The cultures of microorganisms.
The microorganisms group substantially form bacteria, yeast and fungi as well as protozoa and viruses. They are most commonly found microorganisms in the surrounding world, capable of settling and living in various environments such as soil, water, plant and animals tissues and the human body. Some simple methods could be applied to identify and isolate most of the them. The methods include cultivation and microscopic techniques. The previous are associated with the isolation of a group of microorganisms or pure cultures and their growth in certain microbiological media. Microscopic methods are usually used to understand of cell morphology, mobility or classification to specific taxonomic groups. For example, bacteria can be distinguished due to differences in the construction of the cell membrane, using Gram (from authors name) staining method. This method is related to the capacity of dye binding in the cell membrane. Gram positive bacteria have a thick membrane, wherein the dye is retained, while Gram-negative do not have this ability. Based on this simple method and microscopic observation you can easily distinguish these two groups of microorganisms. Examples of Gram-positive bacteria are Staphylococcus aureus, Bacillus cereus; Gramnegative bacteria: Escherichia coli, Pseudomonas fluorescens. Typical examples are the yeast Saccharomyces cerevisiae, Candida albicans. The fungi include molds Penicillium sp. (species), e.g. Penicillium chrysogenum, Penicillium citrinum, and Aspergillus sp., e.g. Aspergillus niger, Aspergillus fumigatus.
To study the properties of microorganisms such as the ability to grow, cell division, the utilization of different substrates and decomposition of the specific compounds, the microbiological culture media are used. The media are typically synthetic of known chemical composition and physical properties. The mixture of different substrates has to provide conditions for the growth of microorganisms. Sometimes a natural substrate (complex medium) is used as cultivation medium, e.g. waste from the sugar industry, dairy or processed fruits and vegetables, which are characterized by variable amounts of nutrients, nevertheless provide conditions for the development of microorganisms. In addition to the specific chemical composition of the medium, the suitable pH and optimal osmotic pressure should be also provided.
Culture media may be liquid, semi-solid or solid. To solidify the liquid media gelatin or agar could be used. Gelatin is an animal protein formed as a hydrolysis product of collagen Unfortunately, this protein can be easily decomposed by microorganisms growing on the solid medium causing its liquefaction. In contrast, agar is a polysaccharide composed of agar and agaropectin sourced from red algae which is not degraded by most of microorganisms. Both of these solidifying factors are further distinguishes by the solidification temperature of the media. The agar medium liquefy at 100 °C and solidify at 45-50 °C, meanwhile the substrate gelatin liquefy at about 25 °C and solidify at 20 °C. In practice, agar is used for solidification.
In the case of mineral media, which contains no organic components, silica gels are applied as solidifying factor.
In laboratory, the solid media culture could be kept mainly on Petri dishes and test tubes (agar slopes). The cultures in liquid media are carried out in test tubes, flasks or in large flatbottomed tanks with a capacity of 1 dm3 called bioreactors.
The cultures of microorganisms.