Basic Characteristics of Bacteria that Cause Foodborne Illness

Bacteria that cause foodborne illness have some basic characteristics:

They are living, single-celled organisms

They may be carried by a variety of means: food, water, humans, or insects

Under favorable conditions, they can reproduce very rapidly

Some can survive freezing

Some turn into spores, a change that protects the bacteria from unfavorable conditions

Some can cause food spoilage; others can cause illness

Some cause illness by producing toxins as they multiply, die, and break down

Bacterial Growth

To grow and reproduce, bacteria need:

Food

Appropriate level of acidity (pH)

Proper temperature

Adequate time

The necessary level of oxygen

Ample moisture

Their growth can be broken down into four progressive stages (phases): lag, log, stationary, and death.

When bacteria are first introduced to food, they go through an adjustment period, called the lag phase. In this phase, their numbers are stable as they prepare for growth. To control their number and prevent food from becoming unsafe, it is important to

prolong the lag phase as long as possible. You can accomplish this by controlling the bacteria's requirements for growth in food: time, temperature, moisture, oxygen, and pH. For example, by refrigerating food, you can keep bacteria in the lag phase. If these conditions are not controlled, bacteria can enter the next phase, the log phase, where they will grow remarkably fast.

Bacteria reproduce by splitting in two. Those in the process of reproduction are called vegetative microorganisms. As long as conditions are favorable, bacteria can grow and multiply very rapidly, doubling their number as often as every twenty minutes. This is called exponential growth, and it occurs in the log phase. Food will rapidly become unsafe during the log phase.

Bacteria can continue to grow until nutrients and moisture become scarce, or conditions become unfavorable. Eventually the population reaches a stationary phase, in which just as many bacteria are growing as are dying. When the number of bacteria dying exceeds the number growing, the population declines. This is called the death phase.

The time required for bacteria to adapt to a new environment (lag phase) and to begin a rapid rate of growth (log phase) depends on several factors, such as temperature. Shows how different temperatures affect the growth rate of Salmonella. As the graph shows, at warmer temperatures (95°F - 35°C), Salmonella grows more quickly than at colder temperatures (44°F and 50°F 7°C and 10°C). At even colder temperatures (42°F - 6°C, Salmonella doesn't grow at all—but notice that it doesn't die either (prolonging the lag phase). This is why refrigerating food properly helps to keep it safe.

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