For bacteria, the term "obligate parasite" can have at least two distinct meanings.
- A parasite that naturally only reproduces within a host and cannot be artificially cultured on cell-free media. Included are all the viruses, most bacterial endosymbionts, and bacteria such as Treponema pallidum, Mycobacterium leprae, the rickettsiae, and the chlamydiae.
- A parasite that naturally only reproduces within a host, whether or not it can be artificially cultured on cell-free media. A large number of bacterial parasites fall into this category, e.g., Pneumococci, Group B strep, gonococci, H. influenzae, mycoplasmas, etc.
In my experience, the prevailing definition for obligate parasite is # 1. A number of people whom I asked said that they call pathogenic bacteria that can grow on artificial media “facultative parasites” rather than "obligate parasites."
What brings this up? A recent paper purports that obligate parasites have genomes smaller than 1Mb. Mycoplasmas and others that can be cultured in cell-free media fall below this arbitrary threshold; thus, they are not obligate parasites by the first definition and are by the second one. But the second definition includes many organisms with genomes much larger than 1 Mb. Obligate parasites and free living organisms can therefore not be divided on the basis of genome size.