Not all bacteria cause disease Over the last hundred years or so, people have grown used to associating bacteria with human or animal disease. However, not all bacteria cause disease. In fact, most bacterial species cannot cause disease.
Many species even play beneficial roles in, say, producing antibiotics and foodstuffs.
The soil teems with free-living bacteria that perform many essential functions in the biosphere, e.g. nitrogen fixation. Our bodies are covered with commensal bacteria that make up the normal flora Even among bacteria that are capable of causing disease, few, if any, species are always pathogenic. For these reasons, great care must be taken in interpreting the results of microbiological investigations. Isolation of potentially pathogenic bacteria may or may not be indicative of infection. Mark Pallen 1995.