And those are? Commensal bacteria (that live in our skin, gut, mouth, etc) are generally beneficial and we live in balance with them... When they are in the right place. In the wrong place they can lead to rapid overproduction and death by sepsis/septic shock. There are no "good" bacteria or evil ones - bacteria lack a moral code.
Yes. While we live in harmony with many bacteria and actually couldn't survive without them, if they get into the wrong area they can cause sepsis. E coli for example is in our gut and helps us break food down into smaller pieces so it can be absorbed. If the bowel perforates however, e coli can kill us by getting into the blood and peritoneal cavity.