Conditions for Bacterial Growth: FAT TOM
Bacteria are a bit like picky customers — they won’t thrive unless the conditions are just right. Food safety experts use the acronym FAT TOM to remember the six main factors that allow bacteria to multiply. If even one of these is missing, growth slows or stops. But if all six are present? Trouble.
Food
Bacteria love nutrient-rich foods. Protein is their favorite, but carbohydrates will do just fine once they’re cooked. This is why meat, poultry, seafood, eggs, dairy, and cooked rice or pasta are considered high-risk foods.
Kitchen examples:
- A tray of raw chicken breasts: packed with protein, perfect for bacteria.
- A pot of rice left overnight on the counter: Bacillus cereus paradise.
- A jug of milk forgotten out of the fridge: bacteria multiply in hours.
Acidity
Microbes prefer environments that are neutral to slightly acidic (pH 4.6–7.5). Very acidic foods like citrus juice or vinegar are naturally hostile to most bacteria, while neutral foods like meat or milk are ideal.
Kitchen examples:
- Chicken soup (pH ~6.5): in the bacterial sweet spot.
- Fresh tomatoes (pH ~4.5): less favorable, but still need care.
- Mayonnaise with vinegar (pH ~3.5): safe as-is, but potato salad made with it is riskier.
Temperature
This is the big one. Between 5–57°C (41–135°F) lies the danger zone, where bacteria can double in number every 20 minutes. Below that, growth slows. Above it, heat begins to kill them.
Kitchen examples:
- Soup cooling slowly on the counter: danger zone for hours.
- Beef stored at 10°C: unsafe, even if it looks fine.
- Chicken cooked to 75°C (EU) / 165°F (U.S.): pathogens destroyed.
Time
Temperature and time work hand in hand. If food stays in the danger zone for too long, bacterial populations explode. After about 4 hours of cumulative exposure, food is considered unsafe.
Kitchen examples:
- Buffet dishes sitting out all evening: must be discarded.
- Thawing chicken on the counter for 6 hours: unsafe to use.
- Delivery left at room temperature for 3 hours, then 2 more hours on the line: over the safe limit.
Oxygen
Some bacteria thrive with oxygen, others without it. This is why vacuum packing and canning can be risky if not done correctly — oxygen is cut off, creating the perfect playground for dangerous anaerobes like Clostridium botulinum.
Kitchen examples:
- Vacuum-packed smoked salmon: safe only when kept very cold.
- Cut lettuce sitting exposed in the prep fridge: aerobes grow on the surface.
- Home-canned beans with poor sterilization: botulism risk.
Moisture
Bacteria need water to grow, but it’s not just about how wet food looks. It’s about available water, measured as water activity (aw). The scale runs from 0 (completely dry) to 1.0 (pure water). Most pathogens need an aw of 0.85 or higher to multiply.
aw values in foods:
- Pure water: 1.00
- Fresh meat and fish: 0.98–0.99
- Bread: ~0.95
- Cheese: 0.90–0.94
- Dried fruit: 0.60–0.70
- Honey: ~0.60
- Crackers, powdered milk: <0.40
Kitchen examples:
- Fresh chicken (aw ~0.99): extremely high risk.
- Jam (aw ~0.70 thanks to sugar): too dry for bacteria, though molds can grow.
- Beef jerky (aw <0.85): safe from most pathogens due to low water activity.
When you put FAT TOM together, it explains why some foods need careful monitoring and others don’t. A raw steak in the fridge is a ticking clock. A jar of honey in the pantry is practically immortal.
Read next: Prevention Strategies