For the case of bacteria that swim, choosing whether to stay on the same direction or change course is quite crucial for survival.
A new study published in the journal eLife reports atomic-level details of the structure that allows swimming bacteria to detect their surroundings and determine when they have to change direction.
The research, led by University of Illinois physics professor Klaus Schulten, opens a big door in trying to
understand how the bacterial brain works.
On the surface of a bacterium are loads of receptors that detect its surroundings for it to determine what
the next step is. This is pretty much similar to the different senses that animals have and process in their
brains. Likewise, bacteria are single-celled organisms and consequently do not possess brains. Despite this,
they are still able to manage, organize and remember the signals that they are able to pick up using
their receptors in such a way that allows them to survive.
The receptors that are found on the cell surface of bacteria can detect different types of things, including
light, chemicals, as well as edible and poisonous stuff, and upon detection, transmit the signals
downstream to proteins called kinases. Depending on the stimulus, the proteins simply give the end
decision of either continuing or changing direction.
If in any case, the decision to change direction is made, a kinase protein in turn activates another one,
called CheY (pronounced key why'), which then detaches and then migrates to the flagella to activate
mechanisms that cause the flagella to spin in reverse.
Previous studies have produced bits and pieces of the structure of this molecular machine which is
responsible for this process, which is called the chemosensory array. However, these studies were not
able to give a clear resolution of its molecular structure.
Peijun Zhang, who is the co-author of the study, and is from the University of Pittsburgh, helped by
devising a technique that was able to purify the proteins that are involved in the array, which he then put
together in such a way that they formed thin layers just enough to allow them to take clear 3-D images
using electron microscopy. This technique was able to greatly increase the resolution of the data. The
picture was then processed using molecular dynamic flexible fitting, which is a computer modeling
technique being used at Schulten's lab.
This study showed the key interactions that occur among the proteins that are involved in the
chemosensory array. One example is for the case of CheA, in which it changes its orientation with respect
to the other proteins, in a motion to which the researchers named dipping. Further experiments showed
that this is very essential to a bacterium's response to its environment.
One significant question that is still unanswered is how the signal is being passed from the receptors to
the kinases. It has to be a motion. It can't be anything else. But what kind of motion?, says Schulten.
Certainly more research is needed to determine the relationships between the components of the system,
as well as the mechanism involved. However, the study surely denotes a huge jump in understanding how
the system works.