The first barrier, and perhaps the most effective, is the skin. After that it depends where the bacteria entered. Infections inside a tooth, for example, are exteremely hard to reach by the body's defenses and will not clear up on their own. The biggest barrier there would be confinement. The infection will likely build up pressure and burst into a new area, often painfully. There are also filters in the body called lymph nodes which trap foreign particles/bodies and can act as barriers to bacteria. Tonsils are an example of lymph nodes.