Enterobacteriaceae — MCQs
A frequent traveler presented with 4 days of continuous fever, abdominal pain, and bradycardia. What is the best diagnostic test to confirm the pathogen?
Which of the following is more frequently associated with Klebsiella pneumoniae than with Pseudomonas aeruginosa?
The following gram stain image shows which bacteria?
A child presented with bloody stools and abdominal pain. Which enrichment medium should be used for processing the fecal sample?
Which of the following is a true statement regarding Enterobacteriaceae?
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 person returns to Delhi from Bangladesh after 2 days and has diarrhea. Stool examination shows RBC's in stool. The likely organism causing is ?
Which of the following Shigella species produces Shiga toxin?
Traveller's diarrhoea is caused by:
All are true about the bacteria shown in the figure except: (Recent NEET Pattern 2016-17)
Enterobacteriaceae Indian Medical PG Practice Questions and MCQs
Question 1: A frequent traveler presented with 4 days of continuous fever, abdominal pain, and bradycardia. What is the best diagnostic test to confirm the pathogen?
- A. Widal test
- B. Blood culture (Correct Answer)
- C. Urine culture
- D. Stool culture
Explanation: ***Blood culture*** - **Blood culture** is the most sensitive and specific test for confirming **typhoid fever** in the first week of illness. - The presence of **continuous fever** (step-ladder pattern), **abdominal pain**, and **relative bradycardia** in a traveler strongly suggests typhoid fever caused by *Salmonella Typhi*. *Widal test* - The **Widal test** detects antibodies against *Salmonella Typhi* antigens and is often positive later in the disease course. - It has **limited sensitivity and specificity**, especially in endemic areas or with prior vaccination, leading to false positives and negatives. *Urine culture* - **Urine culture** has a low yield for *Salmonella Typhi*, as bacteria are intermittently shed in urine, usually later in the disease. - It's primarily useful for diagnosing **urinary tract infections** or in chronic carriers of typhoid. *Stool culture* - **Stool culture** yield is higher in the later stages of typhoid fever, as *Salmonella Typhi* is shed in feces. - Its sensitivity is lower than blood culture in the early acute phase when bacteremia is most prominent.
Question 2: Which of the following is more frequently associated with Klebsiella pneumoniae than with Pseudomonas aeruginosa?
- A. Cystic fibrosis
- B. Diabetes mellitus
- C. Upper lobe cavitation (Correct Answer)
- D. Artificial ventilation
Explanation: Upper lobe cavitation - **Upper lobe cavitation** is a characteristic feature of pneumonia caused by **Klebsiella pneumoniae**, particularly in chronic alcoholics or individuals with poor host defenses [1]. - While *Pseudomonas aeruginosa* can cause severe pneumonia, **cavitary lesions** are less frequently reported and typically occur in immunocompromised patients. *Artificial ventilation* - **Artificial ventilation** is a significant risk factor for nosocomial pneumonia caused by both *Klebsiella pneumoniae* and *Pseudomonas aeruginosa*. - However, the question asks for a feature *more frequently associated* with *Klebsiella pneumoniae*, and ventilation is a common risk for both. *Cystic fibrosis* - **Cystic fibrosis** is overwhelmingly associated with chronic *Pseudomonas aeruginosa* infections, which establish persistent biofilms in the respiratory tract. - While *Klebsiella pneumoniae* can colonize CF patients, it is not the primary or most frequent pathogen linked to this condition. *Diabetes mellitus* - **Diabetes mellitus** increases susceptibility to various infections, including those caused by *Klebsiella pneumoniae* and *Pseudomonas aeruginosa*. - However, it is an underlying condition and not a specific clinical characteristic of the pneumonia itself that differentiates the two organisms.
Question 3: The following gram stain image shows which bacteria?
- A. Staphylococcus aureus (Correct Answer)
- B. Listeria monocytogenes
- C. Vibrio cholerae
- D. Streptococcus pneumoniae
Explanation: ***Staphylococcus aureus*** - This image clearly shows **Gram-positive cocci** arranged in irregular, grape-like **clusters**, which is the classic microscopic appearance of *Staphylococcus* species, including *Staphylococcus aureus* - The purple color indicates they are **Gram-positive** bacteria - **Cluster arrangement** is the key distinguishing feature of staphylococci on Gram stain *Vibrio cholerae* - *Vibrio cholerae* is a **Gram-negative, comma-shaped bacillus** and would appear as pink/red, curved rods, not purple cocci - The morphology in the image is completely different from Vibrio species *Listeria monocytogenes* - *Listeria monocytogenes* is a **Gram-positive rod** (bacillus), not a coccus - It would appear as purple, rod-shaped bacteria, not spherical cocci - Its characteristic morphology is quite different from the cocci seen in the image *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is a **Gram-positive coccus** but typically appears in **pairs (diplococci)** or short chains, often lancet-shaped - The bacteria in the image are predominantly in **clusters**, which differentiates them from *Streptococcus pneumoniae* - Streptococci arrange in chains or pairs, not grape-like clusters
Question 4: A child presented with bloody stools and abdominal pain. Which enrichment medium should be used for processing the fecal sample?
- A. Blood agar
- B. Selenite F broth (Correct Answer)
- C. Alkaline peptone water
- D. Muller Hinton Broth
Explanation: ***Selenite F broth*** - This **enrichment medium** is specifically designed to isolate **Salmonella** and some species of **Shigella**, which are common causes of bloody stools and abdominal pain in children. - It inhibits the growth of commensal gut flora, allowing pathogenic bacteria to proliferate and be subsequently identified on selective media. *Blood agar* - Blood agar is a **general-purpose enrichment medium** that supports the growth of a wide range of bacteria but does not selectively enrich for specific pathogens. - It would be ineffective in outcompeting the normal fecal flora to isolate rarer enteric pathogens causing the symptoms. *Alkaline peptone water* - This medium is primarily used for the enrichment of **Vibrio cholerae** species, which typically cause watery diarrhea, not bloody stools. - While it helps in the isolation of *Vibrio* species, it is not suitable for the suspected pathogens in this clinical scenario. *Muller Hinton Broth* - Muller-Hinton media are primarily used for **antimicrobial susceptibility testing** (antibiotic sensitivity testing) and are not designed for the primary isolation or enrichment of specific pathogens from clinical samples. - It would not provide a selective advantage for the recovery of organisms causing bloody diarrhea from a fecal sample.
Question 5: Which of the following is a true statement regarding Enterobacteriaceae?
- A. Glucose is fermented by most members of the family
- B. Motility in Enterobacteriaceae is via peritrichous flagella
- C. Nitrate reduction positive
- D. All members are oxidase negative (Correct Answer)
Explanation: ***All members are oxidase negative*** - This is the **definitive and universally true** statement for all Enterobacteriaceae, with **no exceptions**. - The **negative oxidase test** is a key defining characteristic that distinguishes Enterobacteriaceae from other Gram-negative bacteria such as *Pseudomonas*. - This enzymatic characteristic is crucial for basic laboratory identification and is **consistently used as a primary screening test** for this family. *Glucose is fermented by most members of the family* - While the fermentation of glucose is indeed a characteristic of Enterobacteriaceae, the statement is **imprecise** because it says "most" when in fact **ALL members** ferment glucose. - The use of "most" makes this statement technically less accurate than the oxidase-negative statement, which correctly uses "all." *Motility in Enterobacteriaceae is via peritrichous flagella* - This statement is **incomplete and potentially misleading** because **not all members are motile**. - While motile members do have **peritrichous flagella** (distributed over the entire cell surface), important members like *Klebsiella* species are **non-motile**. - The statement implies universal motility, which is incorrect. *Nitrate reduction positive* - While all Enterobacteriaceae do reduce nitrates to nitrites, this statement **lacks specificity** compared to the oxidase test. - The oxidase-negative characteristic is more **diagnostically distinctive** and is the **primary screening test** used to differentiate Enterobacteriaceae from other Gram-negative bacteria. - Nitrate reduction is confirmatory but not as definitive for initial identification.
Question 6: 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 7: A person returns to Delhi from Bangladesh after 2 days and has diarrhea. Stool examination shows RBC's in stool. The likely organism causing is ?
- A. Enteropathogenic E. Coli
- B. Salmonella typhi
- C. Enterotoxigenic E. Coli
- D. Shigella dysenteriae (Correct Answer)
Explanation: ***Shigella dysenteriae*** - This organism causes **bacillary dysentery**, characterized by fever, abdominal cramps, and frequent, small-volume stools containing **blood and mucus (RBCs)**, which fits the clinical picture of a traveler experiencing diarrhea with RBCs in stool. - The rapid onset (within 2 days of return) and the presence of **RBCs in stool** are highly suggestive of an invasive bacterial pathogen like *Shigella*. *Enteropathogenic E. Coli* - **EPEC** typically causes **watery diarrhea**, particularly in infants, by disrupting intestinal microvilli. - It does **not usually cause bloody stools** or significant red blood cells in the stool. *Salmonella typhi* - While *Salmonella typhi* causes **typhoid fever**, its initial presentation is typically with fever, headache, and constipation, followed by a **pea-soup diarrhea** in later stages, which is usually not bloody. - The rapid onset of bloody diarrhea within 2 days is less characteristic of typhoid fever, which has a longer incubation period. *Enterotoxigenic E. Coli* - **ETEC** is a common cause of **traveler's diarrhea**, but it produces toxins that lead to **watery, non-bloody diarrhea**. - The presence of **RBCs in the stool** makes ETEC an unlikely cause in this scenario.
Question 8: Which of the following Shigella species produces Shiga toxin?
- A. All Shigella species
- B. Shigella dysenteriae (Correct Answer)
- C. Shigella sonnei
- D. Shigella flexneri
Explanation: ***Shigella dysenteriae*** - This species, specifically **serotype 1 (S. dysenteriae type 1)**, is known for producing the **Shiga toxin (Stx)**, which inhibits protein synthesis in host cells by targeting the 60S ribosomal subunit. - The Shiga toxin is responsible for the severe clinical manifestations, including **bloody diarrhea** and **hemolytic-uremic syndrome (HUS)**, often associated with *Shigella dysenteriae* infections. - This is the only *Shigella* species that produces this potent toxin. *Shigella sonnei* - This species typically causes the **mildest form of shigellosis** and does not produce the classic Shiga toxin. - While it can produce some enterotoxins, they are generally not as potent as the Shiga toxin of *S. dysenteriae*. *Shigella flexneri* - *S. flexneri* is a common cause of shigellosis in developing countries but **does not produce the Shiga toxin**. - It primarily invades and replicates within the intestinal epithelial cells, causing inflammation and damage through direct cellular invasion. *All Shigella species* - This statement is incorrect because only *Shigella dysenteriae* serotype 1 is recognized for producing the potent **Shiga toxin**. - Other *Shigella* species cause disease through invasion and inflammatory mechanisms, although they can still lead to significant gastrointestinal illness.
Question 9: Traveller's diarrhoea is caused by:
- A. E. histolytica
- B. E. coli (Correct Answer)
- C. Giardia lamblia
- D. Shigella
Explanation: ***E. coli*** - **Enterotoxigenic E. coli (ETEC)** is the most common cause of **traveler's diarrhea**, producing toxins that lead to watery stools. - The disease is typically acquired through ingestion of **contaminated food or water**, particularly in regions with poor sanitation. *E. histolytica* - This parasite causes **amoebiasis**, which can lead to **dysentery** (bloody diarrhea) or liver abscesses. - While it can cause diarrhea in travelers, it is not the most frequent pathogen associated with typical traveler's diarrhea. *Giardia lamblia* - **Giardiasis** typically results in **chronic, greasy, foul-smelling stools**, bloating, and malabsorption rather than acute watery diarrhea. - It's a common cause of **protracted diarrhea** in travelers but not the most frequent cause of acute onset. *Shigella* - This bacterium causes **shigellosis**, characterized by **dysentery (bloody, mucoid stools)**, fever, and abdominal cramps. - While a cause of bacterial diarrhea in travelers, it is less common than ETEC and presents with a more severe, invasive illness.
Question 10: All are true about the bacteria shown in the figure except: (Recent NEET Pattern 2016-17)
- A. Commensal in upper respiratory tract of humans
- B. Kovac's method results in formation of deep purple color
- C. Nonhemolytic, gray translucent colonies
- D. Resistant to heat, alteration in pH and disinfectants (Correct Answer)
Explanation: ***Resistant to heat, alteration in pH and disinfectants*** - The bacteria shown in the figure are **Gram-negative diplococci**, characteristic of **Neisseria gonorrhoeae** or **Neisseria meningitidis**. These bacteria are known to be **sensitive to drying, heat, cold, and disinfectants**, not resistant. - Their delicate nature makes them difficult to cultivate outside specific laboratory conditions and contributes to their typical mode of transmission through close contact. *Commensal in the upper respiratory tract of humans* - **Neisseria meningitidis**, a type of diplococci shown and inferred, is a common **commensal** in the nasopharynx of up to 10% of the population. - This colonization can be entirely asymptomatic, but it also serves as a reservoir for potential invasive disease. *Kovac's method results in the formation of a deep purple color* - **Neisseria** species are **oxidase-positive**, meaning they produce cytochrome c oxidase. - The **Kovac's oxidase test** uses N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride, which, in the presence of cytochrome c oxidase, gets oxidized to a **deep purple color**. *Nonhemolytic, gray translucent colonies* - Both *Neisseria gonorrhoeae* and *Neisseria meningitidis* typically form **nonhemolytic** colonies on blood agar. - The colonies are often described as **grayish-white or translucent** with smooth edges, especially after 24-48 hours of incubation.
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