of obese people, their levels quickly taper off when faced with fat-controlled weight-loss diets. Moreover, ulcerative colitis patients show decreased levels of Clostridia and increased levels of other, typically less common, bacteria.

Despite their ubiquity, some Clostridia present as opportunistic pathogens--eg, C. difficile can cause difficult-to-treat infectious diarrhea. Children with autism appear carry higher loads of Clostridium bacteria in their guts as well.

• PMID 20879895

Some species in this genus will cause diarrhea in animals.

• Jandhyala, S. M. et al. Role of the normal gut microbiota. World J. Gastroenterol. 21, 8787–8803 (2015).