Constantly attacked by immune system would make me a pathogen, too.

This week, a new study published in PLOS Pathogens reported how otherwise benign, commensal E. coli can evolve before our eyes into a malicious pathogen.  In this study, researchers cultured E. coli with the mouse immune cells that essentially eat them when found in the body; the macrophages. The co-culture was diluted daily. This would give the bacterial cells a constant source of fresh nutrients. Under constant threat of attack by the immune cells, the E. coli were under ‘continual selective pressures’.  A few days of these living conditions caused positive selection of genetic mutations that now allowed the bacteria to evade macrophages. Seven new types of E. coli were identified through this screen and the genomes were sequenced to find the sources of the mutations.

Here are some problems…

E. coli were continually in contact with macrophages in this study. This is very unrealistic and the authors acknowledge this, saying:

We note that in the context of a real infection repeated contact with macrophages will not likely occur with a similar period as the one in this experimental setup.

Another quick problem noticed is that these bacterial cells were growing in an otherwise optimal environment; nutrients were replenished everyday. Again, this is not going to happen in nature. To put this into focus, it would be like winning the lottery jackpot every drawing for a month. In other words, it is not likely.

Under these experimental conditions, researchers were able to force these bacteria to change or die. As in every other challenge faced for over 2 billion years, the E. coli rose to the challenge and did so quickly.

So, congratulations to the authors. You did the expected and got the expected. But hold on; this study did do something really nice. It allows researchers to see the kind of mutations occurring that allow seemingly nice bacteria to become less so. Yet again, we learn from bacterial superstar, E. coli.