INI CET Microbiology Syllabus

What are the three mechanisms of horizontal gene transfer in bacteria?

Transformation (uptake of free naked DNA from the environment), transduction (DNA transferred by a bacteriophage), and conjugation (direct cell-to-cell transfer via a sex pilus, often of a plasmid).

What is a plasmid and why is it medically important?

A plasmid is a small, extrachromosomal, self-replicating circular DNA molecule. It is important because it often carries antibiotic-resistance genes, virulence factors, and can transfer between bacteria via conjugation.

What is a transposon ("jumping gene")?

A transposon is a mobile DNA segment that can move from one location to another within or between chromosomes and plasmids, often spreading antibiotic-resistance genes.

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

Fermentation uses an organic molecule (e.g., pyruvate) as the final electron acceptor and yields little ATP (~2 per glucose via substrate-level phosphorylation). Aerobic respiration uses O2 as the final electron acceptor and yields much more ATP (~38 per glucose).

Name the three central metabolic pathways by which bacteria catabolize glucose.

Glycolysis (Embden-Meyerhof-Parnas pathway), the pentose phosphate pathway, and the Entner-Doudoroff pathway.

What is the difference between an exotoxin and an endotoxin?

Exotoxins are secreted proteins (from Gram-positive and Gram-negative bacteria), highly potent, often specific, and heat-labile. Endotoxin is the lipid A component of LPS in the Gram-negative outer membrane, released on lysis, heat-stable, and causes fever/shock.

Define virulence factor and give three examples.

A virulence factor is a molecule that helps a pathogen cause disease. Examples: capsules (anti-phagocytic), adhesins/pili (attachment), toxins (exotoxins/endotoxin), and enzymes such as coagulase or hyaluronidase.

What does LD50 measure?

LD50 (median lethal dose) is the dose of a pathogen or toxin required to kill 50% of an inoculated host population; it is a measure of virulence/potency.

List Koch's postulates.

1) The microbe is found in all cases of the disease and absent in healthy hosts; 2) it can be isolated and grown in pure culture; 3) inoculating the cultured microbe into a healthy host reproduces the disease; 4) the same microbe can be re-isolated from the experimentally infected host.

What is the difference between a defined (synthetic) medium and a complex medium?

A defined/synthetic medium has all components in exactly known chemical amounts. A complex medium contains ingredients of unknown exact composition (e.g., peptone, yeast extract, blood).

Distinguish selective media from differential media, with an example.

Selective media inhibit unwanted organisms to favor a target (e.g., MacConkey inhibits Gram-positives). Differential media distinguish organisms by appearance based on metabolic reactions (e.g., blood agar showing hemolysis). MacConkey is both selective and differential.

What are the three types of hemolysis seen on blood agar?

Alpha (α) hemolysis—partial, greenish; Beta (β) hemolysis—complete clearing; Gamma (γ) hemolysis—no hemolysis.

Which catalase and coagulase results identify Staphylococcus aureus among Gram-positive cocci?

Staphylococcus aureus is catalase-positive (distinguishing staph from strep) and coagulase-positive (distinguishing S. aureus from coagulase-negative staphylococci).

What organism causes tuberculosis, and what is its key staining and growth feature?

Mycobacterium tuberculosis—an acid-fast bacillus that is a slow-growing obligate aerobe (grows on Lowenstein-Jensen medium, doubling time ~15–20 hours).

Name the causative organisms of cholera, tetanus, and diphtheria.

Cholera: Vibrio cholerae (cholera toxin). Tetanus: Clostridium tetani (tetanospasmin). Diphtheria: Corynebacterium diphtheriae (diphtheria toxin).

What is the toxin and mechanism of Vibrio cholerae?

Cholera toxin permanently activates adenylate cyclase (via Gs protein ADP-ribosylation), raising cAMP and causing massive secretion of chloride and water, producing watery (rice-water) diarrhea.

Which Gram-negative rod is the most common cause of urinary tract infections and is a lactose fermenter?

Escherichia coli (E. coli), a lactose-fermenting Gram-negative bacillus.

What are the key structural components common to all viruses, and what defines an enveloped virus?

All viruses have a nucleic acid genome (DNA or RNA) enclosed in a protein coat called a capsid (made of capsomeres); together they form the nucleocapsid. An enveloped virus additionally has a lipid bilayer (with glycoprotein spikes) derived from host membranes.

What are the major criteria for classifying viruses (Baltimore/structural)?

Type of nucleic acid (DNA vs RNA), strandedness (single vs double), sense (positive vs negative), presence/absence of an envelope, and capsid symmetry (icosahedral, helical, complex). The Baltimore system groups them into 7 classes by genome type and replication strategy.

What are the three main capsid symmetries of viruses?

Icosahedral (cubic), helical, and complex (e.g., bacteriophages, poxviruses).

Differentiate yeasts from molds in morphology and reproduction.

Yeasts are unicellular, oval/round fungi that reproduce by budding (or fission). Molds are multicellular, filamentous fungi made of hyphae that form a mycelium and reproduce via spores.

What is a dimorphic fungus, and give an example?

A dimorphic fungus grows as a mold (hyphae) at environmental/room temperature (~25°C) and as a yeast in host tissue at body temperature (37°C). Examples: Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides, Sporothrix.

What is the main sterol in the fungal cell membrane, and why is it a drug target?

Ergosterol. It is targeted by antifungals: azoles inhibit its synthesis (ergosterol synthesis), and polyenes (e.g., amphotericin B) bind ergosterol to disrupt the membrane.

Differentiate protozoa from helminths.

Protozoa are single-celled eukaryotic microorganisms (e.g., Plasmodium, Entamoeba, Giardia). Helminths are multicellular parasitic worms (e.g., roundworms/nematodes, tapeworms/cestodes, flukes/trematodes), visible to the naked eye.

What protozoan causes malaria, and which vector transmits it?

Plasmodium species (P. falciparum, P. vivax, P. ovale, P. malariae, P. knowlesi); transmitted by the bite of the female Anopheles mosquito.

What are the four classic locomotor groups of protozoa?

Amoebae (pseudopodia), flagellates (flagella), ciliates (cilia), and sporozoa/apicomplexa (nonmotile in adult forms; gliding/no specialized organelle).

What is the difference between innate and adaptive immunity?

Innate immunity is the rapid, nonspecific first line of defense (skin, phagocytes, complement, NK cells) with no memory. Adaptive immunity is slower, antigen-specific (T and B lymphocytes), and generates immunological memory.

Name the main cellular components of the innate immune system.

Phagocytes (neutrophils, macrophages, dendritic cells), natural killer (NK) cells, mast cells, eosinophils, and basophils—plus physical/chemical barriers and the complement system.

What are the two main arms of adaptive immunity and the cells that mediate them?

Humoral immunity—mediated by B lymphocytes producing antibodies (against extracellular pathogens). Cell-mediated immunity—mediated by T lymphocytes (CD4+ helper and CD8+ cytotoxic) against intracellular pathogens.

List the five classes of human immunoglobulins and the one most abundant in serum.

IgG, IgA, IgM, IgE, and IgD. IgG is the most abundant immunoglobulin in serum and the only one that crosses the placenta; IgM is the first antibody produced in a primary response.