Blog
Sterility, Bioburden and USP 71: What Sterile Means and What Research Grade Does Not
The word sterile gets used loosely. In a laboratory or pharmaceutical setting it has a precise meaning and requires a specific test and manufacturing method. A product is not sterile because it looks clean, and many research-use chemicals are never sold as sterile at all.
Key takeaways
- Sterile means free of viable microorganisms and must be demonstrated by process and test.
- Bioburden is the count of microbes present before sterilization.
- Endotoxin is a separate contaminant from dead bacteria and is tested differently.
- A research-use label does not imply sterility.
How sterility is achieved
- Terminal sterilization of the filled container, by steam or radiation.
- Sterile filtration through a 0.22 micron membrane followed by aseptic filling into sterile containers in a controlled cleanroom. Peptides, which are damaged by heat and radiation, are usually sterilized this way.
- Aseptic process validation such as media fills, in which broth replaces product to prove the process does not introduce microbes.
The accepted standard is a sterility assurance level of 10 to the power of minus 6, meaning a one in a million chance of a non-sterile unit, which is a design target for terminally sterilized products and not something a test alone can prove.
What USP 71 does
The United States Pharmacopeia chapter on sterility tests describes two methods. In membrane filtration the sample is passed through a filter that traps microbes, and the filter is placed in culture media. In direct inoculation the sample is added to the media. Two media cover different organisms: fluid thioglycollate medium, incubated at about 30 to 35 degrees Celsius, supports anaerobes and many aerobes, and soybean-casein digest medium, incubated at about 20 to 25 degrees Celsius, supports aerobes and fungi. Containers are observed for 14 days for turbidity. A sample must also pass a method suitability test, showing that the product does not inhibit growth and so hide contamination.
Limits of the sterility test
Because only a small sample is tested, a sterility test cannot prove that every unit is sterile. It is a check on a process that is itself validated. That is why regulators place so much weight on how a product is made.
Bioburden and preservatives
- Bioburden testing counts viable organisms per unit or gram, usually by plate counts, and may be reported as colony-forming units.
- Preserved products such as bacteriostatic water contain an antimicrobial, typically benzyl alcohol, that suppresses growth in a multi-dose vial. The effectiveness of a preservative is tested under USP 51, by challenging the product with microbes and tracking their decline. A preserved product is not necessarily sterile at the moment of use, and the preservative does not remove endotoxin.
Endotoxin is a different question
Sterile does not mean endotoxin-free. A solution can be sterilized by filtration and still contain bacterial fragments from the water that was in it earlier. That test is covered in endotoxin testing.
What to check on a research product
- Does the label or report say sterile, and by what method?
- Is the claim for the powder, the diluent, or both?
- Is an endotoxin result provided?
- Is the product labeled for injection or only for laboratory research use? Research-use compounds are not intended for human or animal administration.
- What storage and handling does the supplier recommend after the first puncture?
Handling in the lab
If your assay is sensitive to contamination, such as cell culture, you can sterilize a solution yourself by filtering it through a low-binding 0.22 micron filter in a clean hood, accepting some loss of peptide (see syringe filter choice). Water choice is covered in lab water grades.
The bottom line
Sterility is a property proven by process, test and documentation. When a product does not claim it, assume it is not sterile and plan accordingly.
Related reading
- Endotoxin Testing (LAL) and Research-Grade Peptides: What to Know
- Lab Water Grades Explained: Type I, II, III and Why They Matter for Peptides
- Bacteriostatic Water vs Sterile Water in the Lab
- Browse the full Research Library
For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.