Chemotaxis is defined as the orientation or movement of an organism or cell in relation to chemical agents . For years, scientists have studied this behavior in certain species of bacteria. Through experimentation, they discovered that when motile bacterial cells are placed at one end of a closed capillary tube filled with a medium containing an energy source capable of supporting movement, it is very visible to see that bacteria move preferentially toward higher concentrations of oxygen, minerals, and organic nutrients. This chemotatic behavior produces a sharp and clearly visible gradually migrating band . This paper studies and follows the theory of the traveling bands as proposed by Evelyn Keller and Lee Segel, in which they use partial differential equations to describe the consumption of nutrients and the change in bacterial density due to diffusion and chemotaxis. Collectively, they determine the development and movement of migrating bands. This paper will also analyze and discuss different experiments performed to study chemotatic behavior and their respective results.
The theory in question is the Keller-Segel Theory. Developed by Evelyn Keller and Lee Segel, it presents a phenomenological model for the temporal development of a migrating bacterial band. They model the chemotaxis from a one-dimensional biased random walk, the nutrients being the force which prompts bacterial motion. This motion can be broken down into two alternating states: a straight run of an average duration, followed by an uncoordinated tumble which randomizes the direction of the next run[3,4]. When analyzing this motion and in setting up the partial differential equations describing the nutrient and bacterial concentrations, several things need to be taken into account that are directly associated with the motion: the chemotatic response of the bacteria, the non-chemotactic or diffusive motion of the bacteria, and the bacterial consumption of the nutrients. Diffusion, in this case, is the process in which bacteria moves from a region where their concentration is high to a region where their concentration is low.
The concentration of the nutrients is modeled by Keller
and Segel's equation
The concentration of nutrients in a closed region
is . Therefore, the concentration of nutrients is changing
. In this case,
Keller and Segel's corresponding equation for the concentration
of bacteria is
Much can be learned about the role of chemotaxis on bacterial motion by comparing experimental results to the the results produced by the Keller-Segel model. Using their equation for the speed of migrating bands, Keller and Segel compared speed values of migrating bands obtained from previous experiments to calculated values. In once such comparison, a band was observed to be migrating to higher substrate concentration at 0.9 cm/hr. A speed of 1.5 cm/hr was obtained using the equation. These results are reasonably close, with respect to the crude estimates that had to be made for the values of ,, and , showing that the Keller-Segel model provides reliable predictions for band speed.
Keller and Segel's equation for substrate concentration (3), along with the measured width of the migrating band, also provided them with a way to estimate , the ratio of the chemotactic strength to motility. Using proper estimates, they found values of between 1 and 2. These values give a numerical measure of the relative strength of chemotaxis. The values also showed that bands would not appear unless was larger than , because the order of motion produced by chemotaxis must be large enough to overcome the disordering motion due to diffusion. Secondly, the values were accurate in that they also predicted the shape of the bacterial bands. If , the front edge of the band was steeper than the lagging edge, and if , the lagging edge was steeper. These values were in agreement with observation.
Michael Holz and Sow-Hsin Chen also observed migrating bands of chemotactic bacteria. In their experiments, they used a photon correlation spectrometer. This device produced an intensity auto-correlation function and was used so that the development of the bacterial density profile could be observed by the scattered light intensity as the band migrated through a stationary laser beam. Holz and Chen compared the bacterial density profile produced by the photon correlation spectrometer with Keller and Segel's model for travelling bands and made similar comparisons concerning speed. With proper estimates, their calculated value for the speed of the migrating band was 0.9 m/s, and the observed speed was 0.7 m/s. Their results were relatively accurate. Similarly, they also calculated , the ratio of chemotactic strength to motility. For given values of and of the band width, they obtained a ratio = 1.72, compared to the expected ratio of 1.33. They also found that the shape of the band profile was in good agreement with the shape proposed by the model. For a given ratio of 1.33, the calculated curves for bacterial density distribution were very similar to the measured curves predicted by the Keller-Segel model. It also complied with the prediction that for , the back edge was steeper than the front edge. Altogether, they further showed that the model represented certain features of the migrating band very reasonably. However, there were some discrepancies, most likely due to the assumption of = 0 in the Keller-Segel model. The density profile is either too broad or the speed of migration is too high when compared to the experiment. It has been found that when serine was considered as nutrients and , the numerical solution was in good agreement with the Keller-Segel solution. The Keller-Segel model also predicts a rather uniform thickness of the leading edge for all values of . But the experimental line shape is fairly symmetric and the leading edge shows variable thickness. The numerical curves tended to be asymmetric.
Using the photon correlation spectrometer, Holz and Chen also took
into account the presence of tumbling bacteria in formation and movement of migrating bands. They
accomplished this by developing a simple analytical model that
accounts for the contribution of the twiddle motion to the
correlation function. The correlation function was used to analyze
microscopic motility characteristics of the bacteria in the band.
The result was the intermediate scattering function, which
describes the temporal decay of the auto-correlation function and
takes into account both the twiddling and running states of the
bacteria. It allows for the average fraction of bacteria that are
twiddling at a given time to be extracted from the correlation
Holz and Chen furthered the study of migrating chemotactic bands
by continuing their testing of the Keller-Segel model and by
providing means of determining values for the motility () and
chemotactic () coefficients of migrating bacteria. They
used a rapid-scanning, light-scattering densitometer that produced
extensive measurements of band migration speeds and band profiles.
To do this, they extended their analysis of the Keller-Segel model
by taking into account , the substrate diffusion, which was
previously assumed to be zero. They solved the following equation
for the concentration of nutrients, taking into account the
non-zero diffusion coefficient:
In conclusion, the movement of travelling bands of chemotactic bacteria has been successfully discussed and analyzed. Movement of bacteria is influenced by chemotaxis, a process in which bacteria seek and move toward regions of higher substrate concentration. This movement produces highly visible bands of travelling bacteria. Evelyn Keller and Lee Segel wrote a model to describe this movement by using partial differential equations to describe the consumption of the substrate and the change in bacterial density due to random motion and chemotaxis. They tested their model experimentally, and obtained reasonably accurate measurements for the speed and shape of the travelling bands and also for the ratio of chemotactic strength, , to motility, . Michael Holz and Sow-Hsin Chen tested Keller and Segel's model and also produced relatively accurate results for the ratio of chemotactic strength to motility and for band shape. They also observed the effects of the twiddling of bacteria in band movement. They correctly predicted that a large number of the bacteria are twiddling, as compared to running in a straight path, at any given time. Additionally, Holz and Chen took into account the diffusion of the substrate in analyzing the Keller-Segel model. In doing this, they obtained values for , the motility coefficient, and for , the chemotactic coefficient. These values were more accurate than those obtained when neglecting diffusion. Altogether, Keller and Segel reasonably modeled and described the actions of travelling bands of chemotactic bacteria.