Moselio Schaechter


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April 23, 2009

Of Terms in Biology: Obligate Parasite

by Elio

Line-in-the-sand

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

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. 


Perhaps Professor Eisen puts it best, if I can modify it a bit: it's not a rule, but a guideline...

As you (Elio) know, I support the ecological definition. I agree with your assertion however that with either definition we need to not draw the line between free living and obligate parasites at some specific genome size. In the end, assertions of such dramatic divisions rarely hold up in biology and we clearly should not have implied this was a "rule" as much as a semi-trend ...


This is a headscratcher, Elio. I remember reading a review paper by Mortimer Starr that spent 25 or so pages debating the difference between predator and parasite and parasitoid (as it related to my little friend Bdellovibrio). And it is certainly possible to find mutants of absolutely host dependent organisms that can grow under laboratory conditions (again, in Bdellovibrio, and those mutations are not simple nor singular).

Then on top of it, we can worry about the "viable but nonculturable" debate. It is possible to have samples of seawater that do not have detectable levels of Vibrio fisheri in them at all (by culturing), but when an aposymbiotic squid is added, voila! Colonization and lots of Vibrio fischeri. Also the work of Lewis and Epstein with providing "micro-micro" cosms to allow environmental microbes that cannot be cultivated in the lab to at last grow. Something similar to this may be in play with pathogens that appear obligate.

The tiny genome issue, however, is tougher to explain away.

I would almost call this Talmudic....

You bring up a very good point, which I hadn't thought enough about - perhaps a part of the definition needs to acknowledge that the obligate parasite needs a resource from the host; not just a place to live but some material support (ex; synthesized or obtained nutrients, isotonic environment for mycoplasmas). An organism like GAS, which has only humans for a host, occupies an interesting place on the continuum; It is obviously pretty good at living in a culture dish (without much in the way of special provisions), but it isn't found in other places naturally, so is it an obligate parasite? Although I understand the preference, I'm not sure it's right. I think the obligate part is right (it has to be associated with a person) but I think the parasite is problematic.

Elio: ah yes, I hadn't thought of that point at all. It does seem convincing.

It's hard for me to see why anyone would not prefer the ecological definition. That an organism can be invited or coaxed to grow axenically may say something, but how could its behavior in the wild not be more important.


Comment on comment

I agree that this definition is conceptually alluring. However, it depends on knowing that a bug does NOT grow outside a host, something that is patently impossible to prove for most.

Elio

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