Introduction:
Medical Microbiology is the study of microorganisms that cause disease in human beings, the pathogenesis of the disease, diagnosis of these infectious diseases and treatment. (i.e. study of anti infectious agents – antibiotics).
The different branches of medical microbiology are,
1) Bacteriology
2) Immunology
3) Mycology
4) Virology
5) Parasitology
History of Microbiology:
Antony Van Leeuwenhock, a draper of
Louis Pasteur (1822 – 1895): French chemist. Derived the relation between microbes and diseases. He is called as the father of Modern Microbiology. His contributions are:
1) Fermentation experiment.
2) Silkworm disease.
3) Sterilization technique:- Hot air oven
Autoclave
Steam sterilization
4) Pasteurization
5) Studied anthrax, Chicken Cholera, Rabies.
6) Coined the term Vaccine & attenuation.
7) Preparation of live attenuated vaccine for Anthrax, Chicken Cholera rabies.
8) Cotton wool plugs for test tubes
Robert Koch (1843 – 1910): German physician.
Contributions are:
1) Confirmed the association of microbes and diseases.
2) Studied Anthrax
3) Pure culture
4) Isolated bacteria in the pure culture.
5) Sterile techniques & disinfectants
6) Discovered tubercle bacilli, vibrio cholerae
7) Agar-agar seed weed product used for solidification of the culture media.
8) Smear preparation and staining.
Koch’s Postulates.
1) The disease-producing microorganism should be associated with all the disease d cases and should not be present in the healthy animals/ individuals.
2) It should be possible to isolate the disease-causing organism in pure culture from the disease d animal.
3) When this pure culture is injected into an experimental animal, it should produce the same disease
4) It should be possible to isolate the same microorganism in pure culture from the experimental animal.
5) Specific antibodies should be present in the serum of the individual against the pathogenic microorganism.
6) Proposed molecular cause postulate.
a) The phenotypic properties should be seen only in pathogenic microorganism & not in non-pathogenic microorganism.
b) Specific inactivation of the gene responsible for the virulence, should lead to loss of virulence.
c) Reintroduction of the virulent gene will result in gain in virulence.
Koch’s Phenomenon:
A tuberculus inhibin acts differently from non-tuberculus bacterium is introduced, subcutaneouly.
Joseph Lister: Antiseptic technique for surgery.
Edward Jenner: Smallpox vaccination
Walter Read: Virus- yellow fever.
Ignaz Semmelweis: Concept of puerperal sepsis.
Alexander Fleming: Penicillin
Ronald Roes: Malaria parasite.
Cell wall:
Cell wall is the structure of the cell, which lies external to the cytoplasmic membrane & internal to the capsule, if capsule is present. It is about 10-20 u in thickness & it is fully permeable. Cell wall is mainly composed of peptidoglycan. The cell wall, depending upon the stain taken is classified as Gram +ve or Gram –ve.
Gram +ve : Contains large amount of peptidoglycons. Composition of Gram +ve cell wall:
1) Peptidoglycan in large amount
2) Teicoic acid.
Gram –ve: Contains fewer amounts of peptidoglycons. Composition of Gram –ve cell wall:
1) Peptidoglycan
2) Lipoprotein
3) Outer membrane which is made up of :
a) Phospholipid bilayer
b) Lippoly saccharide
c) Outer membrane proteins
Peptidoglycan: It consists of alternating molecule of N- acetyl glucosamine& N- acetyl muramic acid. Each molecule of N-acetyl muramic acid has peptide side chains. These peptide side chains are cross-linked by penta-peptide linkages.
Teicoic acid is present only in Gram +ve cell wall. These are polymers & 2 types of teicoic acid are seen.
a) Wall teicoic acid: which is closely associated with the peptidoglycan
b) Membrane teicoic acid: which is attached to the cytoplasmic membrane.
Gram –ve cell wall
Lipoprotein: made up of 2 parts- Protein part is attached to the peptidoglycan & the lipid part is attached to the outer membrane. Main function is to adhere the outer membrane to the peptidoglycan.
Outer membrane: The outer membrane consists of a phospholipid bilayer, which lies external to the peptidoglycan. It has a very important structure called lipopolysaccharide.
Each lipopolysaccharide has a core polysaccharide that is similar to all Gram -ve organisms. It is attached to lipid A, which has endotoxic activity & also to repeating units, which are antigenic in nature and are specific to a particular species of organism and these species are referred as O-antigen.
The outer membrane proteins like prions, form channels for selective permeability of solutes & other outer membrane proteins may act as receptors.
Uses of cell wall:
Protects the cytoplasmic membrane from osmotic damage. It gives a specific shape to the bacteria. It plays an important role in all division. It is freely permeable except in Gram –ve organism, where they are selectively permeable. In Gram –ve organism, due to the presence of lipid A, it acts as a virulence factor.
Practical uses:
1) Helps to differentiate between Gram +ve and Gram –ve bacteria.
2) O- antigen helps in the identification of certain organisms. Eg: Salmonella.
3) Antibodies to O-antigen can be detected in sereological test.
Clinical significance:
1) The lipid A present in Gram –ve cell wall is responsible for endotoxic shock seen in Gram –ve septicemia.
2) Certain antibodies act on cell wall, like penicillin, Wankomycin.
3) Certain substances like antibodies, damage the cell wall in of osmotically protective medium to form L-forms.
L-forms:
Certain substances damage the cell wall and in the presence of osmotically protective medium, these cell wall deficient bacteria can survive. These cell wall deficient forms are called as L-forms.