CULTURE MEDIA PREPARATION

• The log phase of a bacterial culture is brief, unless the culture is maintained at a constant addition of nutrients and frequent removal of waste products.
• As the nutrients in the liquid medium are used up and the concentration of toxic waste products from the metabolizing bacteria build up, the rate of division slows, such that the number of bacteria that are dividing equals the number that are dying, entering the stationary phase.
• During the stationary phase, the culture is at its greatest population density.
• As overcrowding occurs, the concentration of toxic waste products continues to increase and the nutrient supply decreases, causing the microorganisms to die at a rapid rate, this is the death phase or decline phase.
• The principle of culture media preparation is to provide a balanced mixture of all required nutrients at concentrations that will permit good growth.
• Reduced/Anaerobic Medium contains reducing agents such as cysteine, thioglycollate, Na2S, sodium ascorbate which chemically reduce the concentration of dissolved oxygen, hence, decrease the oxidation-reduction potential of the medium.
• Examples of Reduced/Anaerobic Medium include cooked meat, thioglycollate medium.
• Many microbes cannot digest agar, hence, agar media remain solid even when bacteria and fungi are growing on them.
• Powdered agar dissolves in water at 100°C, a temperature at which most nutrients remain undamaged.
• Agar solidifies at temperatures below 40°C, making it temperature-sensitive, sterile nutrients such as vitamins and blood can be added without detriment, and liquid agar can be poured over most bacterial cells without harming them (pour plate isolation technique).
• Basic components of a culture medium include Beef extract, Peptone, Yeast extract, and Agar.
• Solid agar does not melt below 100°C, making it suitable for culturing many thermophiles.
• The composition of a complex and a chemically defined growth medium includes peptone, which supplies amino acids and peptides, beef extract, which supplies vitamins, minerals, and other nutrients, sodium chloride, which supplies sodium and chloride ions, agar, and distilled water.
• A synthetic growth medium contains glucose as a simple sugar, ammonium phosphate as a nitrogen phosphate, sodium chloride as sodium and chloride ions, magnesium sulfate as magnesium ions and sulfur, potassium phosphate as potassium ions and phosphate, and water.
• Mannitol salt agar is used for staphylococci.
• MacConkey agar is used for gram-negative bacteria.
• Blood agar is used for streptococci.
• Chocolate agar is used for Neisseria species.
• Basal Medium is a basic medium which supports growth of ordinary bacteria such as peptone, nutrient broth, and nutrient agar.
• Enriched Medium is an ordinary basal medium plus blood or serum, for example, blood agar plate.
• Enrichment Medium is a liquid used to increase the number of selective organisms where they may be outnumbered by many unwanted species, the medium should be chosen to take advantage of any known physiological characteristics of a particular organism.
• Differential Medium contains certain requirements or supplements which when incorporated into the culture may allow differentiation of various kinds of bacteria.
• Selective Medium are solid media which contain inhibitory substances to suppress the growth of unwanted organisms, giving an advantage to the required species, improving the chance of isolating these organisms from mixed culture.
• Maintenance Medium is used to maintain the viability and physiological characteristics of a culture over time.