What bacteriostatic water actually is
Bacteriostatic water for injection (often abbreviated BAC water or bac water) is sterile water for injection that contains 0.9% benzyl alcohol as an antimicrobial preservative. The benzyl alcohol concentration is chosen to inhibit the growth of bacteria and fungi without denaturing the peptide or protein being reconstituted. It is supplied in multi-dose vials (commonly 10 ml or 30 ml), each sealed with a rubber septum that allows multiple needle punctures without compromising sterility — the benzyl alcohol preserves the remaining solution after the first entry.
Sterile water for injection (also called SWFI or water for injection, WFI) contains no preservative whatsoever. It is sterile at manufacture and sealed, but once the vial is punctured, there is nothing to prevent bacterial growth in the remaining water. For this reason, single-use SWFI vials are standard in clinical settings, and any unused portion after opening is discarded.
| Property | Bacteriostatic Water (BAC) | Sterile Water for Injection (SWFI) |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Multi-dose use | Yes — designed for repeated needle entry | No — single-use only |
| Post-reconstitution shelf life | ~28 days refrigerated (2–8°C) | 24–72 hours (use immediately or freeze aliquots) |
| pH | 4.5–7.0 (slightly acidic) | 5.0–7.0 |
| Osmolarity | Hypotonic | Hypotonic |
| Typical vial sizes | 10 ml, 30 ml (multi-dose) | 5 ml, 10 ml (single-use) |
| Appropriate for cell culture assays? | Check benzyl alcohol interference at your dilution | Yes — no preservative concern |
Why the shelf life difference matters
The 28-day rule vs the 24-hour rule
An opened vial of sterile water with no preservative is vulnerable to airborne bacterial contamination the moment the septum is punctured. At room temperature, meaningful bacterial growth can establish within hours. Even refrigerated, unpreserved sterile water is not stable for multi-day use.
Bacteriostatic water’s 0.9% benzyl alcohol keeps the reconstituted solution safe for repeated use over approximately 28 days when stored refrigerated. This is not an arbitrary number: it is the standard shelf-life guidance cited in the published pharmaceutical and laboratory-handling literature for benzyl alcohol–preserved aqueous peptide solutions.
For any research protocol that uses the same reconstituted peptide over multiple days or weeks, bac water is the correct choice. Using plain sterile water for a 10 mg vial you plan to use over two weeks means you either waste most of the peptide or risk using a contaminated solution.
How to reconstitute correctly
The reconstitution process is the same regardless of which water you use. The critical variable is technique — especially how you introduce the water to the lyophilised cake:
- Draw the desired volume of bac water into a clean syringe. For most peptides, 1 ml or 2 ml is standard, giving concentrations of 1 mg/ml or 0.5 mg/ml for a 1 mg vial. For 5 mg or 10 mg vials, 1–2 ml of water produces a 5 mg/ml or 2.5 mg/ml solution.
- Do not inject the water directly onto the lyophilised cake. Angle the needle so the water runs slowly down the glass wall of the vial. High-velocity water hitting a lyophilised peptide cake can cause protein aggregation in fragile peptides.
- Do not vortex or shake the vial. Shaking causes foaming which denatures peptide bonds. Instead, gently swirl or roll the vial between your palms until the cake dissolves completely. This may take 30 seconds to several minutes depending on the peptide.
- Allow the solution to sit for 2–3 minutes after initial dissolution to ensure full reconstitution.
- Inspect for clarity. A fully reconstituted peptide in bac water should produce a clear, colourless (or very pale) solution. Cloudiness, visible particles, or significant colour change warrants discarding the vial.
- Label the vial with reconstitution date and store immediately at 2–8°C.
When to use sterile water instead of bac water
There are genuine use cases for plain sterile water over bac water:
- Cell culture assays: If the final concentration of benzyl alcohol in your assay well will be pharmacologically meaningful (above ~0.01%), it may interfere with cell viability or the pathway being studied. Calculate the dilution factor: if you are adding 1 µl of a 1 mg/ml peptide solution to a 1 ml assay volume, you are diluting benzyl alcohol 1000-fold, likely to a negligible level. If you are adding larger volumes, recalculate.
- Single-use protocols: If you are reconstituting a full vial for immediate single-use, there is no preservation advantage to bac water. SWFI is equally suitable for a reconstitute-and-use-immediately protocol.
- Aliquot-and-freeze protocols: If you reconstitute with sterile water and immediately freeze aliquots in low-binding tubes at −20°C or −80°C, each aliquot is effectively single-use. This is a valid alternative to bac water for protocols requiring very long storage (months).
Special cases: peptides with specific reconstitution requirements
GHK-Cu (Copper Peptide)
GHK-Cu is a copper complex and is light-sensitive. Reconstitute in bac water using the standard wall-drip technique, but use amber-covered tubes or aluminium foil for any aliquots. The copper complex is stable in solution when refrigerated and protected from light.
Glutathione
Glutathione (reduced form, GSH) can oxidise to GSSG (glutathione disulfide) if exposed to oxygen during reconstitution. Work quickly, minimise air exposure during reconstitution, and store refrigerated. Bac water is appropriate.
HGH (Somatropin)
Recombinant growth hormone is more shear-sensitive than most peptides. Extra gentle swirling (no shaking) is important. Bac water is standard.
Epithalon
Epithalon (tetrapeptide Ala-Glu-Asp-Gly) dissolves readily. Bac water is appropriate. The vial appearance after reconstitution is typically a clear colourless solution.
UK sourcing: where to buy bacteriostatic water
Bacteriostatic water for injection is available in the UK from:
- Sigma-Aldrich (Merck): pharmaceutical-grade BAC water in multi-dose vials. Appropriate for research labs with accounts.
- Fisher Scientific / VWR: laboratory-grade sterile water and BAC water available to registered research organisations.
- Amazon UK: 30 ml BAC vials from multiple suppliers are available for general purchase. Check that the product description specifies 0.9% benzyl alcohol and pharmaceutical-grade manufacturing.
- Lab77 Peptides: we supply BAC water vials alongside peptide orders. You can add a BAC vial to any order at checkout.