Fig: A generalized bacterial cell
GENERAL STRUCTURE OF BACTERIA
Bacteria are unicellular prokaryotes. They lack true nucleus and other membranous bound organdies. Some species form colonies. Most prokaryotic have diameter between 1-5Am. Bacterial cell is composed of following structures: flagella, cell envelope, cell membrane, cytoplasmic matrix, spores.
The Cell Surface or Envelope
Cell envelope is composed of cell wall. In some bacteria structures lie slime and capsule are also present.
(a) Cell wall
Cell wall is present beneath the extra cellular substances. It is present outside the cytoplasmic membranes. It is rigid structure. It determines the shape of the bacteria. Cell wall also protects the cells from the osmotic lysis. Cell wall is absent in Mycoplasma. The cell wall is essential for the survival of many bacteria. The antibiotic penicillin is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan.
Composition of cell wall
- The cell walls of most bacteria have a unique macromolecule called peptidoglycan. Its amount is different in different types of bacteria. It is composed of long framework of glycan (glucose) chains. These glycan chains are linked with peptide fragments.
- The intact cell wall also contains some molecules of sugar, techoic acid, lipoproteins and Lipopolysaccharides. These molecules are linked to peptidoglycan.
- The cell wall structure present in gram-positive and gram‑
negative bacteria is absent in some bacteria. Some bacteria have no cell wall at all. The cell wall of Archaeobacteria is different from the Eubacteria. Archaeobacteria do not contain peptidoglycan. Their cell wall is composed of proteins, glycoprotein and polysaccharides.
Gram staining technique
This technique was discovered by Christian Gram. There are two dyes used in Gram staining technique: Crystal violet (purple) is primary dye and Red dye (safranine or eosin) is Secondary dye Different bacteria show different response to gram stainin procedure. So bacteria are divided into two groups on the basis this response:
(a) Gram positive bacteria
They are stained purple with this staining technique. They forms CV-1 complex (Crystal violet complex 1) and retain the primary dye colour. Gram positive bacteria have simple cell wall. It has large amount of peptidoglycan. It has lesser amount of lipopolysaccharide. Examples: Staphylococcus, Bacillus, Corynebaclerium
There are following characteristics are generally present in a Gram-positive bacterium:
I. Cytoplasmic lipid membrane
- Gram-positive bacteria have only one membrane layer
- Thick peptidoglycan layer. Teichoic Acids and I ipoids are present. They form lipoteichoic acids. It acts as chelating agents, and also for certain types of adherence.
- Capsule polysaccharides (only in some species)
- Flagellum (only in some species) if present, it contains two rings for support.
(b) Gram Negative bacteria: They are stained pink with this staining technique. They retain secondary dye colour. Gram negative bacteria have lesser amount of peptidoglycan. It has more complex structure. An outer membrane of lipopolysaccharides is present outer to the peptidoglycan layer. Gram negative species are more dangerous than the Gram positive bacteria. The lipopolysaccharides layer is often toxic. This layer protects the bacteria from the immune system of host. Gram negative bacteria are more resistant to antibiotics. Its examples is Pseudomonas.
There are following characteristics of Gram negative bacteria
I. Cytoplasmic membrane
- Thin peptidoglycan layer (which is much thinner than in Gram-positive bacteria)
- Outer membrane containing lipopolysaccharide outside the peptidoglycan layer
- Porins exist in the outer membrane, which act like pores for particular molecules
- There is a space between the layers of peptidoglycan and the secondary cell membrane called the periplasmic space
- The S-layer is directly attached to the outer membrane, rather than the peptidoglycan
- If present, flagella have four supporting rings instead of two
- No teichoic acids or lipoteichoic acids are present
Fig: Gram positive and Gram negative cell wall
Some bacteria produce capsule out side the cell waII. It is made up of repeating units of polysaccharides Or proteins or both. The capsule is tightly bound with the cell wall. It has thick and gummy nature. It makes the encapsulated bacteria sticky. Bacteria can attach with the substrate with their capsule. It also helps bacteria to attach with each other and form colonies. Capsule makes the bacteria resistant to antibiotics.
Some bacteria are coveted with loose, soluble shield of macromolecules called slime. Slime provides greater pathogenicity to bacteria. It protects the bacteria against Phagocytosis.
Fimbriae are fine filaments of protein. These are just 2— 10 nano metres in diameter and up to several micrometers in length. They are distributed over the surface of the cell. They resemble fine hairs when seen under the electron microscope. Fimbriae are involved in attachment to solid surfaces or to other cells. These are also essential for the virulence of some bacterial pathogens.
Pili are hollow, non-helical and filamentous appendages present on cell wall. Pili are smaller than the flagella. True pili are present only in gram-positive bacteria. They are made up of special proteins called pilin. Pili are not involved in locomotion. They are primarily involved conjugation. Conjugation is a mating process. Some pili are used for the attachment of bacteria with various surfaces.