Of Terms in Biology: Obligate Parasite

by Elio
 

Drawing a line in the sand? Source

For bacteria, the term "obligate parasite" can have at least two distinct meanings.

1. 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.
2. 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.
 

 
 

 
 

Comments

[Peter Bonventre]

Elio, I have taught my medical and graduate  students your definition (i.e. those pathogens  that can be cultivated) as facultative intracellular parasites. One nuance slightly different  however is that those pathogens  causing acute infectons like the pneumococcus and the streptococci I differentiated from those causing chronic infections like the TB bacillus and others which live primarily in macrophages as causing chronic infections more difficult to deal with.