Evolution of E. coli - Beneficial, or Bearing a Growth Burden?

Date Published:  November 27, 2014

Source:  BMC Evolutionary Biology

Summary:  

Comparative studies over time have lead scientists to conclude that Escherichia coli (E. coli) cells grow larger in size, through generations of evolution. Testing this hypothesis required the collaboration of over 20 scientists. The Department of Infection Metagenomics, at Osaka University, completed the genome analysis component of this study.

The concern regarding E.coli cell growth stems from the fact that cell size is oftentimes proportionate to cell propagation. It could also indicate that there is a greater number of DNA replicating mechanisms in the cell, thus resulting in more frequent cell division. All of these aspects indicate that a larger cell size means a larger growth rate. This increased size does not come without a cost. There is a possibility, as other types of cells have demonstrated, that a segment of the DNA controlling cell division is mutated. Mutations can result in different shapes and sizes that prevent the cell from functioning normally; this is referred to as a growth burden.

Is E. coli in fact growing in size? If it is, what are the growth burdens? To answer this, researchers combined a series of tests. Using E. coli cells, the first test was done to explore whether or not an increase in cell size would hinder replication and growth. Using technology, cells that were smaller than their ancestors were carefully chosen. Scientists noted that a small number of mutations in the membrane-production genes would result in size evolution and no corresponding growth conflict. What initially seemed to be conclusive results actually contained some unforeseen bias. The technology used to select the cells did so in small samples rather than in a large population. Therefore, more tests had to be used in corroboration.

To confirm this, a similar test was done with cells that were thought to evolve towards a smaller sized cell. BSKY, a derived version of E. Coli, was used in this series of complex tests. Throughout, researchers employed a method of examining generations of cells, monitoring cell size and its relation to density. The cells thought to behave in this way did so regardless of the cell concentrations. In addition to these tests, genome sequencing was necessary to confirm mutations in genes coding for growth rates.

Results confirmed that a growth disadvantage does not directly link to cell size increase. RE Lenski, one of the key researchers in this study, conducted these experiments. In fact, we also see the significance behind the short time span needed for bacterial cell size to evolve. It is clear that cell size holds an important part in survival rates. A cell is less prone to external attack from protists if they are large and have a thick cell membrane. This is one of the results of evolutionary cell size increase. Evolution of cell size may be another form of natural selection, or survival of the fittest.

The diagram above depicts concentration versus time. ACs, T22Cs, Svr22Cs, and Mld22Cs are sample cells that behave similar to E. Coli. Thus, they were used in the empirical studies. They demonstrate a proportional growth rate despite the fact hat their concentration is increasing. As time goes on, their size has evolved as well, without any visible growth burden.

In conclusion, we can see that bacterial cell size evolution is a quick process that occurs in under 400 generations. Because of logistical limitations, scientists used “an empirical method” to test this. Cells thought to decrease in size demonstrated stringent selection, resulting in smaller subsequent generations. They shrunk without any disadvantage to the cell’s fitness. Thus, it can be pragmatically stipulated that E. coli cells are capable of evolving to become larger, without sacrificing fitness.

Citation:

Mari, Y., Saburo, T., Naoko, H., Shigeto, S., Hideo, M., Bei-Wen, Y., & Tetsuya, Y. (2014). Directed evolution of cell size in Escherichia coli. BMC Evolutionary Biology, 14(1), 104-127. doi:10.1186/s12862-014-0257-1