Introduction
Culture media are specially prepared nutrient mixtures that support the growth, isolation and identification of microorganisms. No single medium supports all microbes, so different types of media are used depending on the organism and purpose. A good medium must provide carbon, nitrogen, minerals, energy sources and other essential nutrients.
Common Ingredients of Culture Media
1. Water
Water dissolves nutrients and forms 70–80% of the microbial cell. Pure or distilled water is used in media preparation. Copper-distilled water is avoided because copper inhibits bacterial growth.
2. Peptone
A partially digested protein source obtained from meat, casein, fibrin or soya meal. It supplies nitrogen, amino acids, minerals and growth factors. Peptone also acts as a buffer and must be stored in tightly closed containers.
3. Yeast Extract
Prepared from baker’s yeast cells, rich in carbohydrates, amino acids, vitamins (especially B-group) and inorganic salts. It is used as a vitamin source in media.
4. Meat Extract
Contains gelatin, proteins, creatine, purines, carbohydrates, salts and vitamins. It enriches media for bacterial growth.
5. Agar
A polysaccharide obtained from red algae. It acts as a solidifying agent in media.
- Solid at room temperature
- Melts at 95–98°C and stays liquid till 40–42°C
- Bacteriologically inert and has no nutritional value
- Used at 2% for solid media
Types of Culture Media
Culture media are classified based on physical state, oxygen requirement, chemical composition and functional use.
A. Based on Physical State
- Solid media: 1.5–2.5% agar (e.g., nutrient agar)
- Semi-solid media: 0.2–0.5% agar (e.g., motility media)
- Liquid media: No agar (e.g., thioglycollate broth)
B. Based on Oxygen Requirement
- Aerobic media: e.g., MacConkey broth
- Anaerobic media: e.g., Robertson’s cooked meat medium
C. Based on Chemical Composition
- Simple or basal media: Peptone water, nutrient broth
- Complex or undefined media: Contain biological materials like blood, milk, yeast extract
- Synthetic or defined media: Exact chemical composition known; used for research
D. Based on Functional Use
1. Enriched Media
Used for fastidious bacteria that need additional nutrients.
- Blood agar (Streptococcus)
- Chocolate agar (Neisseria, Haemophilus)
- Loeffler’s serum slope (Corynebacterium diphtheriae)
2. Enrichment Media
Liquid media that favour growth of desired organisms while inhibiting others.
- Tetrathionate broth
- Selenite F broth
Used mainly for isolating Salmonella and Shigella from stool.
3. Selective Media
Solid media containing substances that inhibit unwanted microbes and allow target organisms to grow.
- MacConkey agar – bile salts inhibit Gram-positive bacteria
- DCA (Deoxycholate citrate agar) – Salmonella, Shigella
- Lowenstein–Jensen medium – Mycobacterium tuberculosis
4. Indicator Media
Contain indicators that change colour due to microbial activity.
- Wilson and Blair medium – black colonies of Salmonella typhi
5. Differential Media
Differentiate bacteria based on biochemical properties.
- MacConkey agar – lactose fermenters (pink), non-fermenters (pale)
- Blood agar – hemolysis patterns
6. Sugar Media
Peptone water + 1% sugar + indicator + Durham tube to detect acid and gas production.
7. Transport Media
Used to preserve delicate organisms during transport.
- Stuart’s medium
- Amies medium
8. Assay Media
Used for testing antibiotics, vitamins and disinfectants.
9. Storage Media
Used for long-term preservation.
- Dorset’s egg medium
- Nutrient agar stabs
- Robertson’s cooked meat medium
Media for Fungi
Fungi grow best at pH 5–6 and temperature 15–37°C. Media must contain carbohydrates, nitrogen and vitamins.
Carbon Sources
- Glucose (most common)
- Fructose
- Mannose
Nitrogen Sources
- Peptone
- Yeast extract
- Amino acids
- Ammonium and nitrate salts
Vitamin Requirements
Fungi need trace amounts of vitamins such as thiamin and biotin.
Types of Fungal Media
1. Natural Media
Prepared from natural materials like seeds, leaves, oatmeal, corn meal.
Examples: corn meal agar, potato dextrose agar.
2. Synthetic Media
Known chemical composition.
- Czapek–Dox medium
- Glucose–asparagine medium
- Neurospora minimal medium
General Purpose Fungal Media
- Sabouraud dextrose agar (SDA) – acidic pH, supports dermatophytes and yeasts
- Brain–heart infusion (BHI) agar – supports most fungi
Selective Fungal Media
- Inhibitory mould agar – dimorphic fungi
- Dermatophyte test medium – dermatophytes
- Mycobiotic agar – contains cycloheximide for dermatophytes
Special Purpose Fungal Media
- PDA – enhances pigment production
- Cornmeal agar – sporulation studies
- Malt extract agar – soil and wood fungi
- Potato flake agar – fastidious fungi
Incubation Conditions for Fungi
Fungi are usually incubated at 30°C in humid conditions for at least 21 days. Slow-growing species like Histoplasma may require longer periods.
Key Points
- Different microbes require different media depending on their nutritional and physiological needs.
- Enriched, selective and differential media play major roles in identifying pathogenic bacteria.
- Fungal media differ from bacterial media in pH, carbon sources and additives.
- Proper selection of media ensures accurate recovery, isolation and identification of microbes.
Detailed Notes:
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