Types of Culture Media PDF

Culture Media Performed by : Dr. Ammar ALdabbas  What Is Culture Media? Culture media are mediums that provide essential nutrients and minerals to support the growth of microorganisms in the laboratory. Microorganisms have varying nature, characteristics, habitat, and...

Culture Media Performed by : Dr. Ammar ALdabbas  What Is Culture Media? Culture media are mediums that provide essential nutrients and minerals to support the growth of microorganisms in the laboratory. Microorganisms have varying nature, characteristics, habitat, and even nutritional requirements, thus it is impossible to culture them with one type of culture media. However, there are also microorganisms that can’t grow on a culture media at all in any condition – these are called obligate parasites. Culturing microorganisms is essential for diagnosing infectious diseases, obtaining antigens, developing serological assays for vaccines, genetic studies, and identification of microbial species.  Furthermore, it’s also essential for isolating pure cultures, storing culture stock, studying biochemical reactions, testing microbial contamination, checking antimicrobial agents and preservatives effect, testing viable count, and testing antibiotic sensitivity.  Classification and Types of Culture Media The basic media contains a source of carbon & energy, nitrogen source, growth factors, and some trace elements. However, some additional components or nutrients are added to the media when growing specific microorganisms. Culture media can be classified in three ways: based on their consistency, nutritional component, and applications.  A. Classification of culture media based on consistency Solid media: In these media, the agar is added in the concentration of 1.5-2.0%. Most commonly, 1.3% agar is used to prepare solid media in labs. Solid media are used to grow microorganisms in their full physical form, prepare bacterial pure cultures, or isolate bacteria to study colony characteristics. The bacterial growth on solid media varies in appearance as mucoid, round, smooth, rough, filamentous, irregular, and punctiform. Examples of solid media are blood agar, nutrient agar, McConkey agar, and chocolate agar.  Semisolid media: This media has 0.2-0.5% agar concentration, and due to the reduced agar concentration, it appears as a soft, jelly-like substance. It’s mainly used to study the motility of microorganisms, distinguish between motile and non- motile bacterial strains (through U-tube and cultivate microaerophilic bacteria – bacteria on this media appear as a thick line. Examples of semi-solid media are: Amies media, and Mannitol motility media.  Liquid media: These media do not contain agar.Liquid media are also called broths, they allow for uniform and turbid growth of bacterial strains when incubated at 37ºC for 24hrs. The media is used for the profuse growth of microorganisms and fermentation studies. Examples include Tryptic soy broth, and nutrient broth.  B. Classification based on the nutritional component Simple media: It’s a general-purpose media that supports the growth of non-fastidious microbes, and it is primarily used for the isolation of microorganisms. Examples are nutrient broth, peptone water, and nutrient agar. Complex media: Complex media are rich in minerals and nutrients and are often used for growing a wide variety of microorganisms, including fastidious ones.  C. Classification of culture media based on application/chemical composition Basal media: These are routinely used simple media having carbon and nitrogen sources that boost the growth of many microorganisms. They are also known as general-purpose media and are considered non-selective media. The basal media do not require enrichment sources for the growth of non-fastidious bacteria and are suitable for growing Staphylococcus and Enterobacteriaceae. They are generally used to isolate microorganisms in labs or in sub-culturing processes. Examples are nutrient broth, nutrient agar, and peptone water  Enriched media: This media is prepared by adding additional substances like blood, serum, or egg yolk in the basal medium. It’s used to grow fastidious microorganisms as they require additional nutrients and growth-promoting substances. Examples are chocolate agar, blood agar. Selective media: This media allows the growth of certain microbes while inhibiting the growth of others. It’s an agar-based medium that is used to isolate microorganisms in labs. The selective growth of microbes is decided by adding substances like antibiotics, dyes, bile salts, or by pH adjustments. Examples :Mannitol Salt Agar, MacConkey Agar Thayer Martin Agar.  Enrichment media: It’s a liquid medium, used to increase the relative concentration of certain microbes before culturing them on a solid medium plate. It’s used as a broth medium and inhibits the growth of commensal species of microorganisms (those who live in close association with each other) in the clinical specimen. It’s also used in isolating fecal and soil microorganisms. Examples tetrathionate broth, and alkaline peptone water.  Differential media: It contains certain indicators like dyes or metabolic substrates in the medium composition which gives different colors to colonies of different microbial species when they utilize or react with these components. It allows the growth of more than one microorganism, however, the bacterial colonies are differentiated based on their color when a chemical change occurs in the indicator, such as neutral red, phenol red, methylene blue. Examples are:  Blood agar: In blood agar, three types of blood cell lysis or hemolysis are observed: alpha, beta, and gamma hemolysis. It allows the growth of many microorganisms, however, their ability to lyse blood cells differs, and this helps to distinguish the bacterial colonies. For example, S. pyogenes completely lyse blood cells (beta hemolysis), thus causing total clearing of the media around its colonies. S. pneumoniae partially lyse red blood cells, resulting in a greenish-colored medium, while gamma hemolytic microorganisms like Enterococcus faecalis, Staphylococcus saprophyticus, and Staphylococcus epidermidis, can’t lyse red blood cells, thus causing no color change in the medium. – Mannitol salts agar: The fermentation of mannitol by Staphylococcus aureus causes the media to change to yellow, however, coagulase-negative staphylococci that can’t cause fermentation to appear in pink. – MacConkey agar: It differentiates the gram-negative bacteria based on their lactose metabolism. The lactose fermenting bacteria, such as Escherichia coli, Klebsiella spp, Citrobacter, and Enterobacter forms pink-red colonies, while lactose non-fermenters, like Salmonella, Shigella, Proteus, Providencia, Pseudomonas, and Morganella form pale or colorless colonies.  Nomenclature of Bacteria Nomenclature of bacteria refers to naming and bacteria and other organisms are named according to the binomial system. This means that a bacterium has a species name, which is composed of a genus name that tells you to which genus it belongs and a species epithet which, together with the genus name, is unique to the bacterium. An example of this is Moraxella bovis, where the genus name indicates that the bacterium belongs to the genus Moraxella and the species epithet indicates that the bacterium has been isolated from cattle. The genus name and the species epithet form together the scientific name of the species, which is always written in italics.