Microbiome Practice Questions

Q: A 33-year-old female patient presented with brain abscess. On aspiration, the abscess was foul-smelling and yielded red fluorescence under UV light. What is the likely causative organism?

Bacteroides. ### Explanation The correct answer is **Bacteroides** (specifically the *Bacteroides melaninogenicus* group, now reclassified as *Prevotella* and *Porphyromonas*). **1. Why Bacteroides is correct:** The clinical presentation of a **foul-smelling** brain abscess is a classic indicator of an **anaerobic infection**. Among anaerobes, certain species (like *Prevotella* and *Porphyromonas*, formerly categorized under *Bacteroides*) produce **protoporphyrin**, a pigment that causes the colonies or clinical pus to exhibit a characteristic **brick-red fluorescence** when viewed under ultraviolet (UV) light (Wood’s lamp). These organisms are common components of the oral and gingival flora and can reach the brain via hematogenous spread or contiguous extension (e.g., chronic otitis media or sinusitis). **2. Why the other options are incorrect:** * **Acanthamoeba:** Typically causes Granulomatous Amoebic Encephalitis (GAE) in immunocompromised patients. It does not produce a foul smell or red fluorescence. * **Peptostreptococci:** While these are anaerobic cocci often found in brain abscesses and can produce a foul smell, they do not produce the porphyrin pigments required for red fluorescence. * **Staphylococcus:** *S. aureus* is a common cause of brain abscesses (often post-neurosurgery), but it is an aerobe/facultative anaerobe. It produces "creamy" yellow pus, not foul-smelling pus, and does not fluoresce red. **3. NEET-PG High-Yield Pearls:** * **Foul-smelling discharge/pus:** Always think of **Anaerobes**. * **Red Fluorescence:** Pathognomonic for *Prevotella/Porphyromonas* (formerly *Bacteroides melaninogenicus*). * **Black Pigmentation:** On culture (Blood Agar), these organisms develop dark brown to black colonies due to heme sequestration. * **Brain Abscess Source:** Frontal lobe abscesses are often due to **Sinusitis**; Temporal lobe/Cerebellum abscesses are often due to **Otitis Media**.

Q: In the gut flora, anaerobic bacteria outnumber the aerobes by what ratio?

1000:1. **Explanation:** The human gut microbiome is a complex ecosystem dominated by obligate anaerobes. In the large intestine (colon), the environment is strictly anaerobic due to the rapid consumption of oxygen by facultative anaerobes (like *E. coli*) near the mucosal surface. This creates a highly reduced environment that favors the growth of anaerobic species. **1. Why 1000:1 is Correct:** In the colon, the concentration of bacteria is the highest in the body (approx. $10^{11}$ to $10^{12}$ CFU/g of feces). Within this population, **anaerobes outnumber aerobes (facultative anaerobes) by a ratio of 1000:1**. The dominant genera include *Bacteroides*, *Clostridium*, *Bifidobacterium*, and *Fusobacterium*. **2. Analysis of Incorrect Options:** * **10:1 and 100:1:** These ratios significantly underestimate the anaerobic dominance. While these ratios might be seen in the proximal small intestine where oxygen tension is higher and transit time is faster, they do not represent the overall gut flora. * **10,000:1:** While some specific niches in the distal colon might approach very high ratios, the standard established medical consensus for examination purposes is 1000:1. **Clinical Pearls for NEET-PG:** * **Dominant Species:** *Bacteroides fragilis* is the most common anaerobe isolated from intra-abdominal abscesses, though *Bacteroides vulgatus* is more numerous in the normal gut. * **Functions:** The gut flora aids in Vitamin K and B12 synthesis, prevents colonization by pathogens (colonization resistance), and assists in bile acid metabolism. * **Clinical Correlation:** Disruption of this 1000:1 ratio (dysbiosis) by broad-spectrum antibiotics can lead to the overgrowth of *Clostridioides difficile*, resulting in pseudomembranous colitis.

Q: HeLa cells are derived from which of the following sources?

Carcinoma of the cervix. **Explanation:** **HeLa cells** are the first and most widely used **immortalized human cell line** in medical research. They were derived in 1951 from a biopsy of a **cervical adenocarcinoma** (specifically a glandular cancer of the cervix) from a patient named Henrietta Lacks. * **Why Option C is Correct:** HeLa cells are derived from **carcinoma of the cervix**. These cells are unique because they possess an active version of the enzyme **telomerase**, which prevents the gradual shortening of telomeres during cell division. This allows the cells to bypass the Hayflick limit (cellular senescence), making them "immortal" and capable of dividing indefinitely in laboratory cultures. **Analysis of Incorrect Options:** * **Option A (Sternal bone marrow):** This is the source for the **Detroit-6** cell line. * **Option B (Embryonal lung):** Human embryonic lung fibroblasts are used to create **diploid cell lines** like **WI-38** and **MRC-5**, commonly used for vaccine production. * **Option D (Connective tissue):** Cell lines derived from connective tissues (fibroblasts) include the **L-cell line** (mouse subcutaneous tissue). **High-Yield Clinical Pearls for NEET-PG:** * **Viral Association:** HeLa cells contain **HPV-18** DNA, which integrated into the host genome, inactivating the p53 tumor suppressor protein and driving oncogenesis. * **Classification:** HeLa is a **Continuous (Heteroploid) Cell Line**, meaning it has an abnormal number of chromosomes and can be subcultured indefinitely. * **Applications:** They were instrumental in developing the **Salk Polio Vaccine**, studying cancer, gene mapping, and COVID-19 research. * **Other common lines:** **HEp-2** (Laryngeal carcinoma), **Vero** (Monkey kidney), and **BHK-21** (Baby hamster kidney).

Q: Which of the toxins produced by Clostridium botulinum is non-neurotoxic?

Toxin C2. ### Explanation **Correct Option: D (Toxin C2)** *Clostridium botulinum* produces eight distinct types of toxins, labeled **A through H**. While most are potent neurotoxins that cause flaccid paralysis by inhibiting acetylcholine release, **Toxin C2** is a significant exception. **Why Toxin C2 is the correct answer:** Unlike the neurotropic toxins (A-G), **Toxin C2 is an enterotoxin and a cytotoxin**. It possesses ADP-ribosylating activity that targets cellular actin, leading to increased vascular permeability and tissue damage rather than neurological interference. It does not act on the neuromuscular junction. **Analysis of Incorrect Options:** * **Options A & B (Toxins A and B):** These are the most common causes of human botulism. They are highly potent neurotoxins that proteolytically cleave SNARE proteins (like SNAP-25 or Synaptobrevin), preventing the fusion of acetylcholine vesicles at the synapse. * **Option C (Toxin C1):** While produced by the same strains that produce C2, **C1 is a neurotoxin**. It specifically targets the syntaxin protein at the nerve terminal. --- ### High-Yield NEET-PG Pearls: * **Mechanism of Action:** Botulinum toxins are zinc-dependent endopeptidases that cleave **SNARE proteins**, leading to **flaccid paralysis**. (Contrast with Tetanospasmin, which causes spastic paralysis by inhibiting GABA/Glycine). * **Therapeutic Uses:** Toxin A (Botox) is used for achalasia cardia, strabismus, blepharospasm, and cosmetic wrinkle reduction. * **Infant Botulism:** Associated with **honey** consumption (ingestion of spores), leading to "Floppy Baby Syndrome." * **Food-borne Botulism:** Usually due to ingestion of pre-formed toxins in **canned foods**. * **Detection:** The gold standard for diagnosis is the **Mouse Bioassay** (Mouse Neutralization Test).

Q: Japanese B encephalitis virus is transmitted by which vector?

Culex tritaeniorhynchus. ### Explanation **Correct Option: A. Culex tritaeniorhynchus** Japanese Encephalitis (JE) is caused by a Group B Arbovirus (Flavivirus). In India and Southeast Asia, the primary vector is **Culex tritaeniorhynchus**. These mosquitoes are "rice-field breeders" and are zoophilic, meaning they prefer feeding on animals (pigs and water birds) rather than humans. Humans are **accidental, dead-end hosts** because the viremia in humans is insufficient to infect a biting mosquito. **Analysis of Incorrect Options:** * **B & D (Hard and Soft Ticks):** Ticks are vectors for diseases like Kyasanur Forest Disease (KFD), Crimean-Congo Hemorrhagic Fever (CCHF), and Rickettsial infections. They do not transmit the JE virus. * **C (Culex fatigans):** Also known as *Culex quinquefasciatus*, this is the primary vector for **Bancroftian Filariasis**. While it belongs to the same genus, it is not the principal vector for Japanese Encephalitis. **High-Yield Clinical Pearls for NEET-PG:** * **Reservoir/Amplifier Host:** The **Pig** is the most important amplifier host (shows no symptoms but high viremia). * **Natural Host:** Ardeid birds (Paddy birds, Herons). * **Seasonality:** JE outbreaks usually coincide with the rainy season and intensive rice cultivation. * **Vaccination:** The most commonly used vaccine in the National Immunization Schedule is the **SA-14-14-2** (Live attenuated, Chinese origin), given at 9 months and 16–24 months. * **Diagnosis:** **IgM Capture ELISA** (MAC-ELISA) of CSF or serum is the gold standard.

Q: What is the causative agent of redleg disease?

Aeromonas. **Explanation:** **Correct Answer: D. Aeromonas** Redleg disease (also known as bacterial septicemia) is a common and often fatal infection primarily affecting amphibians, such as frogs and toads. The causative agent is **Aeromonas hydrophila**, a Gram-negative, facultative anaerobic rod commonly found in freshwater environments. The disease is characterized by cutaneous hemorrhages, particularly on the ventral surface of the legs and abdomen (giving it the name "redleg"), resulting from systemic vasculitis and sepsis. In humans, *Aeromonas* is clinically significant as a cause of gastroenteritis and wound infections, especially following exposure to contaminated water. **Analysis of Incorrect Options:** * **A. Pseudomonas:** While *Pseudomonas* species can cause various opportunistic infections in aquatic animals and humans (like "swimmer’s ear" or ecthyma gangrenosum), they are not the specific primary cause of the classic "redleg" syndrome. * **B. Mouldy sugar cane fibre:** This is the causative agent of **Bagassosis**, an occupational hypersensitivity pneumonitis caused by the inhalation of *Thermoactinomyces sacchari* found in stored, mouldy sugar cane. * **C. Coniosporium:** Specifically *Coniosporium corticale*, this fungus is associated with **Maple bark stripper’s disease**, another form of hypersensitivity pneumonitis. **High-Yield Clinical Pearls for NEET-PG:** * **Aeromonas hydrophila** is oxidase-positive, which distinguishes it from the Enterobacteriaceae family. * It is a well-known cause of **medicinal leech-associated infections**; prophylactic antibiotics are often required during leech therapy. * **Zoonotic link:** While redleg is an animal disease, *Aeromonas* can cause severe necrotizing fasciitis in immunocompromised humans following water-related trauma.

Question 1

Which fungus cannot be grown on artificial media?

Accepted Answer

Rhinosporidium seeberi

Answer

Penicillium marneffei

Answer

Aspergillus flavus

Answer

Sporothrix schenckii

Question 2

A 33-year-old female patient presented with brain abscess. On aspiration, the abscess was foul-smelling and yielded red fluorescence under UV light. What is the likely causative organism?

Accepted Answer

Bacteroides

Answer

Acanthameba

Answer

Peptostreptococci

Answer

Staphylococcus

Question 3

In the gut flora, anaerobic bacteria outnumber the aerobes by what ratio?

Accepted Answer

1000:1

Answer

10:1

Answer

100:1

Answer

10,000:1

Question 4

HeLa cells are derived from which of the following sources?

Accepted Answer

Carcinoma of the cervix

Answer

Sternal bone marrow

Answer

Embryonal lung

Answer

Connective tissue

Question 5

Which of the toxins produced by Clostridium botulinum is non-neurotoxic?

Accepted Answer

Toxin C2

Answer

Toxin A

Answer

Toxin B

Answer

Toxin C1

Question 6

Giemsa stain of yeast cells in HIV is associated with which of the following infections?

Accepted Answer

Cryptococcosis

Answer

Aspergillosis

Answer

Blastomycosis

Answer

Histoplasmosis

Question 7

Japanese B encephalitis virus is transmitted by which vector?

Accepted Answer

Culex tritaeniorhynchus

Answer

Hard tick

Answer

Culex fatigans

Answer

Soft tick

Question 8

Which one of the following infections is typically seen one month after renal transplantation?

Accepted Answer

Cytomegalovirus

Answer

Varicella-zoster virus (Chickenpox)

Answer

Coxsackie virus

Answer

Epstein-Barr virus

Question 9

Vincent's angina is caused by Borrelia vincenti along with which of the following microorganisms?

Accepted Answer

Fusobacterium

Answer

Lactobacillus

Answer

Peptostreptococcus

Answer

Bacteroides

Question 10

What is the causative agent of redleg disease?

Accepted Answer

Aeromonas

Answer

Pseudomonas

Answer

Moudly sugar cane fibre

Answer

Coniosporium

Microbiome — MCQs

Microbiome — MCQs

On this page

Practice by Chapter

Want unlimited practice?