In nature, microbes rarely exist in pure cultures. Most often, any environment you encounter will be inhabited by various populations of different microbes coexisting and competing for resources. However, it is difficult to identify or study a particular species of microbe when there are others also present. We call conditions where more than one type of microbe is present a "mixed culture". In such cases, we call the microbe of interest to us the "isolate" or "culture" and the other, undesired microbes the "contaminants". Test results cannot be pinned on a particular microbe when several other species (contaminants) present could be responsible. In cases of disease, it would be difficult to pinpoint the causative microbe when so many other contaminants are also present. The only way to conclusively find answers is to study a culture of a single microbe growing isolated from all others - an unnatural condition called a "pure culture".
Micrographs of a mixed culture (L) and pure culture (R)
For this reason, microbiologists have developed methods for separating microbes and working with them in pure culture. These methods for accomplishing the isolation and for working with it aseptically are collectively called aseptic technique. Aseptic technique is the methods by which (1) pure cultures are created and (2) pure cultures are manipulated to enable their identification and study.
Photos of mixed culture (L) and pure culture (R) streak plates
In activities to follow, we will see that a key method for identifying our isolate is through conducting biochemical tests. A known formulation of nutrients is provided, a pure culture of a microbe is added, and the resulting chemical products from microbial growth are examined. The nutrients are found in the form of bacteriological media, agars and broths in which we aseptically introduce the purified unknown isolate through a process called an aseptic transfer to allow it to grow in pure culture.
Aseptic techniques transfer only the desired microbe into a container of sterile medium, preventing growth of any contaminants found in the surrounding environment. In this way, one can be sure that any changes happening during growth in the medium are the result of the desired microbe and not of any other contaminating microbes.
In the activities for this section, we will first watch videos showing how aseptic transfers are completed in a wetlab, followed by practicing virtual transfers using VUMIEtm 2012 software, and then we will perform aseptic transfers in kitchen microbiology activities.