Bacterial appendages

A bacterial appendage protrudes outward from the surface of the microorganism. Some are highly anchored to the surface, whereas others, like the glycocalyx , are loosely associated with the surface.

The entire surface of a bacterium can be covered with glycocalyx (also known as the slime layer). The layer is made of chains of sugar. Protein can also be present. The exact chemical nature of a glycocalyx varies from one species of bacteria to another. A glycocalyx is easily identified in light microscopy by the application of India ink. The ink does not penetrate the glycocalyx, which then appears as a halo around each bacteria.

A glycocalyx has a number of functions. It aids a bacterium in attaching to a surface. Surface contact triggers the production of a great deal of glycocalyx. The bacteria on the surface can become buried. This phenomenon has been well documented for Pseudomonas aeruginosa, which forms biofilms on surfaces in many environments, both within and outside of the body. The production of glycocalyx is a vital part of the biofilm formation.

By virtue of its chemical make-up, a glycocalyx will retain water near the bacteria, which protects the bacteria from drying out. Protection is also conferred against viruses , antibiotics , antibacterial agents such as detergents, and from the engulfing of the bacteria by immune macrophage cells (a process called phagocytosis ). The mass of glycocalyxenclosed bacteria becomes too large for a macrophage to engulf. For example, encapsulated strains of Streptococcus pneumoniae kill 90% of the animals it infects. Unencapsulated strains, however, are completely non-lethal. As another example of the protection conferred by the glycocalyx, Pseudomonas aeruginosa in an intact biofilm resist for hours concentrations of antibiotics up to one thousand times greater than those which kill within minutes their bacterial counterparts without glycocalyx and bacteria freed from the glycocalyx cocoon of the biofilm.

Glycocalyx material is easily removed from the bacterial surface. A glycocalyx that is more firmly anchored is known called as a capsule. Many disease causing bacteria tend to produce capsules when inside the human host, as a defense against phagocytosis.

Another type of bacterial appendage is the flagella (singular, flagellum). They appear as strings protruding outward from a bacterium. They are long, up to ten times the length of the bacterium. Each flagellum is composed of a spiral arrangement of a protein (flagellin). The flagella are closed off at the end removed from the cell. The end closest to the bacterial surface hooks into the membrane(s), where they are held by two structures termed basal bodies. The basal bodies act as bushings, allowing flagellar tube to turn clockwise and counterclockwise. By spinning around from this membrane anchor, flagella act as propellers to move a bacterium forward, or in a tumbling motion prior to a directed movement in the same or another forward path. These runs and tumbles enable a bacterium to move toward an attractant or away from a repellant. Generally termed taxis, these movements can be in response to nutrients (chemotaxis), oxygen (aerotaxis) or light (phototaxis). The tactic process is highly orchestrated, with sensory proteins detecting the signal molecule and conveying the signal into flagellar action.

Flagella are very powerful. They can propel bacteria at ten times their length per second. In contrast, an Olympic sprinter can propel himself at just over five body lengths per second. Depending upon the type of bacteria, flagella are characteristically arranged singly at only one end of the cell (monotrichous), singly at both ends of the cell (amphitrichous), in a tuft at one or a few sites (lophotrichous), or all over the bacterial surface (peritrichous).

The bacteria called spirochetes have a modified form of flagella, which is termed an endoflagella or an axial filament. In this case, the flagella is not an appendage, in that it is not external to the bacterium, but instead is found in the interior of the cell, running from one end of the cell to another. It is, however, similar in construction to flagella. Endoflagella attach to either end of a cell and provide the rigidity that aids a cell in turning like a corkscrew through its liquid environment.

Two other types of appendages are essentially tubes that stick out from the bacterial surface. The first of these is known as spinae (singular, spina). Spinae are cylinders that flare out at their base. They are a spiral arrangement of a single protein (spinin) that is attached only to the outer surface of the outer membrane. They have been detected in a marine pseudomonad and a freshwater bacterial species. Their formation is triggered by environmental change (pH , temperature, and sodium concentration). Once formed, spinae are extremely resilient, surviving treatment with harsh acids and bases. They are designed for longevity. Their function is unknown. Suggested functions include buoyancy, promoters of bacterial aggregation, and as a conduit of genetic exchange.

The appendages called pili are also tubes that protrude from the bacterial surface. They are smaller in diameter than spinae. Like spinae, pili are constructed of a protein (pilin). Unlike spinae, the functions of pili are well known. Relatively short pili are important in the recognition of receptors on the surface of a host cell and the subsequent attachment to the receptor. These are also known as fimbriae. There can be hundreds of fimbriae scattered all over the bacterial surface. Their attachment function makes fimbriae an important disease factor. An example is Neiserria gonorrheae, the agent of gonorrhea . Strains of the bacteria that produce fimbriae are more virulent than strains that do not manufacture the appendage. Not unexpectedly, such pili are a target of vaccine development. The second type of pili is called conjugation pili, sex pili, or F-pili. These are relatively long and only a few are present on a bacterium. They serve to attach bacteria together and serve as a portal for the movement of genetic material (specifically the circularly organized material called a plasmid) from one bacterium to the other. The genetic spread of antibiotic resistance occurs using pili.

See also Anti-adhesion methods; Bacteria and bacterial infection; Electron microscopic examination of microorganisms