MBBS Microbiology Syllabus

What is pasteurization, and give the two common protocols?

Pasteurization reduces pathogen load (not sterilization) in liquids like milk. Holder method: $63\,^{\circ}\text{C}$ for 30 min. Flash/HTST: $72\,^{\circ}\text{C}$ for 15 s. (UHT: $\sim 140\,^{\circ}\text{C}$ for a few seconds.)

Name the basic bacterial morphologies (shapes) and one arrangement term for cocci.

Cocci (spheres), bacilli (rods), spirilla/spirochetes (spirals), vibrios (comma-shaped), coccobacilli. Coccal arrangements include diplococci (pairs), streptococci (chains), and staphylococci (clusters).

What is acid-fast staining used for, and what makes a cell acid-fast?

Acid-fast (Ziehl-Neelsen) staining identifies Mycobacterium and Nocardia. Their cell walls contain mycolic acids (long-chain fatty acids/waxes) that resist decolorization by acid-alcohol, so they retain carbol fuchsin (red); the background counterstains blue.

Compare the three mechanisms of horizontal gene transfer in bacteria.

Transformation: uptake of naked/free DNA from the environment. Transduction: DNA transfer via a bacteriophage vector. Conjugation: direct cell-to-cell DNA transfer through a sex pilus (often plasmid-mediated, e.g. F factor).

What is a plasmid, and why is the F (fertility) factor important?

A plasmid is a small, circular, extrachromosomal DNA that replicates independently and often carries antibiotic-resistance or virulence genes. The F factor is a conjugative plasmid encoding the sex pilus; $F^{+}$ cells donate DNA to $F^{-}$ recipients.

Distinguish generalized and specialized transduction.

Generalized transduction: any bacterial gene can be packaged into a phage during the lytic cycle (random fragments). Specialized transduction: only genes adjacent to the prophage integration site are transferred, occurring via faulty excision in temperate (lysogenic) phages.

What are transposons ('jumping genes')?

Transposons are mobile DNA segments that move between sites in the genome or between chromosome and plasmid using the enzyme transposase. They can carry antibiotic-resistance genes and contribute to genetic variation and spread of resistance.

Differentiate fermentation from aerobic respiration in terms of final electron acceptor and ATP yield.

Aerobic respiration: final electron acceptor is $\ce{O2}$; complete oxidation of glucose yields up to $\sim 38$ ATP. Fermentation: an organic molecule (e.g. pyruvate) is the final acceptor, no electron transport chain, yielding only $2$ ATP per glucose (substrate-level phosphorylation).

Write the overall equation for the complete aerobic oxidation of glucose.

$$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O} \;+\; \text{energy (ATP)}$$

What does a positive catalase test indicate, and which reaction is involved?

Catalase-positive organisms (e.g. Staphylococcus) produce catalase, which decomposes hydrogen peroxide with visible bubbling: $$\ce{2H2O2 ->[\text{catalase}] 2H2O + O2}$$ Streptococci are catalase-negative.

Distinguish exotoxins from endotoxins.

Exotoxins: secreted proteins, often from Gram-positive (and some Gram-negative) bacteria, highly potent and specific, heat-labile, can be toxoided into vaccines. Endotoxins: lipid A of LPS in the Gram-negative outer membrane, released on lysis, heat-stable, cause fever/shock, poorly antigenic.

List the key stages of bacterial pathogenesis.

1) Transmission/entry into host. 2) Adherence/colonization (adhesins, pili). 3) Invasion and immune evasion (capsule, enzymes). 4) Multiplication. 5) Damage via toxins or host immune response. 6) Exit/transmission to a new host.

Define virulence factors and give three examples.

Virulence factors are microbial traits that enhance the ability to cause disease. Examples: capsules (antiphagocytic), pili/adhesins (attachment), exotoxins and endotoxins (tissue damage), and enzymes like coagulase, hyaluronidase, and collagenase (spread).

In clinical microbiology, distinguish the MIC and MBC of an antibiotic.

MIC (Minimum Inhibitory Concentration): the lowest antibiotic concentration that visibly inhibits bacterial growth. MBC (Minimum Bactericidal Concentration): the lowest concentration that kills $\geq 99.9\%$ of the inoculum. Bactericidal drugs have MBC close to MIC.

How is antibiotic susceptibility tested by the Kirby-Bauer disk diffusion method?

Antibiotic-impregnated disks are placed on a lawn of bacteria on Mueller-Hinton agar. After incubation, the diameter of the zone of inhibition is measured and compared to standard breakpoints to classify the organism as sensitive, intermediate, or resistant.

Differentiate selective and differential culture media with an example.

Selective media inhibit unwanted organisms to favor specific ones (e.g. MacConkey inhibits Gram-positives via bile salts). Differential media distinguish organisms by appearance (e.g. blood agar shows hemolysis patterns). MacConkey is both selective and differential.

How do the coagulase and catalase tests differentiate the main Gram-positive cocci?

Catalase-positive = Staphylococcus; catalase-negative = Streptococcus/Enterococcus. Among staphylococci, coagulase-positive = S. aureus; coagulase-negative = S. epidermidis/S. saprophyticus.

Classify streptococci by hemolysis on blood agar.

Alpha ($\alpha$)-hemolytic: partial hemolysis, green zone (e.g. S. pneumoniae, viridans group). Beta ($\beta$)-hemolytic: complete clearing (e.g. S. pyogenes group A, S. agalactiae group B). Gamma ($\gamma$): non-hemolytic (e.g. Enterococcus).

Which bacterium causes tuberculosis, and what are its key features?

Mycobacterium tuberculosis: an acid-fast, aerobic, slow-growing bacillus with a mycolic-acid-rich cell wall. It is cultured on Lowenstein-Jensen medium, spread by respiratory droplets, and forms caseating granulomas.

Name the causative organisms of cholera, typhoid fever, and tetanus.

Cholera: Vibrio cholerae (comma-shaped, produces cholera enterotoxin causing watery diarrhea). Typhoid fever: Salmonella enterica serovar Typhi. Tetanus: Clostridium tetani (tetanospasmin, a neurotoxin causing spastic paralysis).

What is the basic structure of a virus, and what is a capsid?

A virus consists of a nucleic acid genome (DNA or RNA) enclosed in a protein coat called the capsid (made of capsomeres); together they form the nucleocapsid. Some viruses also have a lipid envelope with glycoprotein spikes derived from host membranes.

Describe the main capsid symmetries used to classify viruses.

Icosahedral (roughly spherical, 20 triangular faces, e.g. adenovirus), helical (rod/filamentous nucleocapsid, e.g. tobacco mosaic virus, rabies), and complex (e.g. bacteriophages with head-tail structure, poxviruses).

Summarize the Baltimore classification basis for viruses.

The Baltimore system classifies viruses by genome type and mRNA-production route into 7 groups: I dsDNA, II ssDNA, III dsRNA, IV (+)ssRNA, V (−)ssRNA, VI ssRNA-RT (retroviruses), VII dsDNA-RT. All must produce mRNA to be translated.

Compare yeasts and molds.

Yeasts: unicellular fungi, reproduce by budding (or fission), form smooth colonies (e.g. Candida, Cryptococcus). Molds: multicellular filamentous fungi made of hyphae forming a mycelium, reproduce via spores (e.g. Aspergillus, Rhizopus).

What is a dimorphic fungus? Give an example.

A dimorphic fungus grows as a mold (filamentous) at environmental temperature ($\sim 25\,^{\circ}\text{C}$) and as a yeast in host tissue ($37\,^{\circ}\text{C}$). Examples: Histoplasma capsulatum, Blastomyces, Coccidioides, Sporothrix schenckii.

What is the principal component of the fungal cell wall, and why is it a drug target?

The fungal cell wall is composed mainly of chitin and glucans; the cell membrane contains ergosterol (instead of cholesterol). Ergosterol is targeted by azoles and polyenes (amphotericin B), and glucan synthesis by echinocandins.

Distinguish protozoa from helminths.

Protozoa are unicellular eukaryotes (e.g. Plasmodium, Entamoeba, Giardia), reproducing by binary fission. Helminths are multicellular parasitic worms, including flatworms (cestodes/tapeworms, trematodes/flukes) and roundworms (nematodes).

Which protozoan causes malaria, and which vector transmits it?

Malaria is caused by Plasmodium species (P. falciparum, P. vivax, P. ovale, P. malariae, P. knowlesi), transmitted by the bite of the female Anopheles mosquito. P. falciparum causes the most severe disease.

Compare innate and adaptive immunity.

Innate immunity: nonspecific, rapid, no memory; includes physical barriers, phagocytes (neutrophils, macrophages), NK cells, complement, and inflammation. Adaptive immunity: antigen-specific, slower onset, has immunological memory; mediated by B lymphocytes (antibodies) and T lymphocytes.

Distinguish the two arms of adaptive immunity: humoral vs cell-mediated.

Humoral immunity: mediated by B cells/plasma cells producing antibodies; targets extracellular pathogens and toxins. Cell-mediated immunity: mediated by T cells — CD8$^{+}$ cytotoxic T cells kill infected cells and CD4$^{+}$ helper T cells coordinate the response (against intracellular pathogens).

List the five immunoglobulin classes and one key feature of each.

IgG: most abundant in serum, crosses placenta, secondary response. IgM: largest (pentamer), first antibody in primary response. IgA: mucosal/secretory immunity (dimer). IgE: allergy and antiparasitic defense (binds mast cells). IgD: B-cell receptor, function less defined.