Introduction
Agar, also known as agar agar, is a thermo-reversible gelling agent extracted from the cell walls of red algae, primarily from genera such as Gracilaria, Gelidium, Ahnfeltia, and Pterocladiella.
Though it’s most famously used in scientific laboratories, about 90% of agar is actually used in food applications—from baked goods and ice cream to jams and puddings. Its tasteless and odourless nature makes it a versatile ingredient. In microbiology, agar is crucial for solidifying culture media, allowing scientists to isolate and grow microorganisms effectively.
How Agar Works (Principle)
Agar solidifies liquid mediums, forming a firm base for microbial growth without interfering with the nutritional content of the medium.
- Melting Point: 85–90°C
- Solidifies at: 40–45°C
- Remains solid up to: ~100°C
These properties make agar ideal for viewing and differentiating microbial colonies.
What is Bacteriological Agar?
Bacteriological agar is a hydrophilic colloid derived mainly from species like Gelidium and other red algae. It’s a key material in microbiological media due to its strong gelling ability.
Key Features:
- Gelling temperature: 32–39°C
- Melting temperature: 90–95°C
- Safe for heat-sensitive additives like blood (added at ~45–50°C)
- Firm gel at 1.5% w/v concentration
- Semi-solid gel at 0.4–0.5% w/v for transfer media like Amies medium
Important: Bacteriological agars must be free from impurities (e.g., trace metals) that could inhibit microbial growth.
Due to limited harvesting locations and rigorous purification processes, the cost of agar is rising, prompting the search for cost-effective alternatives.
Chemical Composition of Agar
Agar is a mix of polysaccharides, primarily galactose monomers, and consists of two main components:
Agarose
- A neutral, linear polysaccharide
- Structure: Alternating D-galactose and 3,6-anhydro-α-L-galactopyranose
- Bonds: α(1→3) and β(1→4)
- Forms strong gels
- Ideal for electrophoresis and chromatography
Agaropectin
- A branched, non-gelling component
- Contributes to the overall chemical stability of agar
Types of Agar and Their Microbiological Uses
| Type of Agar | Primary Use |
| Nutrient Agar | General-purpose; supports many bacterial and fungal species |
| Potato Dextrose Agar | Ideal for fungal growth |
| Blood Agar | Grows most bacteria; differentiates hemolytic activity |
| Chocolate Agar | Cultures Haemophilus and Neisseria species |
| MacConkey Agar | Selective for Gram-negative bacteria |
| Jensen’s Agar | Supports nitrogen-fixing bacteria |
| Pikovskaya’s Agar | Isolates phosphate-solubilizing bacteria |
Precautions When Using Agar
To ensure accurate results and media integrity, follow these precautions:
- Fully dissolve agar before autoclave.
- Avoid overheating to prevent caramelization or nutrient degradation.
- Always use aseptic technique when pouring agar plates.
- Do not re-melt repeatedly; it weakens gelling ability.
- Check the pH and clarity before sterilization.
Limitations of Agar
Despite its versatility, agar has some drawbacks:
- Not suitable for thermophiles (>100°C survival required)
- Can be degraded by some microbes (e.g., Flavobacterium)
- Non-nutritive—only a physical support, not a food source
- Storage issues can lead to cracking, drying, or instability
Applications of Agar
1. Food Industry
Agar is primarily used as a gelling, thickening, and stabilizing agent in:
- Baked goods
- Ice cream
- Jams and jellies
- Processed meats and dairy products like yogurt and cheese
2. Silk Screen Printing
In photographic silkscreen printing, agar acts as a safe and effective emulsifying agent, especially useful in educational and commercial printing due to its accessibility and ease of preparation.
3. Nanotechnology
Agar is used to create biodegradable packaging films. When combined with melanin nanoparticles (MNPs), it forms functional nanocomposite materials with improved structural and thermal properties.
4. Pharmaceutical Industry
As a natural polymer, agar is gaining traction as a sustainable alternative to synthetic polymers. Its strong gel-forming ability makes it suitable for:
- Hydrogels in drug delivery
- Injectable composite gels used in cancer treatment (chemotherapy + photothermal therapy)
Conclusion
Agar agar is more than just a lab staple—its uses span food, pharma, biotechnology, and nanotech. With its unique gelling properties, biocompatibility, and sustainability, it continues to be a valuable resource across multiple industries. However, its growing demand and limited supply call for innovation in finding sustainable alternatives and enhancing agar production technologies.
