In the industry of production of Penicillin and its derivatives, it is customary to grow the producing fungi in liquid media; starting with "germination medium" and continuing with "complex medium for Penicillin production". The "germination medium" is used for the production of primary propagation from spores. The primary mycelium is transferred to "complex medium for Penicillin production" for growth continuation and production of Penicillin. In that manner the fungi is prepared for the students to begin their proposed research.
In case the fungi generated antibiotics, it will be excreted into the medium in which the fungi were grown. To perform the test for presence of antibiotics in this medium, we need to use the medium sample clean of mycelia and spores. Therefore, we would filter the samples of the whole culture (fungi+medium) (It could be done also in the centrifuge) to get clean filtrate.
In order to test whether or not the fungi produce and excrete antibiotics, we test it against bacteria, and therefore it is a "bioassay". When we provide the bacteria with optimum growing conditions they grow rapidly on the medium. If the filtrate of the fungal culture causes bacteria to grow slowly, or not grow at all, we could deduce that it has materials active against bacteria (antibiotics). Since not all bacteria have the same degree of sensitivity to different antibiotics, it is worthwhile to perform the bioassay using different species of bacteria, some of which may differ in characteristics such as being "gram positive", or "gram negative".
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