When using the membrane filtration method to determine bacterial concentrations,
the flow-through contains the bacterial cells
the filter pores are large enough to allow bacteria to pass through
the bacterial cells cannot pass through the membrane filter
a portion of the flow-through is put on agar plates for standard plate count
The correct statement is: a portion of the flow-through is put on agar plates for standard plate count.
When using the membrane filtration method to determine bacterial concentrations, the flow-through contains the bacterial cells.
Explanation: The membrane filtration method involves passing a liquid sample (e.g., water, food, or environmental sample) through a membrane filter. This filter has microscopic pores that are smaller than the bacterial cells. As a result, the liquid sample passes through the filter, while the bacteria get trapped on the surface of the filter.
The filter pores are large enough to allow bacteria to pass through.
Explanation: This statement is incorrect. In the membrane filtration method, the filter pores are actually smaller than the bacterial cells, preventing them from passing through. Only the liquid portion of the sample can pass through the filter.
The bacterial cells cannot pass through the membrane filter.
Explanation: Yes, this statement is correct. The bacterial cells are too large to pass through the membrane filter pores. They get retained and trapped on the surface of the filter.
A portion of the flow-through is put on agar plates for standard plate count.
Explanation: Once the liquid portion of the sample has passed through the membrane filter, it is known as the flow-through. To determine the bacterial concentration in the original sample, a specific volume of the flow-through is plated onto agar plates. The agar plates provide a nutrient-rich environment for the bacteria to grow into colonies visible to the naked eye. These colonies can then be counted to determine the number of bacteria present in the original sample. The result is known as the standard plate count, which gives an estimation of the bacterial concentration.
Step 1: Prepare the membrane filtration apparatus and materials.
- Gather the necessary equipment, including a filtration funnel, vacuum pump, membrane filter with appropriate pore size, sterile petri dishes, agar medium, and a sterile diluent.
- Sterilize the equipment by autoclaving or using a suitable method.
Step 2: Prepare the sample for filtration.
- Collect a representative sample of the liquid or solution containing the bacteria.
- Mix or homogenize the sample to ensure an even distribution of bacteria.
Step 3: Set up the membrane filtration apparatus.
- Place the sterile membrane filter onto the filtration funnel, ensuring it is properly positioned and securely attached.
- Connect the filtration funnel to the vacuum pump.
Step 4: Filter the sample.
- Pour the sample into the filtration funnel.
- Apply a gentle vacuum to facilitate the filtration process.
- The liquid portion of the sample (flow-through) will pass through the filter, containing the bacterial cells.
Step 5: Transfer the bacterial cells onto agar plates.
- Remove the membrane filter from the filtration funnel. The bacteria will be retained on the filter surface.
- Place the membrane filter, with the retained bacterial cells facing upward, onto a sterile petri dish containing agar medium.
- Gently press the filter onto the agar surface to ensure good contact.
Step 6: Incubate the agar plates.
- Place the agar plates in an incubator at the appropriate temperature and conditions for bacterial growth.
- Incubate the plates for a specified period, typically 24-48 hours.
Step 7: Count the bacterial colonies.
- After incubation, observe the agar plates and count the visible colonies formed by the bacterial cells.
- Determine the bacterial concentration by multiplying the colony count by the appropriate dilution factor.
Note: The membrane filtration method is commonly used to determine bacterial concentrations in samples with low counts or where other methods are not suitable. The pore size of the membrane filter is selected to allow the bacteria to pass through while retaining other non-bacterial components. The colonies formed on the agar plates are counted and used for the standard plate count to estimate bacterial concentration.