CFS CFS Food Microbiology 2 — Questions and Answers
Question 1: At what minimum water activity (a_w) can most bacteria grow and pose a food safety risk?
- 0.60
- 0.70
- 0.91 (Correct answer)
- 0.99
Correct answer: 0.91
Most bacteria require a minimum water activity of approximately 0.91 to grow, which is why intermediate-moisture foods formulated below this threshold can suppress bacterial proliferation.
Question 2: Escherichia coli O157:H7 is classified as an enterohemorrhagic pathogen (EHEC). What toxin does it produce that causes severe illness?
- Aflatoxin B1
- Shiga toxin (Stx1 and Stx2) (Correct answer)
- Botulinum neurotoxin
- Enterotoxin A
Correct answer: Shiga toxin (Stx1 and Stx2)
E. coli O157:H7 produces Shiga toxins (Stx1 and Stx2) that damage intestinal cells and can cause hemolytic uremic syndrome (HUS), a potentially fatal kidney complication.
Question 3: HTST (High-Temperature Short-Time) pasteurization of fluid milk in the US is conducted at approximately which conditions?
- 63°C (145°F) for 30 minutes
- 72°C (161°F) for 15 seconds (Correct answer)
- 85°C (185°F) for 3 seconds
- 121°C (250°F) for 15 seconds
Correct answer: 72°C (161°F) for 15 seconds
US HTST pasteurization heats milk to a minimum of 72°C (161°F) for at least 15 seconds, providing a 5-log reduction of Coxiella burnetii, the most heat-resistant non-spore-forming pathogen of concern.
Question 4: Salmonella is most commonly associated with which type of food vehicle in the United States?
- Canned low-acid vegetables
- Poultry, eggs, and egg products (Correct answer)
- Raw fluid milk only
- High-acid fermented products
Correct answer: Poultry, eggs, and egg products
Poultry and eggs are the most common vehicles for Salmonella in the US, with the CDC attributing the majority of salmonellosis outbreaks to these animal-derived foods.
Question 5: Staphylococcus aureus food poisoning differs from most other bacterial foodborne illnesses because:
- The bacteria must be present in the food at the time of consumption
- Illness is caused by a pre-formed heat-stable enterotoxin, not live bacteria (Correct answer)
- It only infects immunocompromised individuals
- Symptoms appear 24–72 hours after consumption
Correct answer: Illness is caused by a pre-formed heat-stable enterotoxin, not live bacteria
S. aureus illness is intoxication, not infection — the bacteria produce heat-stable enterotoxins in food that remain active even after the bacteria are killed by cooking, with symptoms appearing within 1–6 hours.
Question 6: In food fermentation, lactic acid bacteria (LAB) preserve food primarily through which mechanism?
- Producing hydrogen peroxide that sterilizes the product
- Lowering pH via lactic acid production, inhibiting pathogen growth (Correct answer)
- Consuming all available oxygen to create strict anaerobiosis
- Producing antibiotics that kill all competing microorganisms
Correct answer: Lowering pH via lactic acid production, inhibiting pathogen growth
LAB ferment carbohydrates to lactic acid, lowering the product pH to levels (typically ≤ 4.6) that inhibit most pathogens and spoilage organisms, providing natural biopreservation.
CFS CFS Food Microbiology 2
At what minimum water activity (a_w) can most bacteria grow and pose a food safety risk?