What is Bacteriostatic water?
Also called: BAC water, Bacteriostatic Water for Injection
Sterile water containing 0.9% benzyl alcohol, a preservative that suppresses bacterial growth so a vial can be punctured and drawn from more than once.
By the APL Research Team · Updated
Bacteriostatic water is sterile water with 0.9% (w/v) benzyl alcohol added. The alcohol inhibits multiplication of bacteria that might enter through the stopper during withdrawals, which is what allows one vial to be accessed repeatedly. The operative word is "bacteriostatic": it holds growth back but does not reliably kill organisms already present, so it supports aseptic technique rather than replacing it.
Composition in numbers
| Property | Value |
|---|---|
| Benzyl alcohol content | 0.9% w/v, i.e. 9 mg/mL |
| Benzyl alcohol molarity | 83 mM (benzyl alcohol is 108.14 g/mol) |
| Other solutes | None: no buffer, no sodium chloride |
| In-use period on the APL bacteriostatic water label | Discard 28 days after first puncture |
Why the preservative matters for peptides
Benzyl alcohol is a small amphiphilic molecule, and it does not always leave the solute alone. In a formulation study of recombinant human interleukin-1 receptor antagonist, it bound the protein weakly through hydrophobic interactions, shifted it toward partially unfolded forms and accelerated aggregation at 37 °C [1]. Peptides differ from one another. In an NMR study of an acylated 31-residue peptide, benzyl alcohol showed no detectable interaction and did not induce aggregation, whereas 1% m-cresol, another common preservative, produced insoluble aggregates within 24 hours [2]. Compatibility is therefore molecule-specific; for an unfamiliar or hydrophobic sequence it is worth confirming rather than assuming.
Worked example: what reaches the assay
A stock prepared in bacteriostatic water carries its benzyl alcohol into every dilution made from it:
| Dilution of stock into medium | Benzyl alcohol in the well |
|---|---|
| 1:10 | 0.9 mg/mL (8.3 mM) |
| 1:100 | 90 µg/mL (0.83 mM) |
| 1:1,000 | 9 µg/mL (83 µM) |
At a 1:100 dilution, cells see close to a millimolar concentration of a membrane-active alcohol alongside the peptide. The matching vehicle control is bacteriostatic water diluted the same way, not plain medium. Where even that is unwanted, a sterile water stock split into single-use aliquots removes the preservative from the experiment entirely.
Common misunderstandings
- It is not a co-solvent. The benzyl alcohol is there to control microbes, and it is not a fix for a sequence that will not dissolve in water.
- It does not slow chemical degradation. Oxidation, deamidation and hydrolysis proceed as they would in water; the preservative addresses microbial growth only.
- The 28-day figure applies to the diluent vial after its first puncture. It is not a shelf life for a peptide solution made with it, which depends on the peptide.
- It does not make an in-vitro system preservative-free. Any carry-over is part of the treatment unless controlled for.
APL supplies bacteriostatic water in 10 mL and 3 × 10 mL packs, and it is included in the research starter kit. The bacteriostatic water guide goes into more depth, bacteriostatic versus sterile water the choice between them, and the reconstitution guide its use at the bench.
References
- 1.Zhang Y, Roy S, Jones LS, et al. Mechanism for benzyl alcohol-induced aggregation of recombinant human interleukin-1 receptor antagonist in aqueous solution. J Pharm Sci. 2004. PubMed 15514986
- 2.Li M, Falk BT, Lu X, et al. Molecular Mechanism of Antimicrobial Excipient-Induced Aggregation in Parenteral Formulations of Peptide Therapeutics. Mol Pharm. 2022. PubMed 35917158