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Bacteria cells
Bacteria cells. Source: Technology Networks

Bacteria: External Structures

Structures External to the Cell Wall

Among the possible structures external to the prokaryotic cell wall are the glycocalyx, flagella, axial filaments, fimbriae, and pili.

Glycocalyx

Many prokaryotes secrete on their surface a substance called glycocalyx. Glycocalyx (meaning sugar coat) is the general term used for substances that surround cells. The bacterial glycocalyx is a viscous (sticky), gelatinous polymer that is external to the cell wall and composed of polysaccharide, polypeptide, or both.

Star-shaped and rectangular prokaryotes.
Star-shaped and rectangular prokaryotes. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

Its chemical composition varies widely with the species. For the most part, it is made inside the cell and secreted to the cell surface. If the substance is organized and is firmly attached to the cell wall, the glycocalyx is described as a capsule. The presence of a capsule can be determined by using negative staining. If the substance is unorganized and only loosely attached to the cell wall, the glycocalyx is described as a slime layer.

In certain species, capsules are important in contributing to bacterial virulence (the degree to which a pathogen causes disease). Capsules often protect pathogenic bacteria from phagocytosis by the cells of the host. (As you will see later, phagocytosis is the ingestion and digestion of microorganisms and other solid particles.) For example, Bacillus anthracis produces a capsule of d-glutamic acid. (Recall from Chapter 2 that the d forms of amino acids are unusual.) Because only encapsulated B. anthracis causes anthrax, it is speculated that the capsule may prevent it from being destroyed by phagocytosis.

The Structure of a Prokaryotic Cell
The Structure of a Prokaryotic Cell. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

Another example involves Streptococcus pneumoniae (strep′to¯ -KOK-kus noo-MO¯ -ne¯-¯), which causes pneumonia I only when the cells are protected by a polysaccharide capsule. Unencapsulated S. pneumoniae cells cannot cause pneumonia and are readily phagocytized. The polysaccharide capsule of Klebsiella also prevents phagocytosis and allows the bacterium to adhere to and colonize the respiratory tract.

Arrangements of bacterial flagella.
Arrangements of bacterial flagella. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

The glycocalyx is a very important component of biofilms. A glycocalyx that helps cells in a biofilm attach to their target environment and to each other is called an extracellular polymeric substance (EPS). The EPS protects the cells within it, facilitates communication among them, and enables the cells to survive by attaching to various surfaces in their natural environment.

Through attachment, bacteria can grow on diverse surfaces such as rocks in fast-moving streams, plant roots, human teeth, medical implants, water pipes, and even other bacteria. Streptococcus mutans (MU¯ -tanz), an important cause of dental caries, attaches itself to the surface of teeth by a glycocalyx. S. mutans may use its capsule as a source of nutrition by breaking it down and utilizing the sugars when energy stores are low. Vibrio cholerae (VIB-re¯-o¯ KOL-er-¯), the cause of cholera, produces a I glycocalyx that helps it attach to the cells of the small intestine. A glycocalyx also can protect a cell against dehydration, and its viscosity may inhibit the movement of nutrients out of the cell.

Flagella and Archaella

Some bacterial cells have flagella (singular: flagellum), which are long filamentous appendages that propel bacteria. Bacteria that lack flagella are referred to as atrichous (without projections). Flagella may be peritrichous (distributed over the entire cell) or polar (at one or both poles or ends of the cell). If polar, flagella may be monotrichous (a single flagellum at one pole, lophotrichous (a tuft of flagella coming from one pole, or amphitrichous (flagella at both poles of the cell.

The structure of a bacterial flagellum
The structure of a bacterial flagellum. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

A flagellum has three basic parts. The long outermost region, the filament, is constant in diameter and contains the globular (roughly spherical) protein flagellin arranged in several chains that intertwine and form a helix around a hollow core. In most bacteria, filaments are not covered by a membrane or sheath, as in eukaryotic cells. The filament is attached to a slightly wider hook, consisting of a different protein. The third portion of a flagellum is the basal body, which anchors the flagellum to the cell wall and plasma membrane.

The basal body is composed of a small central rod inserted into a series of rings. Gram-negative bacteria contain two pairs of rings; the outer pair of rings is anchored to various portions of the cell wall, and the inner pair of rings is anchored to the plasma membrane. In gram-positive bacteria, only the inner pair is present. As you will see later, the flagella (and cilia) of eukaryotic cells are more complex than those of bacteria.

Each bacterial flagellum is a semirigid, helical structure that moves the cell by rotating from the basal body. The rotation of a flagellum is either clockwise or counterclockwise around its long axis. (Eukaryotic flagella, by contrast, undulate in a wavelike motion.) The movement of a bacterial flagellum results from rotation of its basal body and is similar to the movement of the shaft of an electric motor. As the flagella rotate, they form a bundle that pushes against the surrounding liquid and propels the bacterium. Flagellar rotation depends on the cell’s continuous generation of energy.

Flagella and bacterial motility
Flagella and bacterial motility. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

Bacterial cells can alter the speed and direction of rotation of flagella and thus are capable of various patterns of motility, the ability of an organism to move by itself. When a bacterium moves in one direction for a length of time, the movement is called a “run” or “swim.” “Runs” are interrupted by periodic, abrupt, random changes in direction called “tumbles.” Then, a “run” resumes. “Tumbles” are caused by a reversal of flagellar rotation. Some species of bacteria endowed with many flagella—Proteus (PRO¯ -te¯-us), for example—can “swarm,” or show rapid wavelike movement across a solid culture medium.

One advantage of motility is that it enables a bacterium to move toward a favorable environment or away from an adverse one. The movement of a bacterium toward or away from a particular stimulus is called taxis. Such stimuli include chemicals (chemotaxis) and light (phototaxis). Motile bacteria contain receptors in various locations, such as in or just under the cell wall. These receptors pick up chemical stimuli, such as oxygen, ribose, and galactose. In response to the stimuli, information is passed to the flagella. If the chemotactic signal is positive, called an attractant, the bacteria move toward the stimulus with many runs and few tumbles. If the chemotactic signal is negative, called a repellent, the frequency of tumbles increases as the bacteria move away from the stimulus.

The flagellar protein called H antigen is useful for distinguishing among serovars, or variations within a species, of gramnegative bacteria (see page 304). For example, there are at least 50 different H antigens for E. coli. Those serovars identified as E. coli O157:H7 are associated with foodborne epidemics.

Archaella

Motile archaeal cells have archaella (singular: archaellum). Archaella share similarities with bacterial flagella and pili (discussed on page 79). A knoblike structure anchors archaella to the cell. No basal-body type anchor has been found for pili. Archaella rotate like flagella, an action that pushes the cell through water, and, like pili, archaella use ATP for energy and lack a cytoplasmic core. Archaella consist of glycoproteins called archaellins.

Axial Filaments

Spirochetes are a group of bacteria that have unique structure and motility. One of the best-known spirochetes is Treponema pallidum (trep-o¯ -NE¯-mah PAL-li-dum), the causative agent of syphilis. Another spirochete is Borrelia burgdorferi (bor-RE¯-le¯-ah burg-DOR-fer-e¯), the causative agent of Lyme disease. Spirochetes move by means of axial filaments, or endoflagella, bundles of fibrils that arise at the ends of the cell beneath an outer sheath and spiral around the cell.

Axial filaments, which are anchored at one end of the spirochete, have a structure similar to that of flagella. The rotation of the filaments produces a movement of the outer sheath that propels the spirochetes in a spiral motion. This type of movement is similar to the way a corkscrew moves through a cork. This corkscrew motion probably enables a bacterium such as T. pallidum to move effectively through bodily fluids.

Fimbriae and Pili

Many gram-negative bacteria contain hairlike appendages that are shorter, straighter, and thinner than flagella. These structures, which consist of a protein called pilin arranged helically around a central core, are divided into two types, fimbriae and pili, having very different functions. (Some microbiologists use the two terms interchangeably to refer to all such structures, but we distinguish between them.)

Fimbriae (singular: fimbria) can occur at the poles of the bacterial cell or can be evenly distributed over the entire surface of the cell. They can number anywhere from a few to several hundred per cell. Fimbriae have a tendency to adhere to each other and to surfaces. As a result, they are involved in forming biofilms and other aggregations on the surfaces of liquids, glass, and rocks. Fimbriae can also help bacteria adhere to epithelial surfaces in the body.

Axial filaments.
Axial filaments. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

For example, fimbriae on the bacterium Neisseria gonorrhoeae (n¯-SE- I re¯-ah go-no¯ r-RE¯-I¯), the causative agent of gonorrhea, help the microbe colonize mucous membranes. Once colonization occurs, the bacteria can cause disease. The fimbriae of E. coli O157 enable this bacterium to adhere to the lining of the small intestine, where it causes a severe watery diarrhea. When fimbriae are absent (because of genetic mutation), colonization cannot  happen, and no disease ensues.

Pili (singular: pilus) are usually longer than fimbriae and number only one or two per cell. Pili are involved in motility and DNA transfer. In one type of motility, called twitching motility, a pilus extends by the addition of subunits of pilin, makes contact with a surface or another cell, and then retracts (powerstroke) as the pilin subunits are disassembled. This is called the grappling hook model of twitching motility and results in short, jerky, intermittent movements. Twitching motility has been observed in Pseudomonas aeruginosa, Neisseria gonorrhoeae, and some strains of E. coli. The other type of motility associated with pili is gliding motility, the smooth gliding movement of myxobacteria. Although the exact mechanism is unknown for most myxobacteria, some utilize pilus retraction. Gliding motility provides a means for microbes to travel in environments with a low water content, such as biofilms and soil.

Some pili are used to bring bacteria together, allowing the transfer of DNA from one cell to another, a process called conjugation. Such pili are called conjugation (sex) pili. In this process, the conjugation pilus of one bacterium called an F+ cell connects to receptors on the surface of another bacterium of its own species or a different species. The two cells make physical contact, and DNA from the F+ cell is transferred to the other cell. The exchanged DNA can add a new function to the recipient cell, such as antibiotic resistance or the ability to digest its medium more efficiently.

About Fahmida Akter Bristi

I am currently doing my Bachelor degree. I love to write by exploring knowledge that is new to me. Hope this effort of mine benefits you all. Right now, I am the head of Project R. Franklin & Project Waksman in Society & Science Foundation. Knock me anytime. Email: fahmidabristi683@gmail.com

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