Bacteriostatic Water UK: Uses, Shelf Life and Storage for Peptide Research

Search for “bacteriostatic 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 is often shortened to “bac water” in research and clinical contexts. The product is sterile water with a preservative added, and the preservative is what distinguishes it from sterile water.

This guide explains what bacteriostatic water is, why it is used for peptide reconstitution, how it differs from sterile water, how the volume of diluent affects concentration, 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 (9 mg per mL). 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 should not be used for certain applications where benzyl alcohol is contraindicated.

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.

Concentration (mg/mL) = Mass of peptide (mg) / Volume of diluent (mL)

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 Alta Peptides reconstitution calculator works out the concentration for any combination of mass and volume.

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.

Shelf life and storage

The shelf life of bacteriostatic water depends on the storage conditions and whether the container has been opened.

Unopened 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. The preservative inhibits microbial growth, but it does not prevent chemical degradation or contamination from poor handling practices. The CDC’s injection safety checklist covers safe handling practices for vials, syringes, and other injection supplies in healthcare settings.

Reconstituted peptide solutions have a shorter shelf life than the lyophilised powder. The GenScript peptide storage and handling guide recommends storing reconstituted peptides at 2 to 8 degrees Celsius and using them within a defined window. The exact window depends on the peptide and the diluent.

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. The product should be stored according to the manufacturer’s instructions. The Bachem handling and storage guidelines provide additional detail on the conditions that maintain peptide and diluent integrity.

How Alta Peptides supplies bacteriostatic water

Alta Peptides supplies bacteriostatic water as part of its research accessories range. The bacteriostatic water product page lists the available vial sizes and the documentation that accompanies each order. The company’s certificates page collects the lot-specific reports for all products.

Comparing diluent options

Common questions about bacteriostatic water in the UK

What is bacteriostatic water?

Bacteriostatic water is sterile water containing 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.