When bacteria enter the body, they have a great deal to overcome to colonize the colon. First, they must survive harsh environments with very few salts without bursting (unlike human blood cells within water). Then, they navigate through saliva enzymes and stomach acid, bypass our immune systems within the small intestine, switch from being exposed to oxygen to having none at all, and hang on so that they’re not flushed away by the gut’s constant outward flow.