Bacteria are prokaryotes that lacks well defined nucleuus and membrane bound oranelles like mitochondria and golgiapparatus. These features make them morphologivally and physiologically distinct from eukaryotes.
A prokaryotic cell with three architectural regions :
A cell envelope consisting of a capsule, cell wall and cytoplasmic membrane.
A cytoplasmic region that contains the nuclear material (DNA), ribosomes and various inclusion granules,
Appendages in the form of flagella and fimbria.
The structure of bacterial cell organelles and their functions :
Cell envelope : The envelope is a descriptive term for the several layers of material theat envelope the protoplasm of the cell, it is made up of cytoplasmic membrane, cell wall and capsule.
Capsule : Some bacteria have a polysaccharide layer outside the cell wall. This layer is called a capsule. A true capsule is discrete detectable layer of polysaccharides deposited outside the cell wall as seen in Haemophilus influenzae and Streptococus pneumoniae.
A less discrete structure or matrix which embeds the cells would be called a Slime layer
Another type of capsule found in bacteria called glycocalyx,which is thin layer of interwined polysaccharide fibres.
Capsule are generaly composed of polysaccharide; rarely do they contain amino sugars or peptide as in the case of poly d-glutamate capsules in Bacillus anthracis.
Function of capsule :
It protects bacteria from phagocytosis
It protects bacteria from antimicrobial agents.
It also prefers bacterial adherence from many surfaces.
Cell wall : Most prokayotic have rigid cell wall. The cell wall is an essential structure that protects the cell from mechanical damage, osmotic rupture and lysis.
Cell wall is made up of murein, or peptodoglycan. It is ploymer of disaccharides cross-linked by short chains of amino acids (peptides).
The glycan backbone is made up of alternating molecules of N-ACETYL GLUCOSAMINE (G) and N-ACETYL MURAMICACID (M). This backbone connected to peptide side chain that contains l-alanine, D-glutamate , Diaminopimelic acid, and D-alanine.
|Cell wall of Gram Positive & Gram Negative|
|Ultra Structure of Cell wall of GPC & GNB|
|Ultra Structure of GPC|
|Ultra structure of GNB|
|Chemical structure of Peptidoglycan|
The following cell wall characteristics are generally present in a Gram-positive bacterium:
· cytoplasmic lipid membrane
· thick peptidoglycan layer
· teichoic acid and lipids are present, forming lipoteichoic acid which serve to act as chelating agents, and also for certain types of adherence
· capsule polysaccharides (only in some species)
· flagellum (only in some species)
· if present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer
· The individual peptidoglycan molecules are cross-linked by pentaglycine chains by a DD-transpeptide enzyme. In gram-negative bacteria, the transpeptidase creates a covalent bond directly between peptidoglycan molecules, with no intervening bridge
Plasma membrane : The plasma membrane is also called cytoplasmic membrane, it is most dynamic structure of prokaryotic cell. It functions as a selective permeability barrieer that regulates the passage of substances into and out of the cell. Since prokaryotic lack the membrane that plays an ative role in energy generation and biosynthesis.
Structure of plasma membrane : Bacterial membrane are composed of phosholipid and proteins. The arrangement of proteins and lipids to form a membrane is called the fluid mosaic model. Dispersed within the phoshopholipid bi-layer are various structural and enzymatic protiens which carry out most membrane functions.
Functions of prokaryotic plasma membrane :
Osmotic or permeability barrier.
Location of transport systems for nutrients.
Energy generating functions, involving respiratory and photosynthetic electron transport system.
Synthesis of membrane lipids.
Synthesis of cell wall peptidoglycan.
Assembly and secretion of extracytoplasmic protien.
Cytoplasm : The cytoplasmic consitituents of bacterial cells invaribly include the bacterial chromosome, ribosome and intracellular inclusion granules. Bacteria possess smaller extrachromosomal DNA called plasmid.
Bacterial nuclear material : The total DNA content of a prokayote is referred to as the cell genome. During cell growth and division, the bacterial chromosome is replecated for distribution to progeny cells.
Bacterial chromosome : It is usually a single molecule of double stranded Dna length of the chromosome is often more than the length of the cell. Hence nucleur material is believed to be in a super coiled state. Histone-like proteins are complexed with Dna for structural support.
Plasmid : These are extra chromosomal DNA; much smaller in size than chromosome. Plasmids have non-essential genes that would be advantageous for the bacterium under certain conditions. They code for added virulence like toxin production and antibiotic resistance.
Ribosomes : These are structure involved in the process of protein synthesis. The ribosomes of prokaryotes are smaller than that of eukaryotes. Prokaryotic ribosomes are 70S in size, being composed of 30s and 50s subunits.
Mesosomes : They are multiminated structures formed as invaginations of plasma membrane into the cytoplasm. They are functionally analogous to mitochrondria of eukaryotic cells. They are also thought to co-ordinate nuclear and cytoplasmic division during binary fission.
Inclusion granules : cytoplasm of bacterial celll contains different types of inclusions. Inclusions are distinct granules that may occupy a sustantial part of the cytoplasm. Inclusion granules are usually reserve materials. For example, carbon and energy reserves may be stored as glycogen or as poly betahydroxybutyric acid granules.
Volutin granules are reserves of polymeta phosphate and possibly energy. They are also known as the metachromatic granules and are found in Cornybacterium diptheriae.
Flagella : Flagella are filamentous protein structures attached to the cell surface that provide the swimming movement for most motile bacteria. They are composed of protein subunits called flagellin. Bacterial flagella are powered by proton motive force established on the bacterial membrane. About half of the bacilli and all of the spiral and curved bacteria are motile by means of flagella, very few cocci are motile.
Fimbria : fimbria or pilli are used to designate short, hair-like structure on the surfaces of bacterial cells. They are composed of protein subunits callled pillin. Fimbriae are very common in gram-negative bacteria but occur in some gram positive bacteria as well.
There are two types of pilli; common pilli and sex pilli.
Common pilli : Common pili are usually involved in specific adherence of baceria to surfaces. They are major determinants of bacterial virulance because they allow pathogens to attach to tissue and resist phagocytosis. For example, pathogenic Neisseria gonorrhoeae adhere specifically to the human cervical or urethral epithelium by means of its common pili.
Sex pilli : A specialized type pf pilus., the F or sex pilus, mediates the transfer of DNA between mating bacteria during the process of conjugation.
Spores : some bacteria have the ability to form highly restant resting stages called endospores. It is formed by a few groups of bacteria as intracellular structures. They are highly restiant to environment sresses such as high temperature, irradiation, strong acids and disinfectants. Endospores are formed by vegetative cells in responce to environment l stress such as exhaustion of an essential nutrient. They germinate and become vegetative cells when the environmentqal stress is relieved. Hence endospore-formation is a mechanism of survial rather than a mechanism of reproduction.
Structure of spores : The spore has a core wall of unique peptidoglycan surrounded by several layers, including the cortex, the spore coat and exosporium. The dehydrated core contains the bacterial chromosomes and a few riboisomes and enzymes.
Why endospores is so helpful in bacterial resistance :
less metabolic activity
thick spore coat
presence of calcium dipicolinate; a strong calcium chelator