NEET MDS Lessons
General Microbiology
NON-SPECIFIC KILLER CELLS
Several different cells including NK and LAK cells, K cells, activated macrophages and eosinophils are capable of killing foreign and altered self target cells in a non-specific manner. These cells play an important role in the innate immune system.
A. NK and LAK cells
Natural killer (NK) cells are also known as large granular lymphocytes (LGL) because they resemble lymphocytes in their morphology, except that they are slightly larger and have numerous granules.
NK cells can be identified by the presence of CD56 and CD16 and a lack of CD3 cell surface markers.
NK cells are capable of killing virus-infected and malignant target cells but they are relatively inefficient in doing so.
However, upon exposure to IL-2 and IFN-gamma, NK cells become lymphokine-activated killer (LAK) cells, which are capable of killing malignant cells.
Continued exposure to IL-2 and IFN-gamma enables the LAK cells to kill transformed as well as malignant cells. LAK cell therapy is one approach for the treatment of malignancies.
NK and LAK cells have two kinds of receptors on their surface – a killer activating receptor (KAR) and a killer inhibiting receptor (KIR).
When the KAR encounters its ligand, a killer activating ligand (KAL) on the target cell the NK or LAK cells are capable of killing the target. However, if the KIR also binds to its ligand then killing is inhibited even if KAR binds to KAL.
The ligands for KIR are MHC-class I molecules. Thus, if a target cell expresses class I MHC molecules it will not be killed by NK or LAK cells even if the target also has a KAL which could bind to KAR.
Normal cells constitutively express MHC class I molecules on their surface, however, virus infected and malignant cells down regulate expression of class I MHC. Thus, NK and LAK cells selectively kill virus-infected and malignant cells while sparing normal cells.
B. K cells
Killer (K) cells are not a morphologically distinct type of cell. Rather a K cell is any cell that mediates antibody-dependent cellular cytotoxicity (ADCC).
In ADCC antibody acts as a link to bring the K cell and the target cell together to allow killing to occur. K cells have on their surface an Fc receptor for antibody and thus they can recognize, bind and kill target cells coated with antibody.
Killer cells which have Fc receptors include NK, LAK, and macrophages which have an Fc receptor for IgG antibodies and eosinophils which have an Fc receptor for IgE antibodies.
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.
Radioimmunoassays (RIA)
It is an extremely sensitive technique in which antibody or antigen is labelled with a radioactive material. The amount of radioactive material in the antigen-antibody complex can be measured with which concentration of antigen or antibody can be assayed. After the reaction ‘free’ and ‘bound’ fractions of antigen are separated and their radioactivity-measured.
INNATE (NON-SPECIFIC) IMMUNITY
The elements of the innate (non-specific) immune system include anatomical barriers, secretory molecules and cellular components.
Among the mechanical anatomical barriers are the skin and internal epithelial layers, the movement of the intestines and the oscillation of broncho-pulmonary cilia.
Associated with these protective surfaces are chemical and biological agents.
A. Anatomical barriers to infections
1. Mechanical factors
The epithelial surfaces form a physical barrier that is very impermeable to most infectious agents. Thus, the skin acts as our first line of defense against invading organisms. The desquamation of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surfaces.
2. Chemical factors
Fatty acids in sweat inhibit the growth of bacteria. Lysozyme and phospholipase found in tears, saliva and nasal secretions can breakdown the cell wall of bacteria and destabilize bacterial membranes. The low pH of sweat and gastric secretions prevents growth of bacteria. Defensins (low molecular weight proteins) found in the lung and gastrointestinal tract have antimicrobial activity. Surfactants in the lung act as opsonins (substances that promote phagocytosis of particles by phagocytic cells).
3. Biological factors
The normal flora of the skin and in the gastrointestinal tract can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or attachment to cell surfaces.
B. Humoral barriers to infection
Humoral factors play an important role in inflammation, which is characterized by edema and the recruitment of phagocytic cells. These humoral factors are found in serum or they are formed at the site of infection.
1. Complement system – The complement system is the major humoral non-specific defense mechanism (see complement chapter). Once activated complement can lead to increased vascular permeability, recruitment of phagocytic cells, and lysis and opsonization of bacteria.
2. Coagulation system – Depending on the severity of the tissue injury, the coagulation system may or may not be activated. Some products of the coagulation system can contribute to the non-specific defenses because of their ability to increase vascular permeability and act as chemotactic agents for phagocytic cells. In addition, some of the products of the coagulation system are directly antimicrobial. For example, beta-lysin, a protein produced by platelets during coagulation can lyse many Gram positive bacteria by acting as a cationic detergent.
3. Lactoferrin and transferrin – By binding iron, an essential nutrient for bacteria, these proteins limit bacterial growth.
4. Interferons – Interferons are proteins that can limit virus replication in cells.
5. Lysozyme – Lysozyme breaks down the cell wall of bacteria.
6. Interleukin -1 – Il-1 induces fever and the production of acute phase proteins, some of which are antimicrobial because they can opsonize bacteria.
C. Cellular barriers to infection
Part of the inflammatory response is the recruitment of polymorphonuclear eosinophiles and macrophages to sites of infection. These cells are the main line of defense in the non-specific immune system.
1. Neutrophils – Polymorphonuclear cells are recruited to the site of infection where they phagocytose invading organisms and kill them intracellularly. In addition, PMNs contribute to collateral tissue damage that occurs during inflammation.
2. Macrophages – Tissue macrophages and newly recruited monocytes , which differentiate into macrophages, also function in phagocytosis and intracellular killing of microorganisms. In addition, macrophages are capable of extracellular killing of infected or altered self target cells. Furthermore, macrophages contribute to tissue repair and act as antigen-presenting cells, which are required for the induction of specific immune responses.
3. Natural killer (NK) and lymphokine activated killer (LAK) cells – NK and LAK cells can nonspecifically kill virus infected and tumor cells. These cells are not part of the inflammatory response but they are important in nonspecific immunity to viral infections and tumor surveillance.
4. Eosinophils – Eosinophils have proteins in granules that are effective in killing certain parasites.
BACTERIAL GROWTH
The conversion of a parental cell into two daughters constitutes the bacterial life cycle and the time taken to complete cell cycle is known as generation_time. This is around 15 minutes in vegetative bacteria except mycobacteria.
Bacterial Growth Curve
In the presence of fresh growth medium a bacterium shows following four phases;
The Lag phase -> The Log phase -> The Stationary phase -> The Decline phase
The Lag Phase : short duration , bacteria adapt themselves to new environment
The Log Phase (Exponential Phase) : Regular growth of bacteria occurs The morphology of bacteria is best developed in this phase and organisms manifest typical biochemical characters.
- Most of the cidal Abx work best in this phase
• i.e. Ampicillin
- Best phase for staining bacterial cultures
Chemostat and turbidostat are examples of technique by which this phase can be prolonged.
Stationary Phase : balanced growth and cell division cannot be sustained. The total cell Count remains static till lysis supervenes, but the viable cell count quickly declines.
Decline Phase: death phase. Dyeing bacteria exceed the dividing bacterias.
DISINFECTION AND STERILIZATION
• Sterilization is the best destruction or com removal_of all forms of micro organisms.
• Disinfection is the destruction of many microorganisms but usually the b spores.
• Antisepsis is the destruction or inhibition of microorganisms in living tissues thereby limiting or preventing the harmful effect of infection.
• Astatic Agent would only inhibit the growth of microorganisms (bacteriostatic, fungistatic, sporostatic).
• Acidal agent would kill the microorganism (bactericidal. virucidal, fungicidal)
• Sterilants are the chemicals which under controlled conditions can kill sporinQ bacteria.