Martin Lercher of the University of Bath in England and his colleagues studied a benign strain of Escherichia coli to see how the common intestine dweller picked up new parts for its metabolic network--the internal system of chemical reactions that produce the necessities of life, such as amino acids or cellular structures. By comparing E. coli to its ancestor Salmonella the geneticists found that the former had only one new gene that likely came about through mutation. This means that in the roughly 100 million years since the two organisms diverged only one instance of a copying error led to a significant difference.
But the bacterium had as many as 32 genes in its metabolic network that it had pirated from near relatives, including those that enable it to defend itself against antibiotics. The team revealed that the bacteria do this by using a process known as horizontal gene transfer, in which a cell passes genetic information to another cell that is not its offspring. It is the bacterial equivalent of sex, with two cells sidling up to each other and engaging in conjugation, or the swapping of genes, except that they do not have to wait for progeny to see the benefits. Lercher and his collaborators determined that the genetic material most likely to be exchanged had to do with handling new environments or adding on new functions, rather than affecting E. coli's core processes, such as growth.
Published in the current issue of Nature Genetics, this is the first comprehensive investigation into how a bacterium's metabolic network changes over time. It suggests that scientists studying such evolution must account not just for mutations but also how the environment and a bacterium's relatives play a role in changing its abilities, possibly making it harmful. "Bacteria feel pressure to change in response to a changing world and they react by stealing genetic information from other, better adapted types of bacteria," Lercher says. "Bacteria are just as lazy as humans: why invent the wheel twice if someone else has already found a solution to your problem?"