When living things are provided with the proper nutrients, water, and environmental factors (temperature, humidity, etc.) they will grow and reproduce, often explosively and in surprising ways. To kids, microbes are abstract, invisible germs that mysteriously spread disease, but otherwise have little relevance to their daily lives. However, microbes in the form of bacteria, fungi and viruses are prolific and exist all around us and even inside us. Of the 100 trillion cells found within your skin, only 10 trillion of these (a measly 10%) are human cells! The rest are primarily bacteria living on the surface of your skin, around your eyes, mouth, reproductive organs, and digestive tract, comprising between 500-1,000 different species. Since bacterial cells are generally much smaller than human cells, a great many more bacteria fit into the borders of the human body than human cells.
Nutrient agar plates are a classic tool for culturing microbes in the laboratory. The agar plate provides all the basic organic molecules - water, carbohydrates, fats, and proteins - and will support the growth of many bacteria and fungi species, though by no means will all microbes find the appropriate nutrient and environmental conditions to grow on an agar plate. It is important for students to recognize that many organisms that arrive on the plate will not have the right conditions to grow, thus, an empty plate does not necessarily mean no life exists there – only that this method could not detect life.
Luckily, pathogenic bacteria find it difficult to grow on nutrient agar so it is reasonably safe to use in schools. However, do NOT open the dishes once they have been seeded. High levels of mold and bacterial spores can be released. When you are finished with the experiment, disinfect the plates before disposal.
It is difficult to positively identify specific species from so rough a measure, however, it is possible to determine certain things about the colonies that grow. Bacteria tend to form low-growing buttons or streaks that are glistening and smooth. On the other hand, fungi tend to form fuzzy, irregular patches or fern-like, thread-like patterns. Different varieties can be distinguished by their color and texture. Colonies that were collected on a Q-tip swab from a hard surface will begin their growth where the Q-tip touched the agar. Colonies from the air that landed on the agar while the lid was open will begin as a dot located on part of the plate not touched by the Q tip swab. Each dot represents a single spore that landed on the plate.