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To spore, or not to spore?

http://www.physorg.com/news179521562.html

Surviving harsh environmental conditions is one of the principle barriers to optimizing fitness, or the ability to “pass on” genetic information to the next generation, that bacteria face. Bacteria constantly face “decisions” regarding how to deal with prolonged exposure to extreme (high or low) temperatures, concentration of oxygen, pressure, and pH. One common method by which microbes respond to these stresses is sporulation, or forming a spore. The spore itself is a small, tough capsule in which the bacteria can “pack” some proteins and a copy of its genetic material before the mother cell lyses and releases its remaining DNA and proteins to the environment. Meanwhile, the spores simply wait until the environmental conditions become favorable again, and the spores develop into fully functional bacteria. This is why a lot of medical supplies and lab equipment are “autoclaved,” or simultaneously exposed to extreme heat (~120°C) and pressure, to denature the capsules and to kill off resistant spores.

While many bacteria decide to sporulate in response to unfavorable conditions, some choose to delay sporulation and instead enter an alternative state called “competence.” In this state, their membranes (the surfaces exposed to the environment) become more permeable, thus maximizing the ability to take in higher volumes of proteins, DNA, etc. from the environment. The bacteria that choose to enter this state effectively anticipate that many of their neighbors will release valuable resources as they sporulate, and thereby greatly rewarding the nearby bacteria that decided to delay sporulating.

Within a colony, and sometimes between species, bacteria are able to communicate via chemical messaging pathways, such as quorum sensing, often to convey information about the environment. For example, if several bacteria in one region of a colony sense a drop in pH in their area, they can inform their neighbors of the change, who in turn will continue to spread the information to the rest of the colony. The combination of this communication system and the option of deciding to sporulate or enter competence establish a game for each individual bacterium in a given colony that is similar to the prisoner’s dilemma. In this game, each bacterium must decide whether or not to sporulate based on information that it receives from its neighbors about the declining favorability of their environment. If it decides to sporulate, the mother cell will lose some of its valuable nutrients, but it increases the chances that its genes will be present in the next generation (sporulation is not always successful). If it decides to enter competence while many of its neighbors sporulate and they survive the declining conditions long enough for the surroundings to become more favorable, the mother cell will receive extra packages of nutrients from nearby lysing cells in addition to having a “head start” on their neighbors that have to germinate form the spores.  However, deciding not to sporulate also increases the risk of the mother cell dying in the harsh environment before enclosing its genes in a spore (sporulation can take up to 10 hours in Bacillus subtilis), thereby reducing its fitness to zero.

After observing colonies of Bacillus subtilis “play” this game, researchers have found, is that only about 10% of the bacteria in any given colony decide to enter competence. While the relationships between the endless number of bacteria in these colonies resembles the classic “Prisoner’s dilemma” game on a massive scale, researchers have not discovered how they decide whether or not it’s time to sporulate.

Comments

One Response to “ To spore, or not to spore? ”

  • Brian Jiang

    :D:D:D:D:D:D:D:D:D:D:D:D

    This is a very well-written, insightful, and stimulating blog post. I commend your effort. Top marks!

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