Overview
This lecture covers the main types and classifications of microbial culture media commonly used in medical laboratories, explaining their compositions, purposes, and specific examples.
Classification of Microbial Culture Media
- Culture media are classified by chemical composition (simple, defined, complex), physical nature (solid, liquid), and function (supportive, enriched, selective, differential).
- Most media provide microbes with carbon, nitrogen sources, and salts for growth.
Simple/Basal Media
- Used for routine cultivation of non-fastidious bacteria (e.g., E. coli, Pseudomonas).
- Contains basic nutrients like beef extract, yeast extract, peptone, and salts.
- Examples: nutrient broth, nutrient agar, peptone water.
Defined (Synthetic) Media
- All chemical components and their exact concentrations are known.
- Used when precise nutrient requirements are necessary.
Enriched Media
- Basal media with added nutrients (blood, serum, egg yolk) for fastidious bacteria.
- Blood agar (5-10% sheep blood) supports growth of organisms like Streptococcus pneumoniae.
- Chocolate agar (lysed blood) grows bacteria like Haemophilus influenzae.
Selective Media
- Designed to suppress unwanted microbes and promote the growth of targeted organisms.
- Examples:
- Mannitol salt agar selects for Staphylococcus aureus (yellow colonies from mannitol fermentation).
- Campylobacter agar contains antibiotics to isolate Campylobacter jejuni.
Differential and Chromogenic Media
- Allows differentiation of bacteria based on colony color changes due to substrate metabolism.
- Examples:
- MacConkey agar distinguishes lactose fermenters (pink colonies) from non-fermenters (yellow).
- Mannitol salt agar and chromogenic agars differentiate based on visible color changes.
Media for Fungi
- Sabouraud’s agar is selective for fungal growth, such as Candida albicans.
Physical Nature of Media
- Solid media (e.g., agar plates) are used to isolate colonies.
- Liquid media (e.g., Robertson's cooked meat broth) enable growth from small samples and are important in industrial applications for mass bacterial culture.
Key Terms & Definitions
- Fastidious bacteria — Bacteria with complex nutritional requirements.
- Basal media — Simple media with basic nutrients for non-fastidious organisms.
- Enriched media — Basal media plus extra nutrients to support demanding bacteria.
- Selective media — Media that favors growth of certain organisms while inhibiting others.
- Differential media — Media that distinguishes bacteria based on visible traits, like colony color.
- Defined media — Media with precisely known chemical composition.
- Solid media — Contains a solidifying agent (agar) to allow colony isolation.
- Liquid media — Does not solidify; enables growth in suspension.
Action Items / Next Steps
- Review the examples and purposes of different culture media types.
- Prepare a chart comparing selective, differential, enriched, and basal media.
- Study key media mentioned (nutrient agar, blood agar, MacConkey agar) for lab identification.