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June 8, 2000

Recent revelation about bacteria's evolution could affect thinking on how higher organisms evolved

Recent revelation about bacteria's evolution could affect thinking on how higher organisms evolved

How do bacteria evolve? Primarily, through "lateral gene transfer" — stealing useful genes from other organisms — according to a paper co-written by a Pitt faculty member in the May 18 edition of the journal Nature.

Pitt biological sciences professor Jeffrey G. Lawrence and colleagues Howard Ochman of the University of Arizona and Eduardo A. Groisman of the Washington University School of Medicine studied 20 species of bacteria and found extensive, recent lateral gene transfer in each species.

Scientists have known for years that gene-swapping was one method of bacterial evolution. But until now, it was assumed to be secondary to mutation of DNA already present in the bacteria, Lawrence said.

Bacteria can acquire gene sequences through conjugation (physical contact between donor and recipient cells), transformation (in which bacteria pick up DNA that is no longer part of another organism) and transduction (in which a bacterial virus has replicated itself in the donor microorganism and packaged random DNA fragments).

Conjugation and transformation enable bacteria to acquire gene sequences not only from distantly related microorganisms but also from plants and animals.

Take salmonella bacteria. By obtaining the right genes from higher organisms, this opportunistic pathogen gained the ability to steal vital minerals from where it resides — our guts.

"Non-bacterial organisms hold on to their iron, calcium and magnesium very tightly," Lawrence said. "One of the ways that salmonella became a pathogen was by obtaining genes from other organisms that enabled salmonella to efficiently steal iron and other minerals from its host environment."

Pathogens such as salmonella and E. coli don't typically result from a benign bacteria slowly evolving into a harmful one through a vertical (parent-to-offspring) transfer of genetic material, according to Lawrence.

"Instead, the way that one of these pathogens evolves is that genes are introduced into it from other species all the time, and it happens to arrive at some [genetic] combination that allows it to do a particular job well" — a job that happens to result in sickness or death among humans.

The rapidity with which bacteria pick up new genes explains how pathogenic bacteria can quickly become resistant to antibiotic drugs, Lawrence said.

"Bacteria have been doing gene exchange for billions of years. It's how they diversify," he said. "So, it's not surprising that if any organism develops a protein that allows it to become resistant to an antibiotic, that protein can spread like wildfire through numerous bacterial species."

Why can't geneticists tinker with pathogenic bacteria and render them harmless to humans?

"You're underestimating the population size of the organisms we're talking about," Lawrence explained. "There are billions of species of bacteria. There are 10 to the 10th or 11th power species of bacteria per milliliter in your gut content alone."

Chromosomal evolution in bacteria is too esoteric a subject for most people to appreciate, but it does raise larger questions, Lawrence said.

"Ten years ago, scientists thought that lateral gene transfer was a fairly low impact process in bacterial evolution," he said. "We now know that it's a very high impact process. That raises the question, are we likewise in the dark as to how higher organisms evolved? How important was this [lateral gene transfer] process in the evolution of higher organisms?"

— Bruce Steele


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