Microbiology
General Microbiology and Immunity
General microbiology and immunity for MBBS: staining, sterilisation, bacterial pathogenicity, endotoxins, immunoglobulins and hypersensitivity, mapped to NMC codes MI1.1 to MI1.8.
MedNext Academy | 3 min read
General Microbiology and Immunity
General microbiology and immunity for MBBS: staining, sterilisation, bacterial pathogenicity, endotoxins, immunoglobulins and hypersensitivity, mapped to NMC codes MI1.1 to MI1.8.
General microbiology and immunity lay the groundwork for the entire subject. This chapter covers the structure and staining of bacteria, sterilisation and disinfection, the mechanisms of bacterial pathogenicity, and the innate and adaptive immune responses that determine host defence and vaccination.
High-yield: General Microbiology and Immunity
- The Gram stain separates bacteria into Gram-positive (thick peptidoglycan, stains purple) and Gram-negative (thin peptidoglycan plus outer membrane, stains pink) organisms.
- The Ziehl-Neelsen acid-fast stain detects mycobacteria and Nocardia, whose cell walls are rich in mycolic acid.
- Autoclaving at 121 degrees Celsius and 15 pounds per square inch for 15 minutes is the standard for moist-heat sterilisation and kills bacterial spores.
- Bacterial spores of Clostridium and Bacillus are the most heat-resistant forms and are the reference for testing sterilisation.
- Endotoxin is the lipopolysaccharide of Gram-negative outer membranes, is heat-stable, and its lipid A moiety drives septic shock.
- Exotoxins are heat-labile proteins secreted mainly by Gram-positive bacteria and can be converted to toxoids for vaccines.
- Obligate aerobes such as Pseudomonas need oxygen, while obligate anaerobes such as Clostridium are killed by it.
- Innate immunity is rapid and non-specific, whereas adaptive immunity is slower, specific, and shows memory on re-exposure.
- IgG is the only antibody that crosses the placenta and gives passive immunity to the newborn.
- IgM is the first antibody produced in a primary response and its presence indicates recent or acute infection.
- IgA is the predominant antibody in mucosal secretions, milk and colostrum, protecting mucosal surfaces.
- Type I hypersensitivity is IgE-mediated and immediate, seen in anaphylaxis, atopy and asthma.
- Type IV hypersensitivity is delayed and cell-mediated, illustrated by the tuberculin (Mantoux) reaction and contact dermatitis.
- Live attenuated vaccines such as BCG, oral polio and measles give durable immunity but are avoided in pregnancy and immunodeficiency.
- The ELISA and Widal reactions are examples of antigen-antibody tests used in serological diagnosis.
Immunoglobulins at a glance
- **IgG:** Most abundant, crosses the placenta, mediates the secondary response and neutralises toxins.
- **IgM:** Largest, pentameric, first antibody in a primary response, signals recent infection.
- **IgA:** Chief mucosal antibody, present in secretions, milk and colostrum.
- **IgE:** Binds mast cells and basophils, mediates type I hypersensitivity and antiparasitic defence.
NMC competencies in this chapter
- **MI1.1:** Morphology, classification and structure of bacteria and other microbes
- **MI1.2:** Microscopy and staining techniques including Gram and acid-fast stains
- **MI1.3:** Culture media, bacterial growth and identification methods
- **MI1.4:** Sterilisation, disinfection and biomedical waste handling
- **MI1.5:** Mechanisms of bacterial pathogenicity, endotoxins and exotoxins
- **MI1.6:** Normal microbial flora and host-parasite relationships
- **MI1.7:** Innate and adaptive immunity and their components
- **MI1.8:** Immunoglobulins, antigens and antigen-antibody reactions
Frequently Asked Questions
What is the difference between endotoxin and exotoxin?
Endotoxin is the heat-stable lipopolysaccharide of the Gram-negative outer membrane and causes septic shock through lipid A. Exotoxins are heat-labile secreted proteins, usually from Gram-positive bacteria, and can be inactivated into toxoids for vaccines.
Which antibody crosses the placenta?
IgG is the only immunoglobulin that crosses the placenta, giving the newborn passive immunity during the first months of life.
How is moist-heat sterilisation standardised?
Autoclaving at 121 degrees Celsius and 15 pounds per square inch for 15 minutes is the reference, because it reliably kills the highly resistant bacterial spores.
Why does this chapter matter for NEET-PG?
Staining, sterilisation, toxin mechanisms and immunoglobulin functions are recurring one-liners in NEET-PG, so a firm grasp here supports every later organism-based chapter.
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