Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2461: 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 2462: What is the initial clinical manifestation of Treponema pallidum infection in primary syphilis?

A. Primary chancre (Correct Answer)
B. Secondary rash
C. Neurosyphilis
D. Dark-field microscopy

Explanation: ***Primary chancre*** - *Treponema pallidum* is the causative agent of syphilis, and the **primary chancre** is the initial clinical manifestation at the site of inoculation. - A **chancre** is a painless, indurated ulcer with a clean base that develops 10-90 days after infection, typically on the genitals, rectum, or mouth. - This represents the hallmark of **primary syphilis** and is the first clinical sign of *T. pallidum* infection. *Secondary rash* - A **secondary rash** is a manifestation of **secondary syphilis**, occurring several weeks to months after the primary chancre heals. - This disseminated stage occurs as spirochetes spread throughout the body, but it is not the initial clinical manifestation. *Neurosyphilis* - **Neurosyphilis** refers to infection of the central nervous system by *Treponema pallidum*, which can occur at any stage but typically represents **tertiary syphilis**. - This is a severe complication but not the initial clinical manifestation of the infection. *Dark-field microscopy* - **Dark-field microscopy** is a diagnostic laboratory technique used to visualize motile *Treponema pallidum* directly from chancre exudate. - This is a diagnostic method, not a clinical manifestation of the infection.

Question 2463: The causative organism in a patient suffering from pneumonia with laboratory findings of an acid-fast filamentous bacterium is?

A. M. tuberculosis
B. Actinomyces
C. Nocardia (Correct Answer)
D. Mycobacterium Avium intracellulare

Explanation: ***Nocardia*** - **Nocardia** is an **acid-fast, filamentous bacterium** that commonly causes pulmonary infections, especially in immunocompromised individuals, which aligns with the description of pneumonia with these laboratory findings. - The filamentous morphology and **partial acid-fast staining** are characteristic identifying features of *Nocardia* species in clinical samples. *M. tuberculosis* - While *M. tuberculosis* is an **acid-fast bacterium** and causes pneumonia (tuberculosis), it is typically described as a **rod-shaped bacillus**, not filamentous. - Its staining characteristics are **strongly acid-fast**, differentiating it from the **partially acid-fast** nature of *Nocardia*. *Actinomyces* - *Actinomyces* species are **filamentous bacteria** that can cause pulmonary infections, but they are **not acid-fast**. - They are typically recognized by the presence of **"sulfur granules"** in pus and are anaerobic. *Mycobacterium avium-intracellulare* - *Mycobacterium avium-intracellulare* is an **acid-fast bacterium** that can cause pulmonary disease, particularly in immunocompromised patients. - However, like *M. tuberculosis*, it is described as a **rod-shaped bacillus**, not a filamentous organism.

Question 2464: Staphylococcus is a type of:

A. Gram-positive cocci (Correct Answer)
B. Gram-negative cocci
C. Gram-positive bacillus
D. Gram-negative bacillus

Explanation: ***Gram-positive cocci*** - **Staphylococcus** bacteria are characterized by their **spherical shape** (cocci) and their ability to retain crystal violet stain due to a thick **peptidoglycan layer** in their cell wall, classifying them as Gram-positive. - They typically arrange in **grape-like clusters**. *Gram-negative cocci* - **Gram-negative cocci** do not retain the crystal violet stain, appearing red or pink, and have a thin peptidoglycan layer along with an **outer membrane**. - Examples include species like **Neisseria gonorrhoeae** and **Neisseria meningitidis**, which are distinct from Staphylococcus. *Gram-positive bacillus* - **Gram-positive bacilli** are rod-shaped bacteria that retain the crystal violet stain. - Examples include **Bacillus anthracis** and **Clostridium difficile**, which have a different morphology and staining characteristic than Staphylococcus. *Gram-negative bacillus* - **Gram-negative bacilli** are rod-shaped bacteria that appear red or pink after Gram staining due to their inability to retain crystal violet. - Examples include **Escherichia coli** and **Salmonella species**, sharing neither the morphology nor the Gram staining result of Staphylococcus.

Question 2465: An infant presented with a high-grade fever and respiratory distress at the time of presentation to the emergency room. The sputum sample showed gram-positive cocci with alpha hemolytic activity. The suspected pathogen is susceptible to which of the following agents?

A. Novobiocin
B. Bacitracin
C. Oxacillin
D. Optochin (Correct Answer)

Explanation: **Optochin** - The description of **gram-positive cocci** with **alpha hemolytic activity** in a respiratory sample from an infant with fever and respiratory distress strongly suggests **Streptococcus pneumoniae**. - **S. pneumoniae** is identified by its **susceptibility to optochin** (P-disk) in laboratory testing, which differentiates it from other alpha-hemolytic streptococci like viridans group streptococci. - The optochin test is a key diagnostic tool: **S. pneumoniae is susceptible** (shows a zone of inhibition), while other alpha-hemolytic streptococci are resistant. *Novobiocin* - **Novobiocin susceptibility** or resistance is used to differentiate **coagulase-negative staphylococci** (CNS). - This test is primarily used for identifying **Staphylococcus saprophyticus** (resistant) from other CNS like **Staphylococcus epidermidis** (susceptible), which are gram-positive cocci but typically **non-hemolytic (gamma-hemolytic)** and not the primary cause of this presentation. *Bacitracin* - **Bacitracin susceptibility** is used to identify **Group A Streptococcus** (**Streptococcus pyogenes**), which is gram-positive and typically **beta-hemolytic**. - This organism causes illnesses like streptococcal pharyngitis but would not typically present with alpha-hemolytic activity in a sputum sample. *Oxacillin* - **Oxacillin** is an **antibiotic**, and susceptibility to it is used to determine if **Staphylococcus aureus** is **methicillin-susceptible (MSSA)** or **methicillin-resistant (MRSA)**. - While **S. aureus** is a gram-positive coccus and can cause respiratory infections, it is typically **beta-hemolytic** and not described as alpha-hemolytic.

Question 2466: Which organism is primarily responsible for causing primary atypical pneumonia?

A. Mycoplasma pneumoniae (Correct Answer)
B. Legionella
C. Streptococcus pneumoniae
D. Chlamydophila pneumoniae

Explanation: ***Mycoplasma pneumoniae*** - This organism is the most common cause of **primary atypical pneumonia**, often referred to as "walking pneumonia," due to its mild and insidious presentation. - It lacks a cell wall, making it resistant to many common antibiotics like penicillin and cephalosporins. *Legionella* - **Legionella pneumophila** causes **Legionnaires' disease**, a severe form of pneumonia often associated with contaminated water sources and prominent systemic symptoms. - While it causes an atypical pneumonia, it is typically more severe than that caused by *Mycoplasma pneumoniae*. *Streptococcus pneumoniae* - This bacterium is the most common cause of **typical bacterial pneumonia**, characterized by a more acute onset, high fever, productive cough, and lobar consolidation on chest X-ray. - It is distinguishable from atypical pneumonia by its more severe clinical presentation and response to beta-lactam antibiotics. *Chlamydophila pneumoniae* - This organism can also cause **atypical pneumonia**, but it is less common than *Mycoplasma pneumoniae* as the primary cause. - Infections often have a prodrome of pharyngitis and hoarseness, which is not universally characteristic of primary atypical pneumonia.

Question 2467: Pinta is caused by:

A. Treponema pallidum
B. Treponema endemicum
C. Treponema pertenue
D. Treponema carateum (Correct Answer)

Explanation: ***Treponema carateum*** - Pinta is a **non-venereal treponematosis** caused by *Treponema carateum*, primarily affecting the skin. - It leads to **dyspigmented skin lesions**, including polymorphic erythematous, scaly papules and plaques. *Treponema pertenue* - This bacterium causes **yaws**, another non-venereal treponematosis. - Yaws primarily affects the skin, bones, and joints, causing frambesial lesions. *Treponema pallidum* - *Treponema pallidum* subspecies *pallidum* is the causative agent of **syphilis**, a sexually transmitted infection. - Syphilis presents with various stages, including chancres, rashes, and neurological or cardiovascular complications. *Treponema endemicum* - *Treponema pallidum* subspecies *endemicum* causes **bejel** (endemic syphilis). - Bejel is transmitted non-sexually through close contact and primarily affects mucous membranes and bones.

Question 2468: Virulence or pathogenicity of pneumococcus depends on

A. Capsular polysaccharide (Correct Answer)
B. Cell wall antigen
C. Flagellar antigen
D. Fimbrial antigen

Explanation: ***Capsular polysaccharide*** - The **polysaccharide capsule** is the primary virulence factor of *Streptococcus pneumoniae*, protecting it from phagocytosis by host immune cells. - Its presence allows the bacterium to evade host defenses and establish infection, leading to diseases like **pneumonia**, **meningitis**, and **otitis media**. *Cell wall antigen* - While present in *S. pneumoniae*, the **cell wall antigens** (like teichoic acid and peptidoglycan) are involved in inducing inflammation and host response, they are not the primary determinant of **virulence** in terms of evading phagocytosis. - They also serve as adherence factors but lack the direct anti-phagocytic role of the capsule. *Flagellar antigen* - *Streptococcus pneumoniae* is a **non-motile bacterium** and therefore does not possess flagella. - Thus, **flagellar antigens** cannot contribute to its virulence or pathogenicity. *Fimbrial antigen* - **Fimbriae (pili)** are involved in adherence to host cells for many bacteria, but they are not the primary determinant of **virulence** for *S. pneumoniae*. - Although some strains may possess pilus-like structures, the **capsule** remains the most critical factor for evading the immune system.

Question 2469: Hemorrhagic colitis is caused by which strain of E. coli?

A. Enteropathogenic
B. Enterotoxigenic
C. Enteroinvasive
D. Enterohemorrhagic (Correct Answer)

Explanation: ***Enterohemorrhagic*** - **Enterohemorrhagic E. coli** (EHEC), particularly **O157:H7**, produces **Shiga toxins** that damage gut lining, leading to bloody diarrhea (hemorrhagic colitis). - This strain is also associated with **hemolytic uremic syndrome (HUS)**, especially in children, characterized by hemolytic anemia, thrombocytopenia, and acute kidney injury. *Enteropathogenic* - **Enteropathogenic E. coli** (EPEC) causes **watery diarrhea** primarily in infants in developing countries. - It damages the gut epithelium by forming unique "attaching and effacing" lesions, but typically does not cause bloody stools. *Enterotoxigenic* - **Enterotoxigenic E. coli** (ETEC) is a common cause of **traveler's diarrhea**, producing **heat-labile (LT)** and **heat-stable (ST)** toxins. - These toxins stimulate fluid and electrolyte secretion, resulting in profuse watery diarrhea without blood. *Enteroinvasive* - **Enteroinvasive E. coli** (EIEC) invades and destroys the colonic epithelial cells, leading to symptoms similar to **shigellosis**, including **dysentery** (bloody, mucoid stools) and fever. - While it causes bloody diarrhea, the primary term for the E. coli strain causing "hemorrhagic colitis" is EHEC due to its distinct shiga toxin production and association with HUS.

Question 2470: The single most important laboratory test for determining the virulence of staphylococci is

A. Detection of coagulase (Correct Answer)
B. Mannitol fermentation
C. The catalase test
D. Hemolysis of sheep erythrocytes

Explanation: ***Detection of coagulase*** - The **coagulase enzyme** is a major virulence factor in *Staphylococcus aureus*, distinguishing it from less pathogenic staphylococci (coagulase-negative staphylococci). - Coagulase allows the organism to clot plasma, forming a fibrin layer that protects it from phagocytosis and immune detection. *Mannitol fermentation* - While *Staphylococcus aureus* can **ferment mannitol** (distinguishing it from some other staphylococci on mannitol salt agar), this characteristic is not directly linked to its virulence in the same critical way as coagulase. - Mannitol fermentation is a metabolic property, not a direct virulence factor that mediates host damage or evasion. *The catalase test* - The **catalase test** is used to differentiate staphylococci (catalase-positive) from streptococci (catalase-negative), but it does not differentiate between virulent and avirulent strains of staphylococci. - All staphylococci, both pathogenic and non-pathogenic, produce catalase to neutralize reactive oxygen species. *Hemolysis of sheep erythrocytes* - Many staphylococci (including *S. aureus*) produce **hemolysins**, which can lyse red blood cells. While a virulence factor, it is less specific and less critical than coagulase for determining overall virulence and pathogen identification. - Other bacteria also exhibit hemolysis, and some coagulase-negative staphylococci can also be hemolytic, making it a less definitive virulence marker for *Staphylococcus aureus* compared to coagulase.

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

Practice Questions

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