Bacteriology Practice Questions

Q: A person working in an abattoir presented with a pustule on hand which turned into an ulcer. Which of the following stains will best help in the diagnosis?

Polychrome methylene blue. **Explanation:** The clinical presentation describes a classic case of **Cutaneous Anthrax**, caused by *Bacillus anthracis*. The history of working in an **abattoir** (slaughterhouse) is a significant occupational risk factor. Cutaneous anthrax typically begins as a painless papule/pustule that evolves into a characteristic black necrotic eschar surrounded by edema. **Why Polychrome Methylene Blue is correct:** *Bacillus anthracis* is a capsulated organism. The capsule is composed of **poly-D-glutamic acid**, which is a key virulence factor. **Polychrome Methylene Blue** (also known as the **M’Fadyean reaction**) is the specific stain used to visualize this capsule. When stained, the bacillus appears blue, while the capsule appears as a frayed, disintegrated **pink/purple** halo around the bacteria. This reaction is diagnostic for *B. anthracis*. **Why other options are incorrect:** * **Carbol fuchsin:** This is a primary stain used in Ziehl-Neelsen staining but is not specific for the anthrax capsule. * **Acid-fast stain:** Used primarily for *Mycobacterium tuberculosis* and *Nocardia*. *B. anthracis* is not acid-fast. * **Calcofluor white:** This is a fluorescent stain used to detect **fungal elements** by binding to cellulose and chitin in cell walls. **Clinical Pearls for NEET-PG:** * **M’Fadyean Reaction:** Specifically identifies the polypeptide capsule of *B. anthracis*. * **Morphology:** Large, Gram-positive, non-motile bacilli with "Medusa head" appearance on agar. * **Bamboo stick appearance:** Seen on Gram stain due to chains of bacilli with squared ends. * **Pustule vs. Eschar:** Despite the name "malignant pustule," the lesion is typically **painless** and non-purulent.

Q: What concentration of agar is typically used for solid media to inhibit the swarming of Proteus species?

6%. **Explanation:** The correct answer is **6% (Option C)**. **Understanding the Concept:** *Proteus* species (notably *P. mirabilis* and *P. vulgaris*) are characterized by their high motility due to peritrichous flagella. On standard solid media, this results in **swarming**, a phenomenon where the bacteria migrate across the surface in concentric waves, making it difficult to isolate individual colonies of *Proteus* or other co-existing pathogens. To inhibit swarming in a laboratory setting, the physical consistency of the agar must be altered. Increasing the agar concentration to **6%** (known as **"Stiff Agar"**) creates a surface with high density and reduced moisture, which physically impedes the movement of the flagella, thereby restricting growth to discrete colonies. **Analysis of Options:** * **Option A (1-2%):** This is the standard concentration for routine solid media (e.g., Nutrient Agar or Blood Agar). At this concentration, *Proteus* swarms freely. * **Option B (0.5%):** This is the concentration used for **semi-solid media**, typically employed to *demonstrate* motility (e.g., Mannitol Motility Medium), rather than inhibit it. * **Option D (5%):** While higher than standard, 5% is not the conventional "stiff agar" concentration cited in standard microbiological protocols for total inhibition of swarming. **High-Yield NEET-PG Pearls:** * **Other methods to inhibit swarming:** Addition of chemicals like chloral hydrate, boric acid (1:1000), sodium azide, or alcohol. * **Culture Media:** **CLED agar** (Cystine-Lactose-Electrolyte-Deficient) is the preferred medium in clinical labs to prevent swarming because it lacks electrolytes (salts), which are essential for *Proteus* motility. * **Dienes Phenomenon:** A test used to differentiate strains of *Proteus*; when two different strains meet on an agar plate, a clear line of demarcation forms between them.

Q: All of the following are true about Bacillus anthracis except?

The toxin is a complex of two fractions.. **Explanation:** The correct answer is **D** because it is a false statement. The *Bacillus anthracis* toxin is actually a **tripartite complex** consisting of **three** distinct protein fractions, not two: 1. **Protective Antigen (PA):** The binding unit that facilitates entry into host cells. 2. **Edema Factor (EF):** An adenylate cyclase that causes localized edema. 3. **Lethal Factor (LF):** A zinc metalloprotease that causes cell death and tissue necrosis. **Analysis of other options:** * **Option A (True):** Virulence in *B. anthracis* is mediated by two large plasmids: **pXO1** (encodes the toxin complex) and **pXO2** (encodes the capsule). * **Option B (True):** Cutaneous anthrax (Hide porter’s disease) is the most common form. In 80-90% of cases, the characteristic black eschar heals spontaneously, though antibiotics are given to prevent systemic spread. * **Option C (True):** Unlike most bacteria that have polysaccharide capsules, *B. anthracis* has a unique **poly-D-glutamic acid (polypeptide) capsule** which is strongly anti-phagocytic. **NEET-PG High-Yield Pearls:** * **McFadyean’s Reaction:** Used for presumptive diagnosis; uses polychrome methylene blue to visualize the purple capsule. * **Morphology:** Described as "Bamboo stick" appearance; colonies on agar show a "Medusa head" or "Frosted glass" appearance. * **String of Pearls Reaction:** Occurs when grown on agar containing low concentrations of penicillin. * **Inhalation Anthrax:** Also known as Woolsorter’s disease; characterized by hemorrhagic mediastinitis (widened mediastinum on X-ray).

Q: All of the following are characteristic of Brucella species except:

2-mercaptoethanol (2ME) is used to detect IgA antibodies.. **Explanation:** **Why Option D is the correct answer (The "Except"):** The 2-mercaptoethanol (2ME) test is used to detect **IgG** antibodies, not IgA. In Brucellosis, IgM appears early but declines with treatment. IgG persists in chronic or relapsing cases. 2-mercaptoethanol acts by breaking the disulfide bonds of IgM molecules, neutralizing them. Therefore, any residual agglutination observed after 2ME treatment is attributed solely to **IgG**, making it a crucial marker for monitoring treatment response and identifying chronic infection. **Analysis of Incorrect Options:** * **Option A:** *B. abortus* is indeed **capnophilic**, requiring 5–10% CO₂ for primary isolation, unlike *B. melitensis* or *B. suis*. * **Option B:** While ingestion of unpasteurized dairy is the most common route, **aerosol transmission** is a significant risk, especially in laboratory settings (making it a Tier 1 select agent/biothreat). * **Option C:** Brucella is highly heat-sensitive; standard **pasteurization** effectively kills the bacteria in milk. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Bone marrow culture (more sensitive than blood culture). * **Culture Media:** Castaneda’s medium (biphasic medium) is classically used. * **Serology:** The Standard Agglutination Test (SAT) detects antibodies against *B. abortus*, *B. suis*, and *B. melitensis*, but **not** *B. canis*. * **Drug of Choice:** Doxycycline + Rifampicin (or Streptomycin). * **Rose Bengal Test:** A rapid screening card test used in field settings.

Q: Which bacteria inhibits protein synthesis?

Pseudomonas. **Explanation:** The correct answer is **Pseudomonas** (specifically *Pseudomonas aeruginosa*). **1. Why Pseudomonas is correct:** *Pseudomonas aeruginosa* produces a potent virulence factor called **Exotoxin A**. This toxin acts as an ADP-ribosyltransferase that targets **Elongation Factor-2 (EF-2)**. By ribosylating EF-2, it halts the translocation step of protein synthesis, leading to cell death. This mechanism is identical to the action of the Diphtheria toxin produced by *Corynebacterium diphtheriae*. **2. Why the other options are incorrect:** * **Staphylococcus & Streptococcus:** These Gram-positive cocci primarily cause disease through different mechanisms. *S. aureus* uses toxins like TSST-1 (superantigen) or Alpha-toxin (pore-forming), while *Streptococcus* uses Streptolysins and pyrogenic exotoxins. They do not typically inhibit host protein synthesis as their primary mode of action. * **Klebsiella:** This is a Gram-negative bacterium whose pathogenesis is mainly attributed to its thick polysaccharide capsule (antiphagocytic) and endotoxin (LPS) release, rather than protein synthesis inhibition. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "EF-2 Inhibitor" Duo:** Always remember that *Pseudomonas aeruginosa* (Exotoxin A) and *Corynebacterium diphtheriae* (Diphtheria toxin) share the exact same mechanism of inhibiting protein synthesis via EF-2. * **Shiga Toxin vs. Exotoxin A:** While Exotoxin A inhibits EF-2, **Shiga toxin** (from *Shigella*) and **Shiga-like toxin** (from EHEC) inhibit protein synthesis by clipping the **28S rRNA** of the 60S ribosomal subunit. * **Pseudomonas Characteristics:** It is an obligate aerobe, oxidase-positive, and produces pigments like Pyocyanin (blue-green) and Pyoverdin (fluorescent).

Q: A 4-year-old partially immunized boy presents to the OPD with a history of coughing bouts. Sputum sample staining reveals an organism with bipolar staining. Which of the following organisms is most likely seen?

Bordetella pertussis. **Explanation:** The clinical presentation of a 4-year-old partially immunized child with "coughing bouts" is a classic description of **Whooping Cough (Pertussis)**. The causative agent, *Bordetella pertussis*, is a small, Gram-negative coccobacillus. **1. Why Bordetella pertussis is correct:** *Bordetella pertussis* classically exhibits **bipolar staining** (safety-pin appearance) when stained with special dyes like Toluidine blue or Methylene blue. This occurs because the ends of the bacilli take up the stain more intensely than the center. In the context of a child with paroxysmal cough and incomplete vaccination (missing DTaP/Pentavalent doses), *B. pertussis* is the most likely diagnosis. **2. Why the other options are incorrect:** * **Yersinia pestis:** While it is the "classic" organism associated with bipolar staining (Waysen or Giemsa stain), the clinical history of coughing bouts in a child points towards a respiratory pathogen rather than Plague. * **Streptococcus agalactiae (GBS):** This is a Gram-positive coccus arranged in chains, typically causing neonatal sepsis or meningitis, not paroxysmal cough. * **Klebsiella pneumoniae:** While it can show bipolar staining occasionally, it is a large, encapsulated Gram-negative rod typically causing "currant jelly sputum" in elderly or alcoholic patients, not coughing bouts in children. **3. NEET-PG High-Yield Pearls:** * **Culture Media:** Regan-Lowe medium (preferred) or Bordet-Gengou (potato-blood-glycerol) agar. * **Mercury Drop Appearance:** Colonies on agar look like "bisected pearls" or "mercury drops." * **Specimen of Choice:** Nasopharyngeal swab (using Dacron or Rayon swabs; cotton inhibits growth). * **Bipolar Staining Mnemonic:** "You Better Know Your Pasteurella" (**Y**ersinia, **B**ordetella, **K**lebsiella, **P**asteurella).

Q: Pseudohemoptysis is caused by which of the following microorganisms?

Serratia marcescens. **Explanation:** **Serratia marcescens** is the correct answer because of its unique ability to produce a red/pink intracellular pigment called **prodigiosin**. When this organism colonizes the respiratory tract or causes pneumonia, the red pigment can discolor the sputum, mimicking the appearance of blood. This clinical phenomenon is known as **pseudohemoptysis**. Historically, this pigment has also been associated with "bloody bread" and "miraculous" appearances of blood on communion wafers. **Analysis of Incorrect Options:** * **Histoplasma capsulatum:** While this fungus can cause true hemoptysis by causing cavitary lung lesions or mediastinal lymphadenopathy that erodes into blood vessels, it does not produce a red pigment to cause pseudohemoptysis. * **Proteus:** Known for its "swarming motility" and production of urease (leading to staghorn calculi), *Proteus* species do not produce red pigments. * **Klebsiella:** *Klebsiella pneumoniae* is famous for causing **"currant jelly sputum"** in alcoholics. However, this is **true hemoptysis** caused by tissue necrosis and inflammation, not a pigment-induced mimicry. **NEET-PG High-Yield Pearls:** * **Serratia marcescens** is a Gram-negative, motile rod (Enterobacteriaceae) often associated with nosocomial infections (UTIs, RTI) and IV drug use (endocarditis). * **Prodigiosin** production is temperature-dependent; it is typically produced at room temperature (25°C) rather than 37°C. * **Other causes of Pseudohemoptysis:** *Rifampin* (drug), *Serratia* (pigment), and consumption of large amounts of beets (beeturia/red stools). * **Red diaper syndrome:** *Serratia* colonization in diapers can lead to a reddish discoloration, often mistaken for hematuria.

Question 1

A person working in an abattoir presented with a pustule on hand which turned into an ulcer. Which of the following stains will best help in the diagnosis?

Accepted Answer

Polychrome methylene blue

Answer

Carbol fuchsin

Answer

Acid fast stain

Answer

Calcofluor white

Question 2

What concentration of agar is typically used for solid media to inhibit the swarming of Proteus species?

Accepted Answer

6%

Answer

1-2%

Answer

0.50%

Answer

5%

Question 3

All of the following are true about Bacillus anthracis except?

Accepted Answer

The toxin is a complex of two fractions.

Answer

A plasmid is responsible for toxin production.

Answer

Cutaneous anthrax generally resolves spontaneously.

Answer

The capsular polypeptide aids virulence by inhibiting phagocytosis.

Question 4

All of the following are characteristic of Brucella species except:

Accepted Answer

2-mercaptoethanol (2ME) is used to detect IgA antibodies.

Answer

B. abortus is capnophilic.

Answer

Transmission by aerosol can occur.

Answer

Pasteurization destroys Brucella.

Question 5

Which bacteria inhibits protein synthesis?

Accepted Answer

Pseudomonas

Answer

Staphylococcus

Answer

Streptococcus

Answer

Klebsiella

Question 6

A 4-year-old partially immunized boy presents to the OPD with a history of coughing bouts. Sputum sample staining reveals an organism with bipolar staining. Which of the following organisms is most likely seen?

Accepted Answer

Bordetella pertussis

Answer

Yersinia pestis

Answer

Streptococcus agalactae

Answer

Klebsiella pneumoniae

Question 7

Pseudohemoptysis is caused by which of the following microorganisms?

Accepted Answer

Serratia marcescens

Answer

Histoplasma capsulatum

Answer

Proteus

Answer

Klebsiella

Question 8

Tick-borne relapsing fever is caused by which of the following?

Accepted Answer

Borrelia duttonii

Answer

Borrelia recurrentis

Answer

Borrelia burgdorferi

Answer

All of the above

Question 9

Botulism is a disease of which type of transmission?

Accepted Answer

Neuromuscular transmission caused by the toxin of the bacterium Clostridium botulinum

Answer

Neural transmission caused by the toxin of the bacterium Clostridium botulinum

Answer

Muscular transmission caused by the toxin of the bacterium Clostridium botulinum

Answer

Non-neuromuscular transmission caused by the toxin of the bacterium Clostridium botulinum

Question 10

Botulism is a disease of which type of transmission?

Accepted Answer

Neuromuscular transmission caused by the toxin of the bacterium Clostridium botulinum

Answer

Neural transmission caused by the toxin of the bacterium Clostridium botulinum

Answer

Muscular transmission caused by the toxin of the bacterium Clostridium botulinum

Answer

Non-neuromuscular transmission caused by the toxin of the bacterium Clostridium botulinum

Bacteriology — MCQs

Bacteriology — MCQs

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