In short, Bacteria are not considered parasites, because they do not form a sustained host-parasite interaction. Bacteria multiply and consume the host resources, whereas true parasites exploit their hosts in a manner that allows the host to survive and often prosper. read more
There is an argument that since bacteria can grow independently of a host, they aren’t grouped as parasites. But so are many protozoans, helminths and insects. The corpus of parasitic bacteria is so huge that they are studied under a separate field called ‘bacteriology’. read more
While some bacteria are parasites, not all bacteria are. Neither are all parasites bacteria. Parasites can be any organism that uses another as a host, and sometimes that parasitic organism is bacteria. It depends on the life cycle of the parasitic organism and how it uses the host. read more