Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2511: Which of the following is a motile member of the Enterobacteriaceae family?

A. Shigella
B. Morganella (Correct Answer)
C. Yersinia
D. Klebsiella

Explanation: ***Morganella*** - *Morganella morganii* possesses **peritrichous flagella** and is motile at **25°C** (room temperature), which is the standard temperature for motility testing. - At 37°C (body temperature), motility may be reduced or absent, but in standard microbiology classification, Morganella is considered a **motile organism**. - This distinguishes it from consistently non-motile genera like Shigella and Klebsiella. *Shigella* - All species within the genus *Shigella* are inherently **non-motile** at all temperatures. - Complete absence of flagella is a fundamental differentiating feature from other Enterobacteriaceae. - This lack of motility is one of the key characteristics used in laboratory identification. *Klebsiella* - Members of the *Klebsiella* genus, notably *Klebsiella pneumoniae*, are **non-motile** and possess a prominent polysaccharide capsule. - Their immobility and large capsule are important identifying characteristics in laboratory settings. - The absence of flagella is consistent across all species. *Yersinia* - *Yersinia enterocolitica* exhibits **temperature-dependent motility** (motile at 25°C, non-motile at 37°C). - However, *Yersinia pestis*, the most clinically significant species causing plague, is **non-motile at all temperatures**. - In standard teaching, Yersinia is often represented by Y. pestis and characterized as non-motile, making Morganella the better answer for a consistently recognized motile member.

Question 2512: Cholera is caused by:

A. V.cholerae O1
B. V.cholerae O1 and O139 (Correct Answer)
C. NAG vibrios
D. V.cholerae O139

Explanation: ***V.cholerae O1 and O139*** - Both serogroups, **O1** and **O139**, are known to cause epidemic and pandemic cholera. - They produce the **cholera toxin**, leading to the characteristic severe watery diarrhea. *V.cholerae O1* - While **V. cholerae O1** is historically the most common cause of cholera, it is not the *only* serogroup responsible for the disease globally. - Limiting the definition to O1 would exclude outbreaks caused by the O139 serogroup. *V. cholerae O139* - **V. cholerae O139** emerged as a significant cause of cholera in Asia in the early 1990s and is capable of causing large outbreaks. - However, stating only O139 would omit the more widespread and historically significant O1 serogroup responsible for most cholera cases. *NAG vibrios* - **NAG vibrios** (Non-agglutinable vibrios), also known as non-O1/non-O139 *Vibrio cholerae*, can cause sporadic cases of diarrheal disease. - They typically cause a milder illness and are not commonly associated with the severe epidemic or pandemic cholera caused by O1 and O139 serogroups.

Question 2513: Which anticoagulant is commonly used for bacteriological culture?

A. Heparin
B. Citrate (Correct Answer)
C. EDTA
D. Mechanical defibrination

Explanation: ***Citrate*** - **Sodium citrate** is used as an anticoagulant in some **bacteriological culture systems** as it binds to **calcium ions**, preventing blood coagulation without directly killing bacteria. - However, citrate has limitations: it can **inhibit growth of fastidious organisms** and may affect certain bacterial metabolic processes. - **Note:** Sodium polyanethol sulfonate (SPS) is the **gold standard anticoagulant** for blood cultures in modern practice, but among the listed options, citrate is preferred over the alternatives. *Heparin* - **Heparin** acts by activating **antithrombin III**, which inactivates thrombin and factor Xa. - While effective as an anticoagulant, **heparin can inhibit growth of certain bacteria** (especially Gram-positive cocci) and some fungi, and may interfere with certain laboratory tests. - However, it is occasionally used when other anticoagulants are contraindicated. *EDTA* - **EDTA (ethylenediaminetetraacetic acid)** strongly chelates **calcium and magnesium ions**, effectively preventing coagulation. - **EDTA is toxic to many bacteria** and interferes with bacterial growth, making it **unsuitable for bacteriological cultures**. - It is primarily used for hematology specimens, not cultures. *Mechanical defibrination* - **Mechanical defibrination** physically removes fibrin from blood through agitation, preventing clot formation. - This method is **labor-intensive**, can cause **hemolysis**, and may damage bacterial cells, making it impractical for routine bacteriological cultures.

Question 2514: Exfoliative toxin A, which causes generalized desquamation, is secreted by which of the following?

A. Staphylococci (Correct Answer)
B. Streptococcus
C. Pseudomonas aeruginosa
D. Streptococcus pneumoniae

Explanation: ***Staphylococci*** - **Exfoliative toxins**, specifically ETA and ETB, are produced by *Staphylococcus aureus* and are responsible for **Staphylococcal scalded skin syndrome (SSSS)**, which involves generalized desquamation. - These toxins are **serine proteases** that cleave **desmoglein 1**, a protein essential for cell adhesion in the superficial epidermis. *Streptococcus* - While various *Streptococcus* species produce toxins (e.g., streptolysins, erythrogenic toxins), they are not associated with producing **exfoliative toxins** causing generalized desquamation. - *Streptococcus pyogenes* can cause diseases like **scarlet fever**, which involves a rash and subsequent peeling, but this is distinct from the mechanism of staphylococcal exfoliative toxins. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* produces a range of virulence factors like **exotoxin A**, **elastase**, and **pyocyanin**, which contribute to its pathogenicity in various infections. - However, it does not produce **exfoliative toxins** that cause widespread desquamation of the skin in the manner of *S. aureus*. *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is primarily known for causing **pneumonia**, **otitis media**, and **meningitis**. - Its major virulence factors include its **polysaccharide capsule** and **pneumolysin**, but it does not produce exfoliative toxins.

Question 2515: Which bacterial species is commonly found on the skin?

A. Lactobacillus
B. Streptococcus pyogenes
C. Bacteroides fragilis
D. Cutibacterium acnes (Correct Answer)

Explanation: ***Cutibacterium acnes*** - **_Cutibacterium acnes_** (formerly **_Propionibacterium acnes_**) is a common commensal bacterium found in the sebaceous glands of the skin. - It plays a role in the development of **acne vulgaris** due to its presence in follicles and its ability to produce inflammatory mediators. *Lactobacillus* - **_Lactobacillus_** species are typically found in the **gastrointestinal tract** and **female genital tract**, where they contribute to maintaining a healthy microbial balance. - They are not a primary component of the normal skin flora. *Streptococcus pyogenes* - **_Streptococcus pyogenes_** is a pathogenic bacterium known for causing various infections, including **strep throat**, **scarlet fever**, and **skin infections** (e.g., impetigo, erysipelas). - While it can cause skin infections, it is not considered a normal, common inhabitant of healthy skin. *Bacteroides fragilis* - **_Bacteroides fragilis_** is a prominent **anaerobic bacterium** found in the **human gut** and is a major component of the fecal microbiota. - It is rarely found on the skin unless there is a breach in the integument or contamination from intestinal sources.

Question 2516: Which of the following bacteria is microaerophilic?

A. Campylobacter (Correct Answer)
B. Pseudomonas
C. Salmonella
D. Vibrio cholerae

Explanation: ***Campylobacter*** - **Campylobacter jejuni** is a classic example of a microaerophilic bacterium, thriving in environments with **reduced oxygen (5-10% O2)** and **increased CO2 (5-10%)**. - This specific atmospheric requirement is crucial for its **growth** and **virulence**, often leading to gastroenteritis. *Vibrio cholerae* - **Vibrio cholerae** is a **facultative anaerobe**, meaning it can grow in both the presence and absence of oxygen. - It does not require low oxygen environments; optimal growth occurs **aerobically**. *Pseudomonas* - **Pseudomonas aeruginosa** is an **obligate aerobe**; it requires oxygen for respiration and growth. - It uses oxygen as the **final electron acceptor** in its electron transport chain. *Salmonella* - **Salmonella enterica** is a **facultative anaerobe**, capable of switching between aerobic and anaerobic respiration. - It can grow in the presence of oxygen, as well as in **anaerobic conditions** by fermenting sugars.

Question 2517: Which component of Streptococcus pyogenes is known to mimic human synovial tissue and has cross-reactivity with it?

A. Capsular hyaluronic acid (Correct Answer)
B. Cell proteins
C. Group A carbohydrate antigens
D. Peptidoglycan

Explanation: ***Capsular hyaluronic acid*** - The **hyaluronic acid capsule** of *Streptococcus pyogenes* is chemically identical to hyaluronic acid found in human connective tissues, including **synovial tissue**. - This molecular mimicry helps the bacteria evade the immune system and is implicated in the pathogenesis of **rheumatic fever** and post-streptococcal arthritis by eliciting an autoimmune response. *Cell proteins* - **M protein** is a significant cell surface protein of *Streptococcus pyogenes* known for its antiphagocytic properties and virulence. - While M protein antibodies can cross-react with human tissues (e.g., cardiac myosin in **rheumatic heart disease**), its mimicry is primarily with heart tissue, not directly synovial tissue. *Group A carbohydrate antigens* - The **Group A carbohydrate** is a major component of the cell wall of *Streptococcus pyogenes* and is used for Lancefield classification. - Antibodies against this carbohydrate are common but are not principally associated with molecular mimicry of synovial tissue; the mimicry relates more to certain heart valve components. *Peptidoglycan* - **Peptidoglycan** is a fundamental component of the bacterial cell wall in most bacteria, including *Streptococcus pyogenes*. - While peptidoglycan can induce inflammation, it does not share the specific molecular mimicry with human synovial tissue that hyaluronic acid does.

Question 2518: Which of the following organisms is considered the classic example of polysaccharide capsular resistance to phagocytosis?

A. Neisseria meningitidis
B. Streptococcus pneumoniae (Correct Answer)
C. Klebsiella pneumoniae
D. Cryptococcus neoformans

Explanation: ***Streptococcus pneumoniae*** - It possesses a thick **polysaccharide capsule** that is a primary virulence factor, interfering with **phagocytosis** by preventing the binding of phagocytes. - This organism is the **classic textbook example** of capsular resistance to phagocytosis, with over 90 capsular serotypes that determine virulence. - It is a major cause of pneumonia, meningitis, and otitis media, with its capsular resistance being key to its pathogenesis. *Neisseria meningitidis* - While *N. meningitidis* also has an important **polysaccharide capsule** that contributes to its virulence and resistance to phagocytosis, **Streptococcus pneumoniae** is more widely recognized as the classic example for this mechanism in bacterial pathogenesis. - It causes **meningitis** and **meningococcemia**, and its capsule helps it evade immune surveillance in the bloodstream. *Cryptococcus neoformans* - This is an **encapsulated yeast** with a prominent **polysaccharide capsule** (primarily glucuronoxylomannan) that aids in resistance to phagocytosis. - However, when discussing the **classic example** of capsular resistance to phagocytosis in microbiology, **bacterial capsules**, particularly that of *Streptococcus pneumoniae*, are the primary focus. - *C. neoformans* causes meningitis, especially in immunocompromised patients, and its capsule is visualized with India ink staining. *Klebsiella pneumoniae* - *K. pneumoniae* is known for its prominent **polysaccharide capsule**, which is a significant virulence factor, contributing to its resistance to phagocytosis and making it a common cause of healthcare-associated infections. - However, *Streptococcus pneumoniae* is historically and clinically considered the classic prototype when discussing polysaccharide capsules and their role in evading phagocytosis.

Question 2519: A 23-year-old woman with a history of sickle cell disease presents with fever and severe bone pain localized to her left tibia. X-ray reveals a lytic lesion, and blood cultures reveal infection. A bone culture grows gram-negative rods. Which of the following best describes the infecting organism?

A. It is a facultative intracellular parasite, such as Legionella.
B. It is a non-motile facultative anaerobe, such as Shigella.
C. It is comma-shaped and sensitive to acidic pH, such as Vibrio cholerae.
D. It is motile and does not ferment lactose, such as Salmonella. (Correct Answer)

Explanation: ***It is motile and does not ferment lactose, such as Salmonella.*** - Patients with **sickle cell disease** are at high risk for **Salmonella osteomyelitis** due to impaired reticuloendothelial function in the spleen, which normally clears encapsulated bacteria. - **Salmonella** species are gram-negative rods, typically **motile** due to peritrichous flagella, and most clinically relevant species are **non-lactose fermenters**. *It is a facultative intracellular parasite, such as Legionella.* - **Legionella** is a **facultative intracellular parasite** that causes Legionnaires’ disease (atypical pneumonia) and Pontiac fever, primarily affecting the lungs, not causing osteomyelitis, and is usually diagnosed from respiratory specimens. - While it is a gram-negative rod, it is an unlikely cause of osteomyelitis, especially with a history of sickle cell disease which predisposes to **Salmonella**. *It is a non-motile facultative anaerobe, such as Shigella.* - **Shigella** species are indeed **non-motile** and **facultative anaerobes** that are gram-negative rods, but they primarily cause **dysentery** (bloody diarrhea) and are not a common cause of osteomyelitis, even in immunocompromised patients. - The clinical presentation of severe bone pain and lytic lesion in sickle cell disease points away from **Shigella**. *It is comma-shaped and sensitive to acidic pH, such as Vibrio cholerae.* - **Vibrio cholerae** is a **comma-shaped** (curved) gram-negative rod that causes **cholera**, characterized by severe watery diarrhea, and is indeed sensitive to acidic pH. - It is an extremely rare cause of osteomyelitis and does not fit the typical infectious profile associated with sickle cell disease.

Question 2520: Lyme disease is caused by:

A. T. pertenue
B. B. vincenti
C. B. recurrentis
D. B. burgdorferi (Correct Answer)

Explanation: ***Correct: B. burgdorferi*** - **Lyme disease** is a **tick-borne illness** caused by the spirochete *Borrelia burgdorferi*. - This bacterium is transmitted to humans through the bite of infected ticks, primarily the **black-legged tick** (*Ixodes scapularis*). *Incorrect: B. vincenti* - *Borrelia vincenti* is associated with **Vincent's angina** (fusospirochetal disease), a severe gingivitis, and periodontitis. - It is not implicated in Lyme disease but rather in an oro-dental infection. *Incorrect: B. recurrentis* - *Borrelia recurrentis* is the causative agent of **epidemic relapsing fever**. - This disease is transmitted by the **human body louse** and is characterized by recurring episodes of fever. *Incorrect: T. pertenue* - *Treponema pertenue* is the bacterium responsible for **yaws**, a chronic, disfiguring skin infection. - It is a related spirochete to *Treponema pallidum* (syphilis) but causes a non-venereal disease primarily in tropical regions.

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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