There are many different types of oral bacteria, and they can be divided into two main categories: pathogenic bacteria and commensal bacteria. Pathogenic bacteria are those that cause disease, while commensal bacteria are harmless and often helpful. Some of the most common pathogenic oral bacteria include Streptococcus, Staphylococcus, and Candida. These bacteria can cause a variety of problems, including dental caries, gum disease, and bad breath. Commensal bacteria, on the other hand, are often responsible for keeping the mouth healthy by competing with pathogenic bacteria for space and resources.