Food Microbiology Management

Bacteria in Food

Bacteria are everywhere in our environment. Most are harmless and are used to make foods, such as yogurt. Others are spoilage organisms that sour and rot foods. A few bacteria become a threat to our health when they grow and reproduce; these are commonly known as microbiological pathogens. Sources of these bacteria include soil, water, air, dust, edible plants, plant products, animals, animal products, intestinal tracts of humans and animals, employees’ hands and contaminated food utensils and equipment. A common misconception is that food is free of bacteria that cause food borne diseases when it reaches the establishment or after processing.

Human Bacteria

Another common misconception is that healthy employees do not harbour harmful bacteria. Healthy people commonly carry their own natural population of bacteria, and some of these can cause food borne illness. For example, Staphylococcus aureus is commonly carried on the skin and in the nose of healthy people. Sick employees are carriers of greater numbers of organisms that cause food borne illness, so effective personal hygiene standards and the exclusion of ill employees from food handling are essential controls.

Bacterial Growth

Bacteria require nutrients. These are essentially foods to provide the basic elements for their growth. These nutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, sodium, magnesium, iron and manganese. Before bacterial growth can occur, other essential requirements must be favourable, such as temperature and the level of oxygen of the bacterial growth environment.

Bacterial growth refers to the increase in the number of organisms. This process is accomplished by binary fission, whereby the bacterial cell splits to form two cells. Bacterial growth can be very rapid. It may occur at a frequency of every 20 minutes in optimum conditions, but not until conditions are just right for the type of bacteria involved. There are four phases bacteria go through within their life cycle. It is important to understand what takes place at each phase of the bacterial growth curve to be able to target effective control points for bacteria within a structured food safety and quality program.

Bacterial Lag Phase: Phase 1

When bacteria are introduced to food, there is usually an adjustment or lag period. During this time there is considerable biochemical activity but no increase in the number of cells. The lag phase can be from a few hours to days. When conditions are right for the type of bacteria involved, rapid growth commences.

Bacterial Log Phase: Phase 2

This is called the logarithmic or log phase because the bacteria double their number by cell division, some doubling every 20 minutes. This rapid growth stage is generally not appreciated until it is illustrated. The following theoretical example shows how bacteria can multiply under ideal conditions, starting with 216 cells and a doubling time of 20 minutes:

• 20 minutes: 432 cells;
• 40 minutes: 864 cells;
• 60 minutes (1 hour): 1,728 cells;
• 80 minutes: 3,456 cells;
• 100 minutes: 6,912 cells;
• 120 minutes (2 hours): 13,824 cells;
• 140 minutes: 27,648 cells;
• 160 minutes: 55,296 cells;
• 180 minutes (3 hours): 110,592 cells;
• 200 minutes: 221,184 cells;
• 220 minutes: 442,368 cells;
• 240 minutes (4 hours): 884,736 cells;
• 260 minutes: 1,769,472 cells.

This example demonstrates how a starting population of 216 bacteria with a 20-minute doubling time would grow to more than 1 million bacteria after 4 hours and 20 minutes. Growth is slower when conditions are not ideal.

Stationary Phase: Phase 3

After a period of rapid growth, bacteria numbers reach the levelling-off stage as their nutrients are used up and waste accumulates. Foods contaminated by bacteria at this level and beyond are usually spoiled because of the bacterial activity and are generally unacceptable from a purely organoleptic viewpoint including flavour, aroma, texture and appearance.

Death Phase: Phase 4

At this point, the food is no longer suitable for supporting growth and the bacteria die. Toxins produced earlier, and heat-resistant spores, may remain in the food, so this phase does not make the food safe.

Effects of Temperature on Bacteria

Because of the unique survival capabilities of bacteria, it is important to limit their growth in food as much as possible. Bacteria have a maximum and minimum range of temperature for growth, which varies from one type to the next. Food poisoning bacteria grow in the temperature danger zone of 5°C (41°F) to 60°C (140°F), and growth is fastest at around 37°C (99°F). The following classification is provided as an example of bacterial profiling. The ranges are approximate and vary between sources and species:

• Psychrophiles: Grow best at around 15°C (59°F) and can grow at or below 0°C (32°F), with little growth above about 20°C (68°F). Psychrophiles can grow under refrigerated temperatures;
• Psychrotrophs: Able to grow at refrigeration temperatures and up to about 35°C (95°F). Psychrotrophs are important spoilage organisms in refrigerated foods;
• Mesophiles: Grow best between about 20°C (68°F) and 45°C (113°F), with an optimum near 37°C (99°F). Mesophiles are generally considered the most common bacteria affecting foods;
• Thermophiles: Grow best between about 50°C (122°F) and 80°C (176°F) and do not multiply at normal room temperature. Thermophiles are important to consider in processes such as canning and hot filling, as they are heat resistant.

Microbiological Cross Contamination

Pathogens can be transferred from one food to another, either by direct contact or by food handlers, contact surfaces or the air. Raw, unprocessed food should be effectively separated, either physically or by time, from ready-to-eat foods, with effective intermediate cleaning and where appropriate disinfection. Access to processing areas may need to be restricted or controlled. Where risks are particularly high, access to processing areas should be only via a changing facility. Personnel may need to be required to put on clean protective clothing including footwear and wash their hands before entering. Surfaces, utensils, equipment, fixtures and fittings should be thoroughly cleaned and where necessary disinfected after raw food, particularly meat and poultry, has been handled or processed.

If your food business supplies foodstuffs manufactured to a customer’s specifications, it is important to consider any specific Food Microbiology Management Development requirements in relation to their items.

Common Foodborne Bacteria

The following bacteria are among those most often associated with foodborne illness. Typical sources are provided as examples only, and any food can become contaminated if controls fail:

• Salmonella: Commonly associated with poultry, eggs, meat and fresh produce contaminated through animals or the environment;
• Listeria monocytogenes: Can grow at refrigeration temperatures and is a particular concern in ready-to-eat foods such as deli meats, soft cheeses, smoked fish and pre-cut produce;
• Pathogenic Escherichia coli, including Shiga toxin-producing strains: Associated with undercooked beef, raw milk and fresh produce contaminated by animal faeces;
• Campylobacter: Commonly associated with raw poultry and raw milk;
• Staphylococcus aureus: Carried on human skin and in the nose, and able to produce heat-stable toxins in foods that are handled and then held at unsafe temperatures;
• Clostridium perfringens: Associated with large batches of cooked meat and gravies that are cooled slowly or held warm;
• Bacillus cereus: Associated with cooked rice, pasta and starchy foods held at unsafe temperatures, and capable of forming heat-resistant spores;
• Clostridium botulinum: Forms spores and produces a potent toxin in low-oxygen, low-acid foods that are improperly processed, such as some canned and vacuum-packed foods.

Controls for these bacteria are usually identified through the HACCP hazard analysis and include time and temperature control, effective cleaning, separation of raw and ready-to-eat foods, and personal hygiene.

Microbiological Testing and Criteria

Microbiological testing provides objective evidence of the hygiene and safety of ingredients, products and the processing environment. Testing is a verification tool and should not be relied on to make food safe, because it can only sample a small part of any batch.

Key considerations include:

• Pathogen and indicator tests: Pathogen tests look for specific disease-causing organisms, while indicator tests, such as total plate count or Enterobacteriaceae, show the general level of hygiene and process control;
• Sampling plans: The number of samples, their size and the frequency of testing should be based on risk and be documented;
• Microbiological criteria: Limits should be based on regulatory requirements, customer specifications and recognised guidance, with clear actions defined for results outside the limits;
• Laboratory selection: Where possible, use laboratories accredited for the test methods applied, and confirm that the methods are suitable for the food tested;
• Trend analysis: Results should be reviewed over time, because a gradual rise can show a loss of control before a limit is exceeded.

Results outside criteria should be managed through the corrective action and preventative action process and may require product to be placed on hold.

Regulatory and Industry References

The following official sources support the concepts on this page. Always check the current edition and the requirements that apply in your jurisdiction and to your customers.

• FSANZ Compendium of Microbiological Criteria for Food: Guidance on process hygiene criteria and microbiological guideline criteria for ready-to-eat foods, last updated August 2025;
• Codex Alimentarius guidelines: Includes Codex guidance on the establishment and application of microbiological criteria for foods;
• World Health Organization, Five keys to safer food manual: Includes the core message to keep food at safe temperatures.