Bacteria are everywhere. Some are beneficial, such as those used to make fermented dairy and meat products. Others cause spoilage. And, a small percentage are harmful or pathogenic.

Unlike animals and plants that are composed of many cells, bacteria are single-celled organisms. Each bacterium cell is self-sufficient and so is able to live independently. Bacteria come in a variety of shapes and are impossible to see without a microscope. Because they are about 1/25,000th of an inch long, they must be magnified about 1,000 times to be seen. For example, about 400 million bacteria clumped together would be about the size of a grain of sugar.

When bacteria grow, they increase in numbers not in size. This process is called cell division (or doubling). Under ideal conditions, the number of bacteria can double every 30 minutes. Therefore, one becomes two, two become four, four become eight, and so on. If you start with one bacterial cell, after 12 hours there would be as many as 33,000,000. The rate at which bacteria grow is different for each type or organism and is affected by many factors.

 Factors Affecting Microbial Growth

Many factors affect bacterial growth but the most important ones are:

  1. Water — Bacteria need water to dissolve the food they use for energy and growth. Water allows the food to get into the cells, is used for the many chemical reactions necessary for life and growth, and allows waste products to escape.
  2. Food/Nutrients — All bacteria require energy to live and grow. Energy sources such as sugars, starch, protein, fats and other compounds provide the nutrients.
  3. Oxygen — Some bacteria require oxygen to grow (aerobes) while others can grow only in the absence of oxygen (anaerobes). However, many bacteria grow under either condition and they are facultative anaerobes.
  4. Temperature — Bacteria in general are capable of growing over a wide range of temperatures and are usually classified according to the temperature at which they grow. Psychrotrophic bacteria are those that are capable of growing at 32°F – 45°F but their optimum is from 68°F to 86°F. They cause spoilage in foods stored under refrigeration. Several pathogenic bacteria are psychotrophic — Yersinia and Listeria. Mesophilic bacteria. Most bacteria are capable of growing at 60°F – 110°F and belong in this group. Most pathogenic bacteria grow at these temperatures. Thermophilic bacteria. These microorganisms grow at higher temperatures such as 110°F – 150°F. Temperature is the most widely used method of controlling bacterial growth. Bacteria grow slowly at temperatures below 45°F and thermal destruction occurs at temperatures above 140°F. But in the temperature danger zone — between 40°F and 140°F — many bacteria are not controlled.
  5. pH — pH is a measure of acid or alkali in a product. It is indicated on a scale from 0 to 14, with seven being neutral. If the pH value is below 7, the food is classified as acid; if it is above 7, the food is classified as alkaline. Most bacteria grow well at neutral pH, but many can reproduce in a pH range from 4.5 – 10.0.

Although each of the major factors listed above plays an important role, the interplay between the factors ultimately determines whether a microorganism will grow in a given food. Often, the results of such interplay are unpredictable, as poorly understood synergism or antagonism may occur. An advantage is taken of this interplay with regard to preventing the outgrowth of C. botulinum. Food with a pH of 5.0 (within the range for C. botulinum) and an aw of 0.935 (above the minimum for C. botulinum) may not support the growth of this bacterium. Certain processed cheese spreads take advantage of this fact and are therefore shelf stable at room temperature even though each individual factor would permit the outgrowth of C. botulinum.

Therefore, predictions about whether or not a particular microorganism will grow in a food can, in general, only be made through experimentation. Also, many microorganisms do not need to multiply in food to cause disease.