Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2111: A bacterial culture shows 'tumbling motility' pattern. Which microscopic finding would confirm Listeria monocytogenes?

A. Gram-variable pleomorphic rods
B. Gram-positive coccobacilli (Correct Answer)
C. Gram-negative diplococci
D. Acid-fast bacilli

Explanation: ***Gram-positive coccobacilli*** - *Listeria monocytogenes* classically presents as **Gram-positive coccobacilli** (short rods) and exhibits a characteristic **"tumbling motility"** at **room temperature (20-25°C)** due to peritrichous flagella. - This combination of tumbling motility and Gram-positive coccobacilli morphology is highly characteristic and aids in confirming *Listeria* identification. - The organism shows this distinctive end-over-end tumbling movement in wet mount or hanging drop preparations. *Gram-variable pleomorphic rods* - **Gram-variable** and **pleomorphic rods** describe organisms with inconsistent Gram stain reactions and variable shapes, which does not fit the typical morphology of *Listeria monocytogenes*. - *Listeria* consistently stains Gram-positive and appears as short rods or coccobacilli, not pleomorphic. *Gram-negative diplococci* - **Gram-negative diplococci** are characteristic of *Neisseria* species (*N. meningitidis*, *N. gonorrhoeae*), which have distinctly different morphology and do not exhibit tumbling motility. - This finding would completely rule out *Listeria monocytogenes*. *Acid-fast bacilli* - **Acid-fast bacilli** (AFB) are identified using the Ziehl-Neelsen stain and are characteristic of mycobacteria (e.g., *Mycobacterium tuberculosis*), which have a waxy cell wall rich in mycolic acids. - Mycobacteria are non-motile and morphologically distinct from *Listeria monocytogenes*.

Question 2112: A bacterial culture shows small colonies with narrow zones of β-hemolysis on blood agar and exhibits tumbling motility at 25°C. Which biochemical test would confirm Listeria monocytogenes?

A. CAMP test positivity (Correct Answer)
B. Catalase positivity
C. Oxidase positivity
D. Urease positivity

Explanation: ***CAMP test positivity*** - A positive **CAMP test** result, characterized by enhanced hemolysis when *Listeria monocytogenes* is cultured near *Staphylococcus aureus* on blood agar, is a **key confirmatory test** for identifying this organism. - The test demonstrates the synergistic hemolytic activity between the CAMP factor of *Listeria* and the β-hemolysin of *S. aureus*, producing an arrowhead-shaped zone of enhanced hemolysis. - This test, combined with the characteristic **tumbling motility** at room temperature and **narrow β-hemolysis**, provides strong confirmation of *Listeria monocytogenes*. *Catalase positivity* - While *Listeria monocytogenes* is **catalase-positive**, this characteristic is **not specific** as many bacteria including *Staphylococcus* species, *Bacillus* species, and other gram-positive organisms are also catalase-positive. - Catalase testing alone cannot differentiate *Listeria* from other catalase-positive bacteria. *Oxidase positivity* - *Listeria monocytogenes* is **oxidase-negative**, meaning it does not produce cytochrome c oxidase. - This is a useful negative characteristic but **oxidase negativity alone is not confirmatory** as many bacteria are oxidase-negative. *Urease positivity* - *Listeria monocytogenes* is **urease-negative**, meaning it does not produce the enzyme urease which hydrolyzes urea into ammonia and carbon dioxide. - Urease-positive bacteria include *Proteus* species, *Helicobacter pylori*, and *Yersinia enterocolitica*, making this test useful for ruling out these organisms but not for confirming *Listeria*.

Question 2113: A fluid culture shows 'spoked wheel' colony appearance. Which microscopic finding would confirm Actinomyces species?

A. Pleomorphic rods
B. Branching filaments (Correct Answer)
C. Diplococci
D. Curved bacilli

Explanation: ***Branching filaments*** - **Actinomyces** species are characterized by their **gram-positive, branching filamentous** morphology on microscopy. - The "spoked wheel" (or "molar tooth") colony appearance on culture combined with **branching filaments on Gram stain** is diagnostic for Actinomyces. - These are **anaerobic or facultatively anaerobic** bacteria that can form intricate structures, often visible as "sulfur granules" in clinical specimens, which are microcolonies of the organism. *Pleomorphic rods* - While Actinomyces can appear somewhat pleomorphic, this description alone is **not specific enough** to distinguish it from many other bacterial species. - The key distinguishing feature for Actinomyces is the **branching filamentous** nature, not just pleomorphic rods. *Diplococci* - **Diplococci** refer to cocci that occur in pairs (e.g., *Neisseria* species, *Streptococcus pneumoniae*). - This morphology is distinctly different from the **rod-shaped, branching filamentous structure** of Actinomyces. *Curved bacilli* - **Curved bacilli** are characteristic of bacteria like *Vibrio cholerae* or *Campylobacter jejuni*. - This morphology does not fit the typical appearance of Actinomyces, which are known for their **branching filaments**, not curved shapes.

Question 2114: A bacterial culture shows 'medusa head' colonies. Which biochemical finding is characteristic of Bacillus cereus?

A. Optochin sensitive
B. Catalase positive (Correct Answer)
C. CAMP test positive
D. Oxidase positive

Explanation: ***Catalase positive*** - *Bacillus cereus* is a **catalase-positive**, gram-positive, spore-forming rod. Catalase production is a characteristic shared by many *Bacillus* species. [2] - The **"medusa head" colonies** are a distinctive morphological feature of *Bacillus anthracis* (a close relative) and also seen with *B. cereus*, indicating a large, spreading, rough colony appearance. [1] *Optochin sensitive* - **Optochin sensitivity** is a key characteristic used primarily to differentiate *Streptococcus pneumoniae* from other alpha-hemolytic streptococci. - *Bacillus cereus* is typically **resistant to optochin**. *CAMP test positive* - The **CAMP test** is used to identify **Group B Streptococcus** (*Streptococcus agalactiae*) by detecting an enhanced zone of hemolysis when grown next to *Staphylococcus aureus*. [2] - This test is **not applicable or positive for *Bacillus cereus***. *Oxidase positive* - Being **oxidase positive** indicates the presence of cytochrome c oxidase, an enzyme found in some bacteria. - *Bacillus cereus* is typically **oxidase negative**.

Question 2115: A bacterial culture shows flat, irregular colonies with metallic sheen. Which additional finding would confirm Pseudomonas aeruginosa?

A. Black pigment
B. Beta-hemolysis
C. Yellow pigment
D. Blue-green pigment with grape odor (Correct Answer)

Explanation: ***Blue-green pigment with grape odor*** - *Pseudomonas aeruginosa* characteristically produces **pyocyanin**, a blue-green pigment, and often has a distinct **grape-like odor** due to aminoacetophenone. - The combination of **flat, irregular colonies** with a metallic sheen and this specific pigment and odor is highly indicative of *P. aeruginosa*. *Black pigment* - **Black pigmentation** is typically associated with other bacteria, such as some *Proteus* species or certain anaerobic bacteria like *Prevotella intermedia*. - It is not a characteristic feature used to identify *Pseudomonas aeruginosa*. *Beta-hemolysis* - While *Pseudomonas aeruginosa* can be **beta-hemolytic** on blood agar, this finding is not specific enough to confirm its identity as many other bacteria also exhibit beta-hemolysis. - Other features like pigment production and odor are more unique and definitive for *P. aeruginosa*. *Yellow pigment* - **Yellow pigmentation** is characteristic of various bacteria, including some *Staphylococcus* species (e.g., *Staphylococcus aureus* often produces a golden-yellow pigment) and *Micrococcus* species. - This is not a typical characteristic of *Pseudomonas aeruginosa* and would not help confirm its identification.

Question 2116: In a Gram stain preparation of CSF, which organism appears as pairs of kidney-shaped diplococci with adjacent flattened sides?

A. Neisseria meningitidis (Correct Answer)
B. Streptococcus pneumoniae
C. Moraxella catarrhalis
D. Staphylococcus aureus

Explanation: ***Neisseria meningitidis*** - This description is classic for **Gram-negative diplococci** of *Neisseria meningitidis*, often appearing in pairs with adjacent flattened sides. - Its presence in CSF is a strong indicator of **bacterial meningitis**, especially given the characteristic morphology. *Streptococcus pneumoniae* - This bacterium also appears as **Gram-positive diplococci**, but they are typically **lancet-shaped** (pointed ends), not kidney-shaped with flattened sides. - While a common cause of meningitis, its distinctive morphology differentiates it from *Neisseria*. *Moraxella catarrhalis* - While *Moraxella catarrhalis* is a **Gram-negative diplococcus**, it is typically found in the respiratory tract and rarely causes **meningitis**; when it does, it usually presents as spherical diplococci. - Its morphology is generally not described as kidney-shaped with flattened sides in the context of CSF. *Staphylococcus aureus* - *Staphylococcus aureus* is a **Gram-positive coccus** that forms **clusters resembling grapes** rather than diplococci. - Its morphology is distinctly different from the kidney-shaped diplococci described.

Question 2117: Sporulation occurs in which phase of bacterial growth curve:

A. Decline phase
B. Stationary phase (Correct Answer)
C. Lag phase
D. Log phase

Explanation: ***Stationary phase*** - When **nutrients become limited** and waste products accumulate, bacteria enter the stationary phase, triggering **sporulation** in spore-forming species as a survival mechanism. - In this phase, the rate of **bacterial growth equals the rate of bacterial death**, leading to a plateau in population size. *Decline phase* - The decline phase is characterized by a **net decrease in viable cells** due to continued depletion of nutrients and high accumulation of toxic waste products. - While cells are certainly stressed, sporulation typically occurs *before* this phase, as a preventative measure to survive impending harsh conditions. *Lag phase* - During the lag phase, bacteria are **adapting to new environmental conditions** and synthesizing necessary enzymes and components. - There is no increase in cell number during this phase, and they are preparing for growth, not entering a survival state like sporulation. *Log phase* - The log phase (or exponential phase) is characterized by **rapid binary fission** and exponential increase in bacterial numbers due to optimal growth conditions and abundant nutrients. - Cells are actively dividing; sporulation, which halts active division to form a dormant spore, would be counterproductive at this stage.

Question 2118: Whipple's disease is caused by:

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

Explanation: **Bacteria** - Whipple's disease is caused by the bacterium **_Tropheryma whipplei_**, a rod-shaped, gram-positive actinobacterium. - This bacterium causes a chronic, systemic infection affecting various organ systems, most commonly the gastrointestinal tract. *Virus* - Viruses are intracellular parasites that reproduce inside living cells, and they are not the causative agent of Whipple's disease. - While viral infections can cause gastrointestinal symptoms, they do not lead to the distinct histopathological findings of Whipple's disease, such as **PAS-positive macrophages**. *Helminths* - Helminths are parasitic worms (e.g., tapeworms, roundworms) that can cause intestinal and systemic infections. - These organisms are multicellular eukaryotes and are distinct from the bacterial etiology of Whipple's disease. *Protozoa* - Protozoa are single-celled eukaryotic organisms that can cause a range of infections, often gastrointestinal (e.g., Giardia, Entamoeba). - While parasitic infections can cause malabsorption, the specific **PAS-positive macrophage infiltrates** seen in Whipple's disease are characteristic of bacterial infection, not protozoan.

Question 2119: What is the causative agent for malignant otitis externa?

A. Streptococcus
B. Staphylococcus aureus
C. Influenza
D. Pseudomonas (Correct Answer)

Explanation: ***Pseudomonas*** - **Malignant otitis externa** is a severe, rapidly progressive infection of the external auditory canal and skull base, primarily caused by **Pseudomonas aeruginosa**. - This opportunistic bacterium thrives in moist environments and commonly affects immunocompromised individuals, such as diabetics or the elderly. *Streptococcus* - While various *Streptococcus* species can cause common ear infections (e.g., **otitis media**), they are not the primary causative agent for malignant otitis externa. - Infections by *Streptococcus* typically present differently and affect other parts of the ear or body more commonly. *Staphylococcus aureus* - **Staphylococcus aureus** is a common cause of skin infections, including localized forms of otitis externa (e.g., furuncles), but it is rarely implicated in the aggressive, invasive form known as malignant otitis externa. - When *Staphylococcus aureus* is involved, it often presents differently and is less likely to cause skull base osteomyelitis. *Influenza* - **Influenza** is a viral infection primarily affecting the **respiratory system** and is not a bacterial pathogen responsible for otitis externa, malignant or otherwise. - Viral infections like influenza can predispose individuals to secondary bacterial infections, but they are not directly causative of this condition.

Question 2120: Which of the following is the most likely cause in a case of granuloma with positive AFB?

A. Syphilis
B. Cat scratch disease
C. Leprosy
D. Tuberculosis (Correct Answer)

Explanation: ***Tuberculosis*** - **Mycobacterium tuberculosis** is the classic cause of **granulomatous inflammation** with **positive AFB** staining in histopathology. - TB is the most common mycobacterial infection worldwide, and **AFB staining** is routinely positive in tissue specimens, making it the standard association for granuloma + AFB. *Leprosy* - While **Mycobacterium leprae** is acid-fast, **AFB positivity** is variable and often requires special techniques for demonstration. - Leprosy is much less common globally, and AFB demonstration in routine histopathology is less reliable compared to tuberculosis. *Syphilis* - Syphilis is caused by **Treponema pallidum**, a **spirochete** that is not acid-fast. - Although syphilis can cause granulomatous inflammation (**gummas**), **AFB staining** would be negative. *Cat scratch disease* - Cat scratch disease is caused by **Bartonella henselae**, a **Gram-negative bacterium**. - While it causes **granulomatous lymphadenitis**, **Bartonella** is not acid-fast, so **AFB** would be negative.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

Practice Questions

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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