Tuesday, September 25, 2007

Deadly space bugs

It is known that microgravity (MG) affects the way cells react. For example, astronauts coming back to Earth after a long time spent under very low gravity show signs of bone resorption and muscle mass loss. Bacteria react to low gravity too. In a paper published in 2002, Dr. Cheryl Nickerson and her team (see reference and link to pdf file below) discovered that the expression of many genes is in fact affected by MG (Figure on right). As can be seen, some genes are expressedm or not under normal gravity (1xg) but this expression pattern can be almost completely reversed under MG (or LSMMG - Low Shear Modeled Micro Gravity). Because gene expression seems to be influenced by microgravity, the obvious experiment was now to determine if the virulence of bacteria is increased un der MG...in other words are microbes susceptible to become "superbugs" in space? The answer, sadly, seems to be YES! In another study, to be published in PNAS, Dickerson and fisrt author James Wilson show that some virulence genes are in fact turned on by microgravity. In a mere 12 days in september, during spaceflight STS-115, Salmonella tiphymurium became more virulent. According to the authors, the shape of bacteria did not change but they seem to form a biofilm which is more difficult to eliminate by the immune system. In fact, when these "spacebugs" were fed to mice, they show a 3-times increase in virulence. Space is definitively a weird place to be...even for bacteria! Astronauts beware...bring your Purell!

For audio of this story follow this link


1) James W. Wilson, Rajee Ramamurthy, Steffen Porwollik, Michael McClelland, Timothy Hammond, Pat Allen, C. Mark Ott, Duane L. Pierson, and Cheryl A. Nickerson. Microarray analysis identifies Salmonella genes belonging to the low-shear modeled microgravity regulon PNAS 2002 99: 13807-13812

2) Wilson et al. Space flight alters bacterial gene expression and virulence and reveals a role for global regulator Hfq. PNAS doi/10/1073/pnas.0707155104.

Monday, September 24, 2007

Are we evolving?

Bacteria evolve right before our eyes. Hospital acquired diseases are a very good example. Overexpose bacteria to some antibiotic, toxins, metals etc...they will change in a matter of days, adapting to their new environment. Unicellular organisms have absolute freedom to change because their own evolution, as individuals, can only benefit the population. If they fail to adapt, some individuals will be eliminated BUT those who are successful will become the ancestors of a resistant colony.

Why is it that this principle can not be applied to our own cells? Simply put, because the fate of our own cells is interconnected, a small change in the genome could mean disaster for the other ones. The reason why our own cells do not evolve in our lifetime (under normal circumstances) is discussed in an article by Pepper et al., soon to be published in PLoS computational biology. An article on the Nature website (september 21, 2007)explain why evolution within our own cells is unlikely and not desirable. Our tissues simply DO NOT evolve!

It is known that epithelial tissues have a rather high turnover...old cells are replaced on a constant basis. The speed at which they grow could suggest that these cells are more prone to mutations. This is not the case. As Philip Ball explains in its news feature (Nature): "Why a person doesn't evolve in one lifetime" epithelial cells take a long walk on the way to differentiation. Epithelial stem cells divide just a little before they commit to their final state. Mutations can occur within these cells but since they do not compete against each other because these mutations make differentiation of these stem cells more difficult.

On the other hand, the immune system is made to evolve, it has to adapt to new pathogens every single day but there is a price we pay for this: a higher incidence for cancer!

Reference: http://www.nature.com/news/2007/070917/full/070917-11.html
Image source: http://www.digitalapoptosis.com/archives/science/cells2.jpg