Bacteriology Practice Questions

Q: Toxic shock syndrome is due to the following virulence factor:

Pyrogenic exotoxin. ***Pyrogenic exotoxin*** - **Pyrogenic exotoxins**, specifically **Toxic Shock Syndrome Toxin-1 (TSST-1)** and other streptococcal pyrogenic exotoxins (SPEs), are **superantigens** responsible for the symptoms of **toxic shock syndrome**. - These superantigens bind directly to **MHC class II** and **T-cell receptors**, leading to a massive, non-specific release of **cytokines** that cause severe systemic inflammation and organ failure. *M protein* - **M protein** is a major virulence factor of **Streptococcus pyogenes**, contributing to its antiphagocytic properties and adherence. - While important for streptococcal infections, it is not the primary mediator of the **toxic shock syndrome** associated with *Staphylococcus aureus* or *Streptococcus pyogenes*. *Carbohydrate cell wall* - The **carbohydrate cell wall** is a structural component of bacteria, particularly Gram-positive bacteria, and can have some immunogenic properties. - However, it does not directly act as a specific virulence factor like a superantigen to cause the severe systemic symptoms characteristic of **toxic shock syndrome**. *Streptolysin O* - **Streptolysin O** is an **exotoxin produced by Streptococcus pyogenes** that causes **hemolysis** and is cardiotoxic and cytolytic. - While it contributes to tissue damage and can be involved in severe streptococcal infections, it is not the main superantigen responsible for the widespread systemic effects of **toxic shock syndrome**.

Q: Which of the following statements about tetanus is true?

Caused by C. Tetani. ***Caused by C. Tetani*** - Tetanus is definitively caused by the bacterium **Clostridium tetani**, which is known for producing the potent neurotoxin **tetanospasmin**. - This neurotoxin is responsible for the characteristic **muscle spasms** and rigidity seen in tetanus. *Anaerobic infection* - While *Clostridium tetani* is an **anaerobic bacterium**, stating that tetanus is an "anaerobic infection" is a description of the causative agent, not the most definitive or unique statement about the disease itself. - The unique aspect of tetanus lies in the **neurotoxin production** by this specific anaerobe. *Cannot be prevented* - Tetanus is highly **preventable** through vaccination with the **tetanus toxoid**, which provides active immunity. - Public health efforts globally focus on widespread **immunization** to reduce the incidence of tetanus. *Forms terminal spores* - *Clostridium tetani* forms **spores**, but these are typically **subterminal** or terminal, depending on the strain, which is a characteristic of the bacterium, not the most defining statement about the disease process. - The spores are resilient and allow the bacterium to survive in harsh conditions until they find an **anaerobic environment** to germinate.

Q: Which one of the following statements best characterizes lympho-granuloma venereum (LGV)?

The causative agent is C. trachomatis. **The causative agent is *C. trachomatis*** - **Lymphogranuloma venereum (LGV)** is a sexually transmitted infection caused by specific serovars (**L1, L2, L2a, L2b, L3**) of *Chlamydia trachomatis*. [1] - These serovars are **invasive** and replicate in **mononuclear phagocytes** of lymphatic tissue, leading to the characteristic lymphadenopathy. [1] *It is most common in temperate regions* - LGV is **most common in tropical and subtropical regions**, particularly in parts of Africa, Southeast Asia, South America, and the Caribbean. - Its prevalence is much lower in temperate regions, though outbreaks can occur, especially in specific *high-risk populations*. *In the United States, it is more common among women* - In developed countries like the United States, LGV primarily affects **men who have sex with men (MSM)**, particularly those who are HIV-positive. [1] - While it can occur in women, its incidence is **significantly lower** in this population. *Penicillin is effective in early treatment* - LGV is caused by *Chlamydia trachomatis*, which is a **bacterium that lacks a peptidoglycan cell wall**, making it inherently resistant to penicillin. - The recommended treatments for LGV are **doxycycline** or **erythromycin**, not penicillin.

Q: 45 yrs old Ramlal has intra-abdominal sepsis. The causative organism was found to be vancomycin, gentamicin and ampicillin resistant. It grows well in presence of 6.5% NaCl and arginine. Bile esculin hydrolysis is positive. Which of the following is the organism? -

Enterococcus fecalis. ***Enterococcus fecalis*** - *Enterococcus fecalis* is commonly associated with **intra-abdominal sepsis** and can exhibit resistance to **vancomycin, gentamicin, and ampicillin**. - Its ability to grow in **6.5% NaCl**, hydrolyze **bile esculin**, and utilize **arginine** are classic biochemical characteristics distinguishing it from streptococci. *Streptococcus bovis* - While *Streptococcus bovis* can hydrolyze **bile esculin**, it typically **does not grow in 6.5% NaCl** and is usually susceptible to **ampicillin**. - It is more commonly associated with **bacteremia in patients with colon cancer** or endocarditis, rather than multi-drug resistant intra-abdominal sepsis. *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is known for causing pneumonia and meningitis but **does not grow in 6.5% NaCl** and has a **negative bile esculin test**. - It is also typically **penicillin-sensitive** (though resistance exists) and does not exhibit the multi-drug resistance profile described. *Streptococcus agalactiae* - *Streptococcus agalactiae* (Group B Strep) is known for causing neonatal sepsis and meningitis, but it **does not grow in 6.5% NaCl** and is **bile esculin negative**. - It is generally susceptible to **ampicillin** and does not fit the described multi-drug resistance pattern or biochemical profile.

Q: A patient admitted to an ICU is on a central venous line for the last one week. He is on ceftazidime and amikacin. After 7 days of antibiotics, he develops a spike of fever, and his blood culture is positive for gram-positive cocci in chains, which are catalase negative. Following this, vancomycin was started, but the culture remained positive for the same organism even after 2 weeks of therapy. The most likely organism causing the infection is:

Enterococcus faecalis. ***Enterococcus faecalis*** - The organism is a **gram-positive cocci in chains** and is **catalase negative**, which is consistent with *Enterococcus*. - **Vancomycin resistance** in *Enterococcus* (VRE) is a significant clinical problem, explaining the persistent positive culture despite vancomycin therapy. *Staphylococcus aureus* - *Staphylococcus aureus* is a **catalase-positive** organism, which contradicts the patient's culture results. - While it can be vancomycin-resistant (VRSA), the initial catalase test rules it out. *Viridans streptococci* - *Viridans streptococci* are typically **susceptible to vancomycin**, making a persistent positive culture after 2 weeks of therapy unlikely unless there's a serious underlying issue like endocarditis with large vegetations or an undrained abscess. - They are also **catalase-negative**, but the vancomycin resistance points away from this option. *Coagulase negative Staphylococcus* - **Coagulase-negative Staphylococci** (e.g., *Staphylococcus epidermidis*) are **catalase-positive**, which is inconsistent with the patient's culture results. - While they can cause central line infections and be vancomycin-resistant, the catalase test rules out this group.

Q: Lues maligna is caused by?

Treponema. ***Treponema*** - **Lues maligna** is a severe, ulcerative form of **secondary syphilis**, which is caused by the bacterium **Treponema pallidum**. - This condition involves widespread necrotic skin lesions and systemic symptoms, occurring in individuals with an altered immune response to the spirochete. *Borrelia* - **Borrelia burgdorferi** is the causative agent of **Lyme disease**, characterized by erythema migrans, neurological symptoms, and arthritis. - Other Borrelia species cause relapsing fever, which is distinct from syphilis and lues maligna. *Leptospira* - **Leptospira interrogans** is responsible for **leptospirosis**, a zoonotic infection that can cause flu-like symptoms, liver and kidney damage, and jaundice (Weil's disease). - Its clinical presentation and transmission route are entirely different from syphilis. *Brachyspira* - **Brachyspira species** are primarily associated with intestinal infections in animals, particularly swine dysentery, and are not known to cause human systemic diseases like syphilis. - While some species can colonize the human gut, they do not cause infectious syphilis or its severe manifestations like lues maligna.

Q: The organism causing pseudomembranous colitis:

Clostridium difficile. ***Clostridium difficile*** - This bacterium is the primary cause of **pseudomembranous colitis**, an inflammation of the colon characterized by the formation of membranes on the mucosal surface. - It produces **toxins A and B** which damage the intestinal lining, leading to severe diarrhea, abdominal pain, and fever. *Clostridium botulinum* - This organism is responsible for **botulism**, a severe form of food poisoning or wound infection. - It produces a potent **neurotoxin** that causes flaccid paralysis by blocking acetylcholine release at neuromuscular junctions. *Clostridium tetani* - This bacterium causes **tetanus**, characterized by muscle spasms and rigidity. - It produces the **tetanospasmin neurotoxin** which inhibits inhibitory neurotransmitters in the spinal cord. *Clostridium perfringens* - Primarily known for causing **gas gangrene** (myonecrosis) and certain types of food poisoning. - It produces various **toxins**, including alpha-toxin, which contribute to tissue destruction and gas formation.

Q: Color of diphtheretic membrane is -

Grey. The diphtheritic membrane typically appears **grey** or grayish-white due to fibrin, dead cells, bacteria, and inflammatory exudate [1]. This characteristic color is a classic diagnostic feature of **diphtheria**. [1] *Cream* - A cream color is not typically associated with the diphtheritic membrane, which has a more distinct grayish hue. - While some membranes might have a lighter appearance, it's not the primary descriptor. *White* - Although the membrane can be described as grayish-white, pure white is not the most accurate or common description. - The presence of necrotic tissue and blood often gives it a darker, more mottled appearance. *Yellow* - A yellow color is not typical for a diphtheritic membrane and would suggest other types of exudate or conditions. - Yellow exudates are more often associated with purulent infections.

Q: Which of the following is NOT true about the lepromin test?

It is diagnostic. ***It is diagnostic*** - The **lepromin test** is a measure of **cell-mediated immunity** to *Mycobacterium leprae* and is primarily used for **classification of leprosy**, not for diagnosing the disease itself. - A positive reaction indicates a robust immune response, characteristic of **tuberculoid leprosy**, while a negative reaction is seen in **lepromatous leprosy**. *Used to classify* - The lepromin test helps to **classify the type of leprosy** a patient has, differentiating between **tuberculoid** (positive reaction) and **lepromatous** (negative reaction) forms. - This classification is crucial for understanding the **patient's immune response** and prognosis. *BCG vaccination may convert negative to positive* - **BCG vaccination** can induce a mild, temporary **cell-mediated immune response** to mycobacterial antigens, which may result in a positive lepromin test in some individuals. - This cross-reactivity is due to the shared antigens between *Mycobacterium bovis* (used in BCG) and *Mycobacterium leprae*. *Negative in infants < 6 months* - Infants typically have a **developing immune system** and may not mount a strong cell-mediated immune response to lepromin, leading to a negative result. - This is similar to the **negative PPD test** often observed in very young infants even after BCG vaccination or exposure to tuberculosis due to their immature immune response.

Q: Infection occurring after human bite is?

Anaerobic streptococci. **Anaerobic streptococci** - Human bite infections are characteristically **polymicrobial**, with **anaerobic bacteria being the most predominant** organisms isolated from these wounds. - **Anaerobic streptococci** (including *Peptostreptococcus* species) are commonly found in the human oral flora and are frequently implicated in infections resulting from human bites. - These bacteria thrive in the **anaerobic (low oxygen)** environment created in deep puncture wounds caused by bites. - Other important anaerobes include *Fusobacterium*, *Prevotella*, and *Porphyromonas* species. *Spirochetes* - **Spirochetes** are spiral-shaped bacteria, and while some (*Treponema denticola*) are part of the normal oral flora, they are not the primary cause of acute infections following human bites. - They are more commonly associated with conditions like **syphilis** (*Treponema pallidum*) or **Lyme disease** (*Borrelia burgdorferi*), which are not typically transmitted via human bites in this manner. *Staphylococcus* - **Staphylococcus aureus** is commonly isolated from human bite wounds and can cause significant soft tissue infections. - However, human bite infections are characteristically **polymicrobial with predominance of anaerobic bacteria**, making anaerobic streptococci more characteristic than staphylococci alone. - Note: ***Eikenella corrodens*** is considered pathognomonic for human bite infections but is an **aerobic** organism. *Candida* - **Candida** is a genus of yeast (fungus) and a common cause of fungal infections (e.g., thrush, candidiasis). - While *Candida* can be found in the mouth, it is **not a primary pathogen** causing acute bacterial infection following a human bite wound.

Question 1

Toxic shock syndrome is due to the following virulence factor:

Accepted Answer

Pyrogenic exotoxin

Answer

M protein

Answer

Carbohydrate cell wall

Answer

Streptolysin O

Question 2

Which of the following statements about tetanus is true?

Accepted Answer

Caused by C. Tetani

Answer

Anaerobic infection

Answer

Cannot be prevented

Answer

Forms terminal spores

Question 3

Which one of the following statements best characterizes lympho-granuloma venereum (LGV)?

Accepted Answer

The causative agent is C. trachomatis

Answer

It is most common in temperate regions

Answer

In the United States, it is more common among women

Answer

Penicillin is effective in early treatment

Question 4

45 yrs old Ramlal has intra-abdominal sepsis. The causative organism was found to be vancomycin, gentamicin and ampicillin resistant. It grows well in presence of 6.5% NaCl and arginine. Bile esculin hydrolysis is positive. Which of the following is the organism? -

Accepted Answer

Enterococcus fecalis

Answer

Streptococcus bovis

Answer

Streptococcus pneumoniae

Answer

Streptococcus agalactiae

Question 5

A patient admitted to an ICU is on a central venous line for the last one week. He is on ceftazidime and amikacin. After 7 days of antibiotics, he develops a spike of fever, and his blood culture is positive for gram-positive cocci in chains, which are catalase negative. Following this, vancomycin was started, but the culture remained positive for the same organism even after 2 weeks of therapy. The most likely organism causing the infection is:

Accepted Answer

Enterococcus faecalis

Answer

Staphylococcus aureus

Answer

Viridans streptococci

Answer

Coagulase negative Staphylococcus

Question 6

Lues maligna is caused by?

Accepted Answer

Treponema

Answer

Borrelia

Answer

Leptospira

Answer

Brachyspira

Question 7

The organism causing pseudomembranous colitis:

Accepted Answer

Clostridium difficile

Answer

Clostridium botulinum

Answer

Clostridium tetani

Answer

Clostridium perfringens

Question 8

Color of diphtheretic membrane is -

Accepted Answer

Grey

Answer

Cream

Answer

White

Answer

Yellow

Question 9

Which of the following is NOT true about the lepromin test?

Accepted Answer

It is diagnostic

Answer

Used to classify

Answer

BCG vaccination may convert negative to positive

Answer

Negative in infants < 6 months

Question 10

Infection occurring after human bite is?

Accepted Answer

Anaerobic streptococci

Answer

Spirochetes

Answer

Staphylococcus

Answer

Candida

Bacteriology — MCQs

Bacteriology — MCQs

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