Sterilizing your aqueous plant extract sample through a 0.2 µm filter is indeed a good option to prevent bacterial and fungal contamination. Here's a detailed approach to ensure your extracts remain sterile and stable:

Sterilization via Filtration

1. Filtration Process:

- Use a 0.2 µm filter to remove bacteria and fungi. This pore size is sufficient to eliminate most microbial contaminants without significantly affecting the phytochemical composition of the extract.

2. Equipment:

- Ensure the filtration apparatus (syringe filters, filter units) is sterile. Use aseptic techniques during the filtration process to avoid introducing contaminants.

Preservation After Filtration

1. Preservatives:

- Check and Adjust Preservative Concentration: Evaluate the concentration of preservatives used initially. Common preservatives for aqueous extracts include sodium azide, benzalkonium chloride, or ethanol. Ensure that the concentration is sufficient to prevent microbial growth but not too high to interfere with the biological activity of the extract.

- Common Preservatives and Concentrations:

- Sodium azide: Typically used at concentrations of 0.02% to 0.05%.

- Ethanol: Often used at concentrations of 5% to 20%.

- Benzoic acid or sodium benzoate: Generally used at concentrations of 0.1% to 0.2%.

- Preservative Efficacy Testing: After adding preservatives, it may be prudent to perform a microbial challenge test to ensure the preservatives are effective at the concentrations used.

2. Storage Conditions:

- Store the filtered and preserved extracts at low temperatures (4°C) to slow down any potential microbial growth. For long-term storage, consider freezing the extracts.

3. Handling and Aseptic Techniques:

- Always handle the extracts using aseptic techniques to minimize the risk of contamination. Use sterile containers and tools for transferring and storing the extracts.

Steps to Follow:

1. Filter the Extract:

- Filter the aqueous extract through a sterile 0.2 µm filter using a sterile filtration unit.

2. Add Preservatives:

- Add an appropriate concentration of preservatives to the filtered extract. Ensure thorough mixing.

3. Store Properly:

- Store the sterilized and preserved extract in sterile, tightly sealed containers at 4°C or lower.

4. Monitor for Contamination:

- Periodically check the extracts for any signs of microbial growth. This can be done visually or by plating a small sample on a nutrient agar to detect any contamination.

By following these steps, you can effectively prevent bacterial and fungal growth in your aqueous plant extracts, ensuring their stability and integrity for your experiments.

No. Assuming you used autoclaving correctly, your issue is contamination of a sterilized solution not sterilization per se. Filter sterilization could replace autoclaving if you were concerned for stability through autoclaving.

Your preservative system failed. What are you using ?