Structures in Bacteria: Definition, Diagram, Classification, Microbes, Cells

What Is Bacteria?

Bacteria are single-celled microorganisms that belong to the domain prokaryote since they do not have a true nuclear membrane. Bacteria are present just about everywhere, including in hot hydrothermal vents, and the human gastrointestinal tract. Hence, bacteria are important in the different ecosystems since they are known to contribute to nutrient cycling, decay of organic material and easing the balance of ecosystems. They are essential in human health as commensal organisms that help in digestion and as agents involved in biotechnological uses like antibiotic synthesis and fermentation. But there is another group of bacteria that can be pathogenic, and become carriers of diseases that threaten society.

Classification Of Bacteria

Bacterial classification is discussed below

Based On shape

Based on shape: From the above classification, the bacterial cells are classified by their shapes as:

• Cocci – spherical

• Bacilli – rod-shaped

• Vibrios – comma-shaped

• Spirilla – spiral-shaped

Based on Gram staining

Based on Gram staining: Be converted into two groups; the first category is called Gram-positive which is the one that retains the crystal violet dye and the second category is called Gram-negative which does not retain the dye.

Other classifications

According to the bacterial oxygen requirements, the bacteria can be again divided into aerobic, which require oxygen for their survival, anaerobic, which do not survive in the presence of oxygen, and facultative anaerobes, which can survive in both conditions.

Differences Between Gram-positive and Gram-negative Bacteria

Here are a few differences between gram-positive and gram-negative bacteria:

Basic Bacterial Cell Structure

The cell structure is discussed below:

Cell Wall

It has a thick layer of peptidoglycan in the cell wall and when stained with gram, the purple dye, crystal violet, is retained hence referred to as gram positive. On the other hand, the cell wall of the gram-negative bacteria has a relatively thinner peptidoglycan layer and is located between the inner and the outer membrane that contains lipopolysaccharide in the outer membrane part which makes them differ when stained with gram stain.

Functions Of The Cell Wall

Their structure has the support of the cell wall and protection from pressure changes in osmotic concentration. It also acts as a protective shield for the bacterial cell against unwanted materials and forms part of the structure and firmness of the bacterial cell.

Diagram of Gram-positive vs. Gram-negative bacteria

Plasma Membrane

The outer layer of the bacterial cell includes a plasma membrane consisting of a phospholipid bilayer with proteins integrated. The plasma membrane is a selectively permeable phospholipid bilayer that regulates the movement of substances into and out of the cell while maintaining the internal cellular environment.

Role In Nutrient Transport And Cell Signalling

It allows the transport of nutrients into the cell and the removal of waste products. Some molecular transport across Bacterial membranes occurs through membrane proteins, and about signalling, there are receptors on the bacterial membrane that give the bacteria the ability to note changes in environments leading to phenomena such as Chemotaxis and Quorum sensing.

Cytoplasmic Structures

The cytoplasmic structures are listed below

Cytoplasm

The cytoplasm is a gel-like matrix containing ribosomes, enzymes, metabolites, inclusion bodies, and the nucleoid region. Since bacteria are prokaryotes, they lack membrane-bound organelles.

Role In Metabolic Activities

Biomolecules are assigned to catalyse biochemical reactions including glycolysis, the citric acid cycle and synthesising amino acids and nucleotides in bacteria to produce energy and regulate their cellular processes.

Nucleoid

The nucleoid in bacteria is an area found in the cytoplasm of the bacteria that has a single circular chromosome. Hence while human cells contain genetic DNA this DNA is not contained within a membrane-enclosed nucleus but is more structured and condensed by proteins. For these and other purposes, the nucleoid also contains proteins that are involved in the processes of DNA replication transcription and repair.

Differences from the eukaryotic nucleus

While the eukaryotic nucleus is surrounded by a membrane and has multiple linear chromosomes the nucleoid of bacteria is not membrane surrounded. It is negative for histones and introns and its hereditary materials are characteristically shorter and denser. The nucleoid is also involved in bacterial cell division where through the breakdown of the cell the replicated DNA is divided between the daughter cells.

Ribosomes

Structure and function in protein synthesis

Ribosomes are cell organelles manufactured from RNA and protein. The ribosome is of two types – prokaryotic and eukaryotic – The prokaryotic ribosome is smaller in size than the eukaryotic ribosome. These include a big and a small portion of the ribosome which combine during translation. Ribosomes are very important structures that synthesize proteins through the process of translation, in which the mRNA strand codes for amino acid sequences that in turn fold into functional proteins. Bacterial ribosomes are 70S ribosomes composed of 50S and 30S subunits. They are the target of several antibiotics such as tetracycline and streptomycin.

External Structures

The external structures are listed below

Flagella

Flagella are thin structures that protrude from the bacterial cell surface and usually resemble a whip. The bacterial flagellum rotates using the proton motive force, enabling bacterial movement (motility).

Types Of Flagellar Arrangements

• Monotrichous: This showed only one flagellum located at the end of the cell.

• Lophotrichous: Conclusions All flagella at one pole of the cell or multiple flagella at one end of the cell.

• Amphitrichous: Flagella present in a single number and localized at both ends of the cell.

• Peritrichous: Flagella present all over the surface of the cell

Diagram showing different flagellar arrangements

Pili and Fimbriae

Pili and fimbriae are short, hair-like protein appendages present on the surface of many bacteria. They are composed of the protein pilin.

Role In Adhesion And Conjugation

• Fimbriae and pili allow bacteria to attach themselves to a surface host cell, or another bacterium. These adhesions play a very significant role during colonisation, biofilm formation and the infection processes.

• Pili are also used in conjugation in which DNA (plasmids) is transferred from one bacterial cell to another. Conjugative pili are the structures or tubes which help in the process wherein the donor cell comes very close to the recipient cell and codes that can render resistance to antibiotics or any other positive quality are transferred.

Capsule And Slime Layer

Both capsules and slime layers are external layers of polysaccharides (and in some cases proteins) that surround the bacterial cell wall.

Differences Between Capsule And Slime Layer

Composition: Capsules are layers that are well compacted and close together, while slime layers are individual layers that are not very much in contact with each other.

Function: Capsules shield the bacteria from phagocytosis and drying, while slime layers are useful in holding the bacteria to surfaces and obtaining nutrients.

Importance In Bacterial Virulence And Protection

Capsules increase bacterial virulence by protecting bacteria from phagocytosis, preventing desiccation, and helping them adhere to host tissues. The slime layers assist bacteria in surviving in unfriendly surroundings and combat the antimicrobial opponents.

Specialised Structures

The specialised structures are listed below

Endospores

Endospores are obtained when bacteria go through sporulation since it is a method of surviving unfavourable conditions. They include the DNA body which is heavily compressed and packaged with the spore coat and cortex surrounding it.

Function In Bacterial Survival Under Harsh Conditions

Endospores allow bacteria to be in extreme conditions such as high temperature regimes or lack of humidity and chemical treatments and withstand them because the genetic material and the rest of the cell structures are protected.

Plasmids

Plasmids are small circular, self-replicating pieces of DNA that occur in addition to the bacterial chromosome. It can self-multiply within the bacterial cell and is common in conjunction with chromosomal DNA.

Role In Antibiotic Resistance And Genetic Variation

Thus, plasmids are essential in bacterial adaptation and existence at large. These organisms often hold genes that code for traits that enable bacteria to become resistant to antibiotics and their impact. Such genes can be of metabolic activities, toxins, or other factors making bacteria competitive and suitable for survival in given conditions carried by the plasmids.

Intracellular Inclusions

The intracellular structures are discussed below:

Inclusion Bodies

Inclusion bodies are diverse structures within bacterial cells that serve various functions

• Glycogen granules: Used to store glucose as an emergency energy store during the availability of the nutrients to be used in emergencies when the nutrients are inadequate.

• Gas vesicles: Supply floatation to control the position of the bacterium in water to ensure adequate aeration and irradiation.

Importance In Nutrient Storage

In other aspects, the inclusion bodies serve as nutrient granules where glycogen is stored thus allowing a period of starvation or stress in the bacteria. Gas vesicles help in improving the correct positioning in the environment for efficiency in the uptake of energy and use.

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