2.4 Food Microbiology & Safety Foundations

Food microbiology and safety protect the public and appear on the RD exam in both Principles of Dietetics and Foodservice Systems contexts. You must know what makes bacteria grow, the temperatures that keep food safe, the major pathogens, and the preventive logic of HACCP. These rules come from the FDA Food Code (the 2022 edition is the current model regulation most states adopt) and govern any facility a dietitian manages.

Conditions for Bacterial Growth: FAT TOM

Bacteria need six conditions, remembered as FAT TOM:

• Food — nutrient/protein-rich foods such as meat, poultry, eggs, dairy, and cooked rice.
• Acidity — a near-neutral pH of 4.6-7.5 favors pathogens; high-acid foods (pH < 4.6) inhibit growth, which is why pickling and acidification preserve food.
• Temperature — the danger zone (below).
• Time — limited exposure restricts how far a population can multiply.
• Oxygen — aerobes need it; anaerobes (e.g., Clostridium botulinum) thrive without it.
• Moisture — measured as water activity (aw); pathogens generally need aw above 0.85, so drying and salting (which lower aw) preserve food.

Controlling any one factor limits growth, but time and temperature are the two a foodservice operation controls most directly and the two the FDA Food Code regulates most tightly. Foods that support rapid growth are labeled TCS (Time/Temperature Control for Safety) foods.

The Temperature Danger Zone

The danger zone is 41-135°F (5-57°C) — the range where bacteria multiply most rapidly, doubling roughly every 20 minutes under ideal conditions. Total cumulative time in this zone must not exceed 4 hours before TCS food is discarded.

Safe minimum internal cooking temperatures (FDA Food Code 2022), each held for the required time (15 seconds where noted):

Cooling rule (two-stage): cool hot TCS food from 135°F to 70°F within 2 hours, then from 70°F to 41°F within the next 4 hours — 6 hours total. If either stage is missed, the food must be discarded. Reheating for hot holding requires reaching 165°F within 2 hours. Thaw food safely in the refrigerator, under running water at ≤70°F, in the microwave (then cook immediately), or as part of cooking — never on the counter at room temperature.

Major Foodborne Pathogens (Bacterial)

A key distinction the exam draws is infection versus intoxication: Salmonella and Listeria cause infection (live organisms multiply in the host, longer onset), whereas Staphylococcus aureus and C. botulinum cause intoxication from a preformed toxin (faster onset, and the toxin may survive cooking).

More High-Yield Pathogens

Because Staph aureus toxin is heat-stable and Norovirus spreads from infected workers, both reinforce the same control: rigorous personal hygiene and excluding ill food handlers. The FDA Food Code requires excluding employees with symptoms such as vomiting, diarrhea, jaundice, or a sore throat with fever, and excluding anyone diagnosed with the "Big 6" pathogens (Norovirus, Salmonella Typhi, nontyphoidal Salmonella, Shigella, E. coli O157:H7/Shiga-toxin-producing, and Hepatitis A).

HACCP and Cross-Contamination Control

HACCP (Hazard Analysis and Critical Control Points) is a proactive, science-based system that prevents hazards rather than catching them at final inspection. Its seven principles, in order:

1. Conduct a hazard analysis.
2. Determine the critical control points (CCPs) — steps where a hazard can be controlled.
3. Establish critical limits (e.g., cook poultry to 165°F).
4. Establish monitoring procedures for each CCP.
5. Establish corrective actions when a limit is not met.
6. Establish verification procedures (confirm the system works).
7. Establish record-keeping and documentation.

A CCP is a point where control is essential to prevent, eliminate, or reduce a hazard to an acceptable level — typically the cooking, cooling, or holding step — not merely any place food is touched.

Cross-contamination control: store raw meats below and away from ready-to-eat foods (with raw poultry on the very bottom shelf because of its high cook temperature), use color-coded cutting boards and utensils, clean and sanitize surfaces, and wash hands with soap and warm water for at least 20 seconds. Apply first-in, first-out (FIFO) stock rotation to use older product first. Together these practices control the biological hazards that FAT TOM and the danger zone make possible.