STRUCTURE OF BACTERIA

Bacteria are probably the simplest and smallest living organisms possessing cellular organization. Depending upon the shape, bacteria can be classified into three groups.

Cocci (singular = Coccus):

They are spherical in shape. When they occur in pairs they are known as Diplococci or they can occur in clusters that are punch like, they are known as Staphylococci or they can occur in the form of chain known as Streptococci. They may occur in club shape, fusiform shape or they can be coma shape.

Bacilli (Bacillus):

Bacilli can be rod shaped or can have shape with rounded ends or square end.

Spirochetes or Spirilla:

These may be relaxed or may be tightly coiled. Some bacteria are variable in shape and they are called Pleomorphic (many shapes).

SIZE OF BACTERIA

Bacteria range in size from about 0.2-5.0 micrometer. The smallest bacteria are about the same size as largest viruses (poxviruses). Poxviruses are the largest viruses with brick like shape. Poxviruses range from 20-300 nm in diameter. The largest bacterial rod approaches the size of human red cell or the size of the some yeast.

STRUCTURE OF BACTERIA

Cell wall:

The function of the cell wall is to support underlined plasma membrane, which is subjected to an internal atmospheric pressure of about 5 atmospheres in Gram negative and about 20 atmospheres in Gram positive bacteria.

The cell wall is multi layered structure located external to the cytoplasmic membrane. Plasma membrane and cell wall together comprise envelope.

Cell wall is composed of an inner layer of peptidoglycan surrounded by an outer layer that varies in thickness and chemical composition. The peptidoglycan provides structural shape and maintains characteristic of the bacterial cell, which is very helpful in the identification of the type of bacteria.

The cell wall of Gram positive bacteria differs from Gram negative bacteria.

The peptidoglycan layer is much thick in Gram positive bacteria. It is 50-100 molecule thick whereas in case of Gram-negative bacteria it is much thinner than Gram positive. It is 1-2 molecule thick.

Some Gram-positive bacteria have a layer of teichoic acid on the out side of peptidoglycan whereas gram negatives bacteria do not possess this layer of teichoic acid.

In contrast the Gram-negative bacteria have a complex outer layer or membrane (external to peptidoglycan) consisting of lipopolysaccharides, lipoproteins and phospholipids whereas there is no outer layer or membrane out side the peptidoglycan.

The outer membrane of Gram-negative bacteria contains porins (pore molecules) through which hydrophilic drugs pass more readily.

Lying b/w outer membrane layer and the cytoplasm membrane in gram negatives bacteria is a periplasmic space. This is the site, in some bacteria, where enzymes that degrade penicillin and other Beta-lactam antibiotics are present.

The cell wall has other properties as well e.g.

The cell wall contains endotoxins (lipopolysaccharide)

The polysaccharide and proteins are antigens that are useful in laboratory identification.

Its porin proteins play important role in passage of molecules into the cells.

Mycobacteria have unreal cell wall resulting their inability to Gram stained. These are called Acid-fast bacteria because they resist decolorization with acid-alcohol after being stained with carbol fuchsin. The property is related to high conc. of lipids called Mycolic acids in the cell wall.

MESOSOMES:

Mesosome is cytoplasmic membrane invagination. Its importance during cell division when it functions as an origin of transverse septum that divides the cell in half and as the binding sites of DNA which will become genetic material of each daughter cell.

CYTOPLASM:

Cytoplasm has two distinct areas under electron microscope:

Amorphous matrix containing Ribosomes, nutrients, granules, metabolites and ions.

An inner Nucleoid region composed of DNA.

RIBOSOMES:

Bacterial Ribosomes are the sites of protein synthesis as in Eukaryotic cell but they differ from Eukaryotic cell in size and chemical composition. Bacterial Ribosomes are 70S in size having 50S and 30S subunits whereas Eukaryotic Ribosomes are 80S in size with 60S and 40S.

The difference between both ribosomal RNA and protein constitute the basis of selective action of several antibiotics that inhibit bacterial not the human protein synthesis.

Granules serve as storage area for nutrients and stain characteristically with certain stains or dyes to make identification of bacteria easy.

NUCLEOID:

Nucleoid is the area of cytoplasm in which DNA is located. The DNA of prokaryotes is a single, circular molecule that has a molecular weight of approximately 2 10 and contains about 2000 genes. The Nucleoid contains no nuclear membrane, no mitotic apparatus and no histones.

PLASMIDS:

Plasmids are extra chromosomal double stranded, circular DNA that is capable of replicating independently of the bacterial chromosomes. Although they are extra chromosomal they can be integrated into bacterial chromosomes.

Plasmids can be of two types:

Transmissible

Non-Transmissible

1. Transmissible Plasmids:

Transmissible Plasmids can be transferred from cell to another by conjugation process. They are large b/c they contain about a dozen genes responsible for synthesis of sex pilus and enzyme required for transfer.

2. Non-Transmissible Plasmids:

Non-Transmissible Plasmids are small since they don’t contain transfer genes. Plasmid occurs in Gram-positive and Gram-negative bacteria. The genes for the following functions and structure of medical importance are carried by Plasmids.

Antibiotic resistance, which is mediated by a variety of enzymes.

Resistance to heavy metals by the bacteria e.g. Mercury.

Resistance to U.V. light, which is mediated by DNA repair enzymes.

Pili, which mediate adherence of bacteria to epithelial cells.

Production of exotoxins including enterotoxins. (Exotoxins mean out side of bacterial cell)

The other Plasmid-encoded products of interest are.

Bacteriocins or toxins or enzymes produced by certain bacteria and are lethal to other bacteria.

Nitrogen fixation enzymes in rhizopodium in the root of legumes.

Tumor caused by agrobacterium in plant.

Certain antibiotics produced by streptomycin.

A variety of degraded enzymes.

TRANSPOSONS:

Transposons are pieces of DNA that move readily from one place to another either within or b/w the DNAs of bacterial Plasmids and bacteriophage.

They can code for drug resistance enzymes, toxins or drug metabolizing enzymes. They are named jumping genes.

SPECIALIZED STRUCTURES OUT SIDE THE CELL WALL

1. CAPSULE:

A capsule is a gelatinous layer covering the whole bacterium. It is composed of polysaccharide except in anthrax bacillus, which has a capsule of polymerized d-glutamic acid. The capsule is important for the following reasons:

It limits the ability of phagocytes to engulf the bacteria. Thus it is determinant of virulence of many bacteria. Non-capsulated bacteria are usually non-pathogenic.

Its helps in specific identification of organisms by reacting with antiserum made against the capsular polysaccharide.

Capsular polysaccharides are used as antigens in certain vaccines because they are capable of eliciting protective antibiotics.

Capsule can play an important role in adherence of bacteria to human tissues, which is important step in the occurrence of infection.

2. PILI:

Pili are hair like filaments extending from cell surface. They are found mainly in Gram-negative bacteria. They perform two important functions:

1. They help in the attachment of bacteria to specific receptors on human cell surface, which is a necessary step in initiation of infection for some organisms.

2. A specialized kind of pilus the sex-pilus forms the attachment b/w male (donor) and female (recipient) bacteria during conjugation.

3. FLAGELLA:

Flagella are long whip like appendages that propels bacteria toward food or away from harmful substance. Thus these structures are helpful in the process of chemo taxis.

4. SPORES:

Spores are highly resistant structure which are formed in response to adverse conditions by two genera

1. Bacillus 2. Clostridium