NEET MDS Lessons
General Microbiology
ANTIGENS
Immunogen
A substance that induces a specific immune response.
Antigen (Ag)
A substance that reacts with the products of a specific immune response.
Hapten
A substance that is non-immunogenic but which can react with the products of a specific immune response. Haptens are small molecules which could never induce an immune response when administered by themselves but which can when coupled to a carrier molecule. Free haptens, however, can react with products of the immune response after such products have been elicited. Haptens have the property of antigenicity but not immunogenicity.
Epitope or Antigenic Determinant
That portion of an antigen that combines with the products of a specific immune response.
Antibody (Ab)
A specific protein which is produced in response to an immunogen and which reacts with an antigen.
FACTORS INFLUENCING IMMUNOGENICITY
- Larger the molecule the more immunogenic it is likely to be.
- More complex the substance is chemically the more immunogenic it will be.
- Particulate antigens are more immunogenic than soluble ones and denatured antigens more immunogenic than the native form.
- Antigens that are easily phagocytosed are generally more immunogenic. This is because for most antigens (T-dependant antigens, see below) the development of an immune response requires that the antigen be phagocytosed, processed and presented to helper T cells by an antigen presenting cell (APC).
- Some substances are immunogenic in one species but not in another. Similarly, some substances are immunogenic in one individual but not in others (i.e. responders and non-responders). The species or individuals may lack or have altered genes that code for the receptors for antigen on B cells and T cells or they may not have the appropriate genes needed for the APC to present antigen to the helper T cells.
Method of Administration
1. Dose
The dose of administration of an immunogen can influence its immunogenicity. There is a dose of antigen above or below which the immune response will not be optimal.
2. Route
Generally the subcutaneous route is better than the intravenous or intragastric routes. The route of antigen administration can also alter the nature of the response
3. Adjuvants
Substances that can enhance the immune response to an immunogen are called adjuvants. The use of adjuvants, however, is often hampered by undesirable side effects such as fever and inflammation.
TYPES OF ANTIGENS
T-independent Antigens
T-independent antigens are antigens which can directly stimulate the B cells to produce antibody without the requirement for T cell help In general, polysaccharides are T-independent antigens. The responses to these antigens differ from the responses to other antigens.
Properties of T-independent antigens
1. Polymeric structure
These antigens are characterized by the same antigenic determinant .
2. Polyclonal activation of B cells
Many of these antigens can activate B cell clones specific for other antigens (polyclonal activation). T-independent antigens can be subdivided into Type 1 and Type 2 based on their ability to polyclonally activate B cells. Type 1 T-independent antigens are polyclonal activators while Type 2 are not.
3. Resistance to degradation
T-independent antigens are generally more resistant to degradation and thus they persist for longer periods of time and continue to stimulate the immune system.
T-dependent Antigens
T-dependent antigens are those that do not directly stimulate the production of antibody without the help of T cells. Proteins are T-dependent antigens. Structurally these antigens are characterized by a few copies of many different antigenic determinants as illustrated in the Figure 2.
HAPTEN-CARRIER CONJUGATES
Hapten-carrier conjugates are immunogenic molecules to which haptens have been covalently attached. The immunogenic molecule is called the carrier.
Structure
Structurally these conjugates are characterized by having native antigenic determinants of the carrier as well as new determinants created by the hapten (haptenic determinants). The actual determinant created by the hapten consists of the hapten and a few of the adjacent residues, although the antibody produced to the determinant will also react with free hapten. In such conjugates the type of carrier determines whether the response will be T-independent or T-dependent.
SUPERANTIGENS
When the immune system encounters a conventional T-dependent antigen, only a small fraction (1 in 104 -105) of the T cell population is able to recognize the antigen and become activated (monoclonal/oligoclonal response). However, there are some antigens which polyclonally activate a large fraction of the T cells (up to 25%). These antigens are called superantigens .
Examples of superantigens include: Staphylococcal enterotoxins (food poisoning), Staphylococcal toxic shock toxin (toxic shock syndrome), Staphylococcal exfoliating toxins (scalded skin syndrome) and Streptococcal pyrogenic exotoxins (shock).
Method of Sterilization for common items
Autoclaving : Animal cages, Sugar tubes, Lab. Coats, Cotton , Filters, Instruments Culture media, Rubber, Gloves , Stopper, Tubing, Slides, Syringe and Wax needles , Test tubes, Enamel metal trays ,Wire baskets, Wood, Tongue depressor, Applicator, Endodontic instruments, Orthodontic pliers , Orthodontic kits, Saliva ejector, Handpieces Cavitron heads, Steel burs, Steel tumbler, Hand instruments
Hot air oven
Beakers, Flasks, Petri dish, Slides, Syringes, Test tubes, Glycerine, Needles ,Oil, Paper Saliva ejector, Matrix Band
Ethylene oxide
Fabric, Bedding, Blanket, Clothing, Matteresses, Pillows, Disposable instruments , Instruments, Blades, Knives, Scalpels, Scissors ,Talcum powder, Books, Cups, plates , Plastics., Flask, Petridish, Tubes, Tubing, Rubber , catheters, Drains, Gloves ,Special items - Bronchoscope, Cystoscope, Heart lung machine
Glutaraldehyde
Orthodontic kits, Orthodontic pliers , Steel burrs, 3 in 1 syringe tips ,Cystoscope ,Endoscope
Filtration
Antibiotics, Serum, Vaccines
NUTRITION OF BACTERIA
Nutrients
Chemoheterotrophs: nutrient source is organic material
Bacteria also requires a source of minerals.
Oxygen
On this basis bacteria have been divided into four groups.
Obligate Anaerobes: These grow only under conditions of high reducing intensity. These bacteria catalase peroxidase, superoxide dismutase and cytochrome systems
Clostridium and Bacteroides are important examples.
Facultalive Anaerobes. These can grow under both aerobic and anaerobic conditions and include members of family enterobacteriaceae and many other bacteria.
Obligatory Aerobes. These cannot grow unless oxygen is present in the medium. Pseudomonas belong to this group.
Microaerophillic. These organisms can grow under conditions with low oxygen tension. Clostridium tetani is an important example.
The strict anaerobes are unable to grow unless Eh is as low as 0.2 volt
Temperature
• On the basis of temperature requirements, three groups of bacteria are recognised.
• Psychrophilic : Growth in the range of —5 to 30°C with an optimum of 10-20
• Mesophillic : bacteria grow best at 20-40°C with a range of 10-45°C.
• Medically important bacteria belong to this group
• Myco. leprae is one such important example and it can grow only at reduced temperature such as footpad of mouse
• Thermophillic organisms prefer high temperature (25-80°C) for growth and yield maximum growth at 50-60°C
pH : Most pathogenic bacteria require a pH of 7.2-7.6 for their own optimal growth.
Precipitation Reaction
This reaction takes place only when antigen is in soluble form. Such an antigen when
comes in contact with specific antibody in a suitable medium results into formation of an insoluble complex which precipitates. This precipitate usually settles down at the bottom of the tube. If it fails to sediment and remains suspended as floccules the reaction is known as flocculation. Precipitation also requires optimal concentration of NaCl, suitable temperature and appropriate pH.
Zone Phenomenon
Precipitation occurs most rapidly and abundantly when antigen and antibody are in optimal proportions or equivalent ratio. This is also known as zone of equivalence. When antibody is in great excess, lot of antibody remains uncombined. This is called zone of antibody excess or prozone. Similarly a zone of antigen excess occurs in which all antibody has combined with antigen and additional uncombined antigen is present.
Applications of Precipitation Reactions
Both qualitative determination as well as quantitative estimation of antigen and antibody can be performed with precipitation tests. Detection of antigens has been found to be more sensitive.
Agglutination
In agglutination reaction the antigen is a part of the surface of some particulate material such as erythrocyte, bacterium or an inorganic particle e.g. polystyrene latex which has been coated with antigen. Antibody added to a suspension of such particles combines with the surface antigen and links them together to form clearly visible aggregate which is called as agglutination.
Application of precipitation reactions
Precipitation reaction Example
Ring test Typing of streptococci, Typing of pneumococci
Slide test (flocculation) VDRL test
Tube test (flocculation) Kahn test
Immunodiffusion Eleks test
Immunoelectrophoresis Detection Of HBsAg, Cryptococcal antigen in CSF
ANTIGEN-ANTIBODY REACTIONS
I. NATURE OF ANTIGEN-ANTIBODY REACTIONS
A. Lock and Key Concept
The combining site of an antibody is located in the Fab portion of the molecule and is constructed from the hypervariable regions of the heavy and light chains. Antigen-antibody reactions is one of a key (i.e. the antigen) which fits into a lock (i.e. the antibody).
B. Non-covalent Bonds
The bonds that hold the antigen to the antibody combining site are all non-covalent in nature. These include hydrogen bonds, electrostatic bonds, Van der Waals forces and hydrophobic bonds.
C. Reversibility
Since antigen-antibody reactions occur via non-covalent bonds, they are by their nature reversible.
II. AFFINITY AND AVIDITY
A. Affinity
Antibody affinity is the strength of the reaction between a single antigenic determinant and a single combining site on the antibody. It is the sum of the attractive and repulsive forces operating between the antigenic determinant and the combining site of the antibody .
B. Avidity
Avidity is a measure of the overall strength of binding of an antigen with many antigenic determinants and multivalent antibodies. Avidity is influenced by both the valence of the antibody and the valence of the antigen. Avidity is more than the sum of the individual affinities.
III. SPECIFICITY AND CROSS REACTIVITY
A. Specificity
Specificity refers to the ability of an individual antibody combining site to react with only one antigenic determinant or the ability of a population of antibody molecules to react with only one antigen. In general, there is a high degree of specificity in antigen-antibody reactions.
B. Cross reactivity
Cross reactivity refers to the ability of an individual antibody combining site to react with more than one antigenic determinant or the ability of a population of antibody molecules to react with more than one antigen.
THE PLASMIDS
The extrachromosomal genetic elements, called as plasmids are autonomously replicating , cyclic ,double stranded DNA molecules which are distinct from the cellular chromosome
Classification
Plasmids can be broadly classified as conjugative and nonconjugative.
Conjugative plasmids are large and self-transmissible i.e. they have an apparatus through which they can mediate their own transfer to another cell after coming in contact with the same. Example: RF and certain bacteriocinogen plasmids.
Nonconjugative plasmids are small in size and can be mobilised for transfer into another cell only through the help of a conjugative plasmid. To this group belong some ‘r’ determinants and few bacteriocinogenic plasmids. Plasmids can also be transferred without cell contact by the process of transfection.
Properties of plasmids
Double stranded DNA , Autonomously replicate in host cell, Plasmd specific, Free DNA is transferred b transfection
Significance of Plasmids :The spread of resistance to antibiotics is one such well known example. These also play an important role in the geochemical cycle by spreading genes for the degradation of complex organic compounds.