Study Confirms E. Coli Evolution Via Natural Selection
Jason Mick (Blog) - June 17, 2010 4:45 PM
A 3D-rendering of the E. Coli bacteria (Source: JEFF JOHNSON Hybrid Medical Animation)
Stained E. Coli bacteria (Source: LBL)
GroEL and GroES, E. Coli Heat Shock Proteins (HSPs), visualized (Source: Tulane University)
The Chaperonins play a critical role in bacterial heat shock systems (see the bottom left). (Source: Biomol)
Results offer important insight into evolution, could lead to hardier genetically engineered animals
Natural selection -- the theory that organisms retain favorable traits by surviving and reproducing and conversely removal of unfavorable traits by the opposite effect -- is thought to play a critical role in evolution. As just a handful of modified proteins or protein levels can cause profound anatomical and physiological changes, natural selection may in part have helped mammals to evolve from early reptiles, a road which would eventually lead to ancient hominids and, finally, man.
A new study has provided one of the first compelling examples of what appears to be natural selection in a controlled laboratory setting. Researchers at the Technische Universitaet Muenchen's (TUM) ("Technical University of Munich") Department of Chemistry and Center for Integrated Protein Science Munich, Germany, carried out the study.
They began by taking a single E. coli bacteria, the type of bacteria that resides in the human gut. E. coli typically enjoys life at our pleasant body temperature of around 37 degrees Celsius.
They then slowly turned up the heat on the bacteria, inducing heat shock. Three lines of bacteria, propagated from a common ancestor and living under heat shock conditions were cultured over hundreds of generations. Wild-type E. coli can only survive up to 44 to 46 degrees Celsius. But by carefully bumping up the temperature over two years to 48.5 degrees, the E. coli developed adaptations that allowed them to survive.
A control line of bacteria was also derived form a common ancestor and kept at 37.5 degrees Celsius. All of the bacteria were kept isolated from their sister lines and from any other outside bacterial or viral contaminants (to prevent accidental DNA transfer).
By analyzing extensively the proteins in the adapted cells, the researchers found they only had one key difference -- elevated levels of molecular "chaperone" proteins GroEL and GroES -- intracellular machines that can stabilize folding proteins under stress. Mutations were shown to have allowed for increased transcription of these proteins.
All three lines showed elevated levels of protein -- with an average of a 16-fold increase from the control line. The GroE proteins appear to play a unique role in surviving hotter conditions. Interestingly the study also confirmed the idea that natural selection is a game of tradeoffs -- by increasing the level of heat shock proteins, the cell consumed valuable resources. While this allowed it to survive, it came at the cost of reduced growth.
The changes were shown to persist for over 600 generations -- indicating that E. coli that lost the changes likely died of heat shock and were likely selected out.
Dr. Jeannette Winter, who helped lead the research says that the results not only provide evidence in support of the theory of natural selection, and the broader theory of evolution, but also could yield genetic engineering advances. She states, "The correlation between genetic changes and chaperones has been shown not only in bacteria, but also in eukaryotes such as yeast, fruit flies, and fungi. Better understanding of chaperones might also open the way to targeted generation of organisms for specific purposes -- enhancing their ability, for example, to live under stressful conditions, to break down harmful pollutants, or to produce specific, biotechnologically relevant proteins."
The study is published here in the June 18 edition of the Journal of Biological Chemistry.