Plating- Membrane Filter
Membrane filtration is a quantitative microbiological method used to detect and enumerate microorganisms in clear liquid samples by filtering a known volume through a sterile membrane (typically 0.45 µm pore size). The membrane traps microorganisms, which are then cultured on selective or non-selective agar media.
Step-by-Step Procedure
- Sample Filtration: A measured volume (e.g., 100 mL) is passed through a sterile membrane using a vacuum filtration unit.
- Membrane Transfer: The membrane is aseptically placed onto the surface of a pre-poured agar plate (e.g., m-Endo, PCA, R2A).
- Incubation: Plates are incubated under appropriate conditions (e.g., 35°C for 24–48 hours) depending on the target organism.
- Colony Enumeration: Colonies growing on the membrane surface are counted and reported as CFU per 100 mL or per sample volume.
Applications
- Water Testing: Detection of coliforms, E. coli, and heterotrophic bacteria in drinking and process water
- Beverage Microbiology: Microbial load in bottled water, juices, and clear soft drinks
- Pharmaceutical & Cosmetic Fluids: Bioburden testing of rinse water, raw materials, and final products
- Environmental Monitoring: Air and surface rinse samples in cleanrooms or food production zones
Advantages
- High sensitivity for low microbial loads
- Suitable for large-volume samples
- Allows for selective recovery using differential media
- Faster results compared to enrichment-based methods
Limitations
- Not ideal for turbid or particulate-rich samples
- May under-recover stressed or injured cells
- Requires sterile technique and vacuum equipment
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