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Bacteria species - virus species


I was looking into evolution theory.
Apparently, the definition for species for multicellular (and 2 genders) lifeforms is somewhat fluid, somewhat vague, there are a few definitions. The popular one seems to be that when 2 groups can’t produce children, then it is a new species.
If a horse and a donkey can reproduce together, and the result is the mule, then the mule is counted as a separate species, the horse counts as a separate species and the donkey as well.
Perhaps I misunderstood that part.

There is also the case of the tiger (male) and lion (female) producing a tigron. Also, there is the case of the lion (male) and tiger (female) producing a liger. I think it said that the male liger can’t produce working sperm but the female liger is fertile and can produce a kid with a male lion.

Anyway, my main question was about bacteria and viruses.
What is consider a species? When is a new species produced?

For example:
“Bacterial systematics has not yet reached a consensus for defining the fundamental unit of biological diversity, the species. The past half-century of bacterial systematics has been characterized by improvements in methods for demarcating species as phenotypic and genetic clusters, but species demarcation has not been guided by a theory-based concept of species. Eukaryote systematists have developed a universal concept of species: A species is a group of organisms whose divergence is capped by a force of cohesion; divergence between different species is irreversible; and different species are ecologically distinct. In the case of bacteria, these universal properties are held not by the named species of systematics but by ecotypes. These are populations of organisms occupying the same ecological niche, whose divergence is purged recurrently by natural selection. These ecotypes can be discovered by several universal sequence-based approaches. These molecular methods suggest that a typical named species contains many ecotypes, each with the universal attributes of species. A named bacterial species is thus more like a genus than a species.”

I noticed the Long term E.Coli experiment. I think it is at the 60,000 generation now. There has been a series of mutations that has occurred. If I remember correctly, 20 mutations. 1 of them involves a gene duplication followed by a change of a nitrogenous base and this helps the E.Coli process citrate anions.

Would this be considered a new species?

There is another term: strain.
I read the wiki page. It seems to be a nearly meaningless word.

The wiki page

“As a rule of thumb, microbiologists have assumed that kinds of Bacteria or Archaea with 16S ribosomal RNA”

What’s a 16S? Where is this RNA located? Why is 16S significant?

You have asked a large number of questions, most of which are outside of my knowledge.  However, I can help you with one or two things. "S" stands for a Svedberg unit, and is related to how quickly something is centrifuged.  It is positively related to molecular weight, but Svedberg units are not additive.  16S RNA is one of several kinds of ribosomal RNA, which are integral to the structure and function of bacterial ribosomes.  16S rRNA is a portion of the 30S subunit of a ribosome.  Look up Carl Woese's work for its significance, which is considerable.

I cannot give you a textbook definition of "strain," but I can point you to a review article that might be helpful in understanding its utility:
Li W et al., "Bacterial strain typing in the genomic era" FEMS Microbiology Reviews, Volume 33, Issue 5, September 2009, Pages 892–916,


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