FMGE 2023 — Microbiology
11 Previous Year Questions with Answers & Explanations
The culture media that is used to differentiate lactose fermenters and non-lactose fermenters is
Which of the following is an acid-fast organism?
The stain used for identifying Cryptococcus is
The image shown here is used for the diagnosis of
Which of the following dimorphic fungi causes subcutaneous mycosis?
Plasma sterilization is done by
A cattle farmer presented with a lesion on the arm. Gram staining showed gram-positive rods with square ends with a central spore. Identify the organism.
Which among the following culture media is used in the identification of Salmonella?
Which among the following parasites directly invade the skin?
Which of the following hepatitis viruses is likely to get transmitted via fecal-oral route?
FMGE 2023 - Microbiology FMGE Practice Questions and MCQs
Question 1: The culture media that is used to differentiate lactose fermenters and non-lactose fermenters is
- A. Bile salt agar
- B. Thayer-Martin agar
- C. Sabouraud Dextrose Agar
- D. Mac-Conkey agar (Correct Answer)
Explanation: ***Mac-Conkey agar*** - MacConkey agar is a selective and differential medium containing **lactose** and a **pH indicator** (neutral red) to distinguish gram-negative bacteria based on their ability to ferment lactose. - **Lactose fermenters** (e.g., *E. coli*, *Klebsiella*) produce acid, causing the colonies and surrounding medium to turn pink/red, while **non-lactose fermenters** (e.g., *Salmonella*, *Shigella*) remain colorless or pale. *Bile salt agar* - Primarily used for the isolation and detection of **enteric pathogens** like *Salmonella* and *Shigella*. - It contains bile salts to suppress the growth of many non-enteric Gram-negative rods but is not the primary medium for differentiating lactose fermentation. *Thayer-Martin agar* - This is a **selective medium** used specifically for the isolation of *Neisseria gonorrhoeae* and *Neisseria meningitidis*. - It contains antimicrobial agents (**vancomycin**, **colistin**, **nystatin**, and **trimethoprim**) to inhibit the growth of normal flora, fungi, and other bacteria. *Sabouraud Dextrose Agar* - This is the standard medium used for the selective isolation and cultivation of **fungi** (yeasts and molds). - Its low pH (typically around 5.6) and high glucose concentration inhibit the growth of most bacteria, making it unsuitable for differentiating lactose fermentation among bacteria.
Question 2: Which of the following is an acid-fast organism?
- A. Streptococcus pneumoniae
- B. Escherichia coli
- C. Mycobacterium avium (Correct Answer)
- D. Actinomyces
Explanation: ***Mycobacterium avium*** - Mycobacteria are **classically acid-fast organisms** due to high mycolic acid content in their cell wall - They retain carbolfuchsin stain even after treatment with acid-alcohol (Ziehl-Neelsen or Kinyoun stain) - M. avium is an atypical mycobacterium commonly causing infections in immunocompromised patients (especially AIDS) *Streptococcus pneumoniae* - Gram-positive coccus, NOT acid-fast - Identified by Gram staining, not acid-fast staining *Escherichia coli* - Gram-negative bacillus, NOT acid-fast - Standard Gram staining is used for identification *Actinomyces* - Gram-positive, branching filamentous bacteria - May show partial acid-fastness in modified acid-fast staining, but NOT classically acid-fast like Mycobacterium - Actinomyces is NOT considered an acid-fast organism in standard microbiology
Question 3: The stain used for identifying Cryptococcus is
- A. Albert's stain
- B. Silver staining
- C. Auramine-rhodamine stain
- D. India ink stain (Correct Answer)
Explanation: ***India ink stain***- This stain is a **negative staining** method primarily used for rapid detection of the large **polysaccharide capsule** of *Cryptococcus neoformans*, especially in cerebrospinal fluid (CSF) samples. - The surrounding ink particles are excluded by the capsule, resulting in a characteristic **clear halo** around the yeast cell against a dark background.*Albert's stain*- Albert's stain is specialized for bacterial cytology, specifically used to demonstrate **metachromatic granules** (or volutin granules) in **Corynebacterium diphtheriae**.- It uses a mixture of **toluidine blue** and **malachite green** and is not applicable for fungal identification.*Silver staining*- **Grocott's methenamine silver (GMS) stain** is a histological silver stain used often in pathology to visualize fungal elements, staining the cell walls brown to black.- While effective for detecting *Cryptococcus* in **tissue biopsies**, it is not the standard rapid method for fluid samples like CSF.*Auramine-rhodamine stain*- This is a **fluorescent staining technique** used to identify **acid-fast bacilli (AFB)**, particularly species of **Mycobacterium**.- The stain binds to the **mycolic acid** in the cell wall, a component absent in fungal organisms like *Cryptococcus*.
Question 4: The image shown here is used for the diagnosis of
- A. Human immunodeficiency virus
- B. Hepatitis B virus
- C. Human papillomavirus
- D. Herpes simplex virus (Correct Answer)
Explanation: ***Herpes simplex virus*** - The image is a transmission electron micrograph showing multiple enveloped virions with a characteristic dense central core and an icosahedral nucleocapsid, giving a "target" or "bull's-eye" appearance. - This morphology is a classic feature of the **Herpesviridae** family, which includes Herpes Simplex Virus (HSV), Varicella-Zoster Virus (VZV), and Cytomegalovirus (CMV). *Human immunodeficiency virus* - HIV is a **retrovirus**, and its mature virion has a distinct **cone-shaped** or **bar-shaped** core, which is morphologically different from the spherical, target-like core shown in the image. - The primary diagnostic methods for HIV are serological assays (e.g., **ELISA**) and nucleic acid tests (**PCR**), not electron microscopy. *Hepatitis B virus* - The complete infectious virion of HBV, known as the **Dane particle**, is a double-shelled spherical particle, which does not have the same internal structure as the virions in the image. - Serum from infected individuals also contains non-infectious spherical and filamentous particles of **HBsAg** which lack a nucleocapsid. *Human papillomavirus* - HPV is a **non-enveloped** DNA virus, whereas the virus in the micrograph is clearly surrounded by a lipid envelope. - It has a simple, small **icosahedral capsid** structure, lacking the complex, multi-layered appearance of a herpesvirus.
Question 5: Which of the following dimorphic fungi causes subcutaneous mycosis?
- A. Blastomyces dermatitidis
- B. Penicilliosis marneffei
- C. Histoplasma capsulatum
- D. Sporothrix schenckii (Correct Answer)
Explanation: ***Sporothrix schenckii***- This dimorphic fungus is the classic causative agent of **sporotrichosis**, which is classified as a **subcutaneous mycosis** associated with inoculation injuries (e.g., from roses or hay).- Infection often presents as a primary skin lesion followed by characteristic **lymphocutaneous spread** along lymphatic channels.*Histoplasma capsulatum*- It causes **histoplasmosis**, which is primarily classified as a **systemic or deep mycosis** that is acquired by inhaling microconidia from bird or bat droppings.- Although dissemination can occur, it is not categorized as a primary subcutaneous mycosis initiated by direct wound inoculation.*Blastomyces dermatitidis*- This fungus causes **blastomycosis**, which is considered a **systemic mycosis** primarily affecting the lungs following inhalation of spores.- While disseminated disease frequently involves the skin, causing sharply demarcated verrucous lesions, the infection route remains systemic rather than purely subcutaneous.*Penicilliosis marneffei*- Now known as *Talaromyces marneffei*, it causes **talaromycosis**, which is an **opportunistic systemic mycosis** predominantly affecting immunocompromised individuals in Southeast Asia.- Though it causes diverse skin manifestations (e.g., papules, nodules), the overall clinical picture is one of deep, disseminated infection, not a localized subcutaneous mycosis like sporotrichosis.
Question 6: Plasma sterilization is done by
- A. UV rays
- B. Ethylene oxide
- C. Hydrogen peroxide (Correct Answer)
- D. Gamma rays
Explanation: ***Hydrogen peroxide*** - Plasma sterilization uses **hydrogen peroxide vapor** that is converted into plasma state by electromagnetic energy (radiofrequency or microwave) - The plasma contains **highly reactive free radicals** (hydroxyl and hydroperoxyl) that destroy microorganisms by disrupting cell membranes and DNA - Common system: **STERRAD** (Sterilization by Reactive Gas Discharge) - Advantages: **Low temperature** process (suitable for heat-sensitive instruments), **non-toxic**, leaves no residue, and has a **short cycle time** *UV rays (Incorrect)* - Used for surface disinfection and air sterilization, not plasma sterilization - Limited penetration power *Ethylene oxide (Incorrect)* - Gas sterilization method, not plasma-based - Requires longer aeration time due to toxicity *Gamma rays (Incorrect)* - Ionizing radiation sterilization method - Used for industrial sterilization, not plasma sterilization
Question 7: A cattle farmer presented with a lesion on the arm. Gram staining showed gram-positive rods with square ends with a central spore. Identify the organism.
- A. Bacillus cereus
- B. Bacillus anthracis (Correct Answer)
- C. Pseudomonas aeruginosa
- D. Brucella abortus
Explanation: ***Bacillus anthracis***- The microscopic finding of large, non-motile, **Gram-positive rods** with characteristic **square ends** (often described as 'boxcar' shapes) and a central spore is the classic description of *Bacillus anthracis*.- The clinical history of a **cattle farmer** with a skin lesion strongly suggests **cutaneous anthrax**, the most common form of anthrax acquired through exposure to infected livestock.*Bacillus cereus*- While also a Gram-positive, spore-forming rod, *B. cereus* is generally **motile** and typically causes severe food poisoning (emetic or diarrheal types).- It usually lacks the distinctive **square ends** seen in *Bacillus anthracis*.*Pseudomonas aeruginosa*- This organism is a **Gram-negative rod**, which directly contradicts the finding of a Gram-positive rod in the staining.- Furthermore, *P. aeruginosa* is **non-spore-forming**, ruling it out based on the finding of a central spore.*Brucella abortus*- *Brucella* species are small, obligate intracellular, **Gram-negative coccobacilli** or short rods, not large Gram-positive rods.- They are **non-spore-forming** and typically cause systemic illness (**brucellosis** or undulant fever) rather than localized cutaneous lesions matching this description.
Question 8: Which among the following culture media is used in the identification of Salmonella?
- A. PLET medium
- B. BCYE medium
- C. Wilson and Blair medium (Correct Answer)
- D. Stuart's medium
Explanation: ***Wilson and Blair medium*** - Wilson and Blair medium, also known as **Bismuth Sulfite Agar (BSA)**, is a highly selective and differential medium used primarily for the isolation of **Salmonella** species, particularly *S. Typhi*. - **Salmonella** colonies typically appear **black** on this medium due to the precipitation of **bismuth sulfite** caused by **hydrogen sulfide ($ ext{H₂S}$)** production. *PLET medium* - PLET (Polymyxin-Lysozyme-EDTA-Thallium Acetate) medium is a selective agar strictly used for the isolation of **Bacillus anthracis**. - The medium contains selective agents that suppress the growth of most contaminating bacteria while allowing the growth of *B. anthracis*. *BCYE medium* - **BCYE** (Buffered Charcoal Yeast Extract) medium is the gold standard for culturing **Legionella pneumophila**. - *Legionella* is fastidious and requires **L-cysteine** and **iron salts** which are provided by the BCYE formulation. *Stuart's medium* - Stuart's medium is a **transport medium**, not an identification or culture medium, used for preserving the viability of delicate pathogens during transit. - It is often utilized for transporting specimens containing fastidious organisms such as **Neisseria gonorrhoeae**.
Question 9: Which among the following parasites directly invade the skin?
- A. Schistosoma mansoni (Correct Answer)
- B. Wuchereria bancrofti
- C. Onchocerca volvulus
- D. Dracunculus medinensis
Explanation: ***Schistosoma mansoni***- The infectious stage, the **cercariae**, actively penetrate the intact skin of the human host when swimming or wading in contaminated water. - This stage bypasses the need for an insect vector and directly initiates infection, often causing **'swimmer's itch'** at the site of entry.*Dracunculus medinensis*- This infection is acquired by the oral route through ingestion of **copepods** (water fleas) containing the larval stage in drinking water.- While the mature female worm creates a painful **blister** to exit the skin, it does not invade the skin to initiate the infection.*Wuchereria bancrofti*- This parasite is transmitted by the bite of an **infected mosquito** (e.g., *Culex*), which deposits infective larvae near the bite site.- The larvae enter the host through the **puncture wound** created by the mosquito bite, not by actively penetrating intact dermal layers.*Onchocerca volvulus*- The transmission involves the bite of an infected **blackfly** (*Simulium* species), which transmits the infective larvae (microfilariae) into the host.- The larvae enter through the **wound site** created by the fly bite and subsequent manipulation of the wound by the vector (not direct skin invasion).
Question 10: Which of the following hepatitis viruses is likely to get transmitted via fecal-oral route?
- A. Hepatitis A (Correct Answer)
- B. Hepatitis B
- C. Hepatitis C
- D. Hepatitis D
Explanation: ***Hepatitis A***- Hepatitis A virus (HAV) is primarily spread through the **fecal-oral route**, typically via consumption of food or water contaminated with feces.- It causes an **acute**, self-limiting infection that rarely results in chronic liver disease.*Hepatitis B*- Hepatitis B virus (HBV) is transmitted mainly through **parenteral** (exposure to infected blood), sexual contact, or perinatal (mother-to-child) routes.- Unlike HAV, HBV is a major cause of **chronic hepatitis**, cirrhosis, and hepatocellular carcinoma.*Hepatitis C*- Hepatitis C virus (HCV) transmission is predominantly **parenteral**, most commonly associated with intravenous drug use, blood transfusions (historically), and shared needles.- HCV is the leading cause of chronic viral hepatitis globally and frequently establishes persistent infection.*Hepatitis D*- Hepatitis D virus (HDV) is a defective RNA virus that requires **Hepatitis B surface antigen (HBsAg)** for its replication and assembly.- HDV is transmitted parenterally (same routes as HBV) and not via the fecal-oral route.