Bacteriology Practice Questions

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.

Q: Causative organism of rheumatic fever is:

Group A Streptococci. ***Group A Streptococci*** - **Rheumatic fever** is a serious inflammatory complication that can develop after an untreated or inadequately treated infection of the throat with **streptococcus pyogenes**, which is also known as **Group A Streptococcus (GAS)**. - The immune response to GAS infection mistakenly attacks the body's own tissues, particularly the heart, joints, brain, and skin. *Group B Streptococci* - **Group B Streptococcus (GBS)** is primarily known for causing serious infections in **newborns** (e.g., sepsis, meningitis) and can cause infections in pregnant women. - It is not associated with the development of rheumatic fever. *Group D Streptococci* - **Group D Streptococci** encompasses species such as *Enterococcus faecalis* and *Streptococcus gallolyticus* (formerly *Streptococcus bovis*). - These are typically associated with **urinary tract infections**, **endocarditis**, and **colorectal cancer** (in the case of *S. gallolyticus*), not rheumatic fever. *Staphylococci* - **Staphylococci**, such as *Staphylococcus aureus*, cause a wide range of infections including **skin infections**, **pneumonia**, **sepsis**, and **toxic shock syndrome**. - They are not the causative agents of rheumatic fever; this condition is specifically linked to streptococcal infections.

Q: What is the etiological agent for buruli ulcer which usually occurs in the tropics?

Mycobacterium ulcerans. ***Mycobacterium ulcerans*** - **Buruli ulcer** (also known as Bairnsdale ulcer or Daintree ulcer) is caused by infection with **Mycobacterium ulcerans**. - This bacterium produces a toxin called **mycolactone**, which destroys tissue and suppresses the immune system, leading to characteristic skin lesions. *Mycobacterium fortuitum* - *Mycobacterium fortuitum* is a **rapidly growing mycobacterium** that can cause skin and soft tissue infections, especially after trauma or surgery. - However, it is not the etiologic agent for **Buruli ulcer**, which is associated with a specific slow-growing mycobacterium. *Mycobacterium marinum* - *Mycobacterium marinum* is known to cause **"fish tank granuloma"** or "swimming pool granuloma," an infection acquired from contaminated water, typically presenting as skin lesions on extremities. - While it is a **nontuberculous mycobacterium** with environmental reservoirs, it is not responsible for **Buruli ulcer**. *Mycobacterium kansasii* - *Mycobacterium kansasii* is a **nontuberculous mycobacterium** primarily associated with pulmonary disease, resembling tuberculosis, particularly in individuals with compromised immune systems. - It is not the causative agent of **Buruli ulcer**, which has distinct clinical and epidemiological characteristics.

Q: Vibrio parahaemolyticus is seen in which of the following if undercooked?

Shellfish. ***Shellfish*** - **Vibrio parahaemolyticus** is a common cause of **foodborne gastroenteritis** associated with consumption of raw or undercooked **shellfish**, including both mollusks and crustaceans. - This halophilic bacterium thrives in **warm coastal and estuarine waters** and colonizes various marine shellfish. - Most commonly isolated from **oysters, clams, mussels** (bivalve mollusks that concentrate bacteria through filter feeding), but also found in **shrimp, prawns, and crabs**. - **Shellfish** is the most comprehensive and correct answer as it encompasses all these seafood types. *Prawn* - Prawns (a type of shellfish/crustacean) can indeed harbor **Vibrio parahaemolyticus** and are a recognized source of infection. - However, this answer is **too specific** when a broader category (shellfish) is available. - While medically accurate, "Shellfish" is the better answer as it includes prawns and all other affected seafood. *Crab* - Crabs (also shellfish/crustaceans) can carry **Vibrio parahaemolyticus**, especially in warm coastal waters. - Like prawns, this is **too specific** when the broader category "Shellfish" encompasses crabs. - Multiple outbreaks have been linked to crab consumption, but "Shellfish" remains the more complete answer. *Fish* - While **cross-contamination** can occur, finfish are **less commonly** the primary source of *V. parahaemolyticus* infection compared to shellfish. - The bacterium preferentially colonizes shellfish rather than finfish. - Fish is therefore the least associated option among the choices given.

Q: All of the following organisms are known to survive intracellularly except

Neisseria meningitidis. ***Neisseria meningitidis*** - While *Neisseria meningitidis* can be found in the **phagosomes of neutrophils** in infected individuals, it is not considered a classic intracellular pathogen that replicates significantly or survives for extended periods within host cells. - Its primary mechanism of pathogenesis involves replication extracellularly and the production of a capsule to evade phagocytosis. *Legionella pneumophila* - *Legionella pneumophila* is a **facultative intracellular pathogen** known to replicate within **macrophages** and **amoebae** in aquatic environments. - Its ability to survive and multiply inside host cells is crucial for its virulence and persistence. *Streptococcus pyogenes* - *Streptococcus pyogenes* (Group A Streptococcus) can **invade and survive within non-phagocytic cells** such as epithelial cells, which may contribute to persistent infections and immune evasion. - This intracellular survival mechanism can make it more challenging for antibiotics and the immune system to eradicate the bacteria. *Salmonella Typhi* - *Salmonella Typhi* is a well-known **intracellular pathogen** that survives and replicates within **macrophages and other phagocytic cells** in the host. - This intracellular lifestyle allows it to evade immune surveillance and disseminate throughout the body, leading to systemic infections like typhoid fever.

Question 1

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 2

Lues maligna is caused by?

Accepted Answer

Treponema

Answer

Borrelia

Answer

Leptospira

Answer

Brachyspira

Question 3

The organism causing pseudomembranous colitis:

Accepted Answer

Clostridium difficile

Answer

Clostridium botulinum

Answer

Clostridium tetani

Answer

Clostridium perfringens

Question 4

Color of diphtheretic membrane is -

Accepted Answer

Grey

Answer

Cream

Answer

White

Answer

Yellow

Question 5

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 6

Infection occurring after human bite is?

Accepted Answer

Anaerobic streptococci

Answer

Spirochetes

Answer

Staphylococcus

Answer

Candida

Question 7

Causative organism of rheumatic fever is:

Accepted Answer

Group A Streptococci

Answer

Group B Streptococci

Answer

Group D Streptococci

Answer

Staphylococci

Question 8

What is the etiological agent for buruli ulcer which usually occurs in the tropics?

Accepted Answer

Mycobacterium ulcerans

Answer

Mycobacterium fortuitum

Answer

Mycobacterium marinum

Answer

Mycobacterium kansasii

Question 9

Vibrio parahaemolyticus is seen in which of the following if undercooked?

Accepted Answer

Shellfish

Answer

Prawn

Answer

Crab

Answer

Fish

Question 10

All of the following organisms are known to survive intracellularly except

Accepted Answer

Neisseria meningitidis

Answer

Legionella pneumophila

Answer

Streptococcus pyogenes

Answer

Salmonella Typhi

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?