Mesophiles are organisms that grow best at moderate temperatures. Their optimum growth temperature is between 25C and 40C. Most bacteria are mesophilic. Thermophiles are heat-loving organisms. A colony is a visible mass of microorganisms growing on an agar surface and usually originating from a single organism or arrangement of organisms. Single colonies can be described using standard terms, as listed in Appendix A .
Some microorganisms produce pigment during growth and are said to be chromogenic .
Often, however, formation of pigment depends on environmental factors such as temperature, nutrients, pH and moisture. If the pigment is water insoluble , as in the case of most chromogenic bacteria, it does not diffuse out of the organism. As a result, the colonies are pigmented but the agar remains the normal color. Both the colonies and the agar will appear pigmented. Aseptically inoculate one Trypticase Soy Broth tube with B. subtilis , one with E. coli , one with M. luteus , and one with M. phlei .