Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1921: Gas gangrene is caused by:

A. Clostridium welchii (Correct Answer)
B. Clostridium tetani
C. Clostridium botulinum
D. Clostridium difficile

Explanation: **Explanation:** **1. Why Clostridium welchii is correct:** Gas gangrene (myonecrosis) is primarily caused by **Clostridium perfringens**, historically known as **Clostridium welchii**. It is a Gram-positive, anaerobic, spore-forming bacillus. The pathogenesis is driven by the production of potent exotoxins, most notably **Alpha-toxin (Lecithinase)**, which destroys cell membranes, leading to tissue necrosis, hemolysis, and the characteristic production of gas (crepitus) within the soft tissues. **2. Why the other options are incorrect:** * **Clostridium tetani:** Causes **Tetanus**. It produces *tetanospasmin*, a neurotoxin that blocks inhibitory neurotransmitters (GABA/Glycine), leading to spastic paralysis and "lockjaw," but it does not cause gas gangrene. * **Clostridium botulinum:** Causes **Botulism**. It produces a neurotoxin that inhibits Acetylcholine release at the neuromuscular junction, resulting in symmetrical descending flaccid paralysis. * **Clostridium difficile:** Causes **Pseudomembranous colitis** and antibiotic-associated diarrhea. It affects the gastrointestinal tract rather than deep muscle tissues. **3. High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** A biochemical test used to identify *C. perfringens* based on its lecithinase activity (opalescence on egg yolk agar). * **Stormy Fermentation:** Characteristic reaction in litmus milk media. * **Morphology:** It is a "box-car" shaped bacillus and is unique among Clostridia for being **non-motile** and **capsulated**. * **Target Hemolysis:** On blood agar, it produces a double zone of hemolysis (inner zone of complete hemolysis by theta-toxin and outer zone of incomplete hemolysis by alpha-toxin).

Question 1922: What is the most common aerobic bacterium found in the vagina?

A. Lactobacilli (Correct Answer)
B. Gardnerella
C. Mobilincus
D. Clostridium

Explanation: **Explanation:** The correct answer is **Lactobacilli (Döderlein's bacilli)**. **Why Lactobacilli is correct:** Lactobacilli are the predominant aerobic/facultative anaerobic bacteria in the healthy vaginal flora of reproductive-age women. They play a crucial role in maintaining vaginal health by fermenting glycogen (secreted by vaginal epithelium under estrogen influence) into **lactic acid**. This maintains an acidic vaginal pH (typically 3.8–4.5), which inhibits the overgrowth of pathogenic organisms. **Why the other options are incorrect:** * **Gardnerella vaginalis:** While it can be present in small amounts in healthy women, it is a facultative anaerobe primarily associated with **Bacterial Vaginosis (BV)** when it overgrows due to a decrease in Lactobacilli. * **Mobilincus:** This is a genus of **obligate anaerobic**, motile, curved rods. Like Gardnerella, it is a marker for Bacterial Vaginosis and is not the dominant normal flora. * **Clostridium:** These are obligate anaerobic, spore-forming Gram-positive rods. They are not considered normal vaginal flora and are more commonly associated with the gastrointestinal tract or specific infections (e.g., *C. perfringens* in septic abortions). **High-Yield Clinical Pearls for NEET-PG:** * **Döderlein's Bacilli:** Another name for vaginal Lactobacilli. * **Vaginal pH:** Normal is **<4.5**. A pH >4.5 is a diagnostic criterion for Bacterial Vaginosis (Amsel's Criteria). * **Clue Cells:** Vaginal epithelial cells coated with *Gardnerella vaginalis*, seen on wet mount in Bacterial Vaginosis. * **Whiff Test:** Addition of 10% KOH to vaginal discharge producing a fishy odor (due to amines) is positive in BV.

Question 1923: In Pontiac fever, which antigen is detected in urine?

A. Group specific antigen of Legionella serogroup-1 (LP 1) (Correct Answer)
B. Group specific antigen of Legionella serogroup-2 (LP 2)
C. Group specific antigen of Legionella serogroup-4 (LP 4)
D. Group specific antigen of Legionella serogroup-6 (LP 6)

Explanation: **Explanation:** **Legionella pneumophila** is the causative agent of two distinct clinical entities: **Legionnaires' disease** (severe pneumonia) and **Pontiac fever** (a mild, self-limiting influenza-like illness). The correct answer is **Option A** because **Legionella pneumophila serogroup 1 (LP1)** is responsible for approximately 80–90% of all human infections caused by this genus. The **Urinary Antigen Test (UAT)** is the most common rapid diagnostic tool used in clinical practice. It detects a heat-stable soluble polysaccharide antigen that is specifically secreted by **Serogroup 1**. While there are over 15 serogroups of *L. pneumophila*, the commercially available enzyme-linked immunosorbent assays (ELISA) and immunochromatographic tests are highly sensitive and specific primarily for **Serogroup 1**. **Why other options are incorrect:** * **Options B, C, and D:** While Serogroups 2, 4, and 6 can cause human disease, they are significantly less common. Most standard urinary antigen kits do not reliably detect non-serogroup 1 antigens, leading to potential false negatives if the infection is caused by these rarer strains. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Inhalation of aerosols from contaminated water sources (AC cooling towers, showers, nebulizers). No person-to-person transmission occurs. * **Culture:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with L-cysteine and iron. * **Staining:** Poorly visualized on Gram stain; requires **Silver stains (Dieterle stain)** or Direct Fluorescent Antibody (DFA) staining. * **Clinical Clue:** Legionnaires' disease often presents with "atypical pneumonia" associated with **hyponatremia** and gastrointestinal symptoms (diarrhea). * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin). Pontiac fever usually requires only symptomatic treatment.

Question 1924: Which of the following statements about Haemophilus influenzae is FALSE?

A. It requires both factor X and factor V for growth.
B. It rarely presents as meningitis in children less than 2 months of age.
C. The capsular polysaccharide is responsible for its virulence. (Correct Answer)
D. Meningitis is a common invasive disease caused by Haemophilus influenzae.

Explanation: **Explanation:** The correct answer is **C**. While the capsular polysaccharide (specifically Polyribosyl Ribitol Phosphate or PRP) is a major virulence factor, the statement is technically "False" in the context of this question because it implies it is the *only* factor or uniquely responsible. However, in NEET-PG logic, this is often a "distractor" or "least accurate" question. More accurately, the capsule allows the bacteria to resist phagocytosis, but non-encapsulated (non-typeable) strains also cause significant disease (otitis media, sinusitis) via other virulence factors like IgA protease and pili. *Note: In many standardized exams, if this is marked as the false statement, it is often because the question implies the capsule is the sole determinant of virulence, or it is contrasting it against the fact that non-typeable H. influenzae (NTHi) are also pathogenic.* **Analysis of other options:** * **Option A (True):** *H. influenzae* is fastidious. It requires **Factor X (Hemin)** and **Factor V (NAD)**. This is why it grows on Chocolate agar (where RBCs are lysed to release these factors) but not on Blood agar (unless "Satellitism" occurs around *S. aureus*). * **Option B (True):** Neonatal meningitis (<2 months) is primarily caused by *Group B Streptococcus*, *E. coli*, and *Listeria*. *H. influenzae* typically affects children between **6 months and 5 years** (after maternal antibodies wane). * **Option D (True):** Historically, *H. influenzae* type b (Hib) was the leading cause of bacterial meningitis in children worldwide before the introduction of the Hib conjugate vaccine. **High-Yield Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows around *S. aureus* on blood agar because *S. aureus* provides Factor V via hemolysis. * **Culture:** Best grown on **Chocolate Agar**; inhibited by heating above 75°C (which destroys Factor V). * **Vaccine:** The Hib vaccine is a **conjugate vaccine** (PRP conjugated to a protein carrier like tetanus toxoid) given at 6, 10, and 14 weeks in India (Pentavalent vaccine). * **Drug of Choice:** Ceftriaxone for meningitis; Amoxicillin-clavulanate for mucosal infections.

Question 1925: Tick borne Relapsing Fever is caused by which bacterium?

A. Borrelia recurrentis
B. Borrelia burgdorferi
C. Rickettsia prowazeki
D. Borrelia hermsii (Correct Answer)

Explanation: **Explanation:** Relapsing fever is a vector-borne disease characterized by recurring febrile episodes caused by spirochetes of the genus *Borrelia*. It is classified into two types based on the vector: **Louse-borne** and **Tick-borne**. **1. Why Option D is Correct:** **Borrelia hermsii** (along with *B. turicatae* and *B. parkeri*) is a primary cause of **Tick-borne Relapsing Fever (TBRF)**. It is transmitted to humans via the bite of soft-bodied **Ornithodoros ticks**. The "relapsing" nature of the fever is due to **antigenic variation** of the bacteria’s variable major proteins (VMPs), allowing it to evade the host's immune system repeatedly. **2. Analysis of Incorrect Options:** * **A. Borrelia recurrentis:** This is the causative agent of **Louse-borne Relapsing Fever (LBRF)**, transmitted by the human body louse (*Pediculus humanus corporis*). It typically occurs in crowded, unsanitary conditions (epidemic form). * **B. Borrelia burgdorferi:** This is the causative agent of **Lyme disease**, transmitted by hard-bodied *Ixodes* ticks. It does not cause relapsing fever. * **C. Rickettsia prowazeki:** This causes **Epidemic Typhus**, transmitted by the human body louse. It is an obligate intracellular bacterium, not a spirochete. **3. NEET-PG High-Yield Pearls:** * **Vector Distinction:** Louse-borne = *B. recurrentis*; Tick-borne = *B. hermsii* / *B. duttonii*. * **Diagnosis:** The best time to take a blood film is **during the febrile period** (rising fever). Unlike Lyme disease, these spirochetes are easily visible on **Giemsa or Wright-stained** peripheral blood smears. * **Jarisch-Herxheimer Reaction:** A common complication following the first dose of antibiotics (usually Tetracyclines), caused by the sudden release of endotoxins from dying spirochetes.

Question 1926: What is a virulence factor of group A beta-hemolytic streptococci?

A. Protein M (Correct Answer)
B. Protein T
C. Protein R
D. Lipoteichoic acid

Explanation: **Explanation:** **Group A Streptococcus (GAS)**, or *Streptococcus pyogenes*, possesses several virulence factors, but **Protein M** is the most significant and definitive one. 1. **Why Protein M is Correct:** Protein M is a hair-like projection (fimbriae) extending from the cell wall. It is the primary virulence factor because it is **strongly anti-phagocytic**. It acts by inhibiting the alternative complement pathway (interfering with C3b opsonization) and resisting phagocytosis by polymorphonuclear leukocytes. Furthermore, there are over 100 serotypes of M protein, which accounts for recurrent infections and determines the strain's potential to cause post-streptococcal sequelae (like Rheumatic Fever). 2. **Analysis of Incorrect Options:** * **Protein T and Protein R:** These are surface proteins used primarily for **serological typing** (T-typing) of GAS strains. Unlike Protein M, they have no known role in the pathogenesis or virulence of the organism. * **Lipoteichoic Acid (LTA):** While LTA is present in the cell wall and aids in the **initial attachment** (adhesion) of the bacteria to host pharyngeal epithelial cells, it is not considered the primary virulence factor that allows the bacteria to evade the immune system. **High-Yield Clinical Pearls for NEET-PG:** * **Molecular Mimicry:** Antibodies against certain M protein types (e.g., 1, 3, 5, 6, 18) cross-react with human myocardial tissue (sarcolemma, myosin), leading to **Acute Rheumatic Fever**. * **Hyaluronic acid capsule:** Another virulence factor that provides a "camouflage" against the immune system because it is chemically similar to human connective tissue. * **ASO Titre:** Used to diagnose recent GAS infection; it detects antibodies against **Streptolysin O**, an oxygen-labile hemolysin.

Question 1927: A child presents with a cough and a characteristic inspiratory whoop. What is the most appropriate sample for investigation?

A. Nasopharyngeal swab (Correct Answer)
B. Tracheal aspiration
C. Cough plate culture
D. Sputum culture

Explanation: **Explanation:** The clinical presentation of a cough followed by an inspiratory "whoop" is diagnostic of **Pertussis (Whooping Cough)**, caused by *Bordetella pertussis*. **Why Nasopharyngeal Swab is the Correct Answer:** *Bordetella pertussis* specifically colonizes the **ciliated epithelial cells** of the respiratory tract, with the highest concentration found in the **nasopharynx**. Therefore, a nasopharyngeal swab (using Dacron or calcium alginate, not cotton) or nasopharyngeal aspirate is the gold standard for both culture and PCR. The sample should be collected during the catarrhal or early paroxysmal stage for maximum yield. **Analysis of Incorrect Options:** * **Cough plate culture:** This historical method involves holding a culture plate (Bordet-Gengou medium) in front of the patient’s mouth during a cough. It is no longer recommended due to low sensitivity and the risk of aerosolizing the pathogen. * **Tracheal aspiration:** While it may contain the bacteria, it is an invasive procedure and unnecessary when a nasopharyngeal sample is more accessible and highly sensitive. * **Sputum culture:** *B. pertussis* does not typically reside in the lower respiratory tract secretions that make up sputum; furthermore, sputum is often contaminated with oropharyngeal flora, making it unsuitable for isolating this fastidious organism. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** Regan-Lowe medium (charcoal-horse blood agar) is currently preferred over the classic Bordet-Gengou medium. * **Appearance:** Colonies on agar resemble **"bisected pearls"** or **"mercury drops."** * **Microscopy:** Gram-negative coccobacilli showing a **"thumb-print"** appearance. * **Mercury Arc Phenomenon:** Seen in primary isolation. * **Drug of Choice:** Macrolides (Erythromycin or Azithromycin).

Question 1928: Which of the following microorganisms requires cholesterol and other lipids for growth?

A. Mycobacterium tuberculosis
B. Chlamydia
C. Haemophilus
D. Mycoplasma (Correct Answer)

Explanation: **Explanation:** The correct answer is **Mycoplasma**. **Why Mycoplasma is correct:** Mycoplasmas are unique among prokaryotes because they **lack a cell wall**. To compensate for this structural deficiency and maintain osmotic stability, their cell membrane contains **sterols (cholesterol)**. Since Mycoplasmas cannot synthesize sterols themselves, they must acquire them from the external environment. Consequently, laboratory culture media (like PPLO agar or Eaton’s agar) must be enriched with **20% horse serum**, which provides the necessary cholesterol and lipids for growth. **Why the other options are incorrect:** * **Mycobacterium tuberculosis:** These are acid-fast bacilli characterized by a cell wall rich in **mycolic acids**. While they require complex media (e.g., LJ medium containing egg yolk), they do not have an absolute requirement for exogenous cholesterol for membrane integrity. * **Chlamydia:** These are obligate intracellular bacteria. While they hijack host cell lipids for replication, they are typically grown in cell cultures (like McCoy cells) rather than lipid-enriched agar. * **Haemophilus:** These organisms require specific growth factors found in blood: **Factor X (Hemin)** and **Factor V (NAD)**, but not cholesterol. **NEET-PG High-Yield Pearls:** * **Smallest free-living organisms:** Mycoplasmas are the smallest self-replicating bacteria (0.1–0.3 µm). * **Pleomorphism:** Due to the lack of a cell wall, they are pleomorphic and do not stain with Gram stain. * **Antibiotic Resistance:** They are **innately resistant** to Beta-lactams (Penicillins/Cephalosporins) because these drugs target the cell wall. * **Fried Egg Appearance:** On solid media, Mycoplasma colonies show a characteristic "fried egg" appearance (except *M. pneumoniae*).

Question 1929: Streptococcal toxic shock syndrome is due to the liberation of which substance?

A. TSS-1 toxin
B. Enterotoxin
C. Pyrogenic exotoxin (Correct Answer)
D. Endotoxin

Explanation: **Explanation:** Streptococcal Toxic Shock Syndrome (STSS) is a severe, life-threatening condition primarily caused by **Group A Streptococcus (Streptococcus pyogenes)**. The pathogenesis is driven by the release of **Streptococcal Pyrogenic Exotoxins (SpeA, SpeB, and SpeC)**. These toxins act as **superantigens**, which bypass normal antigen processing by directly binding to the MHC class II molecules on antigen-presenting cells and the Vβ region of T-cell receptors. This leads to a massive, non-specific activation of T-cells and a "cytokine storm" (IL-1, IL-6, TNF-α, and IFN-γ), resulting in capillary leak, hypotension, and multi-organ failure. **Analysis of Options:** * **A. TSS-1 toxin:** Also known as TSST-1, this is the superantigen responsible for Toxic Shock Syndrome caused by *Staphylococcus aureus*, not Streptococcus. * **B. Enterotoxin:** These are heat-stable toxins produced by *Staphylococcus aureus* that cause food poisoning; they are not the primary cause of STSS. * **D. Endotoxin:** These are Lipopolysaccharides (LPS) found in the cell walls of **Gram-negative bacteria**. *Streptococcus* is a Gram-positive organism and does not possess endotoxins. **High-Yield Clinical Pearls for NEET-PG:** * **STSS vs. Staph TSS:** STSS (Streptococcal) is often associated with **necrotizing fasciitis** and bacteremia, whereas Staph TSS is frequently associated with tampon use or focal infections and is less commonly bacteremic. * **M-Protein:** The most important virulence factor of *S. pyogenes* that inhibits phagocytosis; strains with M-types 1 and 3 are most commonly linked to STSS. * **Drug of Choice:** While Penicillin is used for the bacteria, **Clindamycin** is added in STSS because it inhibits protein synthesis, thereby shutting down toxin production.

Question 1930: Which of the following statements about Pneumococcus is false?

A. Pneumolysin, a thiol-activated toxin, exerts a variety of effects on ciliary and PMNs.
B. Autolysin may contribute to the pathogenesis of pneumococcal disease by lysing the bacteria.
C. Anticapsular antibodies are serotype-specific.
D. The virulence of pneumococci is dependent only on the production of the capsular polysaccharide. (Correct Answer)

Explanation: **Explanation** **Why Option D is the correct (False) statement:** While the **capsular polysaccharide** is the most important virulence factor (it is anti-phagocytic and essential for systemic survival), it is **not the only factor**. The virulence of *Streptococcus pneumoniae* is multifactorial, involving surface proteins (PspA, PsaA), enzymes (Neuraminidase, Hyaluronidase), and toxins (Pneumolysin and Autolysin). Strains lacking these factors show reduced virulence even if encapsulated. **Analysis of other options:** * **Option A (True):** **Pneumolysin** is a potent membrane-damaging toxin. It is thiol-activated (oxygen-labile) and acts by creating pores in host cell membranes. It inhibits ciliary movement in the respiratory tract and impairs the chemotaxis and phagocytic activity of Polymorphonuclear leukocytes (PMNs). * **Option B (True):** **Autolysin (LytA)** is an enzyme that degrades the bacterial cell wall. During infection, autolysis releases internal inflammatory mediators like pneumolysin and peptidoglycan fragments, which trigger a massive inflammatory response, contributing to tissue damage. * **Option C (True):** The capsule is the basis for serotyping (over 100 serotypes). Immunity is mediated by **type-specific antibodies** that act as opsonins. This is why the pneumococcal vaccine must include multiple serotypes (e.g., PCV13, PPSV23). **High-Yield Clinical Pearls for NEET-PG:** * **Quellung Reaction:** Gold standard for identification; involves capsular swelling when exposed to specific antisera. * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Bile Solubility & Optochin Sensitivity:** Used to differentiate *S. pneumoniae* (Sensitive/Soluble) from *Viridans streptococci* (Resistant/Insoluble). * **Pneumolysin** is responsible for the **alpha-hemolysis** (greenish discoloration) seen on blood agar under aerobic conditions.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

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Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

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Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

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Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Bordetella and Brucella

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Mycobacteria

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Spirochetes

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