Structure and classification of bacteria || pharmacyteach.com

 Structure of Bacteria and classification 

Introduction

Bacteria are unicellular prokaryotic organisms, lacking well-defined nuclei and membrane-bound organelles. These organisms are essential to life on Earth. A huge variety of shapes and sizes of Bacteria are present ranging from minute spheres to larger filamentous chains.

 

Bacteria are known for their simple body structure. A cell envelope consists of an interior cytoplasmic membrane, the cell wall, and- in some species of bacteria- an outer capsule.

Structure of a bacteria

The structure of a bacterium is relatively simple, consisting of three main parts:

The cell membrane of bacteria:-

The cell membrane of bacteria, also known as the cytoplasmic membrane, is a selectively permeable barrier that surrounds the cytoplasm of bacterial cells.

 It is made up of a lipid bilayer, which is composed of phospholipids, glycolipids, and other lipids. The membrane plays essential roles in maintaining the cell's shape, protecting the cell from its environment, and controlling the movement of molecules in and out of the cell. 

Additionally, it also contains various proteins, such as transporters, pumps, and channels that allow specific molecules to enter and exit the cell. The cell membrane is also an important target for antibiotics, which can disrupt its function and lead to the bacterium's death.

 

label diagram of bacteria 

The cell wall of bacteria:-

Each bacterium is enclosed by a rigid cell wall composed of peptidoglycan. Its function is to give the cell its shape and protect the cytoplasmic membrane from the environment. Call wall composition varies widely amongst bacteria and is used to analyze and differentiate bacterial species.

The capsule is made up of polysaccharides. Its function protects the bacterium from phagocytosis and keeps it from drying out.

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The cytoplasm of bacteria:-

The cytoplasm is the site where the functions of cell growth, metabolism, and replication take place. It is a gel-like matrix composed of water, enzymes, wastes, and gases and contains structures such as ribosomes, a chromosome, and plasmids[2]. The single chromosome is localized in a region called the nucleoid. Plasmids are small genetic structures made up of circular pieces of DNA. Ribosomes of Bacteria are smaller than Eukaryotic Ribosomes. They translate the genetic code from the molecular language of the nucleic acids to that of amino acids and assist in the production of proteins.

Some bacteria also have additional structures, such as pili (short, hairlike projections) that help them adhere to surfaces and flagella (long, whip-like appendages) that help them move. Some bacteria also have specialized structures called spores, which allow them to survive harsh conditions.

Pili of bacteria:-

Many bacterial species have pili which are small hair-like projections. These outgrowths aid in the attachment of Bacteria to other cells and surfaces. Without these structures, bacteria lose the ability to infect because they are unable to attach to host tissue.

There are several types of pili, including:

Fimbriae: Fimbriae are thin, finger-like projections that are found on the surface of many bacteria. They are used for adhesion and are involved in the attachment of bacteria to host cells or tissues.

 

Sex pili: Sex pili are specialized pili that are involved in the transfer of genetic material between bacteria during conjugation. They are found on the surface of certain bacteria and are used to "mate" with other bacteria, transferring a piece of DNA called a plasmid from one bacterium to another.

 

Flagella: Flagella are long, whip-like appendages that are used by bacteria for motility. They are found on the surface of some bacteria and are used to propel the bacterium through a liquid medium.

 

Pili are important for the survival and reproduction of many bacteria, and they have been the subject of much research due to their potential role in the development of bacterial infections.

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Classification of bacteria:-

Classify bacteria on the bases of cell structure:-

Bacteria are classified into two main groups based on their cell structure: gram-positive and gram-negative. The classification is based on the reaction of their cell walls to a staining technique called the Gram stain, developed by the Danish bacteriologist Hans Christian Gram in 1884.

Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which causes them to retain the crystal violet stain used in the Gram stain and appear purple or blue. Examples include Staphylococcus and Streptococcus species.

Gram-negative bacteria have a thinner peptidoglycan layer, as well as an outer membrane containing lipopolysaccharides (LPS)[1]. This outer membrane causes gram-negative bacteria to lose the crystal violet stain and instead take up a counterstain, safranin, which causes them to appear pink or red. Examples include Escherichia coli and Salmonella species.

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 Classify bacteria on the bases of their nutritional characteristics.

These include:

·         Autotrophic bacteria: These bacteria can produce their own food through processes such as photosynthesis or chemosynthesis. Examples include cyanobacteria and purple sulfur bacteria.

·         Heterotrophic bacteria: These bacteria are unable to produce their own food and instead obtain it by consuming organic matter. Examples include E. coli and Streptococcus.

 

Classify bacteria on the bases of oxygen requirement:-

 

·        Facultative anaerobic bacteria: These bacteria can grow in the presence or absence of oxygen. Examples include E. coli and Staphylococcus aureus.

 

·         Aerobic bacteria: These bacteria require oxygen for growth. Examples include Pseudomonas aeruginosa and Streptococcus pneumonia.

 

·         Obligate anaerobic bacteria: These bacteria cannot grow in the presence of oxygen and require an oxygen-free environment. Examples include Clostridium perfringens and Bacteroides fragilis.

Classify bacteria on the bases of the formation of spores:-

Spores are structures that allow bacteria to survive in adverse conditions, such as high temperatures, high salt concentrations, or low moisture. There are two main types of bacteria based on their ability to form spores:

·         Sporulating bacteria: These bacteria can form spores, which are usually highly resistant to environmental stressors. Examples include Bacillus and Clostridium species.

·         Nonsporulating bacteria: These bacteria are unable to form spores and are considered more sensitive to environmental stressors. Examples include E. coli and Staphylococcus aureus.

 Note; It's important to note that not all bacteria that can form spores are pathogenic, and the ability to form spores is not exclusive to pathogenic bacteria. Additionally, the classification of bacteria based on the ability to form spores can be helpful in determining their survival strategies, as well as in identifying them.

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Bacteria can also be classified by their shape:-

which include:

·        Cocci (spherical shaped)

·        Bacilli (rod-shaped)

·        Spirilla (spiral shaped)

·         Cocci: These bacteria are spherical in shape and can be found in clusters, pairs, or chains. Examples include Staphylococcus and Streptococcus species.

structure of cocci 

·         Bacilli: These bacteria are rod-shaped and can be found individually or in chains. Examples include E. coli and Bacillus species.

structure of Bacilli

·         Spirilla: These bacteria are spiral-shaped and can have a variety of forms, such as helical, curved, or hooked. Examples include Vibrio and Spirochaetes species.

Structure of spirilla 

·         Filamentous bacteria: These bacteria are long and thread-like, and can be found in clusters or as single cells. Examples include some species of Actinomycetes and Nocardia.

It's important to note that the shape of a bacterium can be influenced by the growth conditions, and a single species of bacteria can have different forms under different conditions. Additionally, the shape of a bacterium can also be useful in identifying it and determining its properties such as motility, metabolic pathways, and pathogenicity.

 

Bacteria are classified based on their motility

Bacteria can also be classified based on their motility, which refers to their ability to move. The main types of motility in bacteria are:

·         Motile: These bacteria can move using flagella, pili, or other appendages. Examples include E. coli and Salmonella species.

·         Nonmotile: These bacteria cannot move and are considered sessile. Examples include Streptococcus and Staphylococcus species.

Note; It is important to note that the classification based on motility is not absolute and some bacteria may show different motility depending on the growth conditions. Motility can also be useful in identifying a bacteria and determining its properties such as pathogenicity.

All of these classifications provide important information about the biology and behavior of bacteria and can help in identifying and understanding different bacterial species.

References:-

1 . Costerton, J. W., J. M. Ingram, and K. J. Cheng. "Structure and function of the cell envelope of gram-negative bacteria." Bacteriological reviews 38.1 (1974): 87-110.

2. Casjens, Sherwood. "The diverse and dynamic structure of bacterial genomes." Annual review of genetics 32.1 (1998): 339-377.

3 . Jacques M. Chiller, Barry J. Skidmore, David C. Morrison, and William O. Weigle 

y 1, 1973

70 (7) 2129-2133

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