Food Microbiology — MCQs

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?

Q49

A person developed severe vomiting after eating food from a Chinese restaurant, 3 hours prior. What is the most likely causative agent?

Q50

Food poisoning has occurred due to the ingestion of contaminated fried rice. Which organism is most likely responsible?

Food Microbiology Indian Medical PG Practice Questions and MCQs

Question 41: Which of the following organisms is primarily responsible for causing scombroid fish poisoning?

A. Morganella morganii (Correct Answer)
B. Staphylococcus aureus
C. Weissella
D. Salmonella

Explanation: ***Morganella morganii*** - *Morganella morganii* is a common bacterium that converts **histidine** in fish muscle to **histamine** when fish are improperly stored. - The elevated histamine levels in the fish, when consumed, lead to symptoms mimicking an **allergic reaction**, known as scombroid fish poisoning. *Staphylococcus aureus* - *Staphylococcus aureus* is a common cause of food poisoning due to the production of **enterotoxins**, which are not related to histamine formation in fish. - Symptoms typically involve acute **nausea, vomiting, and abdominal cramps**, rather than the histamine-mediated symptoms of scombroid poisoning. *Weissella* - *Weissella* species are lactic acid bacteria commonly found in fermented foods and can sometimes be associated with spoilage, but they are **not the primary causative agent** of scombroid fish poisoning. - While some *Weissella* species can produce biogenic amines, their role in scombroid poisoning through histamine production from fish is **not significant** compared to *Morganella* species. *Salmonella* - *Salmonella* causes food poisoning primarily through **infection** of the gastrointestinal tract, leading to symptoms like **diarrhea, fever, and abdominal cramps**. - It does not produce histamine from histidine in fish muscle and is **not associated** with the allergic-like reaction characteristic of scombroid poisoning.

Question 42: A patient complains about nausea, vomiting and stomach cramps after attending a social gathering party. Which causative organism is likely responsible for food poisoning within 3 hours?

A. Clostridium botulinum
B. Clostridium perfringens
C. Salmonella
D. Staphylococcus aureus (Correct Answer)

Explanation: ***Staphylococcus aureus*** - *Staphylococcus aureus* produces preformed **enterotoxins** in food, leading to rapid onset of symptoms (typically 1-6 hours, often within 3 hours) after ingestion. - The symptoms, including **nausea, vomiting, and stomach cramps**, are characteristic of staphylococcal food poisoning due to the direct action of these toxins on the gastrointestinal tract. *Salmonella* - **Salmonellosis** typically has a longer incubation period, usually 6-72 hours, as it requires the organism to colonize and invade the host's intestinal cells. - While it causes **nausea, vomiting, and cramps**, the rapid onset within 3 hours makes *Staphylococcus aureus* a more likely culprit. *Clostridium botulinum* - **Botulism** is characterized by neurological symptoms such as **flaccid paralysis**, blurred vision, and difficulty speaking or swallowing, rather than prominent nausea and vomiting alone. - The incubation period for botulism is typically 12-36 hours, making a 3-hour onset highly unlikely. *Clostridium perfringens* - *Clostridium perfringens* food poisoning usually has an incubation period of 6-24 hours, longer than the 3 hours specified in the question. - It primarily causes **abdominal cramps and watery diarrhea**, with vomiting being less common than with *Staphylococcus aureus*.

Question 43: Which of the following organisms is MOST associated with histamine production in scombroid fish poisoning?

A. Morganella morganii (Correct Answer)
B. P. aeruginosa
C. Staphylococcus
D. Salmonella

Explanation: ***Morganella morganii*** - ***Morganella morganii*** is the **MOST commonly implicated organism** in **scombroid fish poisoning** - It produces the enzyme **histidine decarboxylase**, which converts **histidine** (abundant in scombroid fish like tuna, mackerel, bonito) to **histamine** - When fish are stored at improper temperatures (>15°C), *M. morganii* proliferates and produces large amounts of **histamine** - Other histamine-producing bacteria include *Klebsiella*, *Proteus*, *Enterobacter*, and *Photobacterium* species - Symptoms mimic an **allergic reaction**: flushing, headache, palpitations, urticaria, diarrhea *Staphylococcus* - ***Staphylococcus aureus*** causes **staphylococcal food poisoning** through preformed **enterotoxins** (not histamine) - Associated with protein-rich foods left at room temperature - Causes rapid-onset nausea, vomiting, and abdominal cramps - **Not a significant cause** of scombroid fish poisoning *Salmonella* - ***Salmonella*** species cause **gastroenteritis** through direct infection and invasion of intestinal mucosa - Symptoms include **diarrhea**, **fever**, and **abdominal cramps** with longer incubation period (6-72 hours) - **Not associated** with histamine production or scombroid poisoning *P. aeruginosa* - ***Pseudomonas aeruginosa*** is an **opportunistic pathogen** causing hospital-acquired infections - Associated with wound infections, pneumonia, and UTIs in immunocompromised patients - **Not a cause** of foodborne histamine poisoning

Question 44: A child presents with vomiting and abdominal pain 5 hours after eating. The most likely causative organism is:

A. Bacillus cereus (Correct Answer)
B. Cl. perfringens
C. V. cholerae
D. Cl. botulinum

Explanation: ***Bacillus cereus*** - This organism produces **emetogenic toxins** such as cereulide, which cause **short-incubation food poisoning** characterized by vomiting and abdominal pain, typically within 1-6 hours after consumption. - Common sources include **fried rice** and other starchy foods that have been improperly re-heated, allowing spore germination and toxin production. *Cl. perfringens* - This bacterium produces an **enterotoxin** that causes diarrheal illness with a longer incubation period, usually 8-16 hours, and is characterized more by **abdominal cramps and diarrhea** rather than prominent vomiting. - It is typically associated with consumption of contaminated meat and poultry. *V. cholerae* - This bacterium causes **cholera**, an acute diarrheal illness characterized by **profuse watery diarrhea** and severe dehydration, with an incubation period of 2 hours to 5 days. - Vomiting can occur but is secondary to the severe fluid loss and is not the primary symptom in short-incubation cases with abdominal pain. *Cl. botulinum* - This bacterium produces **botulinum neurotoxin**, which causes botulism, a rare but severe illness characterized by **flaccid paralysis**. - While gastrointestinal symptoms like vomiting can occur, the hallmark feature is **neurological dysfunction**, and the incubation period is typically 12-36 hours, not 5 hours with isolated vomiting and abdominal pain.

Question 45: Milk ring test is done to detect which organism present in milk?

A. Mycobacterium
B. Bordetella
C. Bartonella
D. Brucella (Correct Answer)

Explanation: ***Brucella*** - The **milk ring test (MRT)** is a rapid serological test used for the detection of **Brucella antibodies** in milk samples, indicating an infection in the herd. - This test is crucial for screening dairy cattle and helps in the control and eradication of **brucellosis**, a zoonotic disease. *Bordetella* - **Bordetella** species are primarily associated with **respiratory infections** in humans and animals, such as whooping cough (B. pertussis). - They are not typically detected in milk using serological tests like the milk ring test. *Bartonella* - **Bartonella** species are known to cause diseases like **cat scratch disease** and **trench fever**, often transmitted by vectors. - They are not commonly associated with milk contamination or detected by the milk ring test. *Mycobacterium* - **Mycobacterium** species, particularly **M. bovis**, can cause tuberculosis in cattle and be transmitted through milk. - While important for milk safety, their detection usually involves different methods like **tuberculin skin tests**, **culture**, or **PCR**, not the milk ring test.

Question 46: An adolescent male developed vomiting and diarrhea 1 hour after having food from a restaurant. The most likely pathogen is?

A. Clostridium perfringens
B. Vibrio parahaemolyticus
C. Staphylococcus aureus (Correct Answer)
D. Salmonella

Explanation: ***Staphylococcus aureus*** - The rapid onset of symptoms (within 1 hour) strongly suggests **pre-formed toxin ingestion**, which is characteristic of *Staphylococcus aureus* food poisoning. - While the typical incubation period is **1-6 hours** (average 2-4 hours), onset within 1 hour can occur with **high toxin loads** in contaminated food. - **Vomiting** is often the predominant symptom, occurring shortly after consuming contaminated food, which distinguishes it from other bacterial causes. *Clostridium perfringens* - Onset of symptoms caused by *Clostridium perfringens* is typically **8-16 hours** after ingestion, which is much longer than observed here. - It primarily causes **diarrhea and abdominal cramps** due to toxin production in the intestine, with minimal vomiting. *Vibrio parahaemolyticus* - Symptoms usually appear **4-96 hours** (average 12-24 hours) after consuming contaminated seafood, which is a longer incubation period than described. - It typically causes **watery diarrhea**, abdominal cramps, nausea, and occasional vomiting, but not within 1 hour. *Salmonella* - The incubation period for *Salmonella* infection is typically **6-72 hours** (average 12-36 hours), making it highly unlikely for symptoms to appear within 1 hour. - **Diarrhea, fever, and abdominal cramps** are common with *Salmonella*, but rapid-onset vomiting from pre-formed toxin is not its mechanism.

Question 47: Vibrio parahaemolyticus is most commonly associated with which type of undercooked seafood?

A. Crustaceans (e.g., crab, prawn)
B. Shellfish (e.g., oysters) (Correct Answer)
C. Fish
D. Vegetables

Explanation: ***Shellfish (e.g., oysters)*** - *Vibrio parahaemolyticus* is a **halophilic bacterium**, meaning it thrives in salty marine environments. **Shellfish**, particularly oysters, filter large volumes of seawater, accumulating these bacteria. - Undercooked raw shellfish is a common source of **foodborne illness** caused by *V. parahaemolyticus*, leading to gastroenteritis. *Crustaceans (e.g., crab, prawn)* - While crustaceans live in marine environments and can host *Vibrio* species, **shellfish** are more commonly and significantly implicated in *V. parahaemolyticus* outbreaks due to their filter-feeding nature. - Contamination from crustaceans is less frequent compared to shellfish when considering *V. parahaemolyticus* infections. *Fish* - **Fin fish** generally do not accumulate *Vibrio parahaemolyticus* to the same extent as filter-feeding shellfish. - While raw fish dishes (e.g., sushi) can be sources of other foodborne pathogens, *V. parahaemolyticus* is less commonly associated with them. *Vegetables* - Vegetables are not marine organisms and do not naturally harbor **halophilic bacteria** like *V. parahaemolyticus*. - Contamination of vegetables would only occur through cross-contamination with seafood or contaminated water, it's not a primary source.

Question 48: 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 49: A person developed severe vomiting after eating food from a Chinese restaurant, 3 hours prior. What is the most likely causative agent?

A. C. difficile
B. Staph. aureus
C. B. cereus (Correct Answer)
D. Pseudomonas

Explanation: ***B. cereus*** - *Bacillus cereus* produces an **emetic toxin (cereulide)** that causes vomiting within **1-6 hours** after consumption, fitting the 3-hour onset perfectly. - This bacterium is commonly associated with **fried rice** in Chinese restaurants due to improperly cooked or reheated rice where spores survive cooking and germinate. - The preformed heat-stable toxin causes rapid-onset vomiting without significant diarrhea. *Staph. aureus* - While *Staphylococcus aureus* food poisoning also causes rapid-onset vomiting within **1-6 hours** due to preformed enterotoxin, making it a close differential. - However, *B. cereus* is more specifically associated with **starchy foods like rice**, making it the most likely culprit in the context of Chinese restaurant food. *C. difficile* - *Clostridioides difficile* infection primarily causes **diarrhea and pseudomembranous colitis**, typically after antibiotic use disrupting normal gut flora. - The onset of symptoms is typically **days to weeks** after exposure, not acute vomiting within hours of a meal. - Not associated with foodborne transmission. *Pseudomonas* - *Pseudomonas* species rarely cause foodborne illness and are not associated with rapid-onset vomiting. - They are primarily opportunistic pathogens causing infections in immunocompromised individuals or healthcare settings. - When implicated in food contamination, they affect raw produce, not typical Chinese restaurant cooked food.

Question 50: Food poisoning has occurred due to the ingestion of contaminated fried rice. Which organism is most likely responsible?

A. Staphylococcus aureus
B. Vibrio parahemolyticus
C. Bacillus cereus (emetic type) (Correct Answer)
D. Clostridium perfringens

Explanation: ***Bacillus cereus (emetic type)*** - The **emetic toxin** produced by *Bacillus cereus* is commonly associated with food poisoning from **reheated fried rice**. - This toxin causes rapid onset of **nausea** and **vomiting** within 0.5 to 6 hours after ingestion. *Staphylococcus aureus* - While *Staphylococcus aureus* also causes **rapid-onset emetic food poisoning**, it is more typically associated with dairy products, meats, and salads, not specifically fried rice. - The symptoms are primarily due to preformed **enterotoxins**. *Vibrio parahemolyticus* - This bacterium is primarily associated with the consumption of **raw or undercooked seafood**, leading to **diarrhea**, abdominal cramps, nausea, vomiting, and fever. - It is not typically linked to fried rice. *Clostridium perfringens* - *Clostridium perfringens* causes **diarrheal-type food poisoning** characterized by abdominal cramps and diarrhea, usually from contaminated meat or poultry dishes. - It typically has a longer incubation period (6-24 hours) and is not primarily emetic or associated with fried rice.

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