B- Cytoplasm membrane
It is a thin elastic semi permeable membrane that surrounds the cytoplasm,
it is formed of two layers of phospholipids in which proteins and enzymes 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 the cross walls during cell division .The most widely accepted model for membrane structure is fluid mosaic model of Singer and Nicholson in 1972.
Functions of plasma membrane
1-Selective permeability to different molecules across the membrane .
2-It supply the cell with energy .
3-Secretion ,excretion of toxins and hydrolytic enzyme outside the cell.
4-It provides the enzymes needed for cell wall synthesis .
5- Bacterial cell deficient of mitochondria so, the plasma membrane is the site of respiration in the cell.
C- Intra cytoplasmic structure
1-nucleoid
It is a primitive nucleus , not surrounded by nuclear membrane and 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 is carries the genetic coded information necessary for growth metabolism and survival of the cell , additional genetic information may be carried in plasmid an extra -chromosomal DNA lies within the bacterial cytoplasm and replicate independently of the chromosome , plasmid are not required for host growth and reproduction but encodes for traits that are given advantage to the cell such as drug resistant antibiotic or give them new metabolic pathway ( 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 several RNA molecules about 60% and proteins 40% , it is an active center for protein synthesis .
3- Storage granules
The cytoplasm contains granules which represent accumulation of food or energy reserve e.g. the metachromatic granules .
D-Appendages of bacterial cell
1-Flagella
An extra cellular long thin filamentous structure responsible for motility of pathogeic bacteria, can play role in production of disease because has an antigenic property. Bacterial cells , may carry a single flagellum described monotrichous , if the single flagellum at one end of a rod – shaped cell it is known as a polar flagellum , 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-Fimbriae
The structure is hair – like projections but shorter and thinner than flagella are 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 .
Gram negative pathogen bacteria may be covered with fine hair called fimbriae helps to stick to body surfaces (Fig.) .
3-Capsule
Most bacteria contain some sort of polysaccharide layer outside of the cell wall , this layer is called capsule , protects the bacterium even within phagocytes , helping to prevent the bacterial cell from being killed . Encapsulated bacteria grow as " smooth " colonies , where as colonies of bacteria that have lost their capsules appear rough.
Some bacteria produce slime to help them to stick to surfaces , usually made up from 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 (predominantly bacteria) found on the tooth surface, embedded in a matrix of polymers of bacterial and salivary origin.
4- Spores
Some bacteria can develop a highly resistant structure called endospore as a response to unfavorable growth environmental condition such as radiation , heat , and desiccation for ex., Clostridium , Bacillus. 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 calcium and is impermeable to water as shown in figure . Spores may vary in :
- Shape : oval or round.
- Site : terminal , sub terminal or central as seen in figure below .
- Size : the same size or bulging of the vegetative cell .