Unit3: Bacteria - Structure, Classification, Importance

by Microbiology Doctor dr (doctor_dr)(doctor-dr)

Plant Diversity - Unit 3: Bacteria -  General Structure, Classification, Biological Importance

Bacteria are the tiniest living things with a cellular structure. They are microorganisms that are unicellular prokaryotic creatures. Bacteriology is the study of bacteria.

Their actions, along with those of fungi, are essential for all other creatures because they induce the destruction of organic matter (decomposition), which results in the recycling of nutrients. Some bacteria cause illness, while others are used in a variety of economically significant biotechnological activities such as fermentation, antibiotic manufacturing, chemical production, and so on. Similarly, they are genetic engineering tools. The bacteria, in collaboration with blue-green algae, can fix atmospheric nitrogen, increasing soil fertility.

Occurrence

Bacteria may be found in a variety of habitats, including soil, dust, water, air, in and on animals and plants, and even in hot springs with temperatures of 60 °C or more. One gramme of rich soil is believed to have 100 million bacteria, whereas one cm3 of fresh milk may contain more than 3000 million.

MORPHOLOGY OF BACTERIA

The morphology refers to size, shape, arrangement and structure of bacterial cells.

Size of Bacterial Cell

Bacteria are single-celled and very tiny. The majority of them have a diameter of 0.5 to 1.0 um and a length of 0.1 to 10 um. Bacteria, on the other hand, have a high surface area to volume ratio, which aids in the exchange of nutrients and wastes. This explains bacteria's very fast growth and metabolism. It is also beneficial. Because 5 microorganisms can obtain nutrients relatively near to the surface, there is no need for a circulatory system.

Shape of Bacterial Cell

The shape of a bacterial cell is governed by its rigid cell wall. Most bacteria have constant shape but some have cells that exhibit a variety of shape.

The bacterial shape is important in classification of bacteria. According to their shape the bacteria can be generally classified as:

a. Cocci: Streptococcus pneumoniae, for example, is a sphere-shaped bacterium that causes pneumonia.

b. Bacilli: These are rodlike bacteria, for example Lactobacillus found in milk.

c. Spirilla: They're helical bacteria with a stiff structure. These are generally twisted , turned and curled. A vibrioid spirillum has fewer than one full twist, while helical spirillums have one or more entire twists.

d. Spirochetes: These are flexible and can twist and control their shape, whereas the spirilla are relatively rigid.

Arrangement of Bacterial Cells

Bacterial cells are arranged in a certain way depending on their type. Cocci have the most complicated arrangement patterns.

The arrangement in cocci depends upon the plane of cell division and whether the daughter cells stay together after the division. They can be classified as:

a. Diplococci: The cells divide in one plane and remain attached in pairs.

b. Streptococci: The cells divide in one plane and remain attached to form chains.

c. Tetracocci: Cells divide in two planes and form groups of four cells.

d. Staphylococci: The cells divide in three planes, in an irregular pattern and produce groups of cocci.

e. Sarcinae: The cells divide in three planes, in a regular pattern, producing a cuboidal arrangements of cells.

The bacilli exhibit simple arrangement patterns and mostly occur singly. The most common arrangement patterns are:

a. Diplobacilli: The bacterial cells arranged in pairs.

b. Streptobacilli: The cells form chains.

c. Hyphae: Some species, for example Streptomyces spp form long, branched, multinucleate filaments called hyphae which collectively form mycelium.

STRUCTURE OF A GENERALIZED BACTERIAL CELL

A bacterial cell is made up of several components. Some of these are exterior to the cell wall, while others are inside. Capsules and slime layers, flagella, and pilli are all exterior components. Some structures are only found in a few species.

i. Capsule and Slime Layer 

Outside the cell wall, many bacteria produce a sticky material that forms protective coverings called capsules. This layer is sometimes referred to as the slime layer since it is diffuse. The majority of bacterial capsules are polysaccharides, with a few polypeptide capsules. They are made up of a web of tiny threads. Generally, the capsular material is not very water-soluble. F capsule's glue-like properties aids bacteria in adhering to their substrate or forming colonies. They also prevent bacteriophages from attaching. Antibiotic resistance is raised in disease-causing microorganisms when the capsule is taken.

Some bacteria, both freshwater and marine, produce chains or trichomes that are encased in a hollow tube called a sheath.

ii. Flagella

Most bacteria are motile, and their locomotion organs are flagella, which are hair-like, helical cytoplasmic appendages. At one or both ends of the bacterial cell, flagella can be found. They might appear around the sides of the bacteria or all around it in certain circumstances. Bacteria use their helical flagella to move themselves.

A basal body linked with the cytoplasmic membrane and cell wall, a small hook, and a helical filament make up each flagellum. The chemical makeup of the basal body is unknown, but the hook and filament are made up of identical flagellin subunits organised in eleven helical spirals to form a hollow cylinder.

iii. Pilli or Fimbriae

Pilli or fimbriae are hollow filamentous rods found in both motile and non-motile species. These are made up of a protein called pillin. Some pilli are engaged in cell-to-cell attachment, while others are involved in surface-to-surface attachment, such as to epithelial cells lining human respiratory surfaces, and yet others, known as F pilus or sex pilus, are involved in bacterial mating during sexual reproduction.

iv. The Cell Wall

The cell wall is present beneath the capsule and external to cytoplasmic membrane. It is a rigid structure that gives shape to the cell and prevents the cell from swelling and bursting as a result of osmosis like plant cells. The cell wall is necessary for bacterial growth and development. The cells whose cell walls have been mpletely removed are incapable of normal growth and division.

Gram-positive and Gram-negative Bacteria

Bacteria can be classified as Gram-positive or Gram-negative depending on their cell wall properties. Gram-positive bacteria can be stained with Gram's stain, while Gram-negative bacteria cannot. When opposed to Gram positive bacteria, Gram negative bacteria have thinner walls. The wall is made up of an insoluble, porous network of peptidoglycan, also known as murein, in both situations. It's a sac-like macromolecule made up of parallel polysaccharide chains that are regularly cross-linked by short peptide chains. Gram-positive bacteria's cell walls often contain far more peptidoglycans than Gram-negative bacteria's. Gram-negative bacteria have a smooth, soft lipid coating on the exterior of their murein layer that protects them from antibacterial enzymes (lysozyme).

In some bacteria, e. g., Arhaeobacteria the cell walls do not contain murein and are composed of proteins, glycoproteins or polysaccharides.

v. Cytoplasmic Membrane 

It is found directly under the cell wall and surrounds the bacterial cell's life contents. It is a semi-permeable membrane made up mostly of phospholipids (20–30%) and proteins (about 60 to 70 percent). Most proteins are encased in a bilayer formed by phospholipids. The flexibility of the lipid bilayer allows proteins to move around laterally (fluid-mosaic model).

Most water-soluble compounds are blocked by the cytoplasmic membrane. These chemicals are carried across the membrane by carrier proteins. It also serves as a location for bacterial DNA binding and energy generation (ATP).

vi. Specialized Membranes

Infoldings generate intricate internal structures and enhance the surface area of the cytoplasmic membrane. The following are two significant specialised membranes:

a. Mesosomes: Mesosomes are membrane infoldings on the cell surface. They appear to be linked to DNA during cell division, allowing the two to be separated. After DNA replication, daughter molecules develop, assisting in the creation of new cross-walls between daughter cells.

b. Photosynthetic Membranes: Tubular or sheet-like infoldings are generated by the cytoplasmic membrane in photosynthetic bacteria. These look like thylakoids in Cyanobacteria (blue-green algae) and contain bacteriochlorophyll, which is a photosynthetic pigment. These are photosynthetic locations.

vii. The Cytoplasm

Cytoplasm is the cell substance that is limited by the cytoplasmic membrane. It is divided into three parts: a liquid termed cytosol, a concentrated region of nuclear material, and a ribosome-rich section.

a. Cytosol: The cytosol is a complex, concentrated solution of inorganic acids, amino acids, proteins, peptides, nitrogenous bases, vitamins, enzymes, coenzymes which provides chemical environment for metabolic and cellular activities.

b. Ribosomes: Ribosomes are RNA-protein structures that are mostly found in the cytoplasm and are involved in protein synthesis. The polysomes are viewed as dense particles. Ribosomes become linked with the inner surface of the cytoplasmic membrane during protein production. Each ribosome is made up of two subunits: a 50S and a 30S subunit, both of which are made up of the same quantity of RNA and protein. They are smaller than ribosomes present in eukaryotic cells and settle at a density of 70 Svedberg units (70S).

c. Nucleoid: Bacterial Chromosome: The bacterial chromosome consists of DNA molecule formed of about 5 x 106 base pairs and about 1 mm in length. Very little protein is associated with it. The bacterial chromosome is primitive and usually referred to as nucleoid.

In most bacteria, the DNA is concentrated as a mass of fibres and the region of the cytoplasm containing it stains less than surrounding cytoplasm. is called nucleoid region or nuclear body. In Escherichia coli, the bacterial chromosome is in the form of a ring of double-stranded DNA molecule. The chromosome contains about 1000 specific sites called loci, each concerned with a particular chemical activity.

viii. Plasmids

A bacterial cell's DNA chromosome may be supplemented by plasmids, which are considerably smaller DNA rings. Each plasmid has a few genes and can self-replicate without the help of the main chromosome. Certain plasmids confer antibiotic or disinfectant resistance to cells. They include genes whose products (enzymes) are responsible for the destruction of these chemicals. Others assist in the cleanup of oil spills as well as the production of protein from petroleum. Episomes are plasmids that are capable of integrating into the bacterial DNA chromosome.

ix. Spores and Cysts

Bacillus and Clostridium spp., for example, generate resistant entities called spores either within cells (endospores) or outside the cell (exospores). These are resistant to heat, desiccation, and radiation, and can assist overcome poor growing circumstances. The spores germinate and create a new cell when favourable conditions arise.

In certain bacteria, such as Azotobacter, the entire cell grows into a thick-walled, inactive, resistant structure known as a cyst. When the conditions are favourable for development, they germinate into new individuals.

x. Gas Vacuole

Some aquatic species form gas vacuoles that provide buoyancy. These are hollow, rigid cylinders with a protein boundary impermeable to water, however dissolved gases can penetrate the boundary.

GROWTH IN BACTERIA

The term "growth" in the context of bacteria refers to changes in the overall population rather than a rise in the size or mass of a single organism. It signifies a growth in the number of people and may be quantified. Because the bacteria have a huge surface area to volume ratio, they may obtain food quickly from their surroundings by diffusion and active transport. Temperature, nutrition availability, pH, and ionic concentrations are all variables that influence growth. Under perfect conditions, cell division might occur every 20 minutes, allowing a single cell to produce 10' daughter cells every 15 hours if conditions remained favourable. At galeach division, the number of cells doubles. The process is known as exponential growth, and the time gap is referred to as generation time.

The initial stage of exponential development, known as the lag phase, appears to be devoid of growth. The bacteria adjust to their new surroundings during this phase. It is followed by the log phase, which is characterised by fast growth. Later on, development appears to halt as resource rivalry becomes more intense. The rate at which new cells are produced comes to a halt. The number of live cells does not change. Finally, there is a decrease in the viable (alive) phe population. This is referred to as the decline phase. The buildup of hazardous waste products and the depletion of vital nutrients cause this phase.