Q1: What’s the core function of a microbial limit tester?

A1: A microbial limit tester is basically a device to measure microbial content.It mainly tests the total amount of aerobic bacteria, molds and yeasts in samples like pharmaceuticals, food and cosmetics.It makes sure their microbial contamination meets regulatory requirements—like Chinese Pharmacopoeia or USP standards.Its core job is to trap or culture microorganisms in samples. It uses precise membrane filtration or direct inoculation.This way, you can do quantitative or qualitative analysis of microbial content.

Q2: What key steps are in the standard operating process?

A2: Let’s take the common membrane filtration method as an example. The steps are:

• Preparations

Turn on the biosafety cabinet or clean bench first. Run UV sterilization for 30 minutes.Check culture media—like TSA and SDA. They must be sterilized and within validity.Prepare sterile filter cups, filter membranes (pore size 0.22 or 0.45 μm usually) and rinsing solution (e.g., buffer with polysorbate).

• Sample Preparation

Liquid samples can be filtered directly.Solid samples need to be dissolved or homogenized first, then filtered.Sample volume follows pharmacopoeia rules. Usually 10 to 100 mL or grams.

• Filtration and Rinsing

Pass the sample through the sterile filter membrane. Use a negative pressure filtration device to finish filtering.Rinse the filter membrane 3 times with rinsing solution. 100 mL each time. This removes antimicrobial components in the sample.

• Incubation and Counting

Move the filter membrane to a culture medium plate.Incubate aerobic bacteria at 30–35°C for 3–5 days.Incubate molds/yeasts at 20–25°C for 5–7 days.Count the colonies. Calculate microbial content per unit sample. Unit is CFU/g or CFU/mL.

Q3: What precautions are most easily overlooked during operation?

A3: These easy-to-miss points need extra attention:

• Environmental Control

Operate only in areas with qualified cleanliness—like Class A environment. This avoids cross-contamination.Verify the biosafety cabinet’s airflow and high-efficiency filter performance regularly. Don’t wait for problems to check.

• Filter Membrane and Rinsing Solution Selection

Filter membrane material must match the sample. For example, hydrophobic samples need pre-treatment with hydrophilic membranes.If the sample has antimicrobial components, rinse thoroughly. Otherwise, results may be falsely negative and inaccurate.

• Culture Medium Quality Control

Check culture media for sterility before use. Do growth promotion tests too. Make sure they’re usable.Don’t let too much condensation form on the culture medium surface. It affects colony dispersion and makes counting inaccurate.

• Data Recording and Deviation Handling

Record operating parameters in real time—like filtration time and rinsing volume. Follow the ALCOA+ principle. Keep data complete.If you find abnormalities—like broken filter membranes or contaminated culture media—stop the experiment right away. Start a deviation investigation. Don’t force the experiment to continue.

Q4: How to verify the recovery rate of a microbial limit tester?

A4: Recovery rate verification ensures test results are reliable. The process is:

First, do a positive control test. Add standard strains of known concentration to the sample—like Staphylococcus aureus and Aspergillus niger.Filter and incubate. Calculate the ratio of actual recovered bacteria to theoretical value. Target recovery rate is no less than 70%.

Then, eliminate interferents. Check if the sample itself affects microbial growth—like antibiotics or preservatives in it.If it does, add a neutralizer. Or do more dilution steps.

Finally, compare methods. Compare results of membrane filtration and direct inoculation. Verify the two methods are consistent. Make sure the method works.

Q5: What are the key points for daily maintenance and calibration?

A5: Daily maintenance and calibration focus on these:

• Equipment Maintenance

Clean the filter cup interface every day. Replace the waste liquid bottle promptly. Avoid biological contamination buildup.Check vacuum pump pressure stability monthly. Recommended range is -0.08 to -0.1 MPa. Adjust if it’s out of range.

• Calibration Requirements

Get a third party to calibrate flowmeters, pressure sensors and timers every year. Keep data accurate.Verify filtration efficiency regularly. Use standard bacterial membranes—like 0.45 μm. Test how well filtration works.

• Consumable Management

Store filter membranes and culture media by storage rules—like protected from light, low temperature. Record the opening date.Never use expired consumables. Or those with damaged packaging. They’ll ruin experiment results.

Q6: How to avoid false positive/false negative results during operation?

A6: Prevent from two sides:

• Preventing False Positives

Follow aseptic operation strictly. Wear sterile gloves. Replace them quickly if they get dirty or broken. Don’t cut corners.Wipe the workbench with 75% ethanol before and after the experiment. Disinfect thoroughly. Avoid external microbial contamination.

• Preventing False Negatives

Make sure no residual antimicrobial components are left in the sample. Add a neutralizer. Or rinse thoroughly to remove antimicrobials.Check the filtration system’s tightness. Don’t let microorganisms leak out. Otherwise, they won’t be detected—causing false negatives.

Q7: What to do if the filter membrane clogs or flow rate is abnormal?

A7: Two situations, clear solutions:

• Filter Membrane Clogging

Stop the experiment. Replace with a new filter membrane.If clogging happens often, check sample pretreatment. See if solid samples have too many particles not removed properly.

• Abnormal Flow Rate

First, check if vacuum pump pressure is enough. Adjust if it’s not.Then check for pipeline leaks. Repair if any are found.If needed, clean the filter pipeline. Or replace it directly. Keep flow rate normal.

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