In response to a list-member's question about the names and strains of bacteria in our yogurt culture, Seth writes:

I've noticed some confusion over the use of the word strain for bacteria. Here is a primer for anyone confused over bacteria and strains etc. Naming bacteria can be a bit fuzzy.

Let's look at lactobacillus acidophilus DDS-1 as an example.

The 'Lactobacillus' part of the name is the genus or family that this bacteria belongs to. It is related to lactobacillus bulgaricus, lactobacillus plantarum, etc. It is not related to Streptococcous.

The 'acidophilus' part is the species of bacteria. Typically, all bacteria called Lactobacillus acidophilus will behave similarly and have very similar genetics. In this example, acidophilus means "acid loving".

The part of the name is given to a specific strain of L. acidophilus. All L. acidophilus DDS-1 bacteria have the same genetic makeup and behave in the same way. They are identical clones of each other.

What gets even trickier, is that sometimes scientists will include the same bacteria in totally different families, because the bacteria may act differently under different conditions.

I hope this helps and didn't confuse anyone further.

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Genus: "The bacterial genus is usually a well-defined group that is clearly separated from other genera, and the thorough descriptions of genera in the 1984 edition of Bergey's Manual exemplify the depth to which this taxonomic group is usually known. However, there is so far no general agreement on the definition of a genus in bacterial taxonomy, and considerable subjectivity is involved at the genus level. Indeed, what is perceived to be a genus by one person may be perceived as being merely a species by another systematist."

Species: "A bacterial species may be regarded as a collection of strains that share many features in common and differ considerably from other strains.One strain of a species is designated as the type strain; this strain serves as the name-bearer strain of the species and is the permanent example of the species, i.e. the reference specimen for the name. The type strain has great importance for classification at the species level, because a species consists of the type strain and all other strains that are considered to be sufficiently similar to it as to warrant inclusion with it in the species."

Taken from http://www.socgenmicrobiol.org.uk/QUA/gest.pdf

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