Flagella and Pili (Fimbriae)
Flagella are long, thin structures that protrude from the surface of some bacteria and allow them to move in liquid environments. They are composed of protein subunits called flagellin, which form a helical filament that rotates like a propeller. Flagella are one of the most distinctive and fascinating features of bacterial cells, as they enable them to sense and respond to their surroundings, colonize new habitats, and evade host defenses.
Flagella are long, thin, whip-like structures that extend from the surface of some bacteria and allow them to move in liquid environments. They are composed of a protein called flagellin, which forms a helical filament that can rotate like a propeller. The rotation is powered by a molecular motor called the basal body, which is embedded in the bacterial cell membrane and cell wall.
Flagella can vary in number, location, and arrangement on the bacterial cell. Some bacteria have only one flagellum at one end (polar flagellum), some have several flagella at one or both ends (lophotrichous or amphitrichous flagella), and some have many flagella distributed all over the cell surface (peritrichous flagella). The number and arrangement of flagella can affect the speed, direction, and pattern of bacterial movement.
Flagella are not only important for locomotion, but also for sensing and responding to environmental stimuli, such as nutrients, toxins, temperature, pH, and light. This ability to move toward or away from certain stimuli is called chemotaxis, and it involves a complex signaling pathway that regulates the rotation of the flagellar motor. Depending on the direction of rotation, the flagella can either push or pull the bacterial cell, or cause it to tumble and change direction.
Flagella are also involved in other biological processes, such as adhesion, biofilm formation, virulence, and conjugation. Some bacteria use their flagella to attach to host cells or surfaces, forming colonies or communities that can resist antibiotics and immune responses. Some bacteria use their flagella to inject toxins or genetic material into other cells, causing disease or transferring resistance genes. Flagella are therefore considered as important factors for bacterial survival and adaptation.
The basal body is the part of the flagellum that anchors it to the cell membrane and cell wall. It also contains the motor that rotates the flagellum and enables bacterial movement. The basal body has a complex structure that varies depending on the type of bacteria and the number and location of flagella.
The basal body consists of several rings that are surrounded by proteins and connected by a rod. The rings are named according to their position in the cell envelope: the C-ring is located in the cytoplasm, the MS-ring is embedded in the inner membrane, the P-ring is attached to the peptidoglycan layer, and the L-ring is associated with the outer membrane (in gram-negative bacteria). The C-ring and the MS-ring form the core of the motor, while the P-ring and the L-ring act as bushings that reduce friction and stabilize the rod.
The rod is a helical structure that extends from the MS-ring to the hook, which connects the basal body to the filament. The rod is composed of different proteins depending on its position: FliE, FlgB, FlgC and FlgF form the proximal rod, while FlgG forms the distal rod. The rod transmits torque from the motor to the filament and also serves as a conduit for the secretion of flagellar components.
The motor of the basal body is powered by a proton gradient across the inner membrane (in most bacteria) or by ATP hydrolysis (in some archaea). The motor consists of a pair of proteins called MotA and MotB (or their homologs) that form a channel for protons to flow through. The flow of protons causes conformational changes in these proteins, which in turn interact with the C-ring and induce its rotation. The C-ring then rotates the MS-ring, which is attached to the rod and drives its rotation. The direction and speed of rotation can be controlled by sensory proteins that modulate the proton flow or by switching proteins that change the orientation of the motor components.
The basal body is a remarkable molecular machine that enables bacteria to swim in various environments. It is also involved in sensing chemical gradients (chemotaxis), forming biofilms, invading host cells, and exchanging genetic material (conjugation). The structure and function of the basal body have been studied extensively using various techniques such as electron microscopy, X-ray crystallography, cryo-electron tomography, fluorescence microscopy, and genetic engineering.
Chemotaxis is the directed movement of an organism toward environmental conditions it deems attractive and/or away from surroundings it finds repellent. For bacteria, chemotaxis is a way of sensing and responding to changes in the concentration of certain chemicals in their environment, such as nutrients, toxins, or signals from other cells.
Bacteria can move toward a higher concentration of an attractant chemical or away from a higher concentration of a repellent chemical by altering their swimming behavior. Bacteria that have flagella, such as E. coli, can swim by rotating their flagella in two ways:
- Counter-clockwise rotation – aligns the flagella into a single rotating bundle, causing the bacterium to swim in a straight line. This is called a run.
- Clockwise rotation – breaks the flagella bundle apart such that each flagellum points in a different direction, causing the bacterium to tumble in place. This is called a tumble.
The overall movement of a bacterium is the result of alternating run and tumble phases. When the bacterium senses a favorable change in the concentration of a chemical (for example, an increase in an attractant or a decrease in a repellent), it will lengthen its runs and reduce its tumbles, resulting in a net movement toward or away from the chemical gradient. This movement is called chemotaxis.
Chemotaxis is important for bacterial survival in a changing environment. It allows bacteria to find food sources, avoid harmful substances, form biofilms, establish symbiosis, and cause infections. Chemotaxis is regulated by a complex signal transduction system that involves receptors, proteins, and enzymes that sense and process the chemical signals and control the flagellar rotation. The details of this system will be discussed in the next section.
Spirochetes are a group of bacteria that have a distinctive spiral shape and a unique mode of locomotion. They belong to the phylum Spirochaetes, which includes about 35 genera and hundreds of species. Some of the most well-known spirochetes are Treponema pallidum, the causative agent of syphilis; Borrelia burgdorferi, the causative agent of Lyme disease; and Leptospira interrogans, the causative agent of leptospirosis.
Unlike most bacteria, spirochetes do not have external flagella that extend from their cell surface. Instead, they have axial filaments or endoflagella that are located within the periplasmic space between the inner and outer membranes. The axial filaments are attached to the ends of the cell and wrap around the cell body in a helical fashion. They are composed of flagellin proteins and have a similar structure and function to regular flagella.
When the axial filaments rotate, they cause the cell body to twist and bend in a corkscrew-like motion. This allows the spirochetes to move through viscous fluids and tissues, such as blood, mucus, and connective tissue. The spirochetes can also change their shape and direction by altering the rotation speed and direction of their axial filaments. This helps them to evade the host immune system and adapt to different environments.
The spirochetes are considered to be periplasmic flagellates, meaning that their flagella are located in the periplasmic space. However, some researchers prefer to use the term axial fibrils or axial filaments to avoid confusion with regular flagella. The term periplasmic flagella is also used to describe the flagella of some other bacteria, such as Campylobacter and Helicobacter, that have a single polar flagellum that runs along the cell body under the outer membrane.
The spirochetes are an example of how bacteria can evolve different ways of using flagella for motility and survival. By having their flagella enclosed within their cell envelope, they can protect them from external factors and increase their efficiency of movement. They can also use their flexible shape and movement to penetrate host tissues and escape immune responses. However, they also face some challenges, such as maintaining their cell integrity and stability under mechanical stress and generating enough energy for their flagellar rotation.
Some bacteria, such as spirochetes, have a unique type of flagella that are located in the periplasmic space between the inner and outer membranes. These flagella are called periplasmic flagella or axial filaments. Unlike the external flagella of other bacteria, periplasmic flagella do not protrude out of the cell surface, but run along the length of the cell under a sheath of outer membrane.
Periplasmic flagella are usually multiple, arising from one or both ends of the cell, and pack together into a helical ribbon whose rotation drives wave-like motion of the cell. This allows the spirochetes to move through viscous environments, such as mucus or blood, and to penetrate tissues and cells. Periplasmic flagella are also involved in cell shape determination and cell division in spirochetes.
Periplasmic flagella have a similar structure and composition to bacterial flagella, but with some differences. They consist of a filament made of flagellin subunits, a hook that connects the filament to the basal body, and a basal body that anchors the flagellum to the cell wall and membrane. However, periplasmic flagella have fewer rings in their basal body than bacterial flagella, and their hook is shorter and curved. Periplasmic flagella also have a different mechanism of rotation than bacterial flagella. They rotate by a torque-generating motor that is powered by proton motive force, but without the involvement of Mot or Fli proteins that are essential for bacterial flagellar motor function.
Periplasmic flagella are found in many pathogenic spirochetes, such as Treponema pallidum (the causative agent of syphilis), Borrelia burgdorferi (the causative agent of Lyme disease), and Leptospira interrogans (the causative agent of leptospirosis). The study of periplasmic flagella in these organisms is important for understanding their virulence and pathogenesis, as well as for developing new strategies for diagnosis, prevention, and treatment.
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