Hundreds of species of bacteria reside in your gut. Some of them are friendly, while others are not.

Most bacteria in the gut belong to one of four groups: Firmicutes, Bacteroidetes, Actinobacteria or Proteobacteria ( 1 , 2 ).

Each group plays a role in your health and requires different nutrients for growth ( 3 ).

The friendly gut bacteria are important for digestion. They destroy harmful bacteria and other microorganisms and produce vitamin K, folate and short-chain fatty acids ( 4 , 5 ).

When the gut flora contains too many harmful bacteria and not enough friendly bacteria, an imbalance can occur. This is known as dysbiosis ( 6 , 7 ).

Both dysbiosis and a reduction in gut flora diversity have been linked to insulin resistance, weight gain, inflammation, obesity, inflammatory bowel disease and colorectal cancer ( 8 , 9 , 10 , 11 ).

Therefore, it’s important to keep your gut bacteria as friendly and abundant as possible.

Without further ado, here are 8 surprising things that can cause harm to your gut bacteria.