Anatomy of Bacterial Cell Outer layer - consist of two components:1. Rigid cell wall2. Cytoplasmic (Cell/ Plasma) membrane

present beneath cell wall

Cytoplasm - gel-like substance enclosed within the

cell membrane contains cytoplasmic inclusions, ribosomes, mesosomes and nucleoid .

Additional structures - plasmid, slime layer, capsule,

flagella, fimbriae (pill), spores, see fig. 1

CELL WALLOutermost layer, encloses cytoplasmic membrane ,It surrounds the cell membrane and provides these cells with structural support and protection, and confers shape and rigidity.

 Peptidoglycan serves a structural role in the bacterial cell wall, giving structural strength, as well as counteracting the osmotic pressure of the cytoplasm. peptidoglycan helps maintain the structural strength of the cell, Peptidoglycan is also involved in binary fission during bacterial cell reproduction.

Chemical nature of the cell wall helps to divide bacteria

into two broad groups — Gram positive & Gram negative.

Gram +ve bacteria have simpler chemical nature than

Gram —ve bacteria

The peptidoglycan layer is substantially thicker in Gram-positive bacteria (20 to 80 nanometers) than in Gram-

negative bacteria (7 to 8 nanometers), Peptidoglycan forms around 90% of the dry weight of Gram-positive bacteria but only 10% of Gram-negative strains. Thus, presence of high levels of peptidoglycan is the primary determinant of the characterisation of bacteria as Gram-positive. In Gram-positive strains, it is important in attachment roles and serotyping purposes. 

Cell wall consist of Mucopeptide (peptidoglycan/ murein):

formed by N-acetyl glucosamine (NAG) & N-acetyl

muramic acid (NAM) alternating in chains, held by

peptide chains.( cross link with tetrapeptide and pentaglycan

bridge. see fig .2

Cell wall can not be seen by direct light microscopy and do not

stain with simple stains, also can be observed by plasmolysis

and carries bacterial antigens — important in virulence &

immunity.Several antibiotics may interfere with cell wall synthesis e.g. Penicillin, Cephalosporins

Gram positive cell wall

• The Gram-positive cell wall is composed of a thick

multilayered peptidoglycan sheath outside of the cytoplasmic

membrane.

• Teichdic 'acids are linked to and embedded in the

peptidoglycan, and lipoteichoic acids extend into the

cytoplasmic membrane

Gram negative cell wall

the Gram-negative cell wall is composed of an outer membrane linked to thin single-layered peptidoglycan by lipoproteins.

the peptidoglycan is located within the periplasmic space that is created between the outer and inner membranes.)

The outer membrane. includes (porins which allow the passage of small hydrophilic molecules across

the membrane and lipopolysaccharide molecules that extend into extracellular space. See table 1.

cytoplasmic(plasma)membrane The boundary of the cell, sometimes called the plasma membrane, thin layer 5-10 nm separates internal metabolic events from the external environment and controls the movement of materials into and out of the cell. This membrane is very selective about what it allows to pass through; this characteristic is referred to as “selective permeability.” For example, it allows oxygen and nutrients to enter the cell while keeping toxins and waste products out. The plasma membrane is a double phospholipid membrane, or a lipid bilayer, with the nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar hydrophilic heads forming the inner and outer surfaces of the membrane with small amounts of Carbohydrates. see fig .3,4

cytoplasm

Colloidal system of variety of organic and enorganic solutes in viscous watery solution

Cytoplasm Contains:

ribosomes,

mesosomes,

inclusions

vacolus

Ribosomes:In most bacteria the most numerous intracellular structure is the ribosome, the site of protein synthesis in all living organisms. All prokaryotes have 70S (where S=Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol. The 70S ribosome is made up of a 50S and 30S subunits. The 50S subunit contains the 23S and 5SrRNA while the 30S subunit contains the 16S rRNA. These rRNA molecules differ in size in eukaryotes and are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms.

Mesosomes:It is a multilaminated structures formed as invagination

of plasma membrane , it is principle sites of respiratory

enzymes, and coordinate nuclear and cytoplasmic division during binary fission.

Intracytoplasmic inclusion :Inclusions are considered to be nonliving components of the cell that do not possess metabolic activity and are not bounded by membranes. It is reserve of energy and phosphate for cell metabolism. The most common inclusions are glycogen, lipid droplets, crystals, and pigments. Volutin granules are cytoplasmic inclusions of

complexed inorganic polyphosphate. These granules are called metachromatic granules due to their displaying the metachromatic effect; they appear red or blue when stained with the blue dyes methylene blue or toluidine blue in diphtheria bacilli.

Nucleus:No nucleolus, no nuclear membrane, oval or elongated bodies generally one per cell.

The genome is single circular double stranded DNA , haploid and divided by binary fission.

Additional organelles

1.Plasmid:Extranuclear genetic elements consisting of DNAAnd transmitted to daughter cells during binary fission . May be transferred from one bacterium to anotherand not essential for life of the cell .the functionof plasmid is to confer certain properties such as drug resistance ,toxicity.

2.. Capsule & Slime layer: Viscous layer secreted around the cell wall Polysaccharide / polypeptide in nature A. Capsule sharply defined structure, antigenic in nature • Protects bacteria from Iytic enzymes , inhibits phagocytosis • Stained by negative staining

using India Ink • Can be demonstrated by Quellung reaction (capsule swelling reaction) .B. Slime layer loose undemarcated secretion Repeated subcultures in vitro lead to loss of capsule and also of virulence.

3. Flagella Flagella (singular: flagellum) are long, thin,

whip-like appendages long (3 to 12 micron ) attached to a bacterial cell that allow for bacterial movement (also known as motility) ,filamentous surface appendages this organs of locomotion is present in motile bacteria. The number and distribution of flagella on the bacterial surface are characteristic for a given species - hence are useful in identifying and classifying bacteria so that different bacterial species have different flagella arrangements, from a single flagellum to one on each end to tufts of many..), Chemically, composed proteins called flagellins ,Each flagellum consist of three part :filament, hook and basal body .Flagella may serve as antigenic determinants (e.g. the H antigens of Gram-negative enteric bacteria( Presence shown by motility e.g. hanging drop preparation.

4 .Fimbriae/ Pili Thin, hairlike appendages

on the surface of many Gram negative bacteria

10 to 20 micron long, acts as organs of adhesion (attachment) -allowing bacteria to colonize environmental surfaces or cells and resist flushing .It is made up of proteins called pilins. • Pili can be of two types — Common pili - short& abundantBordetella pertussis is the bacteria that causes whooping cough. Bordetella has pili coated with adhesins that can surface of the respiratory tract and will stick to only that surface, allowing it to adhere identify the mucosal to and infect those cells.

Sex pil - small number (one to six), very long pili, helps in conjugation (process of transfer of DNA)Pili can also aid in attachment between bacterial cells. Some bacteria are able to produce conjugation pili that allow for the transfer of DNA from one bacterial cell to another. Bacteria have evolved the process of conjugation as a way to increase genetic variability. The cell with the conjugation pilus attaches to another cell, connecting the cytoplasm of each cell and allowing molecules of DNA to pass through the hollow pilus.

5. Spores : Highly resistant resting stages formed during adverse environment-(depletion of nutrients)

• Formed inside the parent cell, hence called Endospores. • Very resistant to heat, radiation and drying and can remain dormant for hundreds of years. • Formed by bacteria like Clostridium, Bacillus. Endospores are bacterial survival structures that are highly resistant to many different types of chemical and environmental stresses and therefore enable the survival of bacteria in environments that would be lethal for these cells in their normal vegetative form. It has been proposed that endospore formation has allowed for the survival of some bacteria for hundreds of millions of years (e.g. in salt crystals) . Endospore formation is limited to several genera of gram-positive bacteria such as Bacillus and Clostridium. It differs from reproductive spores in that only one spore is formed per cell resulting in no net gain in cell number upon endospore germination. The location of an endospore within a cell is species-specific and can be used to determine the identity of a bacterium. See fig 4,5.