The cytoplasm contains granular structures called ribosomes that synthesize proteins. Larger granules called volutin granules store food and energy reserves. The nuclear material of bacteria exists as a single coiled DNA thread. Mesosomes are invaginations of the cell membrane involved in respiration and cell division. Fimbriae are hair-like protein structures that project from the cell and may aid in adherence and pathogenesis. Capsules are gel-like structures outside the cell wall that can provide protection from phagocytosis and aid adherence. Flagella are long thin motility structures that project from the cell and enable bacterial movement.

1. Cytoplasm
The cytoplasm is a granular structure and contains the nuclear material and all the
metabolites and enzymes associated with the life process of the cell .
• It is almost completely filled with granules (10 - 20 nm in diameter, containing 60% RNA
and 40% protein) , which contain ribonucleic acid and protein and are called ribosomes.
• The ribosomes are responsible for protein synthesis and for the production of the
enzymes involved in the metabolic process.
• It also contains much larger granules (600 nm in diameter) named volutin granules,
which are also called metachromatic granules because they stained differently
(metachromatically) from the rest of the protoplasm.
• The volutin granules represent accumulation of food or reserve of energy.
• Within the cytoplasm there are chromatin bodies which constitute the bacterial nucleus .
• The nuclear material of bacteria exists as a single coiled thread ramifying in the
cytoplasm, on this are situated particular sites (genes) composed of DNA which bears for
coded information for protein synthesis (which acts as catalysts of the cell’s activities) and
also controls the characters of the organism.
Mesosomes
• Mesosomes are invaginations of cytoplasmic membrane and are connected to the
membrane .
• They do not occur as free membrane-bound organells in the cytoplasm .
• Mesosomes play a role in respiration and cell division and appear to be associated with
cell wall synthesis .
Fimbriae
• Fimbriae or pili are composed of subunits of a protein & pilin arranged in helical
strands.
• They are external structures on bacterial cell , varying in diameter from 3-30 nm
(thin ) and from 75-100 nm (thick) and up to 2 µm long , which are appendages or
hair – like structural.
• These fimbriae are also called a "fuzzy coat" and they appear to arise from the cell
wall ( recent studies showed that they arise in the cytoplasmic membrane) and can
only be seen by electron microscope.
• Fimbriae can be seen mainly in Gram-negative bacilli such as E. coli and in some
of the Gram-positive cocci such as Strep. salivarious .
Types of fimbriae
1- The ordinary pili which may mediate adherence to specific receptors .
2- The sex pili which are responsible for attachment between male and female
bacteria .
Role of fimbriae
The role of fimbriae is not exactly known but some are associated with:
• The reception of bacteriophage
• The adherence to red cells causing haemagglutination
• The adhesion of bacteria to surfaces.
• The significance of the structures to adhere to mucosal and other body surfaces is
considered to be as the first step in the pathogenicity.
Capsule
• The capsule is a hydrophilic gel - like structure of low optical density that is known

2. to be as an organized accumulation of material outside the bacterial cell wall .
• It is composed of either , polysaccharides (S.pneumonia, N.meningitides,
Klebsiella) , polypeptide (B.anthrax) or hyaluronic acid (S.pyogens).
• It is called shine layer, microcapsule or simply capsule.
• Capsule is usually thicker but less dense than the cell wall.
• It is not essential to the life of bacteria, therefore the capsule can be thought as a
product of metabolism rather than as a structure per se.
Function of capsule
1- May have a protective role against phagocytosis .
2- May aid in the adherence of bacteria to certain sites.
3- May be responsible for the mucoid colonies of some organisms.
Flagella
• Flagella are long ( 3 - 12 µm), thin ( 12- 25 nm), structures that arise in the
cytoplasmic membrane and terminate outside the cell.
• Like fimbriae, flagella are thread- like or wipe-like structures but much longer
than fimbriae.
• They can be seen in motile bacteria as they provide bacteria with a mechanism for
motility a process called chemotaxis .
• Flagella are composed of basel body, hook & filament.
• At the base of the flagellum within the cytoplasmic membrane there are two sets of
rings surrounding the flagella cylinder : an inner set which rotates and an outer set
which minimize friction and leakage .
• Flagella are arranged and distributed outside the bacterial cell (Fig, 4) :
1- Monotrichate : A single flagellum at one pole.
2 - Lophotrichate : A bunch of flagella at one pole.
3 - Peritrichate : Flagella at all over the surface
4 -. Amphitrichate : One or more flagella at each pole.