What is an example of a symbiotic bacteria in humans?

Could you please elaborate on an instance where symbiotic bacteria exist within human beings? I'm intrigued to know about the specific types of bacteria that coexist with our bodies and how they contribute to our overall health and well-being. It would be insightful to hear about the mechanisms through which these bacteria interact with our systems and the benefits they provide. Additionally, are there any potential drawbacks or risks associated with these symbiotic relationships?