Jun 4, 2007

Actinomycetes and Actinobacteria

For some people, Actinomycetes and Actinobacteria are the same thing. Try and search Wikipedia with the word "actinomycete", and you will be redirected to the Actinobacteria page. And the first sentence in the definition is: "The Actinobacteria or Actinomycetes are a group of Gram-positive bacteria with high G+C ratio". OK, that's not authoritative enough for you. Then go to the NCBI Taxonomy Browser and search for "actinomycetes". As a result, a single entry is displayed: Actinobacteria (class). If you dig further into the Actinobacteria (class) entry, you see this:

That's it, Actinomycetes and Actinobacteria (class) are synonyms! So, if we search for "actinobacteria" at the NCBI Taxonomy browser, we will arrive to the same "Actinobacteria (class) = Actinomycete" entry, right? Nope! We come to a different place called: "Actinobacteria, phylum, actinobacteria". Oh, I see, the "Actinobacteria (class) = Actinomycetes" is just a part of a greater Actinobacteria (phylum).

But, wait, there is just a single class in this phylum! Then, under a practical point of view, this means that

Actinobacteria (either class or phylum) = Actinomycetes

If interested, you may visit the List of Prokaryotic Names with Standing in the Nomenclature and search for both words. It seems that "Actinomycetes" was the old name for the class, and was later replaced by "Actinobacteria" (is this correct?).

So, are we happy now? No! We're not, because other people have no doubt that an actinomycete is "any of various filamentous or rod-shaped, often pathogenic microorganisms of the order Actinomycetales that are found in soil and resemble bacteria and fungi" (TheFreeDictionary.com). Actinomycetales is just one of the 4 or 5 orders belonging to the Actinobacteria. Therefore, Actinobacteria such as Acidimicrobium, Bifidobacterium, Coriobacterium and Rubrobacter are not considered to be actinomycetes. This opinion seems to be shared by authoritative microbiologists (see, for instance, this). In other words:

actinomycetes = Actinomycetales (order)

My two cents? For most practical uses, I guess we might use the words "actinomycetes" and "Actinobacteria" (class or phylum) as synonyms. Further precision will be needed when discussing on nomenclature, taxonomy or phylogeny. Or when talking about those Actinobacteria not belonging to the Actinomycetales (order).

What do you think?

[Note added on July 17, 2007: read more on this topic in a later post]

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