How Benign Bacteria Evolve into Virulent Pathogens

Bacteria can evolve quickly in order to adapt to environmental change. But what happens when the "environment" is an immune response from an infected host? Relatively harmless bacteria can quickly turn into virulent pathogens. Now, scientists have taken a closer look at this evolutionary process in order to better understand the creation of pathogens.

In order to better understand and study these pathogens, the scientists created an experimental system to observe and study the evolution of bacteria in response to encounters with cells of a mammalian immune system.

"Escherichia coli bacteria show an extraordinary amount of diversity: Many are benign commensal bacteria, but some are deadly pathogens," said Isabel Gordo, one of the researchers, in a news release. "It is thought that many strains of E. coli that cause disease in humans evolved from commensal strains. We thought that experimental evolution would be a powerful tool to directly observe some of the steps E. coli may take in the transition from commensalism to pathogenesis."

The scientists examined initially benign E. coli bacteria that were continuously confronted with macrophages, which are part of our immune system and can swallow and digest bacteria. The researchers grew a mix of bacteria and macrophages in a liquid culture. Once a day, they diluted the mix and every other day they took a sample of the bacteria for analysis.

So what did they find? On day four, bacteria exposed to macrophages began to show changes in their appearance. In fact, the scientists found that these bacteria were more resistant to being digested by macrophages than the ancestral strain, and the mucoid variant was less likely to be gobbled up. They also found that these changed bacteria had the increased ability to cause disease.

The findings reveal a little bit more about bacteria evolution. More specifically, they show how bacteria could become far more deadly over time.

The findings are published in the journal PLOS Pathogens.