Many samples contain a mixture of microorganisms. A selective medium contains substances that inhibit the growth of some microorganisms while allowing others to grow. It is used to isolate or identify a particular type from a mixed sample.Examples:
• Antibiotic media: adding an antibiotic such as ampicillin means only bacteria resistant to it can grow. This is used in genetic modification to select bacteria that have taken up a plasmid carrying an antibiotic-resistance gene.
• MacConkey agar contains bile salts and crystal violet, which inhibit Gram-positive bacteria. It selects for Gram-negative gut bacteria such as E. coli and Salmonella.
• Mannitol salt agar contains about 7.5% sodium chloride. Most bacteria cannot tolerate this salt concentration, but Staphylococcus can, so it is used to isolate staphylococci from skin swabs.Some media are also differential: they contain an indicator that shows a difference between the microorganisms that grow. MacConkey agar contains lactose and a pH indicator. Bacteria that ferment lactose produce acid and form pink colonies; non-fermenters such as Salmonella form pale colonies. So MacConkey agar is both selective and differential.Selective media are important in hospitals and food testing, because they let a pathogen be picked out quickly from the huge numbers of harmless bacteria in a sample of faeces, food or skin.
• Antibiotic media: adding an antibiotic such as ampicillin means only bacteria resistant to it can grow. This is used in genetic modification to select bacteria that have taken up a plasmid carrying an antibiotic-resistance gene.
• MacConkey agar contains bile salts and crystal violet, which inhibit Gram-positive bacteria. It selects for Gram-negative gut bacteria such as E. coli and Salmonella.
• Mannitol salt agar contains about 7.5% sodium chloride. Most bacteria cannot tolerate this salt concentration, but Staphylococcus can, so it is used to isolate staphylococci from skin swabs.Some media are also differential: they contain an indicator that shows a difference between the microorganisms that grow. MacConkey agar contains lactose and a pH indicator. Bacteria that ferment lactose produce acid and form pink colonies; non-fermenters such as Salmonella form pale colonies. So MacConkey agar is both selective and differential.Selective media are important in hospitals and food testing, because they let a pathogen be picked out quickly from the huge numbers of harmless bacteria in a sample of faeces, food or skin.