Kingdom : Monera

Biological Classification of Class 11

The kingdom Monera (monos-single; Dougherty & Allen, 1960) includes all prokaryotes. The kingdom Monera is divided into two major groups, the Eubacteria (true bacteria) and the Archaebacteria (primitive bacteria). Eubacteria include several sub groups, the most distinctive of which is Cyanobacteria (blue green algae).

Other Monera members include Actinomycetes (filamentous bacteria), Mycoplasma, Rickettsiae etc.

Eubacteria

Antony von Leeuwenhoek (1675), a Dutch naturalist discovered bacteria and interestingly termed those as tiny animalcules. Linnaeus (1758) called them vermes.

Bacteria were traditionally believed to be microscopic unicellular plants without chlorophyll that reproduce by fission.

Ehrenberg (1838) first of all coined the word Bacteria (Gk. bakteron = small rod) for these small organisms.

Occurrence

Bacteria are ubiquitous, i.e., they occur anywhere and everywhere. Some thermophilic bacteria can withstand the temperature up to 78°C while some psychrophilic bacteria occur upto the temperature of -19°C.

Size of bacterial cell

Bacteria are very small and their size generally ranges from 0.2-1.5 mm is length.

Dialister pneumosintes is the smallest bacterium (0.15-0.3 mm long and 100-200 mm diameter).

Longest bacterium - Spirillum volutans (60 µ in length).

A huge bacterium, Epulopscium fishelsoni is as large as 600 µ.

Beggiatoa is a large sulphur bacteria (10 - 12 µ long).

Shape of bacteria

Cohn (1872) recognized four types of shapes in bacteria – Coccus, Bacillus, Spirillum and Vibrio.

Coccus (Gk. Kokkos = berry)

Spherical or aflagellate, sub-divided into following six groups on the basis of cell arrangement.

  • Monococcus : Only single spherical cell represents the bacterium.
  • Diplococcus : Two cocci divide in one plane and remain attached in pairs.
  • Streptococcus : Several cocci divide in one plane and remain attached to form chains of different lengths.
  • Tetracoccus : Four cocci divide in two planes at right angles to one another and form groups of four.
  • Staphylococcus : Cocci divide in several planes resulting in formation of irregular bunches of cells, sometimes resembling a cluster of grapes.
  • Sarcina : Cocci divide in 3 planes at right angles to one another and resemble cubical packets of 8.

Monera

Fig. Different shapes of Bacteria

Bacillus :

Rod like or cylindrical forms, either singly or may be arranged differently. These are generally flagellated and are of the following types:

  • Monobacillus : The bacteria occur singly.
  • Diplobacillus : These are arranged in pairs.
  • Palisade Bacillus : These are lined side by side like matchsticks.
  • Streptobacillus : These form a chain of rods.

Sprillum :

Coiled forms of bacteria exhibiting twists with one or more turns are called spirilla.

Vibrio :

Bacteria with less than one complete twist or turn are called vibrio. These resemble a comma (,) in appearance.

Gram positive and Gram negative bacteria

A Danish physician, Christian Gram (1884) developed a stain for staining bacteria, termed Gram stain. On the basis of stainability with Gram stain, bacteria are classified into two groups; Gram positive and Gram negative.

The procedure involves staining of bacterial smear first with weakly alkaline solution of crystal violet (methyl violet or gentian violet). All the bacterial cells stain blue with this dye. These blue stained cells are then treated with 5% iodine - KI solution (or Lugol’s solution) and washed with 95% alcohol or acetone.

The bacteria which retain the original blue or purple colour are called Gram positive (+ve) bacteria. Those which lose stain and decolourize after the treatment with alcohol are called Gram negative (–ve) bacteria. In Gram (+) bacteria the cell wall has very little lipid content. Therefore, very little stain leaks out of their walls in organic solvent. In Gram (–) bacteria the cell wall has high lipid content. The same dissolves in organic solvent taking out the stain alongwith.

The decolorized Gram (–ve) bacteria may, therefore, be stained with safranine or eosine.

Gram Staining :

It is a differential staining to differentiate bacteria. All bacteria take crystal violet stain. Gram –ve become colourless on treating with alcohol. It is due to thin cell wall and more lipids in cell wall of Gram –ve bacteria which gets dissolved in alcohol.

Gram +ve BacteriaGram –ve Bacteria

(a) Thickness of cell wall is 150-200 ÅThickness of cell wall is 75-120 Å

(b) Cell wall single layered, smooth and in Cell wall two layered, wavy and in contact with cell

contact with the cell membranemembrane only at a few places

(c) Lipid contents in cell wall is 2-4%Lipid contents in cell wall is 20-30%

(d) Muramic acid contents are 70-95%Muramic acid contents are 5-20%

(e) Amino sugars are 10-20%Amino sugars are 2-8%

(f) Mesosome commonMesosome less prominent

(g) Pili rarePili common

(h) Mostly coccus and non-capsulated, Mostly bacilli, capsulated and flagellation common

flagellation less common

(i) Teichoic acid presentIt is absent

(j) May produce exotoxinsMay produce endotoxins

(k) Sensitive to penicillinNot sensitive to penicillin

(l) Diamino pimelic acid is absentIt is present

(m) Porins absent in the cell wallPorins present in the cell wall

(n) Few pathogenic formsPathogenic forms common

Cell wall

It is present outside the membrane and is a rigid structure.

The cell walls of almost all the eubacteria (true bacteria) are made up of peptidoglycan, also called murein or mucopeptide. As the name suggests, the peptidoglycan consists of two components; peptide portion and a glycan or sugar portion.

The glycan portion is composed of alternating units of amino sugar, N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) joined together by , 1-4 linkage.

In Gram (+) bacteria, the cell wall has a thick peptidoglycan layer (90%) and also contains teichoic acids, formed of glucose, phosphate and alcohol.

The cell wall of Gram negative bacteria is much more complex. The peptidoglycan layer is very thin making up only 10% or less. However, the most interesting feature is the presence of an outer membrane that covers a thin underlying layer of peptidoglycan. The outer face of outer membrane contains lipopolysaccharides, a part of which is integrated into the membrane lipids. The inner face has a number of proteins which are anchored into peptidoglycan. The outer membrane of Gram (–) bacteria contains proteins called porins, and those proteins function as channels for the entry and exit of hydrophilic low molecular weight substances.

In Mycobacterium , Corynebacterium and Nocardia, the wall is that of Gram (+) type but a part of their cell wall is made up of a very long chain of the fatty acid called mycolic acid.

Due to the presence of the outer membrane, Gram negative bacteria are rich in lipids that make about 11-12% of the dry weight of the wall. Teichoic acid is absent in this case.

Protoplast

Cell wall encloses the protoplast, the living matter. It is differentiated into cell membrane, cytoplasm and nuclear body.

Cell Membrane

It lies inner to cell wall, actually representing the outermost layer of the protoplast.

Functionally, the cell membrane resembles mitochondria of eukaryotic cells, as some respiratory enzymes are associated with the membrane. Enzymes for lipid synthesis and synthesis of cell wall materials are also found.

The cell membrane gets invaginated and folded to form structure called mesosome (Fitz James 1960) in some bacteria, particularly the Gram positive bacteria. Mesosomes is of two types, septal and lateral. Septal mesosome is in contact with nucleoid while peripheral or lateral mesosome is not attached to nucleoid. Septal mesosome replicates with the replication of nucleoid, takes part in separation of replicated nucleoids and helps in septum formation. Lateral mesosome contains respiratory enzymes.

Cytoplasm

It is homogenous colloidal mass of carbohydrates, fats, proteins, lipids, nucleic acids, minerals and water.

Typical organelles of eukaryotic cell like endoplasmic reticulum, mitochondria, Golgi complex and plastids are absent.

Ribosomes

—Ribosomes lie scattered freely in the cytoplasm but sometimes may form a small helical chain of 4-6 ribosomes called polyribosomes or polysomes by means of mRNA strands. Ribosomes are of 70s type.

Inclusion bodies

—They are nonliving nonstructural materials which lie free in the cytoplasm. They are of three types-food reserve (glycogen and proteins granules), inorganic granules (volutin granules, sulphur granules, etc) and gas vacuoles.

Nuclear Body

Bacterial cell lacks a well organized nucleus. The nuclear material, consisting of single naked, circular double stranded DNA molecule, is identified as nuclear body, nucleoid or genophore. It is attached to plasma membrane directly or by means of mesosomes. Circular DNA ring is often termed as bacterial chromosome. It is not associated with histone proteins.

Plasmids

—They are small, self-replicating, autonomous extrachromosomal single circular DNA present in the cytoplasm. Three forms of plasmids are :

F-plasmid – having fertility factor,

R-plasmid – having resistance to antibiotics,

Col-plasmid – producing colicins or bacteriocins that kill related bacteria.

—Some plasmids can temporarily integrate with nuclear body and then they are called episomes.

—Plasmids are important tool in genetic engineering.

Flagella

—These are fine protoplasmic threads projecting from the cell wall. Each flgellum is
3-12 mm in length and 12-18 nm in width. Each flagellum is made up of three parts-basal body, hook and filament.

—Flagella are made up of specific proteins called flagellin.

Flagellation

—Depending upon the presence or absence, number and position, following types of flagellar arrangement are observed among bacteria.

Atrichous : Flagella absent.

Monotrichous : A single flagellum present at one pole.

Lophotrichous : Two tufts of flagella at both end.

Amphitrichous : One flagellum at both ends.

Cephalotrichous : A tuft of flagella at one end.

Peritrichous : A number of flagella, borne all around the bacterium.

Pili

—These are hollow, non-helical, filamentous appendages projecting from the walls of some Gram (–) bacteria. These are thinner and shorter than the flagella.

—They are 1-4 in number and develop on only donor (or male) cells. They are made up of specific proteins called pilin.

Fimbriae

—They are small bristle like fibres which develop from the surface of bacterial cells. They are quite numerous (300-400 cell). They take part in attachment.

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