Only cells which are motile, that is, those cells which are capable of autonomous movement are able to carry out chemotaxis. The process of chemotaxis is limited to select types of specialised cells which possess the ability to detect changes in their external environment, and, as a consequence of these, to elicit changes in their internal environment which lead to whole – cell movement. Chemotaxis may be defined as “a change in the direction of movement of a motile cell in response to a concentration gradient of a specific chemical”.
This change in the direction of movement may either cause the cell to move towards or away from the sensed chemical. Chemicals which are capable of leading to such a response from chemotactic cells are called Chemotactants and may act as either attractants – in which case the chemotactic cell will move towards them, thereby exhibiting a positive chemotactic response – or as repellents – in which case the chemotactic cell will move away from them, thereby exhibiting a negative chemotactic response.
Bacterial cells achieve motility via the utilisation of specialised structures known as flagella. Similar in structure to spinning tails, these flagella allow the bacterium to show both positive and negative chemotactic responses, based upon the presence of chemotactic molecules. The fine structure of the flagellum is that of a right-handed helical fibre embedded in the bacterial cell membrane. It is composed of multiple repeats of a single type of protein subunit called Flagellin. A single bacterial cell may possess multiple flagella. The bacterial flagellum sits between the inner and outer membranes of the bacterium, and has a number of distinct components.
Rotation of the flagellum follows as a direct result of its structure. The fibre is connected by a short flexible protein hook to a small protein disc embedded in the plasma membrane. This disc constitutes part of the motor, and by utilising the energy stored in the transmembrane proton gradient, the bacterium can cause this disc to rotate, thereby effecting movement of the fibre. Protons move into the cell down the electrochemical gradient established by a proton ATPase and cause the flagellum to rotate at a rate of sixty revolutions per second.
The intrinsic ‘ handedness ‘ which the fibre possesses, due to its helical nature, is vital to its ability to effect whole cell movement in different directions, since rotations of the fibre in opposite directions are not equivalent. The flagellar fibre can be rotated in both clockwise and anti-clockwise fashions. Anti-clockwise rotation results in the numerous flagella of the bacterium being drawn together, so that the bacteria swims uniformly in one direction. By comparison, clockwise rotation causes each flagellum to move away from the others, so that the bacterium tumbles chaotically with no uniform direction of movement. The direction of rotation of the bacterial flagellum is directly dependent upon the absence / presence of a chemotactic molecule.
When the bacterium is exposed to a chemotactic molecule it effectively makes a choice between which mode of flagellar rotation it will use, depending upon the type of chemotactic molecule encountered. In the presence of a chemoattractant, the bacterium spends a greater proportion of its motile time with its flagella rotating in the anti-clockwise fashion. This results in the bacterium exhibiting the tumbling motion less frequently, and consequently travelling in a straight line for longer. Hence, the bacterium can target the source of the chemoattractant, and move towards it. Conversely, in the presence of a chemorepellent, the bacterium will exhibit tumbling motion much more frequently, by rotating its flagella in a clockwise fashion for longer. Hence, the presence of the chemorepellent will cause the bacterium to undergo many changes in the direction of its movement, and this will assist in evading contact with the detected chemorepellent.
Both forms of behaviour shown in the presence of chemotactic molecules show a time dependency of occurrence which is directly related to the concentration of the chemotactic molecule to which the bacterium is exposed. However, there is another aspect to the behaviour of a bacterial cell in the presence of a chemoattractant. When the bacterium encounters nutrient molecules, it begins to travel in a straight line for longer, in an attempt to find the source of this chemoattractant, and hence to expose itself to the maximum concentration possible of nutrients.
As the bacterium gets closer to the source, it will spend less time displaying this chemotactic response and will eventually return to the tumbling motion. This process is called Adaptation and is the mechanism whereby bacteria become desensitised to chemotactic stimuli. This is an important aspect of the chemotactic response, allowing the bacterium to localise itself at the source of the chemoattractant.
Adaptation occurs via a complex mechanism involving a number of specialised bacterial proteins. It is triggered by prolonged exposure to a specific chemoattractant, and ensures that the bacterium does not ‘ overshoot ‘ the source of the chemoattractant which it is seeking. Adaptation is specific for a given chemoattractant, and if the adapted bacterium is then exposed to a new chemoattractant, the observed tumbling motion is abolished, and the bacterium begins to travel for longer periods of time in a straight line – towards the source of the new chemoattractant.
To understand the bacterial chemotactic mechanism, great use has been made of a range of mutants with specific chemotactic deficiencies. The classical scientific approach of isolating mutants in particular cellular processes to elucidate the mode of action of wild type individuals has a number of important aspects. Firstly, by isolating mutants in the process of interest – chemotaxis in this case – from the wild type forms of the organism, genetic components active in the phenotype of interest can be identified. Secondly, the mutant genome may then be compared to that of the wild type so that differences at the genetic level can be studied.
Finally, correlation of the findings with databases on the genome of the organism of interest can then be used to allow understanding of the function of that region of the genome. Studies on these mutants have contributed to the present understanding of the bacterial chemotactic mechanism by allowing the components of the process to be investigated. The crucial components, it seems, are the periplasmic receptor proteins, the MCP’s, and the bacterial flagellum. Together, they facilitate the process of Signal Transduction Chemotaxis.