BACTERIA SHAPES, SIZE AND CELL STRUCTURE NOTES

KINGDOM PROKARYOTAE (MONERA)

Kingdom Prokaryotae / Monera consists of organisms with prokaryotic cells.

In Greek the word Pro means “before” and karyon means nucleus. So prokaryotes are cells that do not have true nucleus structure. It mainly includes bacteria.

Microbiologists place bacteria in two major groups.

  1. Eubacteria (In Greek it means “true bacteria”)
  2. Archaeobacteria (In Greek it means “ancient bacteria”). It is a small division.

SIZE OF BACTERIA

The size is from 0.1 to 600 µm.

The smallest bacteria are present in the genus Mycoplasma with the size of 100 to 200 nm in diameter. It is about the size of the largest viruses (poxviruses).

Escherichia coli is 1.1 to 1.5 µm wide and 2.0 to 6.0 µm long.

Some spirochetes may reach 500 Vim in length.

Staphylococci and Streptococci are 0.75—1.25µ in diameter.

A huge bacterium (Epulopiscium fishelsoni) has been discovered in the intestine of the brown surgeonfish (Acanthurus nigrofuscus). Its size is 600 µm by 80 µm (a little smaller than a printed hyphen). It is now clear that a few bacteria are much larger than the average eukaryotic cell.

SHAPES OF BACTERIA

Most of the bacterial species have constant characteristic shapes. However, some are pleomorphic and present in many shapes. On the basis of shapes bacteria can be classified as cocci, bacilli and spirilli.

  1. Cocci

The Cocci are spherical or oval bacteria.

These are arranged according to planes of division.

Division in One Plane:

If division is in one plane it produces two types of arrangements.

Diplococcus Arrangement: When cocci are present in pairs then arrangement is diplococcus (Diplococcus pneumoniae).

Streptococcus Arrangement: When cocci are present in long chain of cells then the arrangement is streptococci.

Division in Two Planes:

When the division of cell is in two planes it will produce a tetrad arrangement. A tetrad is a square of 4 cocci.

Division in Three Planes:

When the division is in three planes it will produce a sarcina arrangement. Sarcina is a cube of 8 cocci.

Division in Random Planes:

When division occurs in random planes it will produce a staphylococcus arrangement. Here the cocci are arranged in irregular, grape-like clusters (Staphylococcus aureus).

  1. Bacilli

Bacilli are rod-shaped bacteria. They all divide in one plane producing four kinds of arrangements:

Bacillus:

It is a single cell of bacteria.

Streptobacillus:

It is a chain of bacilli.

Diplobacillus:

When rod shaped bacteria occur in pairs then it is called diplobacillus.

Coccobacillus:

When rod shaped bacteria are like coccus.

Examples of Rod Shaped Bacteria:

Escherichia coli, Bacillus subtilis, Pseudomonas

  1. Spirilli (Spiral Shaped Bacteria)

These bacteria are spirally coiled.

Spirals can be in one of three forms, a vibrio, a spirillum, or a spirochete.

Vibrio:

It is curved or comma-shaped rod.

Spirillum:

It is a thick, rigid spiral.

Spirochete:

It is a thin, flexible spiral.

Examples of Spiral Shaped Bacteria:

Vibrio, Hyphomicrobium

Other Shapes of Bacteria:

Some other shapes of bacteria are trichome forming, sheathed, stalked, square, star shaped, spindle shaped, lobe shaped and filamentous bacteria.

All bacteria have cell membrane, cytoplasm, ribosome and chromatin bodies.

Most of bacteria have a cell wall, which gives shape to the bacterial cell.

The structures like capsule, slime, flagella, pili, fimbriae and granules are found in some/different bacteria.

STRUCTURE OF A BACTERIAL CELL:

We will study structure of a bacterial cell under the following headings:

  1. Flagella and their Functions

These are very thin, hair like structures that come out through the cell wall. The flagella originate from basal body. It is a structure just beneath the cell membrane in the cytoplasm. Flagella are made up of protein flagellin.

Classification of bacteria on the basis of presence, attachment and number of flagella.

On the basis of presence, attachment and number of flagella, bacteria are classified as:

Atrichous:

The bacteria without any flagella are called atrichous.

Monotrichous:

These have a single polar flagellum.

Lophotrichous:

These have a tuft of flagella at one pole only,

Amphitrichous:

These have tuft of flagella at both poles.

Peritrichous:

In this condition flagella surrounds the whole cell.

Most of bacilli and spiral shaped bacteria have flagella. Cocci very rarely have flagella.

Functions of Flagella:

Motility:

Primary function of flagella is to help in motility.

Chemotaxis:

They also help to detect and move in response to chemical signals (stimulus). It is a type of behavior called as chemotaxis.

  1. Pili and their Functions

Definition:

These are hollow, non-helical, filamentous appendages.

Presence:

True pili are only present on gram-negative bacteria.

Size:

Pili are smaller than flagella.

Chemical Composition:

They are made up of special protein called pilin.

Functions:

Pili are not involved in motility. Following are their functions:

(a) Mating Process (Conjugation): They are involved in mating process called conjugation.

(b) Attachment: Some pili are used for attachment of bacteria to various surfaces.

  1. The Cell Envelope (The Outer Wrapping of Bacteria)

In bacteria the complex of layers external to the cell protoplasm is called as cell envelope.

It includes capsule, slime and cell wall.

Capsule:

Some bacteria produce capsule. It is tightly bound to the cell.

Capsule is made up of repeating polysaccharide units, or of protein, or of both.

It has a thicker, gummy nature that gives sticky characters to the colony.

Slime:

It is a loose, soluble cover of macromolecules called as slime capsule. It is present in some bacteria.

Slime provides greater pathogenicity to bacteria and protects them against phagocytosis.

Cell Wall:

Beneath the extracellular substances and external to cytoplasmic membrane is cell wall.

It is a rigid structure and determines the shape of bacterium.

Cell wall also protect the cells from osmotic lysis.

Cell wall is only absent in mycoplasmas.

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