Structures & Functions of Bacterial cell
INTRODUCTION
The Bacteria are a group of single-cell microorganisms with prokaryotic cellular configuration.
bacteria are the smallest organisms capable of independent existence.
The genetic material (DNA) of prokaryotic cells exists unbound in the cytoplasm of the bacterial cells with No nucleus, No organelles (mitochondria, chloroplast, Golgi, etc.)
Basic information about Bacteria
- The diameter of the smallest body that can be resolved with naked eye is about 100um
- The size of bacteria range from 0.1 to 10 μm (1 μm = 10−6 m)
- Average bacteria 0.5 - 2.0 um in diam.
- Surface Area ~12 um^2
- Volume is ~4 um
- Food enters through SA, quickly reaches all parts of bacteria
Morphology of the bacterial cell
Bacteria are classified by direct examination with the light microscope through its morphology and aggregation
The basic forms are
- spheres (coccus)
- round-ended cylinders (bacilli)
- vibrio (coma shaped or curved )
- filamentous (long filaments)
- spirochete (many coils)
Bacteria Have One of Three Cellular Shapes:
Coccoid-Shaped
Spirilla
Cocci Bacteria: Bacteria overall shape is spherical or nearly spherical .
Aggregations of Cocci:
- pairs, or diplococci (Neisseria)
- groups of four or eight known as tetrads (Micrococci)
- bead-like chains, or streptococci (Streptococcus)
- grapelike clusters, or staphylococci (Staphylococcus
Arrangement of Cocci bacteria:|
Arrangement of Bacilli bacteria:
Other shapes of bacteria
Prokaryotic Cell
Bacteria is Prokaryotic Cell
The cytoplasm of prokaryotic cell contains:
- 80% Water {20% Salts-Proteins)
- Ribosomes
- Inclusion bodies, glycogen or starch
- DNA is circular, Haploid
- More efficient; grows quicker
- Mutations allow adaptation to environment quicker
- Plasmids; extra circular DNA
- Antibiotic Resistance
- No organelles (Mitochondria, Golgi, etc.)
Structural components of Bacteria
A. Essential structural componentsB. Non essential structural components
A-Essential structural components
- cell wall
- Cell membrane
- a nucleoid (DNA)
- Ribosomes
1-Cell wall
- found in all bacteria ,except mycoplasma .
- Peptidoglycans.
- Peptidoglycans constitute almost 95% of the cell wall in some Gram positive bacteria and as little as 5-10% of the cell wall in Gram negative bacteria
The Cell wall is the determinant of bacteria to be Gram possitive or Nagastive.
Functions of bacterial cell wall:
- protect bacterial from osmotic pressure .
- give the bacteria it is shape .
- division started from the cell wall
2-The bacterial cytoplasmic membrane
- Is a typical phospholipid bilayer and protein .
- Permeability barrier; transport of solutes; energy generation; location of numerous enzyme systems
3-The Nucleoid
- The nucleoid is a region of the cytoplasm which contains the genome and a collection of related proteins.
- The bacterial genome resides on a single chromosome.
- Circular chromosome of supercoiled double-stranded DNA negatively charged.
- Responsible of transfer of cellular information .
4-Bacterial Ribosomes
- Have a sedimentation coefficient of 70S and are composed of 30S and 50S subunits .
- Use for protein biosynthesis.
B-Non essential structural components
- Capsule
- flagella .
- pili (or fimbriae)
- Plasmid
- Endospores
- mesosome
1. Capsule
- Is a well-defined structure of polysaccharide some are simple (though unusual) polypeptides surrounding a bacterial cell and is external to the cell wall.
- protects the bacteria from phagocytosis and plays a role in bacterial adherence and colonization.
- some times, it is referred to as a slime layer.
2. Flagella
- Are protein called flagellin, appendages for locomotion.
- Medical importance of flagella are : movement of bacteria (motility)
- Spreading of infection e. g. urinary tract infection.
NOTE: Cocci do not have flagella
3. Pili (fimbriae)
- Pili are proteinaceous hair-like projections
- They are composed of molecules of a protein called pilin
- There are two general classes, common pili and sex pili .
- Common pili(fimbriae) cover the surface of the cell cases adhesions of bacteria to the human cells and responsible for the ability of bacteria to colonize surfaces and cells.
- The F or sex pilus, apparently stabilizes mating bacteria(male and female) during the process of conjugation (transfer genetic material ).
- The sex pilus is diagnostic of a male bacterium and is involved in exchange of genetic material between some Gram-negative bacteria.
4. Mesosomes
- Are convoluted invaginations of the plasma membrane.
- Function in DNA replication and cell division as well as in secretion.
- Are termed septal mesosomes if they occur at the septum (cross-wall) or lateral mesosomes if they are nonseptal.
5. Plasmids
- Are small, circular, nonchromosomal, double-stranded DNA molecules.
- Are capable of self-replication.
- Are most frequently extrachromosomal, but may become integrated into bacterial DNA.
- Contain genes that confer protective properties, such as antibiotic resistance, virulence factors, or their own transmissibility to other bacteria.
6. Endospores
- Are formed as a survival response to certain adverse nutritional conditions, such as depletion of a certain resource.
- Are metabolically inactive bacterial cells that are highly resistant to desiccation, heat, and various chemicals.
- Germinate under favorable nutritional conditions after an activation process that involves damage to the spore coat.
- Are helpful in identifying some species of bacteria (e.g., Bacillus and Clostridium).
- Are not reproductive structures.
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