MacConkey Agar

🧫 MacConkey Agar Interactive Microbiology Learning System

📖 Intended Use

MacConkey Agar is a selective and differential culture medium used for isolation and differentiation of Gram-negative enteric bacilli based on lactose fermentation.

🧬 Principle

• Peptones supply nitrogen and nutrients.
• Lactose serves as the fermentable carbohydrate.
• Bile salts and crystal violet inhibit Gram-positive bacteria.
• Neutral red acts as the pH indicator.
• Acid production from lactose fermentation changes colonies to pink or red.
• Non-lactose fermenters remain colourless.

🧪 Composition (per litre)

Component	Amount
Peptone	17 g
Proteose Peptone	3 g
Lactose	10 g
Bile Salts	1.5 g
Sodium Chloride	5 g
Neutral Red	30 mg
Crystal Violet	1 mg
Agar	13.5 g
Distilled Water	1000 ml
Final pH	7.1 ±0.2

🧪 Media Preparation

1. Suspend 50 g powder in 1 litre distilled water.
2. Heat with constant stirring until dissolved.
3. Autoclave at 121°C for 15 minutes.
4. Cool to 45–50°C.
5. Pour sterile Petri dishes.

🔬 Inoculation

1. Streak specimen onto MacConkey agar plate.
2. Incubate aerobically at 35–37°C.
3. Observe after 18–24 hours.

🦠 Colony Observation

🧪 Quality Control

Organism	Expected Result
Escherichia coli ATCC 25922	Good growth, pink colonies with bile precipitate
Klebsiella pneumoniae ATCC 13883	Large pink mucoid colonies
Salmonella Typhimurium ATCC 14028	Colourless colonies
Shigella flexneri	Colourless colonies
Enterococcus faecalis ATCC 29212	Inhibited
Staphylococcus aureus ATCC 25923	Inhibited

⚠️ Limitations

• Some Gram-positive organisms may grow weakly.
• Late lactose fermenters may initially appear colourless.
• Not suitable as the sole identification method.
• Biochemical confirmation is recommended.
• Heavy inoculum may reduce selectivity.

📚 References

1. MacConkey A. 1905.
2. MacFaddin J.F. Media for Isolation-Cultivation-Identification-Maintenance of Medical Bacteria.
3. Forbes BA. Bailey & Scott's Diagnostic Microbiology.
4. Koneman's Color Atlas and Textbook of Diagnostic Microbiology.
5. CLSI M22 Quality Control for Culture Media.