Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2451: Which of the following structures is primarily affected by the toxin produced by Vibrio cholerae?

A. Hemi desmosome
B. Gap junctions
C. Zona occludens
D. CFTR channels (Correct Answer)

Explanation: ***CFTR channels*** - The **cholera toxin (CT)** produced by *Vibrio cholerae* primarily affects **CFTR (Cystic Fibrosis Transmembrane conductance Regulator) channels** on the apical membrane of intestinal epithelial cells. - The toxin's **A subunit activates adenylate cyclase**, leading to increased intracellular **cAMP levels**. - Elevated cAMP **directly opens CFTR chloride channels**, causing massive secretion of chloride ions and water into the intestinal lumen. - This results in the **profuse watery diarrhea** characteristic of cholera ("rice-water stools"). *Zona occludens* - **Zona occludens (tight junctions)** maintain barrier function between intestinal epithelial cells. - These are **not the primary target** of cholera toxin, though barrier integrity may be secondarily affected. - The toxin's main mechanism involves ion channel activation, not disruption of cell-cell junctions. *Hemi desmosome* - **Hemi desmosomes** anchor epithelial cells to the basement membrane through integrin-based adhesion. - They provide structural support but are **not targeted** by cholera toxin. *Gap junctions* - **Gap junctions** allow direct cell-to-cell communication through connexin channels. - They are **not involved** in the pathophysiology of cholera toxin action.

Question 2452: Which organism is characterized by a Medusa head appearance?

A. Bacillus subtilis
B. Bacillus stearothermophilus
C. Bacillus cereus
D. Bacillus anthracis (Correct Answer)

Explanation: ***Bacillus anthracis*** - This bacterium is known for forming **non-hemolytic colonies on blood agar** with a characteristic "Medusa head" appearance due to its tenacious, wavy outgrowths. - The "Medusa head" morphology is a key identifier in the laboratory diagnosis of **anthrax**. *Bacillus subtilis* - This organism typically forms **flat, dull, and spreading colonies** on agar, often with a frosted glass appearance, but not the specific "Medusa head" morphology. - It is a common **environmental bacterium** and a frequent laboratory contaminant, rarely pathogenic to humans. *Bacillus stearothermophilus* - This bacterium is a **thermophile**, meaning it grows optimally at high temperatures (around 55°C), and its colony morphology is not described as "Medusa head." - It is often used as a biological indicator for **sterilization processes** due to its heat resistance. *Bacillus cereus* - *Bacillus cereus* colonies are typically **large, feathery, and motile**, often showing **beta-hemolysis** on blood agar. - While it can form large colonies, it does not exhibit the distinctive "Medusa head" morphology observed in *Bacillus anthracis*.

Question 2453: Erysipeloid disease is caused by which type of infection?

A. Whale finger
B. Erysipelothrix infection (Correct Answer)
C. Fish handler's disease
D. Seal finger

Explanation: ***Erysipelothrix infection*** - **Erysipeloid disease** is caused by the bacterium **_Erysipelothrix rhusiopathiae_**. - This is a **zoonotic infection** transmitted through contact with infected animals or animal products (fish, shellfish, poultry, swine) via cuts or abrasions. - Clinically presents as a localized, violaceous, non-suppurative skin lesion, typically on the hands and fingers. - The term **"Erysipelothrix infection"** directly identifies the causative organism, making this the most accurate answer. *Fish handler's disease* - This is actually a **synonym for erysipeloid**, not a separate disease entity. - It refers to the same condition caused by **_Erysipelothrix rhusiopathiae_**, named for the occupational exposure common in fish handlers. - While this term describes erysipeloid, it is a **colloquial name** rather than identifying the specific infectious agent. *Seal finger* - **Seal finger** is a distinct infection occurring in seal handlers, causing painful cellulitis of the fingers. - Causative organisms include various bacteria such as **_Mycoplasma phocacerebrale_**, **_Streptococcus_** species, and occasionally **_Erysipelothrix_**. - This is **not synonymous** with erysipeloid disease. *Whale finger* - Similar to seal finger, this refers to infections from handling whale carcasses. - Caused by various marine-associated bacteria, not specifically **_Erysipelothrix rhusiopathiae_**. - This is a **different clinical entity** from erysipeloid.

Question 2454: Which of the following Shigella species produces Shiga toxin?

A. All Shigella species
B. Shigella dysenteriae (Correct Answer)
C. Shigella sonnei
D. Shigella flexneri

Explanation: ***Shigella dysenteriae*** - This species, specifically **serotype 1 (S. dysenteriae type 1)**, is known for producing the **Shiga toxin (Stx)**, which inhibits protein synthesis in host cells by targeting the 60S ribosomal subunit. - The Shiga toxin is responsible for the severe clinical manifestations, including **bloody diarrhea** and **hemolytic-uremic syndrome (HUS)**, often associated with *Shigella dysenteriae* infections. - This is the only *Shigella* species that produces this potent toxin. *Shigella sonnei* - This species typically causes the **mildest form of shigellosis** and does not produce the classic Shiga toxin. - While it can produce some enterotoxins, they are generally not as potent as the Shiga toxin of *S. dysenteriae*. *Shigella flexneri* - *S. flexneri* is a common cause of shigellosis in developing countries but **does not produce the Shiga toxin**. - It primarily invades and replicates within the intestinal epithelial cells, causing inflammation and damage through direct cellular invasion. *All Shigella species* - This statement is incorrect because only *Shigella dysenteriae* serotype 1 is recognized for producing the potent **Shiga toxin**. - Other *Shigella* species cause disease through invasion and inflammatory mechanisms, although they can still lead to significant gastrointestinal illness.

Question 2455: Which of the following is the PRIMARY causative agent of tuberculosis in humans?

A. M. Bovis
B. M. Tuberculosis (Correct Answer)
C. M. Leprae
D. M. Avium

Explanation: ***M. tuberculosis*** - **_Mycobacterium tuberculosis_** is the principal and most common bacterial agent responsible for causing **tuberculosis** in humans worldwide. - It primarily affects the **lungs** but can also cause extrapulmonary disease in other organs. *M. Bovis* - **_Mycobacterium bovis_** primarily causes **tuberculosis in cattle** and can be transmitted to humans, often through contaminated milk, but it is a less common cause than _M. tuberculosis_. - Human infection by _M. bovis_ usually manifests as **extrapulmonary tuberculosis**, especially in the lymph nodes or bones. *M. Leprae* - **_Mycobacterium leprae_** is the causative agent of **leprosy** (Hansen's disease), a chronic infectious disease affecting the skin, peripheral nerves, upper respiratory tract, eyes, and testes. - It does not cause tuberculosis. *M. Avium* - **_Mycobacterium avium_** is part of the **_Mycobacterium avium_ complex (MAC)**, which commonly causes disseminated disease in individuals with **HIV/AIDS** or other forms of severe immunocompromise. - While it can cause lung disease, it is distinct from tuberculosis caused by _M. tuberculosis_ and is generally not considered the primary causative agent of classic human tuberculosis.

Question 2456: Which of the following organisms does not require IgA protease for infection?

A. Gonococci
B. Meningococci
C. H. influenzae
D. S. pneumoniae (Correct Answer)

Explanation: ***S. pneumoniae*** - *Streptococcus pneumoniae* uses **polysaccharide capsule** as its primary virulence factor to evade immune detection and phagocytosis. - While it may produce IgA1 protease, it does **not require IgA protease for infection** - the capsule is sufficient for virulence. - S. pneumoniae primarily causes **systemic infections** (pneumonia, bacteremia, meningitis) where the capsule, not IgA protease, is the critical virulence factor. *Gonococci* - **IgA protease** is a critical virulence factor for *Neisseria gonorrhoeae* (gonococci), allowing it to cleave IgA and evade mucosal immunity in the genitourinary tract. - This enzyme helps the bacteria colonize and establish infection in the **mucous membranes**. - Gonococci **require** IgA protease for successful mucosal colonization. *Meningococci* - **IgA protease** is produced by *Neisseria meningitidis* (meningococci), enabling it to break down IgA, particularly at mucosal surfaces in the nasopharynx. - This helps the pathogen colonize and potentially invade the bloodstream and central nervous system. - Meningococci **require** IgA protease for nasopharyngeal colonization. *H. influenzae* - **IgA protease** is a significant virulence factor for *Haemophilus influenzae*, facilitating its colonization of the respiratory tract. - By degrading IgA, it helps the bacterium evade host defenses and cause infections like otitis media, sinusitis, and epiglottitis. - H. influenzae **requires** IgA protease for respiratory mucosal colonization.

Question 2457: Glanders is caused by:

A. Protozoa
B. Virus
C. Fungi
D. Bacteria (Correct Answer)

Explanation: ***Bacteria*** - Glanders is a **zoonotic disease** caused by the bacterium *Burkholderia mallei*. - This gram-negative bacterium primarily affects **equids** (horses, mules, donkeys) but can be transmitted to humans and other animals. *Protozoa* - Protozoa are **single-celled eukaryotic organisms** that cause diseases like malaria, Giardiasis, and amoebiasis. - They are distinctly different from bacteria in their cellular structure and mechanisms of infection. *Virus* - Viruses are **acellular infectious agents** that replicate only inside the living cells of an organism. - They cause a wide range of diseases such as influenza, HIV, and COVID-19, but not glanders. *Fungi* - Fungi are **eukaryotic organisms** that include yeasts, molds, and mushrooms. - They cause diseases like candidiasis, aspergillosis, and ringworm, which are distinct from bacterial infections.

Question 2458: Staphylococcus aureus differs from Staphylococcus epidermidis by

A. Is coagulase positive (Correct Answer)
B. A common cause of UTI
C. Causes endocarditis in drug users
D. Forms golden-yellow colonies

Explanation: ***Is coagulase positive*** - All strains of **_Staphylococcus aureus_** produce **coagulase**, an enzyme that clots plasma, which is a key virulence factor and diagnostic marker. - **_Staphylococcus epidermidis_** is **coagulase-negative**, making this enzymatic activity the primary distinguishing characteristic between the two species. *Forms golden-yellow colonies* - While _Staphylococcus aureus_ often produces **golden-yellow colonies** due to carotenoid pigments, this is not a consistent or definitive differentiating feature as some strains may appear white or cream. - _Staphylococcus epidermidis_ typically forms white colonies, but colony color can be variable and influenced by culture conditions. *A common cause of UTI* - **_Staphylococcus saprophyticus_**, not _Staphylococcus aureus_ or _Staphylococcus epidermidis_, is known as a common cause of **urinary tract infections (UTIs)**, particularly in young, sexually active women. - While both _S. aureus_ and _S. epidermidis_ can cause UTIs in specific contexts (e.g., catheter-associated), they are not considered a "common cause" in the same vein as _E. coli_ or _S. saprophyticus_. *Causes endocarditis in drug users* - Both **_Staphylococcus aureus_** and other staphylococcal species, including **_Staphylococcus epidermidis_**, can cause **endocarditis** in intravenous drug users. - **_S. aureus_** is particularly noted for causing **acute endocarditis** on native or prosthetic valves in this population, but it is not unique to _S. aureus_ as _S. epidermidis_ is also a significant pathogen in prosthetic valve endocarditis.

Question 2459: Q fever is caused by:

A. R. quintana
B. C. burnetii (Correct Answer)
C. R. typhi
D. R. akari

Explanation: ***C. burnetii*** - **Coxiella burnetii** is the causative agent of **Q fever**, a zoonotic disease - Transmission typically occurs through **inhalation of contaminated aerosols** from infected animals, particularly livestock (cattle, sheep, goats) - Presents with **acute febrile illness**, pneumonia, and hepatitis; can cause chronic endocarditis - Unique among rickettsial organisms: does **not cause a rash** *R. quintana* - **Rochalimaea quintana** (now **Bartonella quintana**) causes **trench fever** - Transmitted by **body lice**, not aerosols - Characterized by recurrent fevers, headaches, and leg pain *R. typhi* - **Rickettsia typhi** causes **murine typhus** (endemic typhus) - Transmitted by **fleas of rodents**, primarily rats - Presents with fever, headache, and **maculopapular rash** *R. akari* - **Rickettsia akari** causes **rickettsialpox**, a mild rickettsial infection - Transmitted by **mites of house mice** - Characterized by distinctive **eschar** followed by papulovesicular rash

Question 2460: Bacteria with a safety pin appearance are:

A. Yersinia pestis (Correct Answer)
B. Salmonella paratyphi
C. Corynebacterium diphtheriae
D. Vibrio vulnificus

Explanation: ***Yersinia pestis*** - **Yersinia pestis** is the classic organism associated with **"safety pin appearance"** due to its characteristic **bipolar staining** pattern. - When stained with Wayson's stain or Giemsa stain, the bacilli show intense staining at both poles with a clear unstained center, resembling a safety pin. - This Gram-negative coccobacillus is the causative agent of **plague** (bubonic, septicemic, and pneumonic). - The safety pin appearance is best seen in **tissue smears** and **blood films** from infected patients or animal models. *Corynebacterium diphtheriae* - Characteristically appears as **club-shaped rods** with **metachromatic granules** (Babes-Ernst granules). - Shows typical arrangement in **palisades** or **V-shaped formations** ("Chinese letters"), not a safety pin appearance. - Causative agent of **diphtheria**, characterized by pseudomembrane formation. *Vibrio vulnificus* - This bacterium is a **curved rod** (comma-shaped) typical of Vibrio species. - Does not exhibit bipolar staining or "safety pin" morphology. - Associated with **seafood consumption** and can cause severe wound infections and septicemia, particularly in immunocompromised patients. *Salmonella paratyphi* - *Salmonella* species are **straight rods** (bacilli) without distinctive bipolar staining. - Not characterized by a "safety pin appearance." - Causative agent of **paratyphoid fever**, a systemic infection similar to typhoid fever.

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

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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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