Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2441: Which of the following statements about Haemophilus influenzae is true?

A. It is not capsulated
B. Invasive strain causes severe diseases but is less common than non-invasive strains
C. Encapsulated strains are the most common cause of severe H. influenzae diseases (Correct Answer)
D. Typically grown on chocolate agar in CO2-enriched environment but not clinically relevant to severe diseases

Explanation: ***Encapsulated strains are the most common cause of severe H. influenzae diseases*** - **Encapsulated strains**, particularly **type b (Hib)**, are responsible for the majority of severe invasive H. influenzae infections including **meningitis**, **epiglottitis**, and **septicemia**. - While the Hib vaccine has dramatically reduced the incidence of type b disease, encapsulated strains remain the primary cause of severe H. influenzae infections when they occur. - Non-encapsulated (nontypeable) strains more commonly cause **mucosal infections** like otitis media and bronchitis, but rarely cause severe invasive disease. *It is not capsulated* - This is incorrect. H. influenzae exists in both **encapsulated** (types a-f) and **non-encapsulated (nontypeable)** forms. - The **encapsulated strains**, especially **type b**, are most virulent and cause severe invasive diseases. *Typically grown on chocolate agar in CO2-enriched environment but not clinically relevant to severe diseases* - While the growth requirements are correct (H. influenzae requires **X factor (hemin)** and **V factor (NAD+)** found in chocolate agar), the second part is completely false. - H. influenzae is highly clinically relevant and causes severe diseases including meningitis and epiglottitis. *Invasive strain causes severe diseases but is less common than non-invasive strains* - While this statement is epidemiologically true (nontypeable strains are more prevalent than encapsulated strains), it doesn't address which type causes severe diseases most commonly. - The question asks which statement is true, not which strain type is more prevalent in the general population.

Question 2442: What is a key distinguishing feature of meningococci compared to gonococci?

A. Are intracellular pathogens
B. Are catalase positive
C. Ferment maltose (Correct Answer)
D. Possess a capsule

Explanation: ***Ferment maltose*** - *Neisseria meningitidis* ferments both **glucose and maltose**, which is a key biochemical characteristic used for its identification. - *Neisseria gonorrhoeae* only ferments **glucose**, differentiating it from meningococci. *Are intracellular pathogens* - Both **meningococci** (*Neisseria meningitidis*) and **gonococci** (*Neisseria gonorrhoeae*) are facultative intracellular pathogens, meaning they can survive and replicate within host cells. - Therefore, this feature does not distinguish between the two. *Are catalase positive* - Both *Neisseria meningitidis* and *Neisseria gonorrhoeae* are **catalase-positive**, meaning they produce the enzyme catalase. - This characteristic is common to both and thus cannot be used to differentiate them. *Possess a capsule* - While *Neisseria meningitidis* possesses a **polysaccharide capsule**, which is a major virulence factor, *Neisseria gonorrhoeae* typically **lacks a capsule**. - However, the ability to ferment maltose is a more direct and commonly used biochemical distinguishing feature in laboratory settings.

Question 2443: Which of the following statements about B. quintana is false?

A. Tick is the vector (Correct Answer)
B. Recurrence is common
C. Causes trench fever
D. Not detected by Weil-Felix reaction

Explanation: ***Tick is the vector*** - This statement is **false** because *Bartonella quintana* (B. quintana) is primarily transmitted by the **human body louse**, not ticks. - The disease it causes, **trench fever**, was historically associated with crowded and unsanitary conditions where lice thrive. *Causes trench fever* - *B. quintana* is the causative agent of **trench fever**, a louse-borne illness characterized by recurrent febrile episodes. - This symptom was highly prevalent among soldiers in the trenches during World War I. *Recurrence is common* - **Relapsing fever** (recurrence) is a characteristic feature of trench fever, often occurring several times over weeks or months. - This recurrence is due to the **intracellular survival of *Bartonella* species** within erythrocytes and endothelial cells, making clearance difficult. *Not detected by Weil-Felix reaction* - The **Weil-Felix reaction** is a serological test used to detect certain rickettsial infections, which cross-react with *Proteus* antigens. - *Bartonella quintana* infections **do not typically produce antibodies** that cross-react with *Proteus* antigens; therefore, the Weil-Felix test would not be positive for trench fever.

Question 2444: What is the most common bacterial cause of multiple sinus tracts resulting from an infection of the great toe?

A. Tuberculosis
B. Pseudomonas aeruginosa
C. Staphylococcus aureus
D. Actinomyces (Correct Answer)

Explanation: ***Actinomyces*** - **Actinomycosis** is known for causing **chronic suppurative infections** with **multiple draining sinus tracts** and characteristic **sulfur granules**. - The causative organism is **Actinomyces israelii**, an anaerobic, gram-positive, filamentous bacterium. - While rare, it can affect bones, leading to **osteomyelitis**, especially in the feet, presenting with these multifocal sinus tracts. *Tuberculosis* - **Tuberculosis of bone** is typically a **monoarticular affection**, often affecting larger joints, and does not commonly present with multiple draining sinus tracts like actinomycosis. - It is more common in the spine (**Pott's disease**) and large weight-bearing joints, rather than solely the great toe with multiple sinuses. *Staphylococcus aureus* - **Staphylococcus aureus** is the most common cause of **acute osteomyelitis** and can lead to draining sinuses. - However, it typically causes **single or localized sinus tracts** rather than the extensive, multiple sinus formation associated with actinomycosis. *Pseudomonas aeruginosa* - **Pseudomonas aeruginosa** is often associated with **osteomyelitis following puncture wounds** through footwear. - While it can cause chronic infections, it is not primarily known for forming the characteristic **multiple, long-standing sinus tracts** seen with Actinomyces.

Question 2445: Which is the PRIMARY hemolysin of Streptococcus pyogenes?

A. Hyaluronidase
B. Streptolysin S (Correct Answer)
C. Streptodornase
D. Streptolysin O

Explanation: ***Streptolysin S*** - This is the **PRIMARY hemolysin** of *Streptococcus pyogenes*, responsible for the characteristic **beta-hemolysis** visible on blood agar plates. - It is **oxygen-stable** and **non-immunogenic**, causing the surface hemolysis that defines Group A Streptococcus phenotypically. *Hyaluronidase* - This enzyme breaks down **hyaluronic acid**, a component of the extracellular matrix in host tissues. - It facilitates **bacterial spread through tissues** but is **not a hemolysin** and does not directly lyse red blood cells. *Streptolysin O* - This is a **thiol-activated cytolysin** that is **oxygen-labile** and primarily important for its **immunogenic properties**. - While it can cause hemolysis, its main clinical significance lies in the **anti-streptolysin O (ASO) titer** used for diagnosing recent infections. *Streptodornase* - Also known as **DNase**, this enzyme digests **DNA**, reducing the viscosity of pus and facilitating bacterial dissemination. - It is **not a hemolysin** but serves as an important virulence factor for tissue invasion.

Question 2446: What are the characteristics of exotoxins produced by bacteria?

A. Lipid-polysaccharide complex
B. Protein compound (Correct Answer)
C. Lipoprotein
D. None of the options

Explanation: ***Protein compound*** - Exotoxins are **soluble protein compounds** secreted by living bacteria [1]. - Their protein nature allows them to be highly **potent** and specific in their actions, targeting host cell processes [1], [2]. *Lipid-polysaccharide complex* - This describes **endotoxins**, which are components of the **outer membrane of Gram-negative bacteria**, specifically **lipopolysaccharide (LPS)** [1]. - Endotoxins are released primarily upon cell lysis and are not secreted proteins [1]. *Lipoprotein* - Lipoproteins are complexes of **lipids and proteins** that transport fats in the bloodstream, or they can be structural components of bacterial membranes. - While bacteria do contain and produce lipoproteins, these are not the primary structural characteristic of exotoxins. *None of the options* - This option is incorrect as "Protein compound" accurately describes the chemical nature of exotoxins.

Question 2447: What is the primary virulence factor of Neisseria gonorrhoeae?

A. All of the above are incorrect
B. Pili (fimbriae) (Correct Answer)
C. Endotoxin (lipooligosaccharide)
D. Exotoxin

Explanation: ***Pili (fimbriae)*** - **Pili are the PRIMARY virulence factor** of *Neisseria gonorrhoeae*, essential for **initial attachment and colonization** of urogenital mucosa - Enable bacteria to **adhere to non-ciliated epithelial cells**, resisting mechanical clearance by urination and secretions - Undergo **antigenic variation** to evade host immune responses - Without pili, *N. gonorrhoeae* cannot establish infection *Endotoxin (lipooligosaccharide)* - *N. gonorrhoeae* possesses **LOS (lipooligosaccharide)** which causes inflammation and tissue damage - While important for pathogenesis, it is a **secondary virulence factor** - LOS contributes to symptoms but cannot cause infection without prior colonization via pili *Exotoxin* - *N. gonorrhoeae* does **not produce significant exotoxins** - Pathogenicity is mediated through **adherence factors (pili), LOS, and surface proteins** rather than secreted protein toxins - This is not a mechanism of gonococcal virulence *All of the above are incorrect* - This statement is false as **pili (fimbriae)** are definitively the primary virulence factor for *N. gonorrhoeae*

Question 2448: Which of the following is the MOST COMMON clinical manifestation of Neisseria gonorrhoeae infection in males?

A. Highly sensitive to Penicillin
B. Exclusive human pathogen
C. Some strains may cause disseminated disease
D. Acute urethritis is MC manifestation in males (Correct Answer)

Explanation: ***Acute urethritis is MC manifestation in males*** - For males, **acute urethritis** is the most common and often the first noticeable manifestation of *Neisseria gonorrhoeae* infection, typically presenting 2-7 days after exposure. - Symptoms include **purulent urethral discharge** and **dysuria**, which are usually severe enough to prompt patients to seek medical attention. - This early presentation allows for **prompt diagnosis and treatment**, which is crucial in preventing complications and further transmission. *Highly sensitive to Penicillin* - This statement is **incorrect** because *Neisseria gonorrhoeae* has developed significant resistance to penicillin over time. - Penicillinase-producing strains and chromosomally mediated resistance have rendered penicillin ineffective as primary treatment. - Current treatment guidelines recommend **dual therapy with ceftriaxone and azithromycin** due to widespread antibiotic resistance patterns. *Exclusive human pathogen* - While *Neisseria gonorrhoeae* is indeed an obligate human pathogen with no animal reservoir, this characteristic does not represent the most common clinical manifestation. - This is a **microbiological characteristic** rather than a clinical presentation. *Some strains may cause disseminated disease* - **Disseminated gonococcal infection (DGI)** occurs in only **0.5-3%** of untreated infections, making it relatively uncommon. - DGI can manifest as **arthritis-dermatitis syndrome**, tenosynovitis, or rarely endocarditis or meningitis. - While serious, DGI is **far less common** than localized urogenital infections like acute urethritis.

Question 2449: Which of the following organisms is classified as a strongly acid-fast bacterium?

A. Mycobacteria (Correct Answer)
B. Nocardia
C. Cryptosporidia
D. Mycoplasma

Explanation: ***Mycobacteria*** - **Mycobacteria** are characterized by their **mycolic acid-rich cell wall**, which makes them **strongly acid-fast** and resistant to decolorization by strong acid-alcohol (3% HCl in ethanol) after staining with carbol fuchsin. - This property is crucial for their identification in clinical samples, particularly for diagnosing diseases like **tuberculosis** (caused by *Mycobacterium tuberculosis*) and **leprosy** (caused by *Mycobacterium leprae*). - The Ziehl-Neelsen stain is the standard method for identifying strongly acid-fast bacteria. *Nocardia* - **Nocardia** are **Gram-positive, aerobic bacteria** that exhibit **partial acid-fastness**, meaning they resist decolorization with weaker acid solutions (1% H₂SO₄) but NOT strong acid-alcohol. - They are known to cause opportunistic infections, particularly in immunocompromised individuals, leading to pulmonary or systemic disease. - Modified acid-fast staining with weaker acid is used to differentiate them from strongly acid-fast Mycobacteria. *Cryptosporidia* - **Cryptosporidia** are **protozoan parasites**, not bacteria, though they do show acid-fast properties in their oocysts due to unique cell wall structure. - They are commonly associated with **gastrointestinal infections** and are identified using specific staining techniques for parasites. *Mycoplasma* - **Mycoplasma** are unique bacteria due to their complete **lack of a cell wall**, which makes them pleomorphic and resistant to antibiotics that target cell wall synthesis. - They are **not acid-fast** and are typically identified through specialized culture methods or molecular tests.

Question 2450: 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*.

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

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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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