Overview
This lecture explains the Most Probable Number (MPN) method, a statistical technique for estimating microbial contamination, especially coliform bacteria, in water and other samples.
MPN Method Overview
- MPN (Most Probable Number) estimates the number of microbes in a sample, particularly when samples are dilute or microbes won't grow on solid media.
- Commonly used to detect coliforms like E. coli, which indicate fecal contamination in water.
- The method is also applicable in food testing and environmental science.
Procedure Steps
- Use 15 lactose broth tubes (arranged as 3 sets of 5 tubes) for the test.
- Inoculate the first set with 10 mL, the second with 1 mL, and the third with 0.1 mL of the sample.
- Lactose broth contains lactose sugar; coliforms ferment lactose, causing a pH drop and color change.
- Incubate tubes for 24 hours at 37°C (optimal temperature for mesophilic pathogens).
- After incubation, positive tubes turn yellow due to acid produced from lactose fermentation.
Interpreting Results
- Record the number of positive (yellow) tubes in each set.
- Use the pattern of positives (e.g., 5, 2, 0) to consult an MPN table and estimate the microbial concentration.
- Example: 5 positive in 10 mL, 2 in 1 mL, 0 in 0.1 mL corresponds to 49 organisms per 100 mL, with 95% confidence.
- Zero positives indicate very low contamination; all positives indicate very high contamination.
Key Terms & Definitions
- MPN (Most Probable Number) — A statistical method to estimate the concentration of microorganisms in a sample.
- Coliforms — Bacteria, including E. coli, used as indicators of fecal contamination.
- Lactose Broth Tubes — Tubes containing lactose used to detect coliforms via fermentation and color change.
- Mesophilic Organisms — Microbes that grow best at moderate temperatures (e.g., 37°C).
Action Items / Next Steps
- Review the MPN table for interpreting tube results.
- Practice calculating MPN values using sample data.
- Prepare for water testing experiments involving the MPN method.