Which one among the following is a non-pathogenic bacterium, present in the colon?
A. Balantidium coli
B. Entamoeba coli
C. Enterobius vermicularis
D. Escherichia coli

Hint: The microorganisms including various bacteria inhabit the gut or the digestive system in humans. These colonies of microbes are known as the gut flora. They survive through being in a symbiotic relationship with the human gut. These organisms provide various benefits to the human body.

Complete answer: Escherichia coli or E. coli is the gram-negative stained bacteria present in the colon, which is a part of the large intestine in humans. They are facultative aerobic bacteria that is usually the respiration is through aerobic methods for them, but can also survive by anaerobic respiration when oxygen is absent. These organisms exhibit a symbiotic relationship with the host body, by preventing the pathogenic bacteria from forming colonies in the colon and also by producing vitamin K.
Considering the other options:
-Balantidium coli are a kind of pathogenic microbes causing the disease Balantidiasis. The infection of these species occurs via contaminated water and food. It forms colonies in the cecum and colon human beings. The disease develops mostly in immunocompromised patients.
-Entamoeba coli is also a pathogenic protozoan organism causing Amoebiasis. Resulting in loose stools, abdominal cramps, and stomach pain.
-Enterobius vermicularis is also a pathogenic organism belonging to the pinworm family causing the disease Enterobiasis. These organisms can also inhabit the colon and rectum of humans. The infection is spread by fecal-oral routes.
Thus, from the above-given options, the most appropriate option was found to be D, E coli.

Note: The absence of normal gut flora can result in a variety of problems in human beings. These can cause problems constipation, diarrhoea, heartburn, irritable bowel syndrome. The absence of these gut flora can be restored by taking probiotic food materials, which are rich in these beneficial organisms.