6.3 Bacteria Ecology and Diversity
Bacteria have a wide range of habitats. They exhibit and mode of nutrition.
6.3.1 Occurrence
Bacteria are found everywhere on this planet varying in their size, shape living and dead organisms, water, soil, milk, and skin, 6.3.2 Major Groups of Bacteria life exist such as the bodies of forests, etc.
Dr. Hans Christian Gram (1884) divided bacteria into two major groups by using the staining techniques, i.e. Gram-positive and Gram-negative. His grouping depends upon chemical makeup, permeability, metabolism, presence of endospores, physiological characteristics, growth, and nutrition in bacteria.
6.4 Structure, Shape, and Size of Bacteria
A typical bacterium consists of the cell wall, cell membrane, nuclear region, cytoplasm, and also other structures outside the cell wall.
6.4.1 Structure and Chemical Composition of
Bacterial Cell Wall All bacteria possess cell walls except Mycoplasma. The cell wall protects the cell and also gives it a definite shape. It is made up of peptidoglycan (sugar-protein complex found in Prokaryotes) and is rigid.
6.4.2 The cell walls of Gram-positive and Gram-negative bacteria
Based on the variations in the chemical components of the cell walls, Danish physician, Hans Christian Gram, developed a staining technique in 1884 and divided bacteria into two groups i.e., Gram-positive and Gram-negative bacteria.
Gram Positive:
These bacteria are stained blue-purple with crystal violet dye. They have a thick wall of peptidoglycan. They retain dye when cells are washed with an organic solvent like alcohol.
Gram-negative:
These bacteria have thinner layers of peptidoglycan. They lose the dye easily when rinsed with alcohol and stain pink. The thin peptidoglycan layer is externally covered with a layer of lipopolysaccharides, lipoproteins, and phospholipids. Thus they are more resistant than gram-positive against antibiotics (lipopolysaccharide impedes the entry of antibiotics).
Slimy Capsule:
known as a slimy capsule. It is made of polysaccharide which helps in defence and adhering to host tissues. The encapsulated bacteria cause disease while the same bacteria without capsules do not cause disease, e.g., pneumonia causes pneumonia.
6.4.3 Shape and Size of Bacteria
There are three main shapes of bacteria; Spherical, Straight, and Spiral shape.
Spherical or Cocci (Singular Coccus):
Cocci are spherical in shape. They are non-motile because they lack flagella, and may be single or colonial. The colonial may be diplococci (group of two cells) tetrad (group of four cells), octet (packet of eight cells), Streptococci (a long chain of cells), and Staphylococci (a bunch of cells like grapes). Examples of Cocci are Streptococcus pneumoniae, Neisseria meningitidis etc.
Straight Shape or Bacilli (Singular Bacillus):
Bacilli are straight or rod-shaped bacteria. They possess flagella and are motile. Most of them occur either singly or colonially. They are found in pairs (diplobacillus), very short and oval-shaped (coccobacilli), curved and coma-shaped (Vibrio), and stack (Pallisade). Examples of bacilli are Bacillus subtilis, Escherichia coli etc.
Spiral Shaped or Spirochetes:
These are corkscrew-shaped bacteria, flexible, motile, and flagellated. They usually occur singly and seldom form colonies e.g., Helicobacter pylori and Treponema pallidum.
6.4.4 Endospores
Some Gram-positive bacteria produce highly resistant structures known as endospores which during unfavorable conditions serve for the survival of the bacteria. It develops within vegetative cells, so-called endospores. The original cell forms a copy of its chromosome and covers it with a hard wall, water is removed and metabolism stops.
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