Introduction of microbiologyThe term of microbiology compose three word.Micro mean – so small can't seen by kneked eye.Bio mean – living.Logy mean – science.So micro biology is define as the study of microbes or living microorganisms of microscopical size.

Microorganisms were first seen about 1675 by Layven hook. He found many microorganisms in material such as water, saliva and intestinal content of healthy subject.

The term (microbe) was introduced by Louis paster (1857 – 1860) whose demonstration that fermentation was caused by the bacterial and yeast growth.The term microbe was used by Sedillat in 1878 but now is replaced by microorganism.

Robert Koch 1877 described methods for microscopic examination of bacteria in dried fixed films stained dyes and in 1881 devised the simple method for isolating pure culture of bacteria by plating out mixed of single bacteria grow in separate colonies.

Prokaryotes and EukaryotesAll microorganisms that are capable of self multiplication can be differentiated by their cell type into one of two groups.

1 .Prokaryotic 2 .Eukaryotic

ProkaryoticEukaryotes

Cell structure Very simpleComplex

Nuclear membraneAbsentPresent

Genetic materialLies in cytoplasmContained in nuclear membrane

Mitochondria AbsentPresent

EnzymesContain simple enzymeContain complex enzyme

Type of multiplicationBy binary fissionBy mitosis

Examples

This group in include bacteria rikettesia, chiomydia and mycoplasma

This group includes protozoa and fungi

The majority of microorganisms maybe classified in the following biological groups.1 .Algae. 2. Protozoa. 3. Mould.4. Fungi

5 .Bacteria. 6. Spirochaetes. 7. Mycoplasmas. 8. Chlamydiaceae .9 .Rickettsieae. 10. Viruses.

The algae, protozoa, moulds and fungi their cell have the some general type of structure and organization, they are described as eukaryotic.

Bacterial cells structureStructurally, a procaryotic cell (Figure 1 below) has three architectural regions: appendages (attachments to the cell surface) in the form of flagella and pili (or fimbriae); a cell envelope consisting of a capsule, cell wall and plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions .

Structure Flagella

Function(s) Swimming movement

Predominant chemical composition Protein

Pili

Sex pilusMediates DNA transfer during conjugationProtein

Common pili or fimbriaeAttachment to surfaces; protectionProtein against phagotrophic engulfment

Capsules (includes "slime layers" and glycocalyx)

Attachment to surfaces; protection against phagocytic engulfment, occasionally killing or digestion; reserve of nutrients or protection against desiccationUsually polysaccharide; possible polypeptide

Cell wall

Gram-positive bacteriaPrevents osmotic lysis of cell protoplast and confers rigidity and shape on cellPeptidoglycan (murein) complexed with teichoic acids

Gram-negative bacteriaPeptidoglycan prevents osmotic lysis and confers rigidity and shape; outer

membrane is permeability barrier; associated LPS and proteins have various functions

Peptidoglycan (murein) surrounded by phospholipid protein-lipopolysaccharide "outer membrane"

Plasma membranePermeability barrier; transport of solutes; energy generation; location of numerous enzyme systemsPhospholipid and protein

RibosomesSites of translation (protein synthesis)RNA and protein

InclusionsOften reserves of nutrients; additional specialized functionsHighly variable; carbohydrate, lipid, protein or inorganic

ChromosomeGenetic material of cell DNA

PlasmidExtrachromosomal genetic materialDNA

The cell wall.

The layers of the cell envelope lying between the cytoplasmic membrane and capsule.The cell wall provides protection and imports shape to the cell. The cell wall of gram positive (G+) bacteria differ in its structure and composition form that of gram negative (G-). In the Gram-positive Bacteria (those that retain the purple crystal violet dye when subjected to the Gram-staining procedure) the cell wall is thick, consisting of several layers of peptidoglycan as well as teichoic acids. In the Gram negative Bacteria (which do not retain the crystal violet) the cell wall is relatively thin and is composed of a single layer of peptidoglycan (no teichoic acid) surrounded by a membranous structure called the outer membrane. The outer membrane of Gram-negative bacteria invariably contains a unique component, lipopolysaccharide (LPS or endotoxin), which is toxic to animals .

The principle structure of the cell wall is peptidoglycan which consist of 2 hexose sugar. N.accety-glucosamine and N. acetylmoramic acid which are linked to short peptides of 4 amino acid.

Cell wall

Figure 2: (a) The cross-link in Gram-positive cell walls is a pentaglycine bridge.)b (In Gram-negative cell walls, the linkage between the tetrapeptides of adjacent carbohydrate

chains in peptidoglycan involves a direct amide bond between the lysine side chainof one tetrapeptide and D-alanine of the other.

-------------------------------------------------------------------------------FIGURE 3 ● The structures of the cell wall and membrane(s) in Gram-positive and

Gram-negative bacteria. The Gram-positive cell wall is thicker than that in Gram-negativebacteria, compensating for the absence of a second (outer) bilayer membrane.

FIGURE 4● Lipopolysaccharide (LPS) coats the outer membrane of Gram-negativebacteria. The lipid portion of the LPS is embedded in the outer membrane and is linkedto a complex polysaccharide

The function of outer membrane:- prevent leakage of priplasmic, proteins and protect The cell from bile salts and hydrolytic enzyme.

Permeable to low molecular weight solutes large, molecules e.g. antibiotics penetrate slowly.

The permeability of outer membrane varies from one to another G-ve bacteria.

Note: - pseudomonas aeruginosa highly resistant to antibiotics due to less permeability of the outer membrane which is 100 times less

permeable then t. coli.Lipopolysaccharide of G-ve bacteria (LPS).

LPS referred to endotoxin.The Plasma Membrane or Cytoplasmic membrane (Cell membrane or

plasma membrane).Its main function is a selective permeability barrier that regulates the passage of substances into and out of the cell. It is

barrier between interior and exterior of the bacterial cell .

Property Gram-positiveGram-negative

Thickness of wall thick (20-80 nm)thin (10 nm)

Number of layers 1 2

Peptidoglycan (murein) content >50%10-20%

Teichoic acids in wall presentabsent

Lipid and lipoprotein content 0-3%58%

Protein content 09%

Lipopolysaccharide content 0 13%

The Cytoplasm The cytoplasmic constituents of procaryotic cells invariably include the procaryotic chromosome and ribosomes. The chromosome is typically one large circular molecule of DNA, more or less free in the cytoplasm. Procaryotes sometimes possess smaller extrachromosomal pieces of DNA called plasmids. The total DNA content of a procaryote is referred to as the cell genome .

Cytoplasmic componentThe cytoplasm of most bacteria contain DNA, ribosomes RNA and storage granule.Bacterial cell lacks nuclear membrane (it is prokaryotic)>

DNA, double – stranded circular molecule.Plasmid is extra chromosomal circular and smaller then DNA. Plasmids carry genes involved in antibiotic resistance called R. factor.

Ribosomes: composed 2 subunit. One with a sedimentation coefficient of 50 Sved berg units (50s) and other 30s = 70s.RNA: The function of RNA is translation of Genetic code tram DNA for protein synthesis.Storage granuleTemporarily hold excess metabolites storage granule known as volutine and lipid granules (metachromatic granules).

The capsule or glycocolyx .Many bacteria secret around themselves polysaccharide substance, often referred to as a slime layer. This may become sufficiently thick to form a definite capsule around the organism.The pathogencity of organism is increased by possessing a capsule .

Capsule protects the cell from phagocytosis.Capsule plays a major role in adherence of bacterium to surface of the cells (tissue).Flagella: present in many bacteria, it is responsible for motility.Peritrichous flagella: many flagella distributed over the bacterial surface.Monotrichous flagella: bacteria have a single flagellum.Polar flagella: the bacteria have small bundle of flagella located atone end.Pili or Fimbriae:

Protein fibers that cover the entire surface of G-ve bacteria, which originate at the cell membrane and extend through the cell wall.Ploy a major in bacterial adherence to the cell surface.Sex pili involved in bacterial conjugation.Protoplast: is a bacterium without cell wall.The cell wall lost due to the action of lysozyme which destroy peptidoglycan protoplast is metabolically active but un able to reproduce.Spheroplast is bacterium with a damaged cell wall due to action of toxic chemical or antibiotics spheroplast are able to change back to their.Normal form when toxic agent is removed.L-Forms: are mutant bacteria without cell wall. They are produced when the surrounding become un favorable. They are able to reproduce and can grow on special culture media with high osmotic pressure.

Endospores

Endospores: Endospores are formed by a few groups of Bacteria as intracellular structures Endospores are formed by vegetative cells in response to environmental signals that indicate a limiting factor for vegetative growth, such as exhaustion of an essential nutrient. They germinate and become vegetative cells when the environmental stress is relieved. Hence, endospore-formation is a mechanism of survival rather than a mechanism of reproduction. Endospores exhibit no signs of life, being described as cryptobiotic. They are highly resistant to environmental stresses such as high temperature (some endospores can be boiled for hours and retain their viability), irradiation, strong acids, disinfectants, etc. they retain viability indefinitely such that under appropriate environmental conditions, they germinate back into vegetative cells