Bacteria grow in very diverse conditions, which explains why they are found nearly everywhere on Earth. Although bacteria are good at adapting to their environments, certain conditions promote bacterial growth more than others. These conditions include temperature, moisture, pH and environmental oxygen. Understanding the optimal conditions for bacterial growth can potentially help you reduce your risk for bacterial infections and food poisoning.
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Most disease-causing bacteria thrive in warm temperatures, especially those close to body temperature. The human body, therefore, provides an ideal environment for many types of bacteria to grow. Certain strains of bacteria, however, can grow at lower or higher temperatures. Since ideal temperature is crucial for the growth of any given species of bacteria, food must be handled appropriately to avoid food poisoning. In most cases -- but not all -- refrigerating or freezing food is sufficient to suppress the growth disease-causing bacteria, such as Staphylococcus. Thoroughly cooking meats and poultry to the correct internal temperature is also important to kill harmful bacteria that may be present in the food, such as Salmonella and E. coli.
Bacteria need water to grow and die without a water source. Moist areas are particularly prone to bacterial growth, such as bathrooms and kitchens. Water content in food also provides an excellent environment for many types of bacteria to grow. Certain foods can be dehydrated or freeze-dried, which removes most of the water and can allow for longer storage without bacterial growth. Moist tissues in the body, such as the mouth and nose, provide an excellent source of moisture for bacteria and are particularly prone to bacterial growth.
The pH of an environment -- a measure of its acidity or alkalinity -- is important for bacterial growth. Most strains of disease-causing bacteria prefer to grow in conditions with a near neutral pH, similar to the pH of the human body. Some strains of bacteria, however, can live in more acidic or more alkaline conditions. Cleaning solutions are typically highly acidic or basic, which kills bacteria, because they cannot survive at these extremes of pH.
The acidity of food is also an important factor affecting bacterial growth. More acidic foods can typically be stored longer without spoiling. Preserving agents that increase the acidity of food, such as citric acid, are commonly added to help prevent bacterial growth and allow for longer storage. Vinegar and lemon juice have a similar effect.
The presence of oxygen can greatly affect the growth of bacteria. Many types of disease-causing bacteria grow best in an oxygen-rich environment or require oxygen to grow. This is why many commercial foods are vacuum-sealed. Vacuum sealing -- also known as reduced oxygen packaging -- inhibits the growth of many types of bacteria and fungi that cause food spoilage. Once the vacuum seal is broken, exposure to the environment and oxygen limits the shelf life. Keeping food properly sealed while during storage is a good preventive measure against bacterial growth because it restricts the amount of oxygen. Proper sealing is also important when doing home canning for similar reasons.
While reduced oxygen inhibits the growth of many types of bacteria that can spoil food, there are others that thrive in the absence of oxygen. Two important examples are Clostridium botulinum -- the bacteria responsible for botulism -- and Listeria monocytogenes, another food-borne bacteria that is particularly harmful to pregnant women, newborns and people with a weakened immune system.
Reviewed and revised by: Tina M. St. John, M.D.