Bac Water UK: What Bacteriostatic Water Is and Why Researchers Use It
Quick answer: Bacteriostatic water is sterile water containing a small amount of benzyl alcohol as a preservative, typically 0.9 percent. It is used to reconstitute lyophilised peptides for laboratory work. The preservative inhibits microbial growth, which extends the shelf life of the reconstituted solution compared with sterile water. In the US, bacteriostatic water for injection is a prescription-only (Rx) product; research suppliers sell it as a laboratory diluent, and the volume of diluent added determines the concentration of the resulting solution. Alta Peptides supplies bacteriostatic water alongside its research peptide range, with lot-specific certificates of analysis published for every product configuration.
Search for “bac water UK” and you will find a range of products, from 10ml vials to larger bottles, sold as diluents for reconstituting lyophilised peptides. The term “bac water” is shorthand for bacteriostatic water, which is sterile water with a preservative added. The preservative is what distinguishes it from sterile water, and the distinction matters for how the reconstituted solution is stored and how long it remains usable.
This guide explains what bacteriostatic water is, why it is used for peptide reconstitution, how it differs from sterile water, and what to consider when sourcing it in the UK.
What bacteriostatic water is
Bacteriostatic water is sterile water that contains a small amount of benzyl alcohol, typically 0.9 percent by volume. The benzyl alcohol acts as a bacteriostatic agent, which means it inhibits the growth of bacteria but does not necessarily kill them. The preservative extends the shelf life of the solution after the container is opened.
Pfizer’s labelling guidance for bacteriostatic water describes the product as sterile water for injection containing benzyl alcohol as a preservative. The guidance notes that the product is intended for use as a diluent and that it carries a boxed warning that it is not for use in neonates, because benzyl alcohol can be toxic in that population. The label also marks it as a prescription-only (Rx) product.
The distinction between bacteriostatic water and sterile water is the preservative. Sterile water contains no preservative, which means that once the container is opened, microbial contamination can occur. Bacteriostatic water contains the preservative, which inhibits microbial growth and allows the solution to be used over a longer period.
Why researchers use it for peptide reconstitution
Lyophilised peptides are supplied as powders that require reconstitution with a diluent before laboratory use. The diluent dissolves the powder and creates a solution of a defined concentration. Bacteriostatic water is the most common diluent for this purpose.
The reason is the preservative. A reconstituted peptide solution stored in a refrigerator can support microbial growth if the diluent contains no preservative. Bacteriostatic water inhibits that growth, which extends the practical shelf life of the reconstituted solution.
The Alta Peptides guide to bacteriostatic water explains the role of the preservative and the storage conditions that maintain the integrity of the reconstituted solution.
How the volume of diluent affects concentration
The volume of bacteriostatic water added to a peptide vial determines the concentration of the resulting solution. The calculation is simple: the mass of peptide in the vial divided by the volume of diluent added.
For example, if a vial contains 10mg of peptide and 2ml of bacteriostatic water is added, the concentration is 5mg/ml. If 1ml is added, the concentration is 10mg/ml. The mass stays the same; the concentration changes with the volume.
The bacteriostatic water product page lists the available vial sizes and the documentation that accompanies each order.
Bacteriostatic water versus sterile water
The two products are not interchangeable for all applications. Sterile water contains no preservative, so it must be used immediately after opening or discarded. Bacteriostatic water contains the preservative, so it can be used over a longer period, but the preservative itself may be contraindicated for certain applications.
The CDC’s injection safety guidance notes that single-dose vials should be used for a single patient or procedure, and that multi-dose vials require careful handling to prevent contamination. Bacteriostatic water is supplied in multi-dose vials, which means the handling practices matter.
Video: How to reconstitute peptides with bacteriostatic water
This third-party video provides general context on peptide reconstitution. It is not affiliated with Alta Peptides and is not a substitute for laboratory documentation or the published literature.
What to look for when sourcing bac water
When sourcing bacteriostatic water, several factors matter.
Concentration of benzyl alcohol. The standard concentration is 0.9 percent. The concentration should be stated on the label and in the documentation.
Sterility. The product should be sterile and supplied in a sealed vial. Any breach of the seal compromises the sterility of the contents.
Volume. Bacteriostatic water is supplied in various volumes, from 10ml to 30ml and larger. The choice depends on how much diluent is needed and how many reconstitutions a single vial will support.
Storage. Bacteriostatic water should be stored according to the manufacturer’s instructions. The CDC’s injection safety checklist covers safe handling practices for vials and injection supplies in healthcare settings.
How Alta Peptides supplies bacteriostatic water
Alta Peptides supplies bacteriostatic water as part of its research accessories range. The water and accessories category lists the available products, including the different volumes and configurations. The company also publishes a guide to filtration and bacteria in peptide solutions that explains the role of the preservative in maintaining solution integrity.
Comparing diluent options
| Feature | Bacteriostatic water | Sterile water |
|---|---|---|
| Preservative | Benzyl alcohol 0.9% | None |
| Shelf life after opening | Extended | Immediate use |
| Multi-dose use | Yes, with careful handling | No |
| Typical use | Peptide reconstitution | Single-use dilution |
Common questions about bacteriostatic water
What is bacteriostatic water?
Bacteriostatic water is sterile water containing a small amount of benzyl alcohol as a preservative, typically 0.9 percent. The preservative inhibits microbial growth, which extends the shelf life of the solution after the container is opened.
Why is bacteriostatic water used for peptide reconstitution?
A reconstituted peptide solution stored in a refrigerator can support microbial growth if the diluent contains no preservative. Bacteriostatic water inhibits that growth, which extends the practical shelf life of the reconstituted solution.
How does the volume of diluent affect the concentration?
The concentration of the solution is the mass of peptide divided by the volume of diluent added. A smaller volume produces a more concentrated solution; a larger volume produces a more dilute solution.
What is the difference between bacteriostatic water and sterile water?
Bacteriostatic water contains benzyl alcohol as a preservative. Sterile water contains no preservative. The preservative extends the shelf life of the solution after opening, but it may be contraindicated for certain applications.
How should bacteriostatic water be stored?
Bacteriostatic water should be stored according to the manufacturer’s instructions. The product should be kept sealed until use and stored at the recommended temperature. Once opened, the vial should be handled carefully to prevent contamination.
What size of bacteriostatic water should I choose?
The choice depends on how much diluent is needed and how many reconstitutions a single vial will support. Common sizes are 10ml and 30ml. The Alta Peptides product page lists the available configurations.
Disclaimer: This article is for informational purposes only and does not constitute legal, medical, or regulatory advice. Bacteriostatic water for injection is a prescription-only pharmaceutical product in some markets, including the US; the product discussed here is supplied as a laboratory diluent for research use. Research peptides are sold for laboratory research purposes only and are not approved by the MHRA, the FDA, or any other regulatory agency for human or veterinary use. Nothing in this article should be interpreted as guidance for human use, dosing, or administration. Researchers are responsible for complying with all applicable laws and regulations in their jurisdiction. Consult a qualified professional before making decisions about any substance. Alta Peptides is a supplier of research peptides and accessories; this article is editorial content and is not a product endorsement.
Becky Sanders
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