NEET MDS Microbiology Syllabus

Compare bacterial exotoxins and endotoxins.

Exotoxins: secreted proteins, often from Gram-positive and Gram-negative bacteria, highly toxic, specific action, heat-labile, antigenic (toxoids possible). Endotoxins: LPS (lipid A) in Gram-negative cell walls, released on lysis, heat-stable, cause generalized fever/shock, poorly antigenic.

Which bacterium causes tuberculosis, what staining property does it have, and why?

Mycobacterium tuberculosis; it is acid-fast (stains with Ziehl-Neelsen) because its cell wall is rich in mycolic acid (waxy lipids) that resist decolorization by acid-alcohol.

Name the bacterial agents of cholera, tetanus, and diphtheria.

Cholera: Vibrio cholerae; tetanus: Clostridium tetani; diphtheria: Corynebacterium diphtheriae.

What is the difference between an exogenous and endogenous (opportunistic) bacterial infection?

Exogenous infections come from sources outside the body (environment, other people/animals); endogenous/opportunistic infections arise from the host's own normal flora, often when immunity is compromised.

Define the chain of infection and its links.

The sequence required for disease spread: infectious agent → reservoir → portal of exit → mode of transmission → portal of entry → susceptible host.

What are the main components of a virus particle (virion)?

A nucleic acid genome (DNA or RNA), a protein coat (capsid) made of capsomeres, and in some viruses a lipid envelope with glycoprotein spikes.

What is a bacteriophage?

A virus that specifically infects and replicates within bacteria.

Describe the three main capsid symmetries of viruses.

Icosahedral (roughly spherical, e.g., adenovirus), helical (rod/spiral, e.g., tobacco mosaic virus, rabies), and complex (e.g., poxvirus, bacteriophages).

List the stages of the viral replication cycle.

Attachment (adsorption), penetration (entry), uncoating, biosynthesis (replication of genome and synthesis of proteins), assembly (maturation), and release (lysis or budding).

How do enveloped viruses typically exit the host cell compared with non-enveloped viruses?

Enveloped viruses generally exit by budding through the host membrane (acquiring their envelope), often without immediately killing the cell; non-enveloped viruses are usually released by lysing (bursting) the host cell.

What enzyme do retroviruses such as HIV use, and what does it do?

Reverse transcriptase, which synthesizes DNA from the viral RNA genome (reverse transcription); this DNA is then integrated into the host genome by integrase.

Compare the lytic and lysogenic cycles of a bacteriophage.

In the lytic cycle the phage replicates immediately and lyses the host cell. In the lysogenic cycle the phage DNA integrates into the host genome as a prophage and is replicated silently until induction triggers the lytic cycle.

Which virus causes AIDS, and which immune cells does it primarily target?

HIV (Human Immunodeficiency Virus); it primarily infects CD4+ T-helper lymphocytes (also macrophages).

Name the viruses responsible for hepatitis B and the route of transmission.

Hepatitis B virus (HBV), a DNA virus, transmitted parenterally/by blood, sexually, and perinatally (mother to child).

What is the difference between a latent and a persistent (chronic) viral infection, with an example of latency?

In latent infection the virus remains dormant within cells and can reactivate later (e.g., herpes simplex virus, varicella-zoster); in chronic/persistent infection the virus is continuously produced over a long period (e.g., hepatitis B/C).

Distinguish antigenic drift and antigenic shift in influenza virus.

Antigenic drift is gradual minor point mutations in surface antigens (causes seasonal epidemics); antigenic shift is a major abrupt change from genome reassortment producing a new subtype (can cause pandemics).

What are the two basic morphological forms of fungi, and what is a dimorphic fungus?

Yeasts (single-celled, reproduce by budding) and molds (filamentous hyphae forming a mycelium). Dimorphic fungi exist as mold at ambient temperature (~25°C) and as yeast in tissue/at 37°C (e.g., Histoplasma).

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

Ergosterol; it is targeted by azoles (inhibit synthesis) and polyenes like amphotericin B (bind ergosterol), because it differs from human cholesterol, giving selective toxicity.

What polysaccharide makes up the fungal cell wall?

Chitin (along with glucans and mannans).

How do fungi obtain nutrition?

They are heterotrophic and absorb nutrients (absorptive nutrition); they secrete digestive enzymes externally and absorb the soluble products, living as saprophytes or parasites.

Name the fungus that most commonly causes oral and vaginal candidiasis (thrush), and a key risk factor.

Candida albicans; risk factors include antibiotic use, immunosuppression, diabetes, and denture wearing.

Which fungus causes cryptococcal meningitis, and what stain/test aids its diagnosis?

Cryptococcus neoformans; India ink preparation (shows the polysaccharide capsule as a clear halo) and cryptococcal antigen testing.

What are dermatophytes and the general term for the infections they cause?

Dermatophytes (Trichophyton, Microsporum, Epidermophyton) are keratinophilic fungi infecting skin, hair, and nails, causing dermatophytoses called 'tinea' or ringworm (e.g., tinea pedis, tinea corporis).

What is the difference between a superficial mycosis and a systemic (deep) mycosis?

Superficial mycoses affect outer skin, hair, and nails (e.g., tinea, candidiasis); systemic mycoses involve internal organs and can be life-threatening (e.g., histoplasmosis, cryptococcosis), often in immunocompromised hosts.

Which protozoan causes malaria, who transmits it, and what is the infective stage injected into humans?

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

Name the protozoan causing amoebic dysentery and its mode of transmission.

Entamoeba histolytica; transmitted by the faecal-oral route through ingestion of mature cysts in contaminated food/water.

What are the three main groups of medically important helminths (worms)?

Nematodes (roundworms, e.g., Ascaris), cestodes (tapeworms, e.g., Taenia), and trematodes (flukes, e.g., Schistosoma).

Give two examples of ectoparasites of medical importance and a disease each can transmit.

Lice (Pediculus humanus transmits epidemic typhus) and the Sarcoptes scabiei mite causes scabies; fleas can transmit plague (Yersinia pestis).

Compare innate and adaptive immunity in terms of specificity, speed, and memory.

Innate immunity is non-specific, immediate (first line), and has no memory; adaptive immunity is antigen-specific, slower to develop, and generates immunological memory.

List the main components/barriers of innate immunity.

Physical/chemical barriers (skin, mucous membranes, stomach acid, lysozyme), phagocytic cells (neutrophils, macrophages), natural killer (NK) cells, the complement system, and inflammation/fever.

What are the five classes of immunoglobulins and one key feature of each?

IgG (most abundant, crosses placenta, secondary response), IgM (largest/pentamer, first produced in primary response), IgA (secretions/mucosal defense), IgE (allergy and parasite defense), and IgD (B-cell receptor, antigen recognition).

Differentiate active and passive immunity, each with natural and artificial examples.

Active immunity: host makes its own antibodies—natural (recovery from infection) or artificial (vaccination); long-lasting. Passive immunity: pre-formed antibodies given—natural (maternal antibodies/breast milk) or artificial (antiserum/immunoglobulin injection); immediate but short-lived.

Distinguish the roles of helper T cells (CD4+) and cytotoxic T cells (CD8+).

CD4+ helper T cells recognize antigen on MHC class II and coordinate immunity (activate B cells and macrophages via cytokines); CD8+ cytotoxic T cells recognize antigen on MHC class I and directly kill infected or abnormal cells.

Name the four types of hypersensitivity reactions (Gell and Coombs) with one example each.

Type I: immediate/IgE-mediated (anaphylaxis, allergy); Type II: antibody-mediated cytotoxic (haemolytic disease, transfusion reaction); Type III: immune-complex (serum sickness, SLE); Type IV: delayed cell-mediated (tuberculin/Mantoux test, contact dermatitis).

What is the difference between a bactericidal and a bacteriostatic antibiotic?

Bactericidal antibiotics kill bacteria (e.g., penicillins, aminoglycosides); bacteriostatic antibiotics inhibit bacterial growth/multiplication, allowing host defenses to clear them (e.g., tetracyclines, macrolides).

List the five major mechanisms of action of antibacterial antibiotics with a drug class for each.

1) Inhibit cell wall synthesis (beta-lactams, vancomycin); 2) inhibit protein synthesis (aminoglycosides, tetracyclines, macrolides); 3) inhibit nucleic acid synthesis (fluoroquinolones, rifampin); 4) inhibit folate/metabolic pathways (sulfonamides, trimethoprim); 5) disrupt cell membrane (polymyxins).

What are the main mechanisms by which bacteria develop antibiotic resistance?

Enzymatic inactivation of the drug (e.g., beta-lactamases), alteration/modification of the drug target, decreased uptake/increased efflux of the drug, and bypass of the inhibited metabolic pathway.

How is antibiotic resistance commonly spread between bacteria?

By horizontal gene transfer—mainly plasmid-mediated conjugation, as well as transduction (via phages) and transformation (uptake of free DNA)—and by vertical transfer of mutations.

Name two mechanisms of antiviral drugs with a drug example.

Inhibition of viral DNA polymerase by nucleoside analogues (acyclovir for herpes); inhibition of reverse transcriptase (zidovudine for HIV); neuraminidase inhibition (oseltanivir for influenza) also blocks viral release.

Why are antiviral drugs generally harder to develop than antibacterials?

Viruses replicate using the host cell's own machinery, so there are few virus-specific targets, making selective toxicity (harming the virus without harming the host cell) difficult to achieve.

State the targets of the three main antifungal classes: polyenes, azoles, and echinocandins.

Polyenes (amphotericin B, nystatin) bind ergosterol in the membrane; azoles (fluconazole, ketoconazole) inhibit ergosterol synthesis (lanosterol 14-alpha-demethylase); echinocandins (caspofungin) inhibit beta-(1,3)-glucan synthesis in the cell wall.

Name the first-line antimalarial drugs and one drug for amoebiasis/giardiasis.

Antimalarials include chloroquine and artemisinin-based combination therapy (ACT); metronidazole is used for amoebiasis and giardiasis.

What is DNA composed of, and how do its bases pair (Chargaff's rule)?

DNA is a polymer of nucleotides (deoxyribose sugar, phosphate, and a nitrogenous base). Adenine pairs with thymine (A-T, two H-bonds) and guanine pairs with cytosine (G-C, three H-bonds); the two strands are antiparallel.

State the central dogma of molecular biology.

Genetic information flows DNA → RNA → protein: DNA is transcribed into RNA (transcription), and RNA is translated into protein (translation); reverse transcription (RNA→DNA) is an exception.

What is a codon, and what are start and stop codons?

A codon is a sequence of three nucleotides (mRNA) coding for one amino acid; AUG is the start codon (methionine), and UAA, UAG, and UGA are stop codons that terminate translation.

Distinguish the three mechanisms of horizontal gene transfer in bacteria.

Transformation: uptake of free naked DNA from the environment. Transduction: transfer of DNA via a bacteriophage. Conjugation: direct transfer of DNA (often a plasmid) through a sex pilus between cells.

What is a plasmid and why is it medically significant?

A plasmid is a small, circular, extrachromosomal DNA molecule that replicates independently; it often carries antibiotic-resistance and virulence genes that can spread between bacteria by conjugation.

Define mutation and name the major types of point mutations.

A mutation is a permanent change in the nucleotide sequence of DNA. Point mutation types: silent (no amino acid change), missense (different amino acid), nonsense (creates a stop codon), and frameshift (insertion/deletion shifting the reading frame).

What is PCR (polymerase chain reaction) and what are its three temperature steps?

PCR is a technique to amplify specific DNA sequences in vitro. Its cycle has three steps: denaturation (~94–95°C, strands separate), annealing (~50–65°C, primers bind), and extension (~72°C, Taq polymerase synthesizes new DNA).

What are restriction endonucleases and their role in genetic engineering?

Enzymes that cut DNA at specific recognition sequences, often producing 'sticky ends'; they are used to cut DNA fragments and vectors for recombinant DNA cloning.

What general principles govern proper specimen collection for microbiology?

Collect from the actual site of infection before antimicrobial therapy, using aseptic technique to avoid contamination, in adequate quantity, in an appropriate sterile container, and transport promptly with proper labeling.

Why is rapid transport or use of transport media important for specimens, and name a common bacterial transport medium.

To preserve organism viability, prevent overgrowth of commensals, and avoid drying; Stuart's or Amies transport medium is commonly used for swabs.

Differentiate selective, differential, and enriched culture media with an example.

Selective media inhibit unwanted organisms while allowing target growth (e.g., MacConkey for Gram-negatives); differential media distinguish organisms by appearance (e.g., MacConkey shows lactose fermenters as pink); enriched media add nutrients for fastidious organisms (e.g., blood/chocolate agar).

What does ELISA detect and what is its basic principle?

ELISA (enzyme-linked immunosorbent assay) detects and quantifies antigens or antibodies using an enzyme-linked antibody that produces a measurable color change upon binding; widely used in serology (e.g., HIV screening).

How does the Kirby-Bauer disk diffusion test determine antimicrobial susceptibility?

Antibiotic-impregnated disks are placed on a lawn of bacteria on Mueller-Hinton agar; after incubation, the diameter of the zone of inhibition around each disk is measured and compared to standards to classify the organism as susceptible, intermediate, or resistant.

Define MIC and MBC in antimicrobial testing.

MIC (Minimum Inhibitory Concentration) is the lowest concentration of an antimicrobial that visibly inhibits bacterial growth; MBC (Minimum Bactericidal Concentration) is the lowest concentration that kills 99.9% of the bacteria.

Define sterilization, disinfection, and antisepsis.

Sterilization: complete destruction/removal of all microorganisms including spores. Disinfection: killing of most pathogens on inanimate surfaces (usually not all spores). Antisepsis: destruction/inhibition of microbes on living tissue (skin).

What is the standard autoclave condition for sterilization, and what method does it use?

121°C at 15 psi (pounds per square inch) for 15–20 minutes; it uses moist heat (saturated steam under pressure), which kills organisms including spores by protein coagulation.

What are standard precautions in infection control, and which single measure is the most effective for preventing healthcare-associated infections?

Standard precautions treat all blood/body fluids as potentially infectious (hand hygiene, gloves/PPE, safe sharps disposal, proper waste handling); hand hygiene (hand washing) is the single most effective measure to prevent cross-infection.