Monday, May 11, 2009

(Extremely) Hidden paper: An alternative explanation for the lag phase

Everyone will (should, must) remember talking about bacterial growth phases in their undergrad Microbiology courses.
Here's a typical textbook figure about it.

All of this phases have been typically explained on the basis of adaptation to new media, growth conditions, nutrient deprivation, etc.
In fact, a quick search on the web1 will give you the following explanation for the lag phase (which is similar to what is generally found in textbooks):
During lag phase, bacteria adapt themselves to growth conditions.
[Bacterial growth]

This adaptation is evident when cells are transfered from one type of media to another, and this 'lag' in growth is associated to a physiological adaptation to the new media (synthesis of enzymes that will be useful considering the new substrates, metabolic precursors, etc) before the onset of exponential growth. A good example of this 'adaptation' is diaxuie (a word coined by Jaques Monod).

Well, so I don't exactly remember how I came across with this paper, but it sounds interesting (or at least it made me save it to my computer):

Contact-mediated cell-assisted cell proliferation in a model eukaryotic single-cell organism: an explanation for the lag phase in shaken cell culture

Franck C, Ip W, Bae A, Franck N, Bogart E, Le TT.

Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA.

In cell culture, when cells are inoculated into fresh media, there can be a period of slow (or lag phase) growth followed by a transition to exponential growth. This period of slow growth is usually attributed to the cells' adaptation to a new environment. However, we argue that, based on observations of shaken suspension culture of Dictyostelium discoideum, a model single-cell eukaryote, this transition is due to a density effect. Attempts to demonstrate the existence of implicit cell signaling via long-range diffusible messengers (i.e., soluble growth factors) through cell-medium separation and microfluidic flow perturbation experiments produced negative results. This, in turn, led to the development of a signaling model based on direct cell-to-cell contacts. Employing a scaling argument for the collision rate due to fluid shear, we reasonably estimate the crossover density for the transition into the exponential phase and fit the observed growth kinetics.

Phys Rev E Stat Nonlin Soft Matter Phys. 2008 Apr;77(4 Pt 1):041905. (Now you understand the name of this post?).

The abstract didn't really tell me all that much, and I have kept this article in my computer desktop. Nevertheless, I haven't got around to reading it yet, as I'm swamped studying for my qualification exam. I'd really like it if someone who has read it (of has the time to read it) could comment on it and let us know if we should read it ('if it's worth our time" as a professor once told me).

Why were this guys (Laboratory of Atomic and Solid State Physics) working on this? I wonder...

1 Ok, on Wikipedia. Although not a reliable source for many things, it helps me on the point I'm trying to make here.

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