What is bacteria made of and how is it used in the environment?

The bacteria that is in our bodies I do understand some help us and some can hurt us please explain to me.

What is bacteria made of and how is it used in the environment?
Bacteria are prokaryotic, which means no membrane bound organelles. They are unicellular organisms that are comprised of a cytoplasm surrounded by a plasma membrane like all cells plus a number of other bacteria specific structures like a peptidoglycan cell wall. They have a circular genome and may also contain extrachromosomal DNA in the form of circular structures called plasmids. Bacteria are everywhere. In (and on) the human body, they serve to help us produce vital nutrients from food such as Vitamin K and help to keep potentially pathogenic bacteria in check so that we do not develop infections among others. The majority of bacteria do not harm us. However, some, especially Gram-negative bacteria, possess endotoxins in their outer membrane. When your body destroys these bacteria through lysis, these toxins are released thus triggering an immune response, which in some cases, may cause your immune system to attack your own cells destroying them (necrotizing fasciitis is one case where this occurs). Additionally, some bacteria can colonize tissues and use your body as a food source or a place to replicate. An interesting example is Listeria monocytogenes, which invades cells, grows and replicates using nutrients present in the cell, and hijacks the cell's actin machinery to propel itself around the cell eventually breaking out and invading other cells. This process eventually leads to cell death.