Microbial colony counting is a fundamental technique in microbiology used to estimate the number of viable microorganisms in a sample. This method is crucial for various applications, including quality control in food production, assessing environmental samples, and conducting clinical microbiology tests. Accurate colony counting enables researchers to monitor microbial populations and their implications in health, industry, and ecology.
In this example, we will use the plate count method to assess the microbial load in a water sample collected from a local river. This technique helps determine whether the water is safe for recreational use.
To begin, a 10 mL sample of river water is diluted in a series of sterile saline solutions (1:10, 1:100, 1:1000). Each dilution is plated onto nutrient agar plates and incubated at 37°C for 24 hours. After incubation, colonies are counted on the plates.
To calculate the number of CFU (colony-forming units) per mL of the original water sample, use the formula:
Number of CFU/mL = (Number of colonies × dilution factor) / volume plated.
For the 1:100 dilution:
Number of CFU/mL = (30 colonies × 100) / 0.1 mL = 30,000 CFU/mL.
This indicates a potentially high microbial load, suggesting the water may not be safe for recreational activities.
This example focuses on the enumeration of total aerobic bacteria in a food sample, such as raw chicken, to evaluate its microbiological safety. The method employed here is the spread plate technique.
A 25 g sample of raw chicken is mixed with 225 mL of sterile peptone water to achieve a 1:10 dilution. From this solution, a series of further dilutions (1:100, 1:1000) are prepared.
A 0.1 mL aliquot from each dilution is spread onto plate count agar and incubated at 30°C for 48 hours. After the incubation period, the following colony counts are recorded:
Using the same CFU calculation as before, we analyze the 1:100 dilution:
Number of CFU/g = (50 colonies × 100) / 0.1 = 50,000 CFU/g.
This level of bacteria indicates that the raw chicken may pose a health risk if not cooked properly.
In this example, we will determine the fungal load in a soil sample from a garden. This assessment can help evaluate soil health and its suitability for planting.
A 10 g soil sample is suspended in 90 mL of sterile saline, creating a 1:10 dilution. Subsequent dilutions are prepared (1:100, 1:1000). A 0.1 mL aliquot from each dilution is inoculated onto potato dextrose agar (PDA) plates, which are incubated at room temperature (25°C) for 5-7 days.
After the incubation, the following fungal colony counts are observed:
Calculating the CFU for the 1:100 dilution yields:
Number of CFU/g = (60 colonies × 100) / 0.1 = 60,000 CFU/g.
This indicates a high fungal presence, which may affect plant growth and soil health.