Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2461: Which of the following is a catalase-positive, beta-hemolytic staphylococcus?

A. S. epidermidis
B. S. saprophyticus
C. S. aureus (Correct Answer)
D. None of the above

Explanation: ***Correct Option: S. aureus*** - *Staphylococcus aureus* is **catalase-positive** and characteristically produces **beta-hemolysis** (complete hemolysis with clear zones) on blood agar - S. aureus produces several hemolysins including **alpha-toxin** (alpha-hemolysin), which causes complete lysis of red blood cells, resulting in the characteristic beta-hemolytic pattern - It is also **coagulase-positive**, a key differentiating factor from other *Staphylococcus* species - This is the most clinically significant staphylococcus species, causing skin infections, pneumonia, endocarditis, and toxin-mediated diseases *Incorrect Option: S. epidermidis* - *Staphylococcus epidermidis* is **catalase-positive** but is **gamma-hemolytic** (non-hemolytic) on blood agar - It is **coagulase-negative** and a common cause of foreign body and prosthetic device infections due to its ability to form **biofilms** - Part of normal skin flora *Incorrect Option: S. saprophyticus* - *Staphylococcus saprophyticus* is **catalase-positive** but is **gamma-hemolytic** (non-hemolytic) on blood agar - It is **coagulase-negative** and well-known as a cause of **urinary tract infections (UTIs)**, especially in young, sexually active women - Resistant to **novobiocin**, a key distinguishing characteristic from S. epidermidis *Incorrect Option: None of the above* - This is incorrect because S. aureus clearly meets both criteria: catalase-positive and beta-hemolytic

Question 2462: 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 2463: 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 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: 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 2468: 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 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: Which of the following is the most diagnostically significant characteristic of Campylobacter jejuni?

A. Grows at 42°C (Correct Answer)
B. Obligate aerobe
C. Oxidase negative
D. Non-motile

Explanation: ***Grows at 42°C*** - The ability to grow optimally at **42°C (thermophilic growth)** is a key discriminating characteristic for *Campylobacter jejuni*, differentiating it from other common enteric pathogens. - This property is exploited in selective media (e.g., Skirrow agar) incubated at this higher temperature to aid in its isolation from fecal samples. *Obligate aerobe* - *Campylobacter jejuni* is actually **microaerophilic**, meaning it requires reduced oxygen levels (typically 5% O2) for optimal growth, not completely aerobic conditions. - Obligate aerobes require atmospheric oxygen concentrations for survival, which is contrary to the growth requirements of *C. jejuni*. *Oxidase negative* - *Campylobacter jejuni* is **oxidase positive**, which means it produces the enzyme cytochrome c oxidase. - An oxidase negative result would rule out *Campylobacter jejuni* and is characteristic of other organisms like *Escherichia coli*. *Non-motile* - *Campylobacter jejuni* is **highly motile** due to its characteristic single polar flagellum, which contributes to its invasiveness in the intestinal mucosa. - Non-motility would be inconsistent with the typical microscopic appearance and pathogenic mechanisms of *C. jejuni*.

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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