Bacteria are typical members of the Kingdom Monera thatincludes prokaryotic organisms. The structures present inthe bacteria may be internal or External. Firstly, theexternal structures are:

FlagellaThey are hair like projections coming out of the cell wall and help in the

movement of the bacteria. It has 3 parts; viz.,

1. Basal Body: Associated with plasma membrane and cell wall.

2. Hook: It connects the basal body to the filament.

3. Filament: It is long & slender. It is made up of protein called Flagellin.

These bacteria are divided on the basis of number of bacteria present.

1. Monotrichous

2. Amphitrichous

3. Cephalotrichous

4. Lophotrichous

5. Peritrichous

(a)Basal Body in Gram Negative(b)Basal Body in Gram Positive

Pilli

Also called as fimbrae. They are small hollow tubular structure and are not

involved in movement. They are more numerous than Flagella. They are also made

up of protein called Pillin. They generally are involved in transfer of genetic

material during bacterial mating.

Cell Wall

It is a rigid structure that surrounds the bacterial membrane. It maintains the

shape of the cell and even protects the cell from lysis. The pathogenicity of the cell

depends on the presence or absence of capsule. The cell wall is made up of

peptidoglycan (N-Acetylglucosamine) or murein

(N-Acetylmuramic Acid). The proteins are of 4 types and are present besides

peptidoglycan or murein: L-alanine, D-alanine, D-glutamic acid and L-lysine or

DAP (Diaminopimellic Acid). The capsule protects the bacteria against desiccation

and attack of Bacteriophage by preventing their attachment.

Further the bacteria can be classified into two on the basis of Gram staining. They

are:

1.Gram Positive bacteria: eg. Bacillus, Staphylococcus

2.Gram Negative bacteria: eg. E.coli

Character Gram Positive Bacteria

Gram Negative Bacteria

Thickness Thicker layer of about 20-25 Nano-meter

Thinner layer of 10-15 Nano-meter

Layer Single layer made of peptidoglycan

Double layer. Presence of an outer-membrane

Peptidoglycan Accounts for 50% of dry weight

Accounts for only 10% of the dry weight. It has presence of oligosaccharides & phospholipids on outer-membrane

Techoic acid Present and act as receptor

absent

Susceptibility More susceptible to antibiotics and less to mechanical disintegration

Less susceptible to antibiotics but more to mechanical disintegration.

Gram Negative Bacteria’s Membrane

Gram Positive Bacteria’s Membrane

The structures internally present in the cell are as follows:

Cytoplasmic membrane

This is about 7.5 ηm thick and is immediately beneath the cell wall. This is

primarily composed of phospholipids (20-30%) and proteins (60-70%). On

the inner side of this layer are present respiratory enzymes that help in

ATP synthesis and also are present enzymes that help in formation of cell

wall component and capsule. Damage to this membrane may result in the

death of the cell due to unavailability of energy and hence, no movement.

Mesosomes

It is a structure that is commonly found in Gram positive bacteria, formed

by the in-folding of the plasma membrane and nearly touches the nucleoid.

This structure helps in division and DNA replication. Its absence in Gram

negative is mainly due to the presence of thick layer of outer-membrane.

Bacterial Chromosome

Bacteria lacks the presence of true nucleus. The bacterial nucleus, present, is not

enclosed in a well-defined membranous structure. The nuclear material is

generally confined to the centre of the cell. It consists of single circular double

stranded DNA molecule in which all the genes are linked. This nuclear material is

generally designated as nucleoid or chromatin body or nuclear equivalent.

Ribosomes

Ribosomes are 70 S type consisting of 50 S and 30 S sub-units. Some ribosomes are

free in the cytoplasm and some are attached to inner surface of the cytoplasmic

membrane. There are some ribosomes that attach themselves to the RNA strand

called as Polyribosomes.

Cytoplasm

It’s a dense semi-liquid substance contained inside the cell membrane. All the

organelles are present inside cytoplasmic fluid. It provides them with the food

present in the form of fat and Volutin granules (reserve source of phosphate),

poly-β-hydroxybutyrate (PHB) and glycogen (both serving as source of carbon and

energy) are some of the granules present in the cytoplasm of some bacteria. Gas

vesicles are present in bacteria that grow in aquatic habitat.

Plasmids

These are the extra-chromosomal structures present in the bacteria. This structure

may some time fuse with the Nucleoid of the cell to form Episomes that further

increases the recombination efficiency. These plasmids are of 3 types; viz.,

1.F-plasmid

2.R-plasmid

3.Col-plasmid

Transformation

It is the process by which a donor DNA molecule is taken up from the external

environment and incorporated into the genome of the recipient cell. It involves the

use of Plasmids.

Transduction

It involves the transfer of genetic material from one bacterium to another by a

bacteriophage, acting as a vector. The virus carries its own genome plus a

fragment of DNA from the bacterium that survives its attack & recombination

occurs.

Conjugation

It is the temporary direct contact between two bacterial cells leading to an

exchange of genetic material. In this one bacterial cell is the donor of the DNA

called as Male(F+)and other is the recipient called as Female (F-). In this way

genome is transferred laterally through conjugation tube.

SporesSpore is a metabolically dormant form, which under appropriate conditions can

undergo germination and grow out to form a vegetative cell. Spores produced

within the cell are called endospores and the spores produced outside to cell are

called exospores.

Endospores are thick walled, highly refractile bodies that are produced (one per

cell) by Bacillus, Clostridium and few other genera. They are generally

formed at the end of the active growth or during stationary phase. They are

extremely resistant to desiccation, staining, disinfecting chemicals, radiation and

heat. This is due to the presence of Ca+ Dipicolinic acid in the spore coat.

Exospores are formed external to the vegetative cell by budding at one end of the

cell in the methane oxidizing genus Methylosinus. They are resistant to desiccation

and heat.

CystsCysts resemble endospores in some ways, but their structure and chemical

composition are different. Cysts are thick walled, desiccation resistant, dormant

forms that develop by differentiation of vegetative cells. Azotobacter and some

other genera produce cysts.