Medical microbiology

Introduction to microbiology

The term of microbiology compose three word.

1-    Micro mean – so small can't seen by neked eye.

2-    Bio mean – living.

3-    Logy mean – science.

So micro biology is defined 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 Eukaryotes

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

1. Prokaryotic     2. Eukaryotic

	Prokaryotic	Eukaryotes
Cell structure	Very simple	Complex
Nuclear membrane	Absent	Present
Genetic material	Lies in cytoplasm	Contained in nuclear membrane
Mitochondria	Absent	Present
Enzymes	Contain simple enzyme	Contain complex enzyme
Type of multiplication	By binary fission	By mitosis
Examples	This group in include bacteria rikettesia, chiomydia and mycoplasma	This group includes protozoa and fungi, moulds& algae

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

Viruses

1.     Viruses are the smallest intracellular microorganism that containing only one kind of nucleic acid (DNA or RNA) as their genome.

2.     Viruses can pass through bacteria stopping filter. All species are strictly parasites.

3.     Can grow only in living cells, non can grow on an inanimate nutrient media.

4.     Viruses are distinguished from other many bacteria by having an entirely different method of growth and reproduction.

The Bacteria are a group of single-cell microorganisms with procaryotic cellular configuration. The genetic material (DNA) of procaryotic cells is not contained within a nucleus, which is the definitive characteristic of eukaryotic cells, such as those that make up plants and animals.

Bacterial cells structure

Structurally, 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. 

Table 2. Summary: Characteristics of typical bacterial cell structures
Structure	Function(s)	Predominant chemical composition
Flagella	Swimming movement	Protein
Pili
Sex pilus	Mediates DNA transfer during conjugation	Protein
Common pili or fimbriae	Attachment to surfaces; protection	Protein 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 desiccation	Usually polysaccharide; possible polypeptide
Cell wall
Gram-positive bacteria	Prevents osmotic lysis of cell protoplast and confers rigidity and shape on cell	Peptidoglycan (murein) complexed with teichoic acids
Gram-negative bacteria	Peptidoglycan 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 membrane	Permeability barrier; transport of solutes; energy generation; location of numerous enzyme systems	Phospholipid and protein
Ribosomes	Sites of translation (protein synthesis)	RNA and protein
Inclusions	Often reserves of nutrients; additional specialized functions	Highly variable; carbohydrate, lipid, protein or inorganic
Chromosome	Genetic material of cell	DNA
Plasmid	Extrachromosomal genetic material	DNA

 

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+ve) bacteria differ in its structure and composition form that of gram negative (G-ve)

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 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.

The Cytoplasm

The cytoplasm of most bacteria contain DNA, ribosomes RNA and storage granule.

Storage granule

Temporarily hold excess metabolites storage granule known as volutine and lipid granules (metachromatic granules).

DNA, double – stranded circular molecule.

Plasmid is extra chromosomal circular and smaller then DNA. Plasmids carry genes involved in antibiotic resistance called.

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.

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.

1.     protects the cell from phagocytosis.

2.     adherence of bacterium to surface of the cells (tissue).

3.     Virulence factor

Flagella     

-         Arrangement basis for classification

-         Monotrichous; 1 flagella

-         Lophotrichous (polar flagella); tuft at one end

-         Amphitrichous; both ends

-         Peritrichous; all around bacteria

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:

1.     Protein fibers that cover the entire surface of G-ve bacteria

2.     Ploy a major in bacterial adherence to the cell surface.

3.     Sex pili involved in bacterial conjugation (and gene transfer).

Classification of microorganisms

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

Morphological classification of bacteria

Morphologically bacteria car resemble.

Cocci                    (singular: coccus)

Coccobacilli          (singular: coccobacillus)

Rods (bacilli)        (singular: rod, bacillus)

Vibrios                 (singular: vibrio)

Spirilla                 (singular: spirillum)

Spirochetes                  (singular: spirochete)