FOOD MICROBIOLOGY: TEMPERATURE AND GROWTH
Diagram
Two bacteria, two temperatures
Psychrophilic bacteria
Like cold, from about 0 to 15 C. They grow in the fridge, where most bacteria stop.
↓
Mesophilic bacteria
Like moderate warmth, about 20 to 45 C. Most illness-causing bacteria are mesophiles.
↓
The gap
Between the two ranges sits the danger zone, often described as 5 to 60 C, where growth is fast enough to matter within hours.
Figure 1: Psychrophiles grow in the cold and mesophiles grow in moderate warmth. Drawn in our own style from the FTN Food Microbiology notes, Unit 1, and from Gerday and Glansdorff, Physiology and Biochemistry of Extremophiles.
Introduction
Psychrophilic and mesophilic bacteria are two groups of bacteria sorted by the temperature they prefer, and both grow in food, but at opposite ends of the temperatures a kitchen knows. Psychrophiles like cold, from about 0 to 15 C, and grow in the fridge, while mesophiles like moderate warmth, about 20 to 45 C, and grow at room temperature and at body temperature. That is why cold storage stops one group and feeds the other: the fridge is a home for psychrophiles and a pause for mesophiles.
The FTN Food Microbiology notes, Unit 1, call temperature the strongest extrinsic factor and give the ranges for both groups. Gerday and Glansdorff, Physiology and Biochemistry of Extremophiles, cover the cold-adapted microbes in depth. This article follows those sources.
Quick comparison
Table 1: Psychrophilic bacteria compared with mesophilic bacteria
| Aspect | Psychrophilic bacteria | Mesophilic bacteria |
|---|---|---|
| Preferred temperature | Cold, from about 0 to 15 C | Moderate warmth, about 20 to 45 C |
| Growth in the fridge | Grow well, even in chilled food | Grow slowly or not at all in the cold |
| Growth at room temperature | Grow, but warmth is not their best range | Grow fast, and fastest between 20 and 45 C |
| Growth at body temperature | Do not grow well at 37 C | Grow well: most illness-causing bacteria are mesophiles |
| Example | Pseudomonas species, which spoil chilled milk even in the fridge | Salmonella, which is a mesophile, so warm food left out is its best chance to grow |
| Food risk | Spoilage of chilled food: milk, meat and produce kept cold still spoil | Illness: warm food left out grows mesophiles fast enough to matter within hours |
| Danger zone | Below the danger zone, which is often described as 5 to 60 C | Across the danger zone: growth in this range is fast enough to matter within hours |
| Effect of freezing | Freezing stops growth but does not kill most microbes, and thawing lets them start again | The same: freezing stops growth but does not kill most microbes |
| Effect of cooking | Killed by cooking, like most vegetative cells | Cooking kills most vegetative cells, but not all spores |
| Control | Keep chilled food cold and use it in time, because cold alone does not stop psychrophiles | Keep food out of the danger zone: chill it fast, keep it hot, and do not leave it warm |
Scientific explanation
Definition
A psychrophile is a microbe that grows well in the cold, from about 0 to 15 C, and the food example in the FTN notes is Pseudomonas species, which spoil chilled milk even in the fridge. A mesophile is a microbe that grows best at moderate warmth, about 20 to 45 C, and the food example is Salmonella, which is a mesophile, so warm food left out is its best chance to grow.
Working principle
Temperature is the strongest extrinsic factor for microbes, and the groups are named by the temperature they prefer: psychrophiles like cold, mesophiles like moderate warmth, and thermophiles like heat, from 45 to 70 C. Most illness-causing bacteria are mesophiles and grow fastest between 20 and 45 C, with a danger zone often described as 5 to 60 C, where growth is fast enough to matter within hours. That is why the same food behaves differently at different temperatures: in the fridge, mesophiles pause and psychrophiles grow, so chilled milk still spoils; on a warm counter, mesophiles grow fast, so food left out becomes a risk within hours. Freezing stops growth but does not kill most microbes, and thawing lets them start again. Cooking kills most vegetative cells but not all spores.
Apparatus
The books used here name no laboratory apparatus for telling the two apart, so no apparatus list is given.
Measurement procedure
The books name no single standard procedure for telling the two apart. The distinction is shown by growth at different temperatures, using the plate count methods of the FTN Food Microbiology notes, Unit 3: a plate held cold grows the psychrophile, and a plate held warm grows the mesophile.
Factors affecting readings
Temperature is the strongest factor, and it is the factor that sorts the two groups. Other factors in the FTN notes, such as water activity, pH and oxygen, decide how fast each group grows once the temperature suits it, but they do not change which group prefers the cold and which prefers warmth. Time matters too: growth in the danger zone is fast enough to matter within hours, so a food left warm for a short time is a smaller risk than the same food left warm all day.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Psychrophilic bacteria: spoilage of chilled milk, meat and produce, which must be used in time even when kept cold.
- Psychrophilic bacteria: the reason chilled food has a shelf life, and cold storage slows spoilage without stopping it.
- Psychrophilic bacteria: testing of chilled foods, where a cold plate count finds the microbes that a warm count would miss.
- Mesophilic bacteria: the main cause of foodborne illness, because most illness-causing bacteria grow best at moderate warmth.
- Mesophilic bacteria: the reason warm food left out is a risk within hours, and the reason the danger zone is watched.
- Mesophilic bacteria: fermentation at moderate warmth, where mesophilic starter cultures turn milk into cheese.
Advantages and limitations
- Psychrophilic bacteria, advantage to the maker: none, they are spoilage organisms; but knowing them explains why chilled food still spoils.
- Psychrophilic bacteria, limitation as a risk: they grow slowly in the cold, so chilled food keeps for days, and they are killed by cooking.
- Mesophilic bacteria, advantage to the maker: mesophilic starter cultures ferment foods at moderate warmth, turning milk into cheese and butter.
- Mesophilic bacteria, limitation as a risk: most illness-causing bacteria are mesophiles, so any food held warm is their best chance to grow.
FAQ
What is the main difference between psychrophilic and mesophilic bacteria?
The temperature they grow best at: psychrophiles like cold, from about 0 to 15 C, while mesophiles like moderate warmth, about 20 to 45 C.
Why does chilled milk still spoil in the fridge?
Because psychrophiles such as Pseudomonas grow well in the cold, even in chilled milk.
Why is warm food left out a risk?
Because most illness-causing bacteria are mesophiles, and they grow fastest between 20 and 45 C, so food left warm grows them fast enough to matter within hours.
What is the danger zone?
The range often described as 5 to 60 C, where growth is fast enough to matter within hours.
Does freezing kill psychrophiles and mesophiles?
No: freezing stops growth but does not kill most microbes, and thawing lets them start again.
References and further reading
- FTN subject: Food Microbiology, Unit 1: Microorganisms in Food, Sources and Factors.
- Charles Gerday and Nicolas Glansdorff (eds.), Physiology and Biochemistry of Extremophiles, ASM Press, 2007 (Google Books page read)
