Morphology of Bacteria: Size, Shape, and Arrangement
Since the time of the classification of living organisms, prokaryotic organisms have remained the point of interest for scientists worldwide. The members of the prokaryotic world make up a vastly heterogeneous group of microscopic unicellular organisms. Prokaryotes include bacteria and archaea. Bacterias are both autotrophic and heterotrophic. Some are even parasites, that is depending on the host for nutrition. Thousands of species of bacteria are classified on a different basis (size, shape and arrangement of bacteria). This includes morphology (shape), biochemical activities, chemical composition, nutritional requirements, and sources of energy.
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Size, shape and arrangement of bacteria:
Classification Of Bacteria based on size
Bacteria are small microscopic prokaryotes approximately 0.5 to 2.0 micrometres in diameter and 2 to 8 micrometres in length. An important significance of such a small size range of bacteria is that the surface area volume ratio of bacteria is exceedingly high compared to the same ratio for larger organisms of similar shape. Thus they have a relatively large surface area for gathering the nutrients compared to the small volume of cess substance to be nourished. This accounts for the unusually high rate of growth and metabolism in bacteria. Also, because of the high surface area/ volume ratio, the cell mass can be nourished very close to the surface. Thus no circulatory mechanism is required to redistribute the nutrients taken in. Despite these advantages, it also limits the size of bacteria to microscopic dimensions.
Classification of Bacteria On The Basis Of Shape And Arrangement
Bacteria come with great variations in their shape. The morphology of a bacterium is governed by its rigid cell wall. However, what attribute of this rigid material determines that a cell will have a particular shape is not determined yet understood. They have a few basic shapes:
Cocci (singular coccus)
Cocci are usually round in shape but can also be oval, elongated, or flattened on one side. Typically these types of bacterial cells are spherical in shape. Cocci appear in several characteristic arrangements, depending on the plane of cellular division, and whether the daughter cells stay together following division.
Diplococcus: Cocci divide to reproduce the cells in various arrangements. The cells that remain in pairs after division are called Diplococcus.
Streptococci: Those that divide and remain attached in chain-like patterns are called streptococcus bacteria.
Tetrad: When the bacteria gets divided into two planes and remain in a group of four, they are called tetrad.
Sarcinae: Those that divide into three planes and remain attached in cube-like groups of eight, are known as circinate.
Staphylococci: When the bacterias divide into multiple planes to form clusters or broad sheet-like arrangements.
Bacilli (singular bacilli )
These are rod-shaped bacteria. “Bacillus” has two meanings in microbiology. As we have just used it, bacillus refers to a bacterial shape. When capitalized and italicized, it refers to a specific genus. For example, the bacterium Bacillus anthracis is the causative agent of anthrax. Bacillus cells often form long, twisted chains of cells.
- Bacilli are not arranged in complex patterns as compared to coccus. Most bacilli occur in pairs and are called Diplobacilli. But some species also occur in chain-like forms called Streptobacilli, for example, Bacillus subtilis.
- Some bacilli also have to staw shape. Some have tapered ends like cigars, still, there are ones that look so much like coccus and are called Coccobacilli.
Spirilla (singular Spirillum)
Spirilla are helically spiral-shaped bacteria. They have one or more twists and are never straight. They are basically rods with helical curves like a corkscrew and have fairly rigid bodies. Spirilla uses propeller-like external appendages called flagella for movement and locomotion.
Vibrio (singular Vibrion)
These are comma-shaped bacterias which are basically rods curved at one end. They are the causative agents of many diseases.
Example: Vibrio cholerae, Vibrio vulnificus.
There are also various other classifications of bacteria on the basis of shape and arrangement.
- There is yet another group of bacteria much like spirals are helical and flexible, called spirochetes. Spirochetes move by means of axial filaments, which resemble flagella but are contaminated within a flexible external sheath.
- In addition to these, there exist star-shaped bacteria (genus Stella ), rectangular and flat cells (Halophilic archaea ) of the genus Haloarcula and triangular cells.
Experiments have shown that the shape f bacterium is defined by their genetic makeup. Genetically, most bacteria are monotropic that is they maintain a single shape in their life span. Defining them becomes difficult once their shape gets altered. This happens mainly because of varying environmental conditions. Moreover, some bacteria are genetically pleomorphic that is they can have many shapes, not just one. Such as Rhizobium and Corynebacterium.
We will discuss its components according to the following organization:
- The external structure of the cell wall
- The cell wall itself
- Structures internal to the cell wall
EXTERNAL STRUCTURE OF THE BACTERIAL CELL
Apart from different size, shape and arrangement of bacteria, the external structure of a bacterial cell consists of the following:
Most of the prokaryotes secrete on the surface a substance called the glycocalyx. The glycocalyx is the term used for substances that surround cells. It is a gelatinous polymer external to the cell wall. It basically has protective effects on the bacteria. In some, it is described as a capsule and is a viscous covering attached firmly to the cell wall.
Bacterial flagella are hair-like, helical appendages protruding through the cell wall. They are responsible for bacteria motility or swimming in the medium. On the basis of the location of the flagella, they are classified as
- peritrichous (distributed over the entire cell)
- polar (at one or both poles or ends of the cell)
- monotrichous (a single flagellum at one pole)
- lophotrichous (a tuft of flagella coming from one pole)
- amphitrichous (flagella at both poles of the cell)
Fimbriae And Pilli
Fimbriae are also hair-like appendages but smaller than flagella. They can occur both ways as on the poles or may be evenly distributed over the surface of the cell. Whereas, pilli are usually longer than fimbriae and vary from few to numerous. They are least involved in motility but are involved in DNA transfer and are thus also offered as sex pilli.
THE CELL WALL OF BACTERIA
The bacterial cell wall is a complex, semi-rigid structure responsible for the shape and rigidity of the cell. It is the protective casing around the plasma membrane of the bacteria. Its main function is to prevent the bacteria from expanding and eventually bursting out because of the uptake of water because most of the bacteria live in a hypotonic environment. It prevents the bacteria from the external environment and excessive pressure and temperature.
To sum up, the internal structures of the bacterial cell including the cell membrane help in various metabolic functions of the cell. Different organelles are responsible for different metabolic reactions in bacterial cells. The prokaryotes have no well-defined nucleus thus the genetic material is present as a nucleoid. The genetic material (nucleoid), ribosome, mesosome, vacuole, plasmid, mesosome, etc all function together for the survival and proper functioning of the bacterial cell.
Therefore, there is a lot to explore about bacterial morphology. There are various sizes, shapes and arrangements of bacteria. This field has great scope. Keep reading!