Notes:

The growths or cultures observed in lab are named according to the microbe present, the source, or the number of different microbes. A microbiologist may identify a culture as a yeast culture, Fusarium (mold) culture, Spirogyra (algae) culture, or viral culture. Also a culture may be named according to its initial source; for example a throat culture, a water culture, an air culture, a soil culture, or a skin culture. After observing a culture, you might observe only one type of microbe and call your culture a "pure" Staphylococcus culture. Only in the laboratory are you likely to find pure cultures. In the natural environment you usually observe "mixed" cultures. If a microbe from the air grows on your throat culture or a microbe from the soil grows on your wheat culture, the culture is called "contaminated."

There are many observations and methods or procedures that assist microbiologists in estimating the presence of microbes and determining actual counts. Microbes are known to grow in liquid and solid media. Sometimes you can observe colony growths on solid media or the formation of microbe growths on the top of a liquid (pellicle) or growth on the bottom of a liquid (sediment). Also microbes may grow throughout a liquid causing turbidity or cloudiness. Since microbes vary in size, the average number of microbes needed to make a liquid appear cloudy is estimated to be 1 x 107 cells per milliliter or cubic centimeter. Instruments like the spectrophotometer and coulter counter determine the absorbance and/or actual number of cells per volume. Both dead and living microbes are involved in the counts; so viable (living) microbe counts require plating on media to determine the number of colonies produced. It is sometimes possible to make a direct microscope count of microbes in milk and other materials; however, when numbers are large, it becomes necessary to dilute (reduce the concentration) the sample for more accurate counting of the living microbes. During the growth of microbes, many biochemical activities occur; and the number of microbes can be estimated by the change in biomass, pH, or amount of gas produced. The changes in food (such as color, texture, smell) are biochemical changes we associate with spoiled food due to microbial growth and activity.