Once discrete, well-separated colonies develop on the surface of the streak plate, selected ones may be picked up with an inoculating needle and transferred to separate culture tubes such as tryptic soy agar slants (the type of agar will depend on the microorganism). Where possible, bacteria from the center of a colony are transferred, because the center is less likely to be contaminated than the edges. Each slant now represents the growth of a single species of microorganism and is called a pure or stock culture.
One of the more important problems in a microbiology laboratory is the maintenance of pure stock cultures over a long period. Ideally, one should employ a technique that minimizes the need for subculturing the microorganism. This is achieved by storing the microorganism in a state of dormancy either by refrigeration or desiccation.
Short-term maintenance (generally between one to three months) of aerobic bacteria can often be achieved by storing slant cultures in the refrigerator at 4℃ to 10℃. The use of screw cap tubes for these slants will minimize desiccation during storage.
These cultures are maintained as agar slants, agar stabs, spore preparations or broth cultures and they are held under refrigeration. They must be checked constantly for possible changes in growth characteristics, nutrition, productive capacity and contamination.· Primary stocks are cultures that are held in reserve for practical or new fermentations, for comparative purposes, for biological assays or for possible later screening programs.