How bacteria multiply

Pathogenic bacteria, also known as pathogens, are the organisms responsible for most foodborne illnesses. When pathogenic bacteria find themselves in the right conditions they multiply simply by each cell dividing in two, repeatedly. In this way, a single bacteria cell becomes two cells. Two cells become four, then four become eight (and so on). In ideal conditions, bacteria are capable of dividing in this way once every 10-15 mins;  enabling them to multiply very quickly.

Pathogenic bacteria need ALL of the following factors to be present to multiply:

1. Food - Like us, pathogenic bacteria need food to survive and multiply. They thrive in raw and cooked foods containing meat, dairy, fish and seafood. They also thrive in ready-to-eat foods including filled sandwiches, pies and quiches, and foods containing moist, protein rich ingredients such as mayonnaise and pate. 
2. Moisture - Pathogenic bacteria need water to live. They cannot multiply in dried (dehydrated) foods, for example, pasta, rice, and powdered milk. 
3. Warmth - Pathogenic bacteria multiply fastest between 8°C and 63°C. This is known as the Danger Zone.  Understanding the threat of the Danger Zone is vitally important to food safety. 
4. Time - When pathogenic bacteria are given sufficient time, food, warmth and moisture, they multiply to dangerous levels. 

Danger Zone

You’ve just discovered two vitally important facts about food safety: 

1. Pathogenic bacteria multiply fastest between 8°C and 63°C (the Danger Zone). 
2. Pathogenic bacteria require time in the Danger Zone to multiply.

However, the danger doesn't stop there - pathogens are not the only organisms in food that can cause us serious harm.

Toxins and spores

Many pathogenic bacteria are known to release poisonous substances called toxins. Some release them during their life, and some when they are destroyed by cooking heat. Cooking kills most pathogenic bacteria but the toxins left behind can still cause illness. 

Several pathogenic bacteria are also resistant to extreme temperatures.  They achieve this by developing a protective shell known as a spore. They simply wait until they return to the Danger Zone and start multiplying again. Rice is particularly susceptible to spore forming pathogenic bacteria. So, restricting the growth of pathogens is vital in order to minimise the risk of the development of both toxins and spores 

How to limit the growth of pathogens

We now know that pathogens need four factors to multiply. By restricting these four factors - food, moisture, warmth and time - you restrict the pathogen's opportunity to multiply to dangerous levels. Therefore - keeping food out of the warm Danger Zone for as much time as possible is a major factor in controlling pathogenic growth.

It is likely that the temperature of your kitchen sits comfortably within the Danger Zone. Every minute food spends outside the chiller, cooker, hot-hold or freezer, it is exposed to the Danger Zone and pathogenic bacteria will be able to multiply. Your job is to monitor and reduce the time food spends in the Danger Zone during its journey.

Think about the processes that happen to a joint of meat within the course of food production.

• Delivery - the meat will be transferred from a delivery vehicle to your chilled or frozen storage.
• Processing - the meat may be portioned, wrapped and labelled for further use, placed in a marinade, trimmed or otherwise prepped for future use, before returning to chilled storage.
• Cooking - the meat may be left to come to temperature before placing in the oven.
• Resting, re-chilling - the meat may be left to rest after cooking or rechilled for future use.
• Slicing, serving - the meat could be left at ambient temperature for some time during the serving process.

At all of these stages, the meat spends time in the danger zone. As part of your HACCP plan, you will need to assess the time spent in the danger zone and reduce wherever practical. At an absolute maximum, high risk foods should spend no longer than 2 hours within danger zone temperatures, and this time is further reduced on warmer days.

To understand more about pathogenic bacteria and how to keep them out of the Danger Zone, have a look at our Level 2 Food Safety and Level 2 HACCP courses. Less bacteria means safer food, happier customers and a better business!