Structure of bacterial cell

Dentistry College - first class
Medical biology- Lec.5
Structure of bacterial cell
The structure of Bacterial cell composed of: cell wall, cytoplasm membrane, intra - cytoplasm structure and cell surface appendages such as capsule, flagella, pili and spore as fallow:

A- Cell wall
Cell wall of bacteria is an essential structure consists of peptidoglycan is a polymer of two sugar derivatives N- acetylglucosamine and N- acetylmuramic acid cross linked by short chains of amino acids (peptide). Bacteria were divided into two separated groups Gram positive and Gram negative based on variation in chemical composition of bacterial cell wall. Gram stain developed by Christian Gram 1884 that widely employed in bacteriology Laboratory.












Fig. bacterial cell structure


Chemical composition of the cell wall

A- Gram positive bacteria cell wall composed of:
1-Peptidoglycan
This layer is very thick in G +ve bacteria constituting (50-80) nm of cell wall and responsible for rigidity of cell wall and retain the crystal violet stain.
2-Teichoic acid and thin layer of lipid

B- Gram negative bacteria cell wall composed of:

1- Outer layer of lipopolysaccharides (LPS)
Containing of lipid A (endotoxin) and polysaccharide
2-Inner layer of peptidoglcan
This layer is thin constituting of (5-10) nm of cell wall which can not retain the crystal violet stain.





3-Periplasmic space between the inner and outer layers
It is filled with gel and is crossed by lipoprotein molecules to link the peptidoglycan layer and outer layer.

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Fig. Structure of cell wall in G –ve and G+ ve bacteria

Functions of bacterial cell wall
1-It maintains the shape of bacterial cell.
2- It protects the bacterial cell from osmotic lyses.
3-It is responsible for endotoxic activity of G- ve bacteria and Gram stain reaction.
4- It plays a role in cell division.

Example of cell wall deficient bacteria
a- Mycoplasma
Mycoplasma naturally deficient cell wall so, it is polymorphic shape and not
affected by penicillin treatment .
b- L- forms
Some of bacteria under certain condition are failing in synthesis of cell wall when the cells are subjected to penicillin drug or lysozymes

B- Cytoplasm membrane
It is a thin elastic semipermeable membrane that surrounds the cytoplasm; it is formed of two layers of phospholipids in which proteins are embedded. Mesosomes are invagination of plasma membrane in the shape of vesicles; it is thought has a role in cell division because they are the origin of cross walls during cell division. The most widely accepted model for membrane structure is fluid mosaic is of Singer and Nicholson in 1972.















Fig. fluid mosaic model of plasma membrane

Functions of plasma membrane
1- Selective permeability to different molecules across the membrane.
2-It supplies the cell with energy and site of respiration, because deficient of mitochondria in bacterial cell.
3- Secretion, excretion of toxins and hydrolytic enzyme outside the cell.
4-It provides the enzymes needed for cell wall synthesis.


C- Intra cytoplasmic structure
1-nucleoid
It is a primitive nucleus, not surrounded by nuclear membrane and posses a single circular double stranded DNA molecule located in a region of the cell known as the nucleoid and a small amount of RNA, it carries a genetic information necessary for growth, metabolism and survival of the cell. There is an additional genetic information may be carried in plasmid an extra - chromosomal (DNA) lies within the bacterial cytoplasm and replicate independently. Plasmid are not required for cell growth and reproduction but encodes for traits that are given advantage to the cell such as antibiotic resistant, give them new metabolic pathway and tolerance toxicity of mineral ( Fig).















2 - Ribosome
They are spherical, non membranous particles in cytoplasm of bacterial cell; the collections of ribosome are called polysomes. Chemically they are complex structures composed of 60% RNA molecules and 40% proteins; it is an active center for protein synthesis.

3- Storage granules
The cytoplasm contains granules which represent accumulation of food or energy reserve like volutin or metachromatic granules.


D-Appendages of bacterial cell
1-Flagella
An extra cellular long thin filamentous structure responsible for motility of pathogenic bacteria, can play role in production and detection of disease because has an antigenic property. Bacterial cells, may carry a single flagellum described monotrichous, if the bacterium carries a single tuft of flagella it is said to be lophotrichous . When the tuft appears at both ends of the cell, the bacteria is amphitrichous. Bacteria that are covered all over body in flagella are said to be peritrichous as shown in the figure below.










2- Pili
The structure is hair – like projections but shorter and thinner than flagella , it is found mainly in G-ve bacteria .They are two types of pilli divided according to their functions :
1- Ordinary pili which play a role in attachment of mucous membrane.
2- Sex pili their function was transfer DNA between conjugated bacteria.
















Fig. fimbriae and flagella of bacterial cell

3-Capsule
Most bacteria contain sort of polysaccharide layer outside of cell wall, this layer is called capsule (Fig.), protects the bacterium even within phagocytes from being killed. Bacteria with capsule grow as “smooth colonies”, where as colonies of bacteria that have lost their capsules appear rough.









Fig. capsule of bacteria


Some bacteria produce slime are usually made up of polysaccharides produced by Streptococcus mutants enables stick to the surface of teeth, were helps to form plaque leading to dental carries. Dental plaque is a general term for the diverse microbial community found on tooth surface.











Fig. dental carries Fig. slime of bacteria


4- Spores
Some bacteria like genus Clostridium and Bacillus can develop a highly resistant structure called endospore as a response to unfavorable growth environmental condition such as radiation, chemicals, heat, starvation and desiccation. The spore is formed inside the parent vegetative cell incorporating the nuclear material acquiring a thick covering layer is called cortex and an outer spore coat that contains complex of dipicolinic acid – calcium - peptidoglycan was impermeable to water and highly resistant to external environment conditions. Spores may vary in:
- Shape: oval or round.
- Site: terminal, sub terminal or central.
- Size: the same size or bulging of the vegetative cell.











Reproduction in bacteria
Bacteria can reproduce asexually through:

Binary fission
Binary fission occurs normally under favorable conditions, the cell elongate and becomes constricted at middle, the chromosome replicated then constriction increases until the cell is split into two cells , each receiving one chromosome . The time needed for cell to divide into two cells is called generation time and differs according to species and prevailing condition, may be range 20 minute to 6 hours.














Fig. binary fission in bacteria