Morgan Chambers
Biology 1615
4/4/2011
Swim or Chill: lifestyles of a bacillus summary
Bacillus subtilis, also known as grass bacillus, is a bacterium commonly found
in the region of soil surrounding plant roots. It is a model organism for lab studies
because of its sporulation and capability of genetic manipulation. B. subtilis is
subject to many environmental factors that have the potential to harm the bacteria.
To protect itself, B. subtilis produces an endospore through cell division that resists
those factors. Prior to sporulation, B. subtilis may become motile by producing
flagella allowing the bacteria to move quickly. This gene expression is also displayed
in a sessile state, which happens when the daughter cells remain attached to each
other because of the lack of enzymes needed to degrade the cell wall. A sessile state
is a stationary one since the daughter cells do not produce flagella.
Bacillus subtilis has two lifestyles, swimming and sessile. These cell states
coexist in the same population. The deciding process on the cell’s state is thought to
be stochastic; meaning that it has a random variable causes the difference in states.
The cell can either form biofilm or remain pelagic. This is believed to happen in the
basal expression of ComK, which drives the transcription of DNA uptake genes and
to its own gene. This creates a positive autoregulatory loop. ComK binds to its own
promoter and combined with the loop causes a two state behavior (ex. motile and
sessile). The scientists conducting experiments on the behavior of these cells have
discovered that SinR is a repressor of a regulatory gene present in B. subtilis, s1rR,
and that S1rR protein sequesters SinR preventing it from inhibiting the formation of
biofilm. This causes a double-negative feedback loop, which is the basis of the
motile-sessile decision. If Sin1 is synthesized in the cell and binds to SinR, it will
repress s1rR. Then the S1rR-SinR complex will bind to promoters of autolysin and
flagellar genes. This will inhibit expression and prevent the daughter cells from
separating and forming flagella remaining sessile. The motile cells are only created
in random cells where sinl expression remains low and will exist in a state with
active SinR and a low concentration of S1rR, which will allow the daughter cells to
separate and move.
The switching of the B. subtilis cell states is a result of evolution. The motile
cells evolved to escape detrimental environmental factors, while the sessile cells
remain stationary to ensure the perpetuation and reproduction of the shared
genome. As the cells enter the stationary phase a basal expression of comK occurs
and then decreases. This adjusts the threshold for transition and provides a
deterministic mechanism that modulates the stochastic decision and regulates the
transition rate of competence as cells cease growing.
After discovering the decision process of B. subtilis new questions arise. It
was found that the sessile bias is due to a mutation in he swr gene. What is the role
and mechanism of action of the Swr protein? Also, is this system reversible? How do
sessile cells in biofilms become motile? In order to answer these questions and
others that arise the scientific community should continue experimenting with B.
subtilis and start using real world models.