Introduction

Microorganisms have a considerable impact in the environment as well as in our daily lives, as they can gradually modify an ecosystem, cause disease to plants, animals, and humans or they may be used for the production of compounds such as antibiotics, vaccines, enzymes added to detergents, or food such as wine and yoghurt.

Thanks to the genetic manipulation of microorganisms, they can be used to synthesize products foreign to their natural metabolism (e.g., the human hormone insulin, bovine growth hormone, and the factor VIII involved in coagulation). It is important to isolate bacteria, yeasts, and filamentous fungi from their natural habitats, and to be able to purify, characterize and classify them.

An important distinction is their capacity to live under various environmental conditions, sometimes even in extreme conditions of temperature, pH, and salinity that other organisms cannot tolerate.

In this experiment, students will isolate microorganisms from different environments and observe them through an optical microscope. The method used here is the agar plate method using selective media that allows for the growth of specific types of microorganisms [i.e., nutrient agar is used for the growth of bacteria, while potato dextrose agar (PDA) is preferred for yeast growth, and Sabourad medium for filamentous fungi].

Although this isolation method has several limitations, it is simple and can give an idea of the diversity of microorganisms in different environments. For example:

• anaerobes in soil fail to grow on the surface of agar exposed to air

• non-symbiotic, nitrogen-fixing organisms grow under these conditions only to a limited extent

• many of the cellulose-digesting forms fail to grow or grow poorly on nutrient agar

Therefore, the observed microorganisms represent only a fraction of the total viable bacterial or yeast population.

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