Nutrient agar plates are one of the most widely used and effective methods for growing microbial cultures. A nutrient agar plate consists of a petri dish partially filled with a warm liquid containing both agar and nutrients. The nutrients encourage microorganisms to grow, whereas the agar solidifies once it cools to create a gel structure that most microorganisms cannot digest. This simple contraption makes the perfect environment for growing several different microbial cultures.
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Bacteria are single-celled organisms that can grow practically anywhere providing they have the basic nutrients they need to survive. A host of bacterial colonies can grow on a single nutrient agar plate. You should be able to easily spot the different bacterial colonies as each colony will grow in patches on the plate. Bacterial colonies are usually quite easy to distinguish from other organisms. They are often a white, yellow, or creamy colour -- although they can sometimes appear red, brown, green or blue -- and almost all form a circular shape.
Like bacteria, yeasts are also single-celled organisms that love to grow on nutrient agar plates. Yeasts are similar to bacteria in many ways and often have the same appearance. Most yeasts will appear circular, grow in patches, and have a white or creamy coloured appearance. Nutrient agar plates exist that specifically promote the growth of yeasts. These plates, known as YPD plates, contain a special blend of yeast extract, peptone, and dextrose to encourage microbial growth.
Unlike bacteria and yeasts, moulds are fungi that are made of multicellular filaments known as "hyphae." Moulds often grow in damp areas and on decaying foods. However, they also grow very easily a nutrient agar plate. Moulds are relatively easy to distinguish from yeast and bacteria because their mutlicellular and filamentary structure gives them a fuzzy appearance. They are also rarely uniform in colour and will have an outer ring that is a different colour to the centre.
Aside from mould, there are many other types of fungi that can grow on a nutrient agar plate. These fungi grow in all shapes and sizes. Scientists often identify a species of fungi based on three different factors in appearance; known as "colony morphology." These factors are the general shape and form of the colony, the shape and extent of elevation from the agar plate and the shape and appearance at the margin of the colony. You can use these techniques to distinguish fungi from bacteria and yeasts.
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