Bacteriostatic water (BAC): why, how and how much
Bacteriostatic water is the reference solvent for reconstituting research peptides. This guide explains its composition, its advantages over plain sterile water, the volumes to use, and the rare cases where alternatives are necessary.
What is bacteriostatic water?
L’bacteriostatic water for injection (BAC Water, Bacteriostatic Water for Injection) is a sterile water solution for injection containing 0.9% benzyl alcohol as an antimicrobial preservative. It is produced according to USP (United States Pharmacopeia) and Ph.Eur standards.
The formulation is simple: sterile ultrapure water + 0.9% benzyl alcohol. Nothing else. The pH is close to neutral (pH 5-7 depending on the formulation). It is packaged in multi-dose glass bottles with a perforated septum stopper, allowing for multiple punctures without compromising the sterility of the remaining contents.
Benzyl alcohol (C₆H₅CH₂OH, 0.9%) inhibits bacterial growth by disrupting the bacterial cell membrane and inhibiting certain respiratory enzymes. At this concentration, it is bacteriostatic (inhibits growth) but not sterilizing (does not immediately kill spores). This is sufficient to prevent contamination of multi-dose peptides stored for 28 days under refrigeration.
BAC water vs sterile water: a complete comparison
| Criteria | BAC water (0.9% benzyl alcohol) | Sterile Water Only (WFI) |
|---|---|---|
| Conservative | Yes — benzyl alcohol 0.9% | No |
| Multiple punctures (multidose) | Yes — safe for 28 days | No — single use |
| Shelf life after reconstitution | 28 days (2-8°C) | 5-7 days max |
| Typical bottle volume | 10 ml, 30 ml | 2 ml, 5 ml, 10 ml |
| pH | ~5-7 | ~5-7 |
| Peptide compatibility | Very large | Large (but limited duration) |
| Sterility guaranteed | Yes (USP) | Yes (USP) |
When to use plain sterile water?
Sterile water without preservatives is preferable in two cases:
- The peptide will be used in single dose and the vial consumed entirely in one go
- Some very rare peptides exhibiting a incompatibility with benzyl alcohol (peptide sensitivity to alcohols) — check the product information sheet
BAC volumes to use: rules and examples
Calculation principles
The volume of BAC to be added determines the final concentration. The basic rule:
Concentration (mg/ml) = Mass of peptide (mg) ÷ Volume of added BAC (ml)
Insulin U-100 syringe: 1 ml = 100 units → 1 unit = 0.01 ml = 10 µl
Table of practical configurations
| peptide quantity | BAC added | Concentration | Dose 0.25 mg | Dose 0.5 mg | Dose 1 mg |
|---|---|---|---|---|---|
| 5 mg | 1 ml | 5 mg/ml | 5 units | 10 units | 20 units |
| 5 mg | 2 ml | 2.5 mg/ml | 10 units | 20 units | 40 units |
| 10 mg | 2 ml | 5 mg/ml | 5 units | 10 units | 20 units |
| 10 mg | 5 ml | 2 mg/ml | 12.5 units | 25 units | 50 units |
| 20 mg | 4 ml | 5 mg/ml | 5 units | 10 units | 20 units |
| 20 mg | 10 ml | 2 mg/ml | 12.5 units | 25 units | 50 units |
Choose a tank volume that yields whole numbers of units for your target dose. For example, for a dose of 0.5 mg with 10 mg of peptide, adding 2 ml (= 10 units) is much more practical and precise than adding 3 ml (= 6.67 units, not directly readable).
BAC Water Use Protocol
Store the container in the refrigerator
Once opened, BAC water can be kept refrigerated (2-8°C) for up to 28 days. After this time, its preservative properties decrease and there is a risk of microbial growth. Mark the opening date on the bottle.
Disinfect the stopper before each puncture
Even though the BAC is bacteriostatic, disinfecting the septum with 70% isopropyl alcohol before each needle insertion remains good practice. Let it dry for 30 seconds.
Use a clean syringe for each puncture.
Never reinsert a used needle into the BAC vial. This will introduce contaminants into the vial and compromise all future reconstitutions.
Gently suck
Draw up the desired volume slowly to avoid bubbles. If bubbles do form, tap the syringe and gently push the plunger upwards to expel them before injecting the peptide into the vial.
Alternatives to BAC water: when and which one?
Dilute acetic acid (0.6% — 0.1 M)
Used for peptides that do not dissolve in neutral water or that precipitate at neutral pH. Typical of basic peptides (such as DAC-free CJC-1295, GHRH analogue). Dilute acetic acid slightly lowers the pH and improves the solubility of these peptides.
- Prepare: 0.35 ml of pure glacial acetic acid + 99.65 ml of sterile water = ~0.6% solution
- Or use ready-made 0.1 M acetic acid (pH ~2.9)
- Must be sterile and water-grade for injection
DMSO (dimethyl sulfoxide) at low concentration
A universal solvent used for highly hydrophobic (lipophilic) peptides. Rarely used for standard research peptides. Concentration typically ≤ 10% in sterile water. DMSO is itself biologically active and can affect experimental models—use with caution.
PBS (Phosphate Buffered Saline)
For certain cell culture protocols, PBS can be used. Disadvantage: no antimicrobial preservative, immediate use only after reconstitution.
BAC water available at MyPeptide
Our USP grade bacteriostatic water is available in 10 ml bottles with a multi-dose septum. It is produced according to European and American pharmacopoeia standards, guaranteeing sterility and the correct concentration of benzyl alcohol.
Water tank and accessories available
USP bacteriostatic water, U-100 insulin syringes, 30G needles — all the accessories for your research protocol.