The newest bacterium: Part 2

2 Jul

Myco what?

In the introduction to the article “synthetic genome” article in Science, the authors explain that they had begun work towards this end with Mycoplasma genitalium over 15 years ago. As the name suggests, this bacterium is a human parasitic organism, often found on linings of the urogenital tracts. However, the final, published, “synthetic genome” work used two different Mycoplasma species, M. mycoides and M. carpricolum, neither of which is a human pathogen, but both of which can cause a variety of problems in cattle and goats.

As an environmental microbiologist, I’m not particularly familiar with this genus and I had to ask myself: what’s a Mycoplasma and why use these organisms?

It didn’t take digging too deep to find out that members of the genus Mycoplasma in general lend themselves to this work for several reasons.  For one, they are relatively easy to grow in the lab.  This is important considering that over 99% of bacteria that exist in our world won’t grow on a Petri dish in the lab at all.  The trick to grow these guys in the lab is that they need a source of cholesterol; in nature they would get this from their host.  Secondly, the entire genus lacks a cell wall.  These amorphous little creatures have only a cell membrane, which makes transporting anything in or out of the cell a lot less complicated than if they were to also have a cell wall (like most bacteria).

These organisms are also exceedingly small;  most Mycoplasmas average around 0.3 µm in diameter.  Take my word for it, that’s small even for a bacterium (a typical E. coli cell is about 1.1 µm long by 2.0 to 6.0 µm long). (For those of you unfamiliar with the unit micrometer, µm, here’s a useful site on size comparisons). Of note, M. genitalium is the smallest known free-living life form at around 0.1 µm in diameter.

Being so small has two important consequences: 1.) they grow slowly.  2.) they carry the smallest set of genes (a.k.a. genome) of any known self-replicating organism capable of growth in the lab.  I say “any self-replicating organism” here to specifically exclude viruses which carry even smaller amounts of genetic material.

This first side-effect (slow growth) is a distinct disadvantage when considering other possible biotech applications and is actually the reason the scientist at the J. Craig Venter Institute (JCVI) ended up using different species for the final research.  The second side-effect (small genome) and the fact that Mycoplasmas only have one chromosome are what Dr. Venter’s group utilized to their advantage.

Even as small as the genome of each of these bacteria already is, the authors discovered over the last 10 years that more than 100 of their genes aren’t necessary to grow and reproduce.  This means the scientists could actually eliminate those 100 genes to reveal the most minimal, streamlined set of genes, genuinely the smallest amount of genetic material, which is absolutely essential for survival (as a bacterium on a Petri dish in the lab).

On the final post in this series (Part 3): how did they synthesize this streamlined set of genes for transplant and assemble it into a genome?

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 Gibson, D.G. et al. (2010) Creation of a bacterial cell controlled by a chemically synthesized genome.  Science, 329 (5987): 52-56.  DOI: 10.1126/science.1190719

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