Bacteria (singular: bacterium) are prokaryotic, generally unicellular organisms, which exist as single cells or as cell clusters. They are among the organisms that are too small to be visible to the naked eye. Thus, they can only be seen with the aid of a microscope. An exception to this is Thiomargarita and Epulopiscium which are visible without a microscope.
The presence of bacteria may be considered ubiquitous. They are, easily, the most abundant of microorganisms, and are present virtually everywhere. Bacteria grow in a multiplicity of environments ranging from hot sulfur springs (65 ° C) to deep freezers (−20 ° C), from high (pH 1) to low (pH 13) acidity and high (0.7 m) to low osmolarity (water). In addition, they can grow in both nutritionally rich (compost) and nutritionally poor (distilled water) situations. Hence, although each organism is uniquely suited to its own particular environmental niche, and rarely grows out of it.
Majority of bacteria do not cause disease but serve as useful scavengers in the breakdown of organic matters and its re-synthesis into living organisms; they are essential in the cycle of life. The power to invade tissues and cause diseases in humans, other animals, and plants is regarded by most bacteriologists as a form of aberration because it is self-defeating; that is the host is destroyed by the invasion.
This article shall encompass briefly the major cellular structures usually encountered in the bacteria. It will provide answers to the following questions.
- What is the structure of a bacterial cell?
- What is found in a bacterial cell?
- What are the general characteristics of bacteria?
- What is the typical size of a bacterial cell?
- What are the functions of slime layers and capsules?
- How do capsules and slime layers differ?
- How does the bacterial capsule or slime layer contribute to pathogenicity?
- What is the function of the cell wall?
- What is the function of flagella in a bacterial cell?
- What are the different arrangements of bacterial flagella?
- How are bacteria classified according to the number and arrangement of flagella?
- What is the functional difference between flagella and pili?
- What are Fimbriae and what is their function?
- What is the function of the fimbriae in a bacterial cell?
- What is the function of the pili?
- What is the difference between pili and fimbriae?
- What is the cytoplasmic membrane made of?
- Where is the cytoplasmic membrane found in the cell?
- What is the function of the cytoplasmic membrane?
- What is mesosome in prokaryotes?
- What is the function of mesosomes in bacteria?
- Where is cytoplasm found?
- What is the function of cytoplasm?
- What is the function of a ribosome in a bacterial cell?
- Which types of ribosome are found in bacteria?
- What is the function of the nucleoid?
- What is difference between nucleus and nucleoid?
- What are the types of inclusion bodies?
- What is the function of inclusion bodies?
- What is plasmid and its function?
Typical bacterial cell
Bacteria being prokaryotic in nature are much simpler in comparison to the eukaryotic cell. In addition to this, they have three distinct characteristic features, namely:
- an extensive endoplasmic reticulum
- essentially lack a membrane-bound nucleus and
Much of bacterial cell structure is well understood, thanks to the successes recorded in special histological techniques and electron microscopy.
Different species of bacteria have varying shapes and sizes but generally, bacteria have a size range of 0.5 µm to 0.3 µm in diameter and 0.5 µm to 10 µm in length. The diagram below portrays the structures and features of a typical bacterial cell.
Bacterial Cell Structure and Function
1. Slime layers and capsules
The outermost surface of a bacterial cell consists of a layer of excreted polysaccharide material. This viscous material that essentially forms a covering layer or a sort of envelope around the cell wall material is called slime layer if it is loose and slimy. In some bacterial species, the material is compact and rigid, in which case it is called capsule.
The slime layer/capsule provides protection to the organism against adverse environmental conditions. Thus, it protects the cell against desiccation, chemical antimicrobial agents, and antibiotics as well as phagocytosis by immune cells.
In addition to its protective function, the extracellular polysaccharides (EPS) may invariably promote attachment of bacteria to each other and to the substratum in biofilms (for example, Streptococcus mutans). It can also function in virulence. Streptococcus pneumoniae provides a classic example as it depends upon its polysaccharide capsule for pathogenicity.
2. Cell wall
Directly below the cell surface layer (slime layer/capsule) is the cell wall. The cell wall is one of the most important structures of the bacterial cell and one of the things which sets it apart from animal cells.
The bacterial cell wall has two major roles to play:
- It protects the cell against osmotic rupture particularly in diluted media, and also against certain possible mechanical damage(s). Any breakdown in, or malformation of, the cell wall structure would lead to the loss of cytoplasmic contents and, consequently, death of the cell.
- It is responsible for the rigidity and shape of the cell, their subsequent major division into Gram-positive and Gram-negative microorganisms, and their antigenic attributes.
Although in some bacterial species, the cell wall may be flexible, it is generally associated with a rigid nature.
Bacterial cell wall consists of a specific alternating arrangement of molecules of N-acetylglucosamine and N-acetylmuramic acid to form a glycan chain. A tetrapeptide string of amino acids attached to each molecule of N-acetylmuramic acid, while pentaglycine bridges interlink the tetrapeptide strings of adjacent glycan chain. The resulting polymeric structure is called peptidoglycan or muran.
The peptidoglycan-based cell wall is unique to bacteria. It is absent in eukaryotic cells. It is the target of action of some antibiotics, which can selectively kill bacteria with little or no harm to the host.
Read Also: Microorganisms of Pharmaceutical Interest
Flagella (singular: flagellum) refer to thread-like structures attached to the surface of some bacteria. They enable bacteria to move and cells lacking flagella are nonmotile. Flagella are approximately 0.01 µm in diameter and 15 µm in length. They are made up of repeating units of a simple protein called flagellin.
Where the filament enters the surface of the bacterium, there is a hook in the flagellum, which is attached to the cell surface by a series of complex proteins called the flagellar motor. This rotates the flagellum like a propeller up to 300x per second, causing the bacterium to move 200 times their own length per second through the environment.
The numbers of flagella per bacterial cell vary with bacterial species. Some have a single, polar flagellum and thus are described as monotrichous, whereas others are flagellate over their entire surface (peritrichous). A single flagellum (or multiple flagella) can extend from both ends of the cell – if so, the bacterium is said to be amphitrichous. A tuft intermediate form (lophotrichous) that extend from one end or both ends of the cell also exist.
Much like flagella, fimbriae consist of protein subunits. Fimbriae are, however, shorter (3 µm), finer, and more rigid than flagella. They are not involved in motility rather they serve as instruments of attachment of bacteria to surfaces and tissues, especially in the course of infection. Fimbriae are also responsible for haemagglutination and cell clumping in bacteria. Among the best-characterized fimbriae are the type I fimbriae of enteric (intestinal) bacteria.
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