Field of Science

Salmonella in space

A paper I've been waiting to see finally appeared, several days after the news articles about its findings. (Larry Moran has a post today about PNAs's wicked practice of letting the media gush about results of papers that aren't yet available.)

Space flight alters bacterial gene expression and virulence and reveals a role for global regulator Hfq. J. W. Wilson et al. (many authors), PNAS Sept. 27 2007. Link

Here's what the news reports said:
"The researchers found 167 genes had changed in the salmonella that went to space..."

Dr. Cheryl Nickerson, the PI on the project, is quoted (misquoted?) as saying "These bugs can sense where they are by changes in their environment. The minute they sense a different environment, they change their genetic machinery so they can survive."

This left me thinking that the genotypes of the bacteria had consistently mutated (cosmic rays) in a way that made them more virulent. From an evolutionary perspective this seemed very improbable - both in having similar mutations arise in many bacteria and in having the bacteria direct their mutations to fit their changed environment.

Now I read the paper I discover that the genes hadn't changed at all. As the title indicates, what had changed was the expression of some genes (some turned up and some down) - this is a transient response to the altered culture conditions in space, not a genetic change. The cells in the culture that had been in space were more clumped together, in what may be the zero-gravity broth-phase equivalent of a biofilm. These bacteria caused more serious disease when injected into mice, probably because the clumping made it harder for the mouse immune system to kill them.

1 comment:

  1. Although 167 genetic changes surely did not occur it is entirely possible that a few changes did occur, which then resulted in the expression changes. The bacteria were able to grow while they were there so evolution could have occurred in that amount of time. If the zero gravity environment is similar to that of a mouse (and the authors mention that this is the case in the paper although in a different context) then it is possible that adaptations to the space environment pre-adapted the cells to the mouse. But their claims of the space environment sending a "signal" to cells telling them to change gene expression ignores evolution.


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