NEET MDS Lessons
General Microbiology
STRUCTURE AND SOME PROPERTIES OF IG CLASSES AND SUBCLASSES
A. IgG
1. Structure
All IgG’s are monomers (7S immunoglobulin). The subclasses differ in the number of disulfide bonds and length of the hinge region.
2. Properties
IgG is the most versatile immunoglobulin because it is capable of carrying out all of the functions of immunoglobulin molecules.
a) IgG is the major Ig in serum – 75% of serum Ig is IgG
b) IgG is the major Ig in extra vascular spaces
c) Placental transfer – IgG is the only class of Ig that crosses the placenta. Transfer is mediated by a receptor on placental cells for the Fc region of IgG. Not all subclasses cross equally well; IgG2 does not cross well.
d) Fixes complement – Not all subclasses fix equally well; IgG4 does not fix complement
e) Binding to cells – Macrophages, monocytes and neutrophils and some lymphocytes have Fc receptors for the Fc region of IgG. A consequence of binding to the Fc receptors on such cells is that the cells can now internalize the antigen better. The antibody prepares the antigen for killing by the phagocytic cells. The term opsonin is used to describe substances that enhance phagocytosis. (Coating of the surface of pathogen by antibody is called opsonization).IgG is a good opsonin. Binding of IgG to Fc receptors on other types of cells results in the activation of other functions.
IgM
1. Structure
IgM normally exists as a pentamer (19S immunoglobulin) but it can also exist as a monomer. In the pentameric form all heavy chains are identical and all light chains are identical. Thus, the valence is theoretically 10. IgM has an extra domain on the mu chain (CH4) and it has another protein covalently bound via a S-S bond called the J chain. This chain functions in polymerization of the molecule into a pentamer.
2. Properties
a) IgM is the third most common serum Ig.
b) IgM is the first Ig to be made by the fetus and the first Ig to be made by a virgin B cells when it is stimulated by antigen.
c) As a consequence of its pentameric structure, IgM is a good complement fixing Ig. Thus, IgM antibodies are very efficient in leading to the lysis of microorganisms.
d) As a consequence of its structure, IgM is also a good agglutinating Ig . Thus, IgM antibodies are very good in clumping microorganisms for eventual elimination from the body.
e) IgM binds to some cells via Fc receptors.
f) B cell surface Ig
Surface IgM exists as a monomer and lacks J chain but it has an extra 20 amino acids at the C-terminus to anchor it into the membrane . Cell surface IgM functions as a receptor for antigen on B cells.
IgA
1. Structure
Serum IgA is a monomer but IgA found in secretions is a dimer as presented in Figure 10. When IgA exits as a dimer, a J chain is associated with it.
When IgA is found in secretions is also has another protein associated with it called the secretory piece or T piece; sIgA is sometimes referred to as 11S immunoglobulin. Unlike the remainder of the IgA which is made in the plasma cell, the secretory piece is made in epithelial cells and is added to the IgA as it passes into the secretions . The secretory piece helps IgA to be transported across mucosa and also protects it from degradation in the secretions.
2. Properties
a) IgA is the 2nd most common serum Ig.
b) IgA is the major class of Ig in secretions – tears, saliva, colostrum, mucus. Since it is found in secretions secretory IgA is important in local (mucosal) immunity.
c) Normally IgA does not fix complement, unless aggregated.
d) IgA can binding to some cells – PMN’s and some lymphocytes.
IgD
1. Structure
IgD exists only as a monomer.
2. Properties
a) IgD is found in low levels in serum; its role in serum is uncertain.
b) IgD is primarily found on B cell surfaces where it functions as a receptor for antigen.
c) IgD does not bind complement.
E. IgE
1. Structure
IgE exists as a monomer and has an extra domain in the constant region.
2. Properties
a) IgE is the least common serum Ig since it binds very tightly to Fc receptors on basophils and mast cells even before interacting with antigen.
b) Involved in allergic reactions – As a consequence of its binding to basophils and mast cells, IgE is involved in allergic reactions. Binding of the allergen to the IgE on the cells results in the release of various pharmacological mediators that result in allergic symptoms.
c) IgE also plays a role in parasitic helminth diseases. Since serum IgE levels rise in parasitic diseases, measuring IgE levels is helpful in diagnosing parasitic infections. Eosinophils have Fc receptors for IgE and binding of eosinophils to IgE-coated helminths results in killing of the parasite.
d) IgE does not fix complement.
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.
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.
Immunology:
The branch of life science which deals with immune reaction is known as immunology.
Components of Immune System:
The immune system consists of a network of diverse organs and tissue which vary structurally as well as functionally from each other. These organs remain spreaded throughout the body. Basically, immune system is a complex network of lymphoid organs, tissues and cells.
These lymphoid organs can be categorized under three types depending upon their functional aspects:
i. Primary lymphoid organ.
ii. Secondary lymphoid organ.
iii.Tertiary lymphoid organ.
White blood cells or leukocytes are the basic cell types which help to give rise to different types of cells which participate in the development of immune response . WBC are classified into granulocytes and agranulocytes depending on the presence or absence of granules in the cytoplasm.
Agranular leukocytes are of two types, viz., lymphocytes and monocytes. Lymphocytes play pivotal role in producing defensive molecules of immune system. Out of all leukocytes, only lymphocytes possess the quality of diversity, specificity, memory and self-non self recognition as various important aspects of immune response.
Other cell types remain as accessory one; help to activate lymphocytes, to generate various immune effector cells, to increase the rate of antigen clearance
All cells of the immune system have their origin in the bone marrow
myeloid (neutrophils, basophils, eosinpophils, macrophages and dendritic cells)
lymphoid (B lymphocyte, T lymphocyte and Natural Killer) cells .
The myeloid progenitor (stem) cell in the bone marrow gives rise to erythrocytes, platelets, neutrophils, monocytes/macrophages and dendritic cells whereas the lymphoid progenitor (stem) cell gives rise to the NK, T cells and B cells.
For T cell development the precursor T cells must migrate to the thymus where they undergo differentiation into two distinct types of T cells, the CD4+ T helper cell and the CD8+ pre-cytotoxic T cell.
Two types of T helper cells are produced in the thymus the TH1 cells, which help the CD8+ pre-cytotoxic cells to differentiate into cytotoxic T cells, and TH2 cells, which help B cells, differentiate into plasma cells, which secrete antibodies.
Function of the immune system is self/non-self discrimination.
This ability to distinguish between self and non-self is necessary to protect the organism from invading pathogens and to eliminate modified or altered cells (e.g. malignant cells).
Since pathogens may replicate intracellularly (viruses and some bacteria and parasites) or extracellularly (most bacteria, fungi and parasites), different components of the immune system have evolved to protect against these different types of pathogens.
ANTIGEN-ANTIBODY REACTIONS
Affinity of the antigen-antibody reaction refers to the intensity of the attraction between antigen and antibody molecule.
Antigen-antibody reactions
Reaction test Modified test
Precipitation -> Immunoelectrophoresis, Immunoprecipitation
Agglutination -> Latex agglutination, Indirect, Haemagglutination , Coagglutination ,Coombs test
Neutralization -> Measurement of LD, Plaque assays
Complement fixation -> Conglutination
Immunofluorescence -> Indirect immunofiuorescence, Immunoofluoremetric Assay
Enzyme immunoassay -> Enzyme linked, Immunosorbent assay
Radioimmunoassay -> Immunoradiometric assay
Avidity is the strength of the bond after the formation of antigen-antibody complex.
Sensitivity refers to the ability of the test to detect even very minute quantities of antigen or antibody. A test shall be called as highly sensitive if false negative results are absent or minimal.
Specificity refers to the ability of the test to detect reactions between homologous antigens and antibodies only, and with no other. In a highly specific test, false positive reactions will be minimal or absent.
Autoantibodies
Anti-nuclear antibodies (ANA) Systemic Lupus
Anti-dsDNA, anti-Smith Specific for Systemic Lupus
Anti-histone Drug-induced Lupus
Anti-IgG Rheumatoid arthritis
Anti-neutrophil Vasculitis
Anti-centromere Scleroderma (CREST)
Anti-Scl-70 Sclerderma (diffuse)
Anti-mitochondria 1oary biliary cirrhosis
Anti-gliadin Celiac disease
Anti-basement membrane Goodpasture’s syndrome
Anti-epithelial cell Pemphigus vulgaris
Anti-microsomal Hashimoto’s thryoiditis
Bacteria
A bacterial cell has a nuclear apparatus which is a loose arrangement of DNA This is surrounded cytoplasm which contains ribosomes, mesosomes and inclusion granules. The cytoplasm is enclosed within a cytoplasmic membrane. Bacterium has a rigid cell wall Fimbriae and flagella are the surface adherents. Some bacteria may have a capsule (or loose slime) around the cell wall.
Shape and Size of Bacteria
The bacteria can be spheroidal (coccus), rod or cylindrical (bacillus) and spirillar (spirochaete). Very short bacilli are called as coccobacilli Some of the bacilli may be curved or comma shaped (Vibrio cholerae).
Arrangement of Bacterial Cells
Streptococci are present in chains; staphylococci in grape-like clusters Cocci in pairs (diplococci) are suggestive of pneumococci, gonococci or menigococci.
Bacilli do not exhibit typical arrangement pattern except the Chinese letter arrangement shown by Corynebacterium diphtheriae
Surface Adherents and Appendages
CAPSULE The gels formed by the capsule adhere to the cell Capsule can be detected by negative staining ,with specific antiserum and observing the capsular swelling phenomenon called as Quellung reaction
Usually weakly antigenic Capsule production is better in vivo as compared to in vitro environment.
Eg. Capsules seen in Pneumococci, Klebsiella, Escherichia coli, Haemophilus influenzae
Flagella : provide motility to the bacterium.
Motile organisms: vibrios, pseudomonas, Esch.coli, salmonellae, spirochaetes and spirilla.
Pathogenic cocci are nomotile.
Flagella measure in length from 3 to 20 µm and in diameter from 0.01 to 0.0 13 µm.
Arrangement
Bacteria with one polar flagellum are known as monotrichous;
Tuft of several polar flagellae is known as lophotrichous
Presence of Flagellae at both the ends of organism is amphitrichous
Flagellae distributed all over the surface of the bacterium, it is called peritrichous.
• Filament is composed of a protein-flagellin. The flagellar antigen is called as H (Hauch) antigen in contrast to somatic antigen which is called as O (Ohne haunch)
PILI (fimbriae) : hair like structures help in attachment also called sex pilli, transfers genetic material through conjugation , Present in Certain Gram negative bacteria. Only Composed of protein pilin
Gram positive bacterium that has pili is Cornebacterium renale
The Cell Wall
The cell wall of bacteria is multilayered structure. The external surface of cell wall is smooth in Gram positive bacteria Gram negative bacteria have convoluted cell surfaces. The average thickness of cell wall is 0.15 to 0.50 .µm. Chemically composed of mucopeptide scaffolding formed by N acetyl glucosamine and N acetyl muramic acid
The cell wall is a three layered structure in Gram negative bacteria: outer membrane middle layer and plasma membrane. The outer membrane consists of lipoprotein and 1ipoppolysaccaride component
Functions of bacterial cell wall
Provides shape , Gives rigidity , Protection, Surface has receptor sites for phages, Site of antibody action, Provides attachment to complement, Contains components toxic to host
Cytoplasmic Structures
The Plasma Membrane: This delicate membrane separates rigid cell wall from cytoplasm. It accounts for 30% of total cell weight. Chemically, it is 60% protein, 20-30% lipids and remaining carbohydrates.
Mesosomes:
Principal sites of respiratory enzyme , Seen well in Gram positive bacteria as compared to Gram negative batcteria. Attachement of mesosomes to both DNA chromatin and membrane have been noticed thus help in cell division
Ribosomes:
sites of protein synthesis. These are composed of RNA and proteins and constitute upto 4 of total cell protein and 90% of total cellular RNA.
Cytoplasmic Granules: Glycogen : Enteric bacteria
Poly-beta & hydroxy Butyrate : Bacillus & Pseudomonas
Babes-Ernst :Corynebacterium & Yersinia pestis
Nuclear Apparatus
Bacterial DNA represents 2-3% of the cell weight and 10% of the volume of bacterium. Nucleous can be demonstrated by staining it with DNA specific Fuelgen stain .Consists of a single molecule of double stranded DNA arranged in a circular form. Bacterial chromosome is haploid and replicates by binary fission, the bacteria may have plasmid an extrachromosomal genetic material.