Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2531: What mechanism does Chlamydia use to evade killing by the immune system?

A. Inhibiting phagolysosome fusion (Correct Answer)
B. Causing cell membrane perforation
C. Molecular mimicry in immune evasion
D. Producing immune-modulating factors

Explanation: ***Inhibiting phagolysosome fusion*** - *Chlamydia* **prevents the fusion of the phagosome** (containing the bacterium) with the lysosome, thus avoiding the degradative enzymes and low pH environment that would kill it. - This mechanism allows *Chlamydia* to **replicate intracellularly** within a protected vacuole called an **inclusion body**. - This is the **most direct mechanism** by which *Chlamydia* evades intracellular killing. *Molecular mimicry in immune evasion* - **Molecular mimicry** involves pathogens displaying antigens similar to host tissues, leading to **autoimmune damage** rather than evasion of killing. - While this may contribute to chronic complications of *Chlamydia* infection, it is not the primary mechanism *Chlamydia* employs to directly **evade intracellular destruction** by immune cells. *Causing cell membrane perforation* - **Cell membrane perforation** would typically lead to cell lysis and release of intracellular contents, which is counterproductive for an **obligate intracellular pathogen** like *Chlamydia*. - Instead, *Chlamydia* **maintains host cell integrity** to establish its replicative niche within the inclusion body. *Producing immune-modulating factors* - While *Chlamydia* does produce **immune-modulating factors** to suppress host immune responses, this is a **secondary mechanism** that complements its survival strategy. - The question specifically asks about evading **killing**, which is most directly achieved by **preventing phagolysosome fusion** and the resulting exposure to lysosomal enzymes. - Immune modulation affects the broader immune response but does not directly prevent the bacterium from being killed once inside a phagosome.

Question 2532: A 65-year-old diabetic presents with necrosis of the external auditory meatus and foul-smelling discharge. What is the probable organism associated with this condition?

A. Streptococcus pyogenes
B. E. coli
C. Pseudomonas aeruginosa (Correct Answer)
D. Haemophilus influenzae

Explanation: ***Pseudomonas aeruginosa*** - This clinical presentation, involving a **diabetic patient** with external auditory canal **necrosis** and **foul-smelling discharge**, is classic for **malignant otitis externa**. - **Pseudomonas aeruginosa** is the most common causative organism for malignant otitis externa, thriving in moist environments and common in immunocompromised individuals like diabetics. *Haemophilus influenzae* - While it can cause otitis media, **Haemophilus influenzae** is not typically associated with **malignant otitis externa** or the severe necrosis and foul-smelling discharge seen in this patient. - It primarily causes **respiratory tract infections**, including acute otitis media, and is less common in severe, necrotizing external ear infections. *Streptococcus pyogenes* - **Streptococcus pyogenes** is a common cause of pharyngitis and skin infections (e.g., cellulitis), but it is not typically linked to **malignant otitis externa**. - This organism does not usually cause the extensive tissue necrosis and foul-smelling discharge characteristic of this condition. *E. coli* - **E. coli** is a common pathogen in urinary tract infections and intra-abdominal infections, but it is rarely a cause of **otitis externa**, especially the severe, necrotizing form. - It would not typically produce the specific clinical picture of external auditory meatus necrosis and foul-smelling discharge in a diabetic.

Question 2533: Polyribosyl ribitol phosphate (PRP) antigen is present in the capsule of which H. influenzae serotype?

A. Serotype b (Correct Answer)
B. Serotype A
C. Serotype C
D. Serotype D

Explanation: ***Serotype b*** - **Polyribosyl ribitol phosphate (PRP)** is the unique capsular polysaccharide of *Haemophilus influenzae* **serotype b (Hib)**. - The presence of this antigen is crucial for diagnostic tests and vaccine development against Hib infections. *Serotype A* - *H. influenzae* serotype A possesses a **different capsular polysaccharide** structure than PRP. - Infections by serotype A are far **less common** and typically less severe than those caused by Hib. *Serotype C* - The capsule of *H. influenzae* serotype C is composed of a **distinct polysaccharide**, not PRP. - Serotype C infections are relatively **rare** and do not exhibit the same level of invasive disease as Hib. *Serotype D* - *H. influenzae* serotype D has a **capsular polysaccharide distinct** from PRP. - This serotype is infrequently associated with human disease and is **not targeted by the Hib vaccine**.

Question 2534: A forceps-delivered premature baby develops fever, abnormal behavior, and a bulging fontanelle 5 days after birth. What is the likely causative organism?

A. Staphylococcus aureus
B. Listeria monocytogenes (Correct Answer)
C. Neisseria meningitidis
D. Streptococcus pneumoniae

Explanation: ***Listeria monocytogenes*** - Among the options provided, *Listeria monocytogenes* is the most appropriate answer for **neonatal meningitis** in a **premature infant** at 5 days of life. - *Listeria* accounts for approximately **5-10% of neonatal meningitis** cases and is associated with **vertical transmission** during birth, particularly in premature infants. - The symptoms of fever, abnormal behavior, and bulging fontanelle are classic signs of **bacterial meningitis** in neonates. - **Note**: While *Listeria* is the correct answer here, **Group B Streptococcus (GBS)** and **E. coli** are actually the most common causes of early neonatal meningitis (first week of life), with GBS being the leading pathogen. However, these are not among the given options. *Staphylococcus aureus* - *S. aureus* is not a common cause of early neonatal meningitis acquired during birth. - It typically causes **skin and soft tissue infections**, **osteomyelitis**, or **catheter-related sepsis** in neonates. - Meningitis due to *S. aureus* usually occurs following **neurosurgical procedures**, **shunt infections**, or in the setting of bacteremia, not as primary birth-related infection. *Neisseria meningitidis* - *N. meningitidis* is **rare in the neonatal period** (first 28 days of life), particularly in the first week. - It becomes a more significant cause of meningitis in **older infants (>3 months), children, and adolescents**. - Typically associated with **respiratory transmission** and close contacts, not birth-related transmission. *Streptococcus pneumoniae* - *S. pneumoniae* is **uncommon in neonatal meningitis**, especially in the first week of life. - It becomes a leading cause of bacterial meningitis in **infants >2-3 months** and children. - Neonatal meningitis is predominantly caused by organisms acquired during birth: **GBS (most common), E. coli**, and *Listeria monocytogenes*.

Question 2535: Which of the following is an acid-fast organism?

A. All Actinomycetes
B. Mycobacterium (Correct Answer)
C. Streptococcus
D. Corynebacterium

Explanation: ***Mycobacterium*** - **Mycobacterium** species, such as *Mycobacterium tuberculosis*, are characterized by a **mycolic acid**-rich cell wall that retains carbolfuchsin stain even after decolorization with acid-alcohol, making them **acid-fast**. - This unique cell wall structure confers resistance to many disinfectants and antibiotics, and it is a key diagnostic feature in identifying these organisms. *All Actinomycetes* - While some **actinomycetes**, like *Nocardia*, can be partially acid-fast due to smaller amounts of mycolic acid, not **all actinomycetes** exhibit this property. - Genera such as *Actinomyces* are typically **non-acid-fast** and are distinguished by their filamentous, branching forms. *Streptococcus* - **Streptococcus** species are **Gram-positive** bacteria that lack **mycolic acid** in their cell walls and are therefore **not acid-fast**. - They are typically identified by Gram staining (appearing as purple cocci in chains) and biochemical tests. *Corynebacterium* - **Corynebacterium** species are **Gram-positive**, club-shaped bacilli that lack **mycolic acid** and are **not acid-fast**. - They are known for their characteristic "Chinese letter" or palisade arrangements and are typically stained with Gram stain or special diphtheria stains.

Question 2536: A patient presents with a history of unresponsive fever and cough. Chest X-ray reveals pneumonia. Sputum examination shows gram-positive, partially acid-fast bacteria with branching filaments that grows on sheep blood agar. What is the most likely etiologic agent?

A. Nocardia (Correct Answer)
B. Aspergillus
C. Pneumococci
D. Actinomyces

Explanation: **Nocardia** - *Nocardia* species are **gram-positive**, **partially acid-fast, branching filamentous bacteria** that can cause pneumonia, especially in immunocompromised individuals. - They grow well on standard laboratory media like **sheep blood agar**, and their **branching morphology** is a key diagnostic feature. *Actinomyces* - *Actinomyces* species are also **gram-positive, branching filamentous bacteria** but are **not acid-fast** and are typically **anaerobic**, not growing well on sheep blood agar in the presence of oxygen. - They are commonly associated with cervicofacial abscesses and draining sinuses, often described as having "sulfur granules." *Aspergillus* - *Aspergillus* is a **fungus**, characterized by **septate hyphae with acute angle branching**, and would not be described as gram-positive bacteria. - It is a common cause of pneumonia, particularly in immunocompromised patients, but the sputum microscopic description clearly points to a bacterial etiology. *Pneumococci* - *Streptococcus pneumoniae* (Pneumococci) are **gram-positive cocci** that typically appear in pairs (**diplococci**) or short chains, **not branching filamentous structures**. - They are a very common cause of bacterial pneumonia but do not exhibit partial acid-fastness or the filamentous morphology described.

Question 2537: Which of the following statements most accurately describes the characteristics of exotoxins?

A. Can provoke a strong immune response
B. Secreted by certain bacteria into the surrounding environment (Correct Answer)
C. Generally heat-stable and resistant to antibodies
D. A component of the outer membrane of gram-negative bacteria

Explanation: ***Secreted by certain bacteria into the surrounding environment*** - **Exotoxins** are proteins actively produced and **secreted** by living bacteria into their external environment, often causing specific localized or systemic effects. - This secretion mechanism distinguishes them from endotoxins, which are part of the bacterial cell structure. *Can provoke a strong immune response* - While exotoxins can provoke an immune response, this statement is not the *most accurate* defining characteristic, as severity of response varies. - Their primary characteristic is their nature as **secreted proteins** with specific toxic activities. *Generally heat-stable and resistant to antibodies* - **Exotoxins** are generally **heat-labile** (destroyed by heat) and are highly **antigenic**, meaning they readily stimulate antibody production (antitoxins). - This contrasts with **endotoxins**, which are heat-stable. *A component of the outer membrane of gram-negative bacteria* - This description applies specifically to **endotoxins** (lipopolysaccharide, LPS), which are integral parts of the **outer membrane of Gram-negative bacteria**. - **Exotoxins** are secreted proteins, not structural components of the bacterial cell wall.

Question 2538: All of the following statements about Staphylococcus aureus are true, except:

A. About 30% of general population is healthy nasal carriers.
B. Methicillin resistance is chromosomally mediated.
C. Epidermolysin and TSS toxin are superantigens.
D. Most common source of infection is cross infection from infected people. (Correct Answer)

Explanation: ***Incorrect: Most common source of infection is cross-infection from infected people.*** - The most common source of *Staphylococcus aureus* infection, particularly in community-acquired cases, is from the patient's **own endogenous flora**, especially from the **nasal passages or skin**. - While cross-infection in healthcare settings (nosocomial transmission) is a significant issue, it is NOT the primary or most common source for the majority of *S. aureus* infections overall. - **Endogenous infection from colonized sites** is the predominant mode of acquisition. *Correct: About 30% of the general population is healthy nasal carriers.* - This statement is accurate; approximately **20-40%** (with an average often cited as 30%) of healthy individuals carry *Staphylococcus aureus* asymptomatically in their **anterior nares**. - This asymptomatic carriage is a significant reservoir for both self-infection and transmission to others. *Correct: Epidermolysin and TSS toxin are superantigens.* - This statement is correct. **Toxic Shock Syndrome Toxin-1 (TSST-1)** and the **exfoliative toxins (Epidermolysins)** are both classified as **superantigens**. - Superantigens bypass normal antigen processing and directly bind to MHC class II molecules and T-cell receptors, causing **widespread T-cell activation** (up to 20% of T cells) and massive cytokine release, leading to systemic symptoms like fever, rash, and shock. *Correct: Methicillin resistance is chromosomally mediated.* - This statement is true. Methicillin resistance in *Staphylococcus aureus* (MRSA) is mediated by the ***mecA* gene**, which encodes for an altered penicillin-binding protein (**PBP2a** or **PBP2'**). - The *mecA* gene is located on a mobile genetic element called the **staphylococcal cassette chromosome *mec* (SCCmec)**, which integrates into the bacterial **chromosome** (hence chromosomally mediated). - PBP2a has low affinity for β-lactam antibiotics, allowing cell wall synthesis to continue despite antibiotic presence.

Question 2539: Which of the following statements about the lepromin test is true?

A. It is a diagnostic test
B. BCG vaccination may convert lepra reaction from negative to positive
C. It is an important aid to classify type of leprosy disease (Correct Answer)
D. It is negative in most children below 2 years of age

Explanation: ***It is an important aid to classify type of leprosy disease*** - The **lepromin test** assesses the host's cellular immune response to *Mycobacterium leprae* antigens, which is crucial for differentiating between **tuberculoid (positive)** and **lepromatous (negative)** forms of leprosy. - A positive lepromin test indicates a strong cell-mediated immune response, characteristic of the more contained and less infectious **tuberculoid leprosy**, while a negative test suggests a weak or absent response, typical of the widespread and highly infectious **lepromatous leprosy**. - The test is **not diagnostic** but helps in **classification and prognosis** of leprosy. *It is negative in most children below 2 years of age* - The lepromin test is typically **negative in young children** (usually below 2 years) because their cell-mediated immune system is still developing and may not have fully matured. - This negativity reflects **immunological immaturity** rather than disease status, and the response gradually develops with age and potential exposure to mycobacterial antigens. *It is a diagnostic test* - The lepromin test is **not a diagnostic test** for active leprosy infection; it indicates the **immunological response** of the host, not the presence of disease. - Diagnosis of leprosy relies on clinical features (skin lesions, nerve involvement) and detection of **acid-fast bacilli** in skin smears or biopsy specimens. *BCG vaccination may convert lepra reaction from negative to positive* - While BCG vaccination can provide cross-immunity against mycobacteria, the statement oversimplifies the relationship between BCG and lepromin reactivity. - The lepromin test uses specific *Mycobacterium leprae* antigens, and while some cross-reactivity may occur, BCG vaccination **does not reliably convert** lepromin negativity to positivity in a clinically predictable manner.

Question 2540: Endotoxins have all but one of the following properties?

A. Heat stable
B. Produced by gram positive bacteria (Correct Answer)
C. Produced by gram negative bacteria
D. Cannot be toxoided

Explanation: ***Produced by gram positive bacteria*** - **Endotoxins** are exclusively associated with the **outer membrane of gram-negative bacteria**, making this statement incorrect. - **Gram-positive bacteria** produce **exotoxins**, not endotoxins. *Produced by gram negative bacteria* - **Endotoxins are integral components of the outer membrane** of gram-negative bacteria, specifically their **lipopolysaccharide (LPS)** layer. - Upon bacterial cell lysis or growth, these endotoxins are released and can cause a potent immune response. *Heat stable* - **Endotoxins** are known for their **heat stability**, meaning they can withstand high temperatures (autoclaving for several hours) without losing their toxic activity. - This property makes them particularly challenging to eliminate from contaminated materials. *Cannot be toxoided* - **Endotoxins cannot be effectively toxoided** (inactivated to lose toxicity but retain immunogenicity) to produce vaccines. - This is a key difference from **exotoxins**, which can often be toxoided to create vaccines (e.g., diphtheria and tetanus toxoids).

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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