How E. Coli Evolves To Adapt To Changing Acidity

Dr. James Hicks, Editor in Chief of PBZ, explains the momentum behind this collection of papers: "This exciting approach -- experimental evolution -- allows scientists to investigate the fundamental mechanisms of evolution. Prior to the advent of contemporary laboratory techniques, inferences about evolution were based on observation. Now, we can study evolutionary change as it is happening, by selecting organisms that change rapidly, such as the fruit fly or E. coli. This advantage allows scientists to investigate how changes occur and how they affect an organism's individual physiology and overall community."

In the July/August 2007 issue, the first of three issues that will contain articles from the collection, Bradley S. Hughes, Alistair J. Cullum, and Albert F. Bennett (University of California, Irvine) explore the effect on E. coli of fluctuating acidity, an especially important environmental factor for the bacteria.

E. coli may spend hundreds or thousands of generations in the relatively neutral-acidity colon, with brief exposure to the extreme acidity of the stomach and modest alkalinity in the small intestine during colonization of a new host. With modern sewage handling (or mishandling), the bacteria may also experience exposure to the ocean, with a pH near 8.0, before infecting a new host.

To assess how E. coli might adapt to different environmental conditions, the researchers observed four groups of bacteria. One group was exposed to constant acidity (pH of 5.3) and another to constant alkalinity (pH of 7.8). A third group was exposed to randomly fluctuating pH levels, and the fourth was exposed to pH levels that cycled daily between acidic and basic conditions.

After at least 1,000 generations, the researchers exposed the groups to either an acidic or basic environment. The groups exposed to acid or base for the entire period had developed into specialists -- that is, they displayed significant fitness gains when transitioning into their preferred environment.

In contrast, the groups that evolved in variable pH environments exhibited generalist fitness patterns, with neither group having any significant fitness loss in any of the environments. Interestingly, the researchers also found that there was no significant cost to being a generalist at any tested pH level: "Overall, these comparisons suggest that the jack-of-all-trades may be a master of at least some as well," the researchers write.

"What is interesting here is that the complex patterns of adaptation in the various pH regimes were so different among the groups and revealed the first empirical characterization of the intricacies of evolution in response to variable pH," explain the authors. "Plans for future studies include the extension of this experimental evolution system applied to . . . ways in which E. coli may be evolving fitness to survive within the coastal ecosystem or the human host."