Vibrio, Aeromonas, and Plesiomonas — MCQs

10 questions

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 disease is characterized by severe watery diarrhea and is associated with a toxin-producing bacterium?

In a patient presenting with fever and suspected systemic infection, which of the following specimens is the most appropriate for the isolation of microorganisms in laboratory diagnosis?

Which of the following Shigella species produces Shiga toxin?

A healthy 21-year-old woman develops a profuse, watery diarrhea one day after a meal of raw oysters. On physical examination, her temperature is 37.5°C. A stool sample is negative for occult blood. There is no abdominal distension or tenderness, and bowel sounds are present. Diarrhea subsides over the next three days. Which of the following organisms is most likely to produce these findings?

Pseudomonas is which type of bacteria?

All of the following statements about cholera are true except -

A patient was admitted with bloody diarrhea after consumption of oysters. The organism exhibits the Kanagawa phenomenon. What is the correct organism?

An 18-year-old girl presents with watery diarrhea. Most likely causative agent -

Q10

Which enzyme does Vibrio cholerae toxin activate to cause profuse watery diarrhea?

Vibrio, Aeromonas, and Plesiomonas 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 disease is characterized by severe watery diarrhea and is associated with a toxin-producing bacterium?

A. Gastroenteritis
B. Cholera (Correct Answer)
C. Dysentery
D. Typhoid fever

Explanation: ***Cholera*** - Cholera is known for causing **severe, watery diarrhea** (often described as "rice water stools") due to the action of **cholera toxin** produced by *Vibrio cholerae* [1], [2]. - The toxin stimulates excessive fluid and electrolyte secretion in the small intestine, leading to rapid **dehydration** and electrolyte imbalance [2]. *Gastroenteritis* - This is a general term for **inflammation of the stomach and intestines**, which can be caused by various pathogens (viruses, bacteria, parasites) and toxins. - While it often presents with diarrhea, it doesn't specifically imply the **severe watery diarrhea** linked to a specific toxin as seen in cholera. *Dysentery* - Dysentery is characterized by **bloody diarrhea**, often accompanied by **fever and abdominal cramps**, indicating inflammation and damage to the intestinal lining [3]. - It is typically caused by bacteria like *Shigella* or *entamoeba histolytica*, distinct from the purely watery diarrhea of cholera [3]. *Typhoid fever* - Typhoid fever is a systemic illness caused by *Salmonella typhi*, characterized by **sustained fever, headache, malaise**, and can include **constipation or mild diarrhea**. - It does not primarily present with **severe watery diarrhea** induced by a specific toxin, unlike cholera.

Question 3: In a patient presenting with fever and suspected systemic infection, which of the following specimens is the most appropriate for the isolation of microorganisms in laboratory diagnosis?

A. Blood culture for isolation of bacteria (Correct Answer)
B. Stool sample in cases of gastroenteritis
C. Throat swab for suspected pharyngitis
D. Urine sample for urinary tract infection

Explanation: ***Blood culture for isolation of bacteria*** - For **systemic infection** and **fever**, **blood culture** is the most direct method to isolate and identify the causative microorganism disseminated throughout the body. - It helps guide **appropriate antibiotic therapy** by determining the pathogen's **susceptibility profile**. *Stool sample in cases of gastroenteritis* - This specimen is appropriate for diagnosing **gastrointestinal infections** where the pathogen primarily affects the digestive tract. - It is not the primary choice for suspected **systemic infection** unless GI symptoms are prominent and dissemination is suspected. *Throat swab for suspected pharyngitis* - A throat swab is specific for diagnosing **pharyngitis** or upper **respiratory tract infections**, localizing the infection to the pharynx. - It would not sufficiently identify a **systemic infection**, as the pathogen may not be present in the throat in such cases. *Urine sample for urinary tract infection* - A urine sample is indicated for diagnosing **urinary tract infections (UTIs)**, where the pathogen is concentrated in the urinary system. - While a UTI can lead to systemic symptoms, a urine sample alone is insufficient to confirm a generalized systemic infection unless the infection has specifically localized there.

Question 4: 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 5: A healthy 21-year-old woman develops a profuse, watery diarrhea one day after a meal of raw oysters. On physical examination, her temperature is 37.5°C. A stool sample is negative for occult blood. There is no abdominal distension or tenderness, and bowel sounds are present. Diarrhea subsides over the next three days. Which of the following organisms is most likely to produce these findings?

A. Cryptosporidium parvum
B. Entamoeba histolytica
C. Staphylococcus aureus
D. Vibrio parahaemolyticus (Correct Answer)

Explanation: ***Vibrio parahaemolyticus*** - **Raw seafood consumption** (raw oysters) is a classical risk factor for *Vibrio parahaemolyticus* infection, which commonly causes **watery diarrhea** and abdominal cramps. - The symptoms, including the self-limiting nature and absence of fever or severe abdominal findings, are consistent with a typical onset of 24 hours after exposure to *Vibrio parahaemolyticus*. *Cryptosporidium parvum* - This parasite typically causes **prolonged watery diarrhea**, especially in immunocompromised individuals, and is not usually associated with rapid onset following consumption of raw oysters. - While it causes watery diarrhea, its incubation period can range from 2 to 10 days, and the illness often lasts for weeks, not just three days. *Entamoeba histolytica* - *Entamoeba histolytica* causes **amoebic dysentery**, characterized by **bloody diarrhea**, fever, and abdominal pain, which is not seen in this patient (stool negative for occult blood). - Transmission is typically via contaminated food or water, but raw oysters are not a common source, and the symptoms described do not match amoebiasis. *Staphylococcus aureus* - *Staphylococcus aureus* causes **food poisoning** due to preformed toxins, leading to very rapid onset (1-6 hours) of severe nausea, vomiting, and abdominal cramps, with diarrhea sometimes present. - The 24-hour incubation period and predominantly diarrheal presentation make *Vibrio parahaemolyticus* a more likely culprit than *Staphylococcus aureus*, where vomiting is usually prominent.

Question 6: Pseudomonas is which type of bacteria?

A. Anaerobic
B. Aerobic (Correct Answer)
C. Microaerophilic
D. Obligate anaerobe

Explanation: ***Aerobic*** - **Pseudomonas aeruginosa** is a classic example of an **aerobic bacterium**, meaning it requires oxygen for growth and metabolism. - Its ability to thrive in aerobic environments contributes to its prevalence in hospital settings and its capacity to cause a wide range of infections. *Anaerobic* - Anaerobic bacteria grow in the absence of oxygen and often die in its presence. **Pseudomonas** does not exhibit this characteristic. - Examples of anaerobic bacteria include *Clostridium* species. *Microaerophilic* - Microaerophilic bacteria require oxygen but only in low concentrations (5-10%), higher concentrations are inhibitory. - **Pseudomonas** grows optimally in atmospheric oxygen levels, not restricted to low concentrations. *Obligate anaerobe* - Obligate anaerobes are severely inhibited or killed by oxygen. - This is the opposite of **Pseudomonas**, which strictly requires oxygen for survival.

Question 7: All of the following statements about cholera are true except -

A. Culture medium is TCBS Agar
B. O & H antigens measure carrier state (Correct Answer)
C. Produces indole and reduces nitrate
D. Synthesize neuraminidase

Explanation: ***O & H antigens measure carrier state*** - **O and H antigens** are primarily involved in serotyping *Vibrio cholerae* and are crucial for the initial classification of different strains, particularly differentiating between toxigenic and non-toxigenic strains. - The detection of **carrier states** in cholera is typically achieved through culturing stool samples for the presence of *Vibrio cholerae*, rather than by measuring O and H antigens, as these antigens reflect the bacterial surface components. *Culture medium is TCBS Agar* - **Thiosulfate-citrate-bile salts-sucrose (TCBS) agar** is a selective and differential medium widely used for isolating *Vibrio* species, including *Vibrio cholerae*, from clinical samples and environmental sources. - It works by inhibiting the growth of most enteric bacteria while allowing *Vibrio* species to grow and produce distinct colonies (e.g., yellow colonies for sucrose-fermenting *V. cholerae*). *Produces indole and reduces nitrate* - *Vibrio cholerae* is biochemically characterized by its ability to **produce indole** from tryptophan and to **reduce nitrates** to nitrites, which are important diagnostic markers. - These metabolic activities are part of the standard battery of biochemical tests used to identify and confirm the presence of *Vibrio cholerae* in laboratory settings. *Synthesize neuraminidase* - *Vibrio cholerae* produces **neuraminidase**, an enzyme that cleaves **sialic acid** residues from mucin, potentially enhancing the binding of cholera toxin to intestinal epithelial cells by exposing GM1 ganglioside receptors. - This enzyme contributes to the pathogen's virulence by modifying the host's intestinal environment, although its direct role in disease pathogenesis is still under investigation.

Question 8: A patient was admitted with bloody diarrhea after consumption of oysters. The organism exhibits the Kanagawa phenomenon. What is the correct organism?

A. Staphylococcus aureus
B. Vibrio parahaemolyticus (Correct Answer)
C. Campylobacter jejuni
D. Salmonella typhi

Explanation: ***Vibrio parahaemolyticus*** - This organism is a common cause of **gastroenteritis** linked to consumption of **raw or undercooked seafood**, especially oysters, and is known for causing **bloody diarrhea**. - The **Kanagawa phenomenon** refers to the production of **thermostable direct hemolysin (TDH)** by *V. parahaemolyticus*, which is a key virulence factor causing hemolysis and is detectable by a specific agar assay. *Staphylococcus aureus* - While *S. aureus* can cause **food poisoning** through its toxins, it typically presents with **nausea, vomiting**, and **abdominal cramps**, not bloody diarrhea. - Food poisoning from *S. aureus* is usually rapid-onset (within hours) and is associated with **improperly handled foods**, particularly those requiring no cooking after preparation (e.g., salads, custards). *Campylobacter jejuni* - This bacterium is a common cause of **bacterial gastroenteritis** and can lead to **bloody diarrhea**, but it's typically acquired from **poultry** or contaminated water, not specifically oysters. - **Guillain-Barré syndrome** is a serious complication that can follow *C. jejuni* infection, which is not characteristic of *Vibrio* infections. *Salmonella typhi* - *Salmonella typhi* causes **typhoid fever**, a systemic illness characterized by **sustained fever, headache, malaise**, and can lead to a "rose spot" rash. - While it can cause gastrointestinal symptoms, **bloody diarrhea is not the predominant symptom**, and raw oyster consumption is not its typical mode of transmission; it's usually spread through contaminated food or water by asymptomatic carriers.

Question 9: An 18-year-old girl presents with watery diarrhea. Most likely causative agent -

A. Rotavirus
B. V. cholerae (Correct Answer)
C. Salmonella
D. Shigella

Explanation: ***V. cholerae*** - *Vibrio cholerae* is a classic cause of **acute, severe watery diarrhea** that can lead to rapid dehydration. - While other agents can cause watery diarrhea, *V. cholerae* is primarily associated with large-scale outbreaks of this symptom. *Rota virus* - While rotavirus causes **watery diarrhea**, it primarily affects **infants and young children** and is less common as the most likely cause in an 18-year-old in many settings due to widespread vaccination programs. - The diarrhea, though watery, is often accompanied by **fever and vomiting**. *Salmonella* - *Salmonella* typically causes **inflammatory diarrhea** (dysentery-like symptoms with blood/mucus in stool) or **food poisoning**, with diarrhea that may be watery but is often not as profuse or severe as cholera. - It is more commonly associated with **fever and abdominal cramps**. *Shigella* - *Shigella* causes **bacillary dysentery**, characterized by **bloody, mucoid stools**, abdominal cramps, and fever. - It is not typically associated with solely profuse watery diarrhea.

Question 10: Which enzyme does Vibrio cholerae toxin activate to cause profuse watery diarrhea?

A. Sodium channels
B. Adenylate cyclase (Correct Answer)
C. Potassium channels
D. Guanylate cyclase

Explanation: ***Adenylate cyclase*** - The **Vibrio cholerae toxin** (cholera toxin/choleragen) causes activation of **adenylate cyclase** through ADP-ribosylation of the Gs alpha subunit of G-proteins. - The toxin's A subunit **permanently activates the Gs protein** by preventing its GTPase activity, keeping it in an active GTP-bound state. - The constitutively active Gs protein continuously activates **adenylate cyclase**, leading to sustained elevation of intracellular **cyclic AMP (cAMP)**. - This increased cAMP causes **excessive secretion of chloride ions and water** into the intestinal lumen, resulting in profuse watery "rice-water" diarrhea characteristic of cholera. *Sodium channels* - Sodium channels are ion channels, not enzymes. - While sodium and chloride transport is affected in cholera, the toxin does not directly activate sodium channels. - The increased cAMP affects the **CFTR (cystic fibrosis transmembrane conductance regulator)** chloride channel, which indirectly affects sodium absorption. *Potassium channels* - Potassium channels are ion channels, not enzymes. - The cholera toxin does not activate potassium channels. - Potassium loss occurs as a consequence of the massive fluid secretion, not through direct channel activation. *Guanylate cyclase* - **Guanylate cyclase** is activated by different enterotoxins, notably **E. coli heat-stable enterotoxin (STa)**, leading to increased cyclic GMP (cGMP). - Increased cGMP also causes secretory diarrhea, but this is **not the mechanism of cholera toxin**. - Heat-labile E. coli toxin (LT) has a similar mechanism to cholera toxin (activating adenylate cyclase).

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