Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 141: Which of the following is NOT true for Bordetella pertussis?

A. It is a strict human pathogen.
B. It can be cultured from the patient during the catarrhal stage.
C. It leads to invasion of the respiratory mucosa.
D. Infection can be prevented by an acellular vaccine. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**, but there is a nuance in the question's framing: In the context of *Bordetella pertussis* microbiology, the statement "Infection can be prevented by an acellular vaccine" is technically **false** because the vaccine (both whole-cell and acellular) prevents **clinical disease** (symptoms), but does **not** prevent colonization or transmission (infection) of the bacteria. **1. Why Option C is the "NOT true" statement (Conceptual Clarification):** *Bordetella pertussis* is a **non-invasive** organism. It attaches to the ciliated epithelium of the respiratory tract via adhesins (Filamentous hemagglutinin and Pertactin) and releases toxins (Pertussis toxin, Adenylate cyclase toxin) that cause local tissue damage and systemic symptoms. It does **not** invade the mucosal cells or the bloodstream. **2. Analysis of Other Options:** * **Option A:** True. Humans are the **only known reservoir** for *B. pertussis*. There is no zoonotic transmission or environmental source. * **Option B:** True. The **catarrhal stage** (1–2 weeks) is the period of maximum communicability. Bacterial culture yield is highest during this stage and drops significantly during the paroxysmal stage. * **Option D:** True (in clinical practice). The **acellular vaccine (aP)**, containing purified components like PT, FHA, and pertactin, is the standard in many countries to prevent the clinical manifestations of Whooping Cough. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Nasopharyngeal swab (Dacron/Rayon) cultured on **Regan-Lowe** or **Bordet-Gengou** medium ("Mercury drops" or "Bisected pearls" appearance). * **Drug of Choice:** Erythromycin (or other Macrolides) is used to reduce communicability, though it doesn't alter the clinical course if started late. * **Lymphocytosis:** A high absolute lymphocyte count in a child with a cough is a classic diagnostic clue for Pertussis.

Question 142: Fletcher's medium containing rabbit serum is used for the isolation of which organism?

A. Streptococcus
B. Bacillus anthracis
C. Leptospira (Correct Answer)
D. Borrelia

Explanation: ### Explanation **Correct Answer: C. Leptospira** **Why it is correct:** *Leptospira* are highly fastidious, obligate aerobic spirochetes that cannot be grown on routine agar. They require specialized enriched media containing long-chain fatty acids and vitamins. **Fletcher’s medium** is a semi-solid medium specifically designed for the isolation of *Leptospira*. It contains **rabbit serum**, which provides the necessary lipids (albumin-bound fatty acids) and vitamins required for growth. Other similar media used for *Leptospira* include **EMJH (Ellinghausen-McCullough-Johnson-Harris)** medium and **Korthof’s medium**. **Why the other options are incorrect:** * **A. Streptococcus:** Most species are grown on Blood Agar (to observe hemolysis). Selective media like Pike’s medium may be used for transport. * **B. Bacillus anthracis:** It grows well on routine media like Nutrient Agar (Medusa head colonies) or PLET medium (Polymyxin, Lysozyme, EDTA, Thallous acetate) for selective isolation. * **D. Borrelia:** While also a spirochete, *Borrelia burgdorferi* (Lyme disease) requires **BSK-II (Barbour-Stoenner-Kelly)** medium, which is much more complex than Fletcher’s. **High-Yield Clinical Pearls for NEET-PG:** * **Specimen Timing:** In Leptospirosis, the organism is isolated from **blood and CSF** during the first week (leptospiremic phase) and from **urine** after the second week (leptospiruric phase). * **Dark Ground Microscopy (DGM):** This is the rapid method used to visualize the characteristic hooked ends ("shepherd's crook" appearance) of *Leptospira*. * **Gold Standard Test:** The **Microscopic Agglutination Test (MAT)** is the reference serological standard. * **Clinical Presentation:** Watch for **Weil’s Disease**, characterized by the triad of jaundice, renal failure, and hemorrhage.

Question 143: Which of the following organisms produces enterotoxin?

A. Shigella dysenteriae
B. Shigella sonnei
C. Shigella flexneri
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. All of the above.** While *Shigella* species are primarily known for their ability to invade the colonic mucosa (intracellular invasion), they also produce enterotoxins that contribute to the initial stages of diarrhea. 1. **Underlying Concept:** All four species of *Shigella* (*S. dysenteriae, S. flexneri, S. boydii,* and *S. sonnei*) produce **Shigella Enterotoxins (ShET-1 and ShET-2)**. These toxins act on the small intestine, leading to the secretion of water and electrolytes. This explains why shigellosis often begins with a period of watery diarrhea before progressing to the classic bloody, mucoid stools (dysentery) caused by mucosal invasion and destruction. 2. **Analysis of Options:** * **Shigella dysenteriae (Type 1):** Unique for producing the potent **Shiga Toxin (Stx)**, which has cytotoxic, enterotoxic, and neurotoxic activities. It is the most virulent species. * **Shigella sonnei and Shigella flexneri:** While they do not produce the Shiga Toxin (Stx), they both possess the chromosomal and plasmid-encoded genes for **ShET-1 and ShET-2**, respectively. Therefore, they are capable of enterotoxin production. **High-Yield Clinical Pearls for NEET-PG:** * **Virulence Marker:** The primary virulence factor for *Shigella* is the **Large Virulence Plasmid (pINV)**, which encodes the Type III Secretion System (T3SS) required for invasion. * **Infective Dose:** *Shigella* has a very low infective dose (**10–100 organisms**), making it highly contagious. * **Complications:** *S. dysenteriae* Type 1 is most commonly associated with **Hemolytic Uremic Syndrome (HUS)** due to Shiga toxin-mediated endothelial damage. * **Epidemiology:** *S. sonnei* is the most common cause in developed countries, while *S. flexneri* is most common in developing nations.

Question 144: Yersinia enterocolitica, formerly a Pasteurella, has more than 50 serotypes that can be isolated from rodents, sheep, cattle, swine, dogs, and cats, and from water contaminated by them. Which of the following best describes this organism?

A. Commonly inhabits the canine respiratory tract and is an occasional pathogen for humans; strongly urease-positive
B. Gram-negative bipolar stained bacilli that cause diarrhea by means of a heat-stable enterotoxin, with abdominal pain that may be mistaken for appendicitis (Correct Answer)
C. Pits agar, grows both in carbon dioxide and under anaerobic conditions, and is part of the normal oral cavity flora
D. Typically infects cattle, requires 5% to 10% carbon dioxide for growth, and is inhibited by the dye thionine

Explanation: **Explanation:** *Yersinia enterocolitica* is a Gram-negative coccobacillus that exhibits characteristic **bipolar staining** (safety-pin appearance). It is a significant cause of enterocolitis, particularly in children. The organism produces a **heat-stable enterotoxin (Yst)**, which increases cGMP levels, leading to secretory diarrhea. A hallmark clinical feature is **pseudoappendicitis**, where mesenteric lymphadenitis causes right lower quadrant pain that mimics acute appendicitis. **Analysis of Options:** * **Option B (Correct):** Accurately describes the morphology (bipolar staining), pathogenesis (heat-stable toxin), and classic clinical presentation (pseudoappendicitis). * **Option A:** Describes ***Bordetella bronchiseptica***, which is urease-positive and inhabits the canine respiratory tract. * **Option C:** Describes ***Eikenella corrodens***, a member of the HACEK group known for "pitting" the agar and being part of the normal oral flora. * **Option D:** Describes ***Brucella abortus***, which is capnophilic (requires $CO_2$) and is inhibited by the dye thionine (unlike *B. suis*). **High-Yield Clinical Pearls for NEET-PG:** * **Psychrotrophic:** *Yersinia* can grow at cold temperatures ($4^\circ C$), making it a risk for transfusion-related sepsis (it can multiply in refrigerated blood bags). * **Cold Enrichment:** Used in the lab to isolate *Yersinia* from stool samples. * **Selective Media:** Grows on **CIN agar** (Cefsulodin-Irgasan-Novobiocin), producing "bull’s eye" colonies. * **Iron Overload:** Patients with hereditary hemochromatosis or those on desferrioxamine therapy are at increased risk for *Yersinia* septicemia.

Question 145: What is true about Helicobacter pylori?

A. Culture and Gram staining of a biopsy is the gold standard investigation.
B. Urea breath test is negative with massive infection.
C. Anti-urease antibodies are produced only by invasive strains.
D. Urease activity provides a protective environment to the bacilli. (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is correct:** *Helicobacter pylori* is an acid-sensitive bacterium that survives the harsh acidic environment of the stomach through its potent **urease enzyme**. This enzyme hydrolyzes urea into ammonia and carbon dioxide. The ammonia produced acts as a base, neutralizing the surrounding gastric acid and creating a **"protective alkaline cloud"** (ammonia jacket) around the bacilli, allowing them to colonize the gastric mucosa. **2. Why other options are incorrect:** * **Option A:** While biopsy with histology or culture is highly specific, the **Endoscopic Biopsy with Rapid Urease Test (RUT)** is considered the "invasive gold standard" for rapid diagnosis. However, for overall diagnostic accuracy and non-invasiveness, the **Urea Breath Test (UBT)** is often preferred. Gram staining alone is not the gold standard. * **Option B:** The Urea Breath Test (UBT) relies on the detection of labeled $CO_2$ produced by bacterial urease. Therefore, a **massive infection** would result in high urease activity and a **strongly positive** test, not a negative one. * **Option C:** All strains of *H. pylori* produce urease as it is essential for survival. Anti-urease antibodies are not restricted to "invasive" strains; rather, virulence is often associated with **CagA** (Cytotoxin-associated gene A) and **VacA** (Vacuolating cytotoxin) markers. **Clinical Pearls for NEET-PG:** * **Morphology:** Gram-negative, S-shaped or spiral bacilli (showing "seagull wing" appearance). * **Most sensitive non-invasive test:** Urea Breath Test (uses $^{13}C$ or $^{14}C$ isotopes). * **Test of choice for post-treatment follow-up:** Urea Breath Test or Stool Antigen Test. * **Culture Media:** Skirrow’s medium, Chocolate agar, or Pylori agar. * **Associations:** Type B Gastritis, Peptic Ulcer Disease (Duodenal > Gastric), Gastric Adenocarcinoma, and MALT lymphoma.

Question 146: Which of the following conditions can be caused by Staphylococcus?

A. Pharyngitis (Correct Answer)
B. Tuberculosis
C. Migraine
D. Dermatitis

Explanation: **Explanation:** **Staphylococcus aureus** is a versatile human pathogen capable of causing a wide range of pyogenic infections. While *Streptococcus pyogenes* (Group A Strep) is the most common bacterial cause of **Pharyngitis**, *Staphylococcus aureus* is a recognized, albeit less frequent, causative agent of bacterial sore throat and peritonsillar abscesses. In clinical practice, it is often isolated from the oropharynx in cases of secondary bacterial infection or in chronic tonsillitis. **Analysis of Options:** * **B. Tuberculosis:** This is caused exclusively by *Mycobacterium tuberculosis*, an acid-fast bacillus. Staphylococcus does not cause granulomatous pulmonary disease. * **C. Migraine:** This is a primary neurovascular headache disorder, not an infectious disease. While systemic infections can cause secondary headaches (fever-induced), Migraine itself is non-infectious. * **D. Dermatitis:** This is a general term for skin inflammation (like eczema or allergic contact dermatitis). While Staphylococcus can cause **Pyoderma** (Impetigo, Folliculitis, Furuncles), "Dermatitis" is typically an inflammatory/allergic condition rather than a primary staphylococcal infection. **High-Yield NEET-PG Pearls:** * **Coagulase Test:** The gold standard to differentiate *S. aureus* (positive) from Coagulase-negative Staphylococci (CoNS like *S. epidermidis*). * **Toxin-Mediated Diseases:** Remember that *S. aureus* also causes Toxic Shock Syndrome (TSST-1), Scalded Skin Syndrome (Exfoliatin toxin), and Food Poisoning (Preformed Enterotoxin). * **Protein A:** A key virulence factor of *S. aureus* that binds to the Fc portion of IgG, inhibiting phagocytosis.

Question 147: What does 'pleomorphic' mean in the context of bacteria?

A. Having variable shapes (Correct Answer)
B. Characterized by a rod shape
C. Having a dumbbell shape
D. Arranged in clusters

Explanation: **Explanation:** **Pleomorphism** (derived from the Greek *pleion* meaning "more" and *morphe* meaning "form") refers to the ability of certain bacteria to alter their shape or size in response to environmental conditions or due to the inherent lack of a rigid cell wall. 1. **Why Option A is correct:** Pleomorphic bacteria do not conform to a single standard morphology (like a sphere or a rod). Instead, they exhibit **variable shapes**—ranging from coccoid to filamentous—within a single culture. This is often due to the absence of a peptidoglycan cell wall or variations in cell wall synthesis. 2. **Why other options are incorrect:** * **Option B:** A rod shape is specifically termed **Bacillus**. * **Option C:** A dumbbell shape (or club shape) is characteristic of *Corynebacterium diphtheriae*, which is a specific type of pleomorphism, but the term "pleomorphic" itself is the broader definition of variability. * **Option D:** Arrangement in clusters (like grapes) is the definition of **Staphylococci**, referring to the plane of division rather than the shape of the individual cell. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Examples:** *Mycoplasma* (lacks a cell wall entirely), *L-forms* (cell wall-deficient variants), *Haemophilus influenzae*, and *Corynebacterium diphtheriae*. * **Diagnostic Tip:** *C. diphtheriae* is often described as "pleomorphic" because it shows club-shaped swellings and "Chinese letter" arrangements. * **Culture Note:** Pleomorphism is often more pronounced in aging cultures where nutrients are depleted and toxic metabolic products accumulate.

Question 148: In a patient with orbital cellulitis, cultures show greenish colonies and optochin sensitivity. What is the most likely causative organism?

A. Streptococcus viridans
B. Staphylococcus
C. Pseudomonas
D. Streptococcus pneumoniae (Correct Answer)

Explanation: The correct answer is **Streptococcus pneumoniae**. ### **Explanation of the Correct Answer** The diagnosis is based on two key microbiological characteristics provided in the question: 1. **Greenish colonies:** This indicates **alpha-hemolysis** (partial hemolysis) on blood agar, where hemoglobin is converted to biliverdin. 2. **Optochin Sensitivity:** This is the definitive biochemical test to differentiate *S. pneumoniae* from other alpha-hemolytic streptococci. *S. pneumoniae* is **inhibited** by optochin (ethylhydrocupreine hydrochloride), whereas *Viridans streptococci* are resistant. Additionally, *S. pneumoniae* is a common cause of orbital cellulitis, often spreading from an underlying sinusitis. ### **Analysis of Incorrect Options** * **A. Streptococcus viridans:** While these also produce greenish (alpha-hemolytic) colonies, they are **Optochin Resistant** and **Bile Insoluble**, which distinguishes them from *S. pneumoniae*. * **B. Staphylococcus:** *Staphylococcus aureus* typically produces **golden-yellow** colonies and shows **beta-hemolysis** (complete clearing). It is catalase-positive and clusters in "grape-like" formations. * **C. Pseudomonas:** *Pseudomonas aeruginosa* produces a characteristic **blue-green pigment** (pyocyanin/pyoverdin) and a fruity odor, but it is a Gram-negative rod and does not show optochin sensitivity. ### **NEET-PG High-Yield Pearls** * **Quellung Reaction:** The gold standard for serotyping *S. pneumoniae* (capsular swelling). * **Bile Solubility:** *S. pneumoniae* is bile soluble (colonies disappear/lyse), while *Viridans* is not. * **Morphology:** Classically described as Gram-positive, **lancet-shaped** diplococci. * **Clinical:** *S. pneumoniae* is the #1 cause of **MOPS**: **M**eningitis, **O**titis media, **P**neumonia, and **S**inusitis.

Question 149: Which serum is used for testing streptococcal pyrogenic toxin?

A. Convalescent human serum (Correct Answer)
B. Serum from patient of acute scarlet fever
C. Horse serum
D. None of the above

Explanation: **Explanation:** The correct answer is **Convalescent human serum**. This is based on the principle of the **Schultz-Charlton Reaction**, a diagnostic skin test used to identify the rash of Scarlet Fever caused by *Streptococcus pyogenes*. 1. **Why Convalescent Human Serum is Correct:** *Streptococcus pyogenes* produces **Streptococcal Pyrogenic Exotoxins (SPE)**, specifically Erythrogenic toxin, which causes the characteristic scarlet fever rash. Patients who have recovered from scarlet fever develop specific **antitoxin antibodies** in their blood. When this **convalescent serum** (containing antibodies) is injected intradermally into a patient with an active erythematous rash, it neutralizes the toxin locally. A positive test is indicated by "blanching" (fading) of the rash at the injection site, confirming the rash is scarlatinal. 2. **Why Other Options are Incorrect:** * **Serum from patient of acute scarlet fever:** During the acute phase, the patient is currently suffering from toxin-induced symptoms and has not yet developed a high enough titer of neutralizing antitoxins. * **Horse serum:** While antitoxins for some diseases (like Diphtheria) were historically raised in horses, the specific diagnostic test for Scarlet Fever (Schultz-Charlton) traditionally utilizes human convalescent serum to ensure specificity and avoid hypersensitivity. 3. **High-Yield Clinical Pearls for NEET-PG:** * **Dick Test:** The opposite of the Schultz-Charlton reaction. It involves injecting a minute dose of the toxin intradermally to determine **susceptibility** to scarlet fever. (No immunity = Redness/Inflammation). * **Erythrogenic Toxin:** It is a **Superantigen** (Type I Exotoxin) that causes massive T-cell activation and cytokine release. * **Scarlet Fever Triad:** Strawberry tongue, Sandpaper-like rash, and circumoral pallor. * **Schultz-Charlton Reaction = Neutralization Test.**

Question 150: Which of the following is true about exotoxins?

A. Non-antigenic
B. Enzymatic (Correct Answer)
C. Non-protein
D. Heat stable

Explanation: **Explanation:** Exotoxins are potent, soluble proteins secreted by both Gram-positive and Gram-negative bacteria. The correct answer is **Enzymatic** because most exotoxins function as enzymes (e.g., ADP-ribosyltransferases, proteases, or phospholipases) that catalyze specific biochemical reactions within host cells, leading to cellular dysfunction or death. **Analysis of Options:** * **Enzymatic (Correct):** Many exotoxins act enzymatically. For example, Diphtheria toxin and Cholera toxin act as ADP-ribosyltransferases, while Tetanus and Botulinum toxins are zinc-dependent endopeptidases. * **Non-antigenic (Incorrect):** Exotoxins are highly **antigenic**. They induce the production of high-titer antibodies called antitoxins. This property is utilized to create **toxoids** (detoxified toxins that retain antigenicity) for vaccines (e.g., DTaP). * **Non-protein (Incorrect):** Exotoxins are almost exclusively **proteins** (polypeptides). In contrast, endotoxins are lipopolysaccharides (LPS). * **Heat stable (Incorrect):** Being proteins, most exotoxins are **heat-labile** (destroyed at 60°C), with the notable exception of *Staphylococcal* enterotoxin and *E. coli* heat-stable toxin (ST). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Most exotoxins follow an **A-B subunit structure**, where 'B' binds to the cell surface and 'A' (Active) exerts the enzymatic effect. * **Potency:** Exotoxins are among the most poisonous substances known (e.g., Botulinum toxin has the lowest LD50). * **Genetics:** Often encoded by **plasmids or bacteriophages** (e.g., Diphtheria toxin via the Beta-phage). * **Endotoxin vs. Exotoxin:** Remember that Endotoxins (LPS) are heat-stable, weakly antigenic, and do not form toxoids.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

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