The New York Times,
May 30, 2006Link
About 5,000 years ago, cattle herders discovered how to make yogurt. Scientists are now discovering that they also mounted an unplanned experiment in evolution.
Certain species of bacteria are responsible for turning milk into yogurt. As the microbes feed on sugar in the milk, they produce acidic wastes that cause the milk to clot. They also make the yogurt uninhabitable for other bacteria, preventing it from spoiling. And by breaking down some milk proteins into smaller pieces, they give yogurt its distinctive flavor.
Americans bought 2.7 billion pounds of yogurt in 2004. To better understand the bacteria that drive that business, scientists have begun sequencing their genomes. A team of French scientists published the genome of a common inhabitant of yogurt, Lactobacillus bulgaricus. Their research appears in the current Proceedings of the National Academy of Sciences.
The study reveals a microbe in the midst of adapting to a new environment. ''We think it may have originally been bacteria that lived on plants,'' said Maarten van de Guchte of France's National Institute for Agricultural Research, the lead author of the study. Its genome contains a number of genes for breaking down plant sugars.
Most of those genes appear to be broken, however. Mutations to the genes prevent the bacteria from reading their codes and making the corresponding proteins. Lactobacillus bulgaricus has 270 such broken genes, known as pseudogenes, in its genome. ''It was surprising to find that many,'' Dr. van de Guchte said. The microbe's genome has only 1,562 working genes. It also shows signs of having lost many other genes altogether.
This transformation may have begun when an ancestor of Lactobacillus bulgaricus fell into someone's fresh milk. ''Maybe somebody was curious and tasted it and thought it was O.K.,'' said Dr. van de Guchte. ''And then he noticed that he could keep the milk longer because it wasn't spoiling.''
The bacteria in the yogurt gradually adapted to this new home. Milk is rich in the sugar lactose, but lacks other sugars. As a result, mutations that destroyed the ability of Lactobacillus bulgaricus to digest plant sugars did no harm to its reproductive success.
It also lost the ability to make many of the building blocks of proteins, known as amino acids. Instead, it split milk proteins and absorbed the amino acids.
Lactobacillus bulgaricus not only lost genes, but it gained others. A large section of its genome appears to have been acquired from a different species of bacteria. Dr. van de Guchte said that more research was necessary to determine what these new genes did for the bacteria.
Howard Ochman, a professor at the University of Arizona who was not involved in the study, observed that bacteria that have adapted to living inside host cells also show the same trend. These include pathogens that cause diseases like dysentery, as well as beneficial bacteria that supply insects with amino acids.
''This fits in the common scheme of things,'' he said. ''The bacteria are getting accustomed to us, and they're getting rid of things they don't need.''
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