BRAINCAKESelective Media
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.
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Key terms in this lesson
- gram-positive
- Describes bacteria with a thick peptidoglycan cell wall that retains crystal violet stain and appears purple in Gram staining.
- gram-negative
- Describes bacteria with a thin peptidoglycan wall and an outer membrane that does not retain crystal violet, appearing pink in Gram staining.
- plasmid
- A small, circular piece of DNA, separate from the main bacterial chromosome, that can be transferred between bacteria and often carries genes such as antibiotic resistance.
More in Bacteria and Disease
- Culturing Microorganisms
- Aseptic Culture Techniques
- Growing a Pure Culture
- Counting the Number of Cells
- Turbidity
- Dilution Plating
- Area and Mass of Fungi
- Growth of Bacterial Colonies
All 30 lessons in Bacteria and Disease · All Edexcel A-level Biology topics