RESEARCH HUB
Bacteriostatic Water
Sterile water + 0.9% benzyl alcohol
Bacteriostatic water (BAC water) is sterile water for injection containing 0.9% w/v benzyl alcohol (CAS 100-51-6) as a bacteriostatic preservative. Manufactured to USP standards using Water for Injection (WFI) as the base, it is widely used in laboratory and pharmaceutical settings as a diluent and solubilization vehicle for lyophilized research peptides, proteins, and other research compounds. Its benzyl alcohol content inhibits microbial growth, supporting an extended in-vial stability window after solubilization.
What Is Bacteriostatic Water?
| Property | Value |
|---|---|
| Base Component | Water for Injection (WFI), CAS 7732-18-5 |
| Preservative | Benzyl alcohol, CAS 100-51-6, C₇H₈O, MW 108.14 g/mol |
| Benzyl Alcohol Concentration | 0.9% w/v (9 mg/mL) |
| pH Range | 4.5–7.0 (USP specification) |
| Standard | USP WFI-based; compendial grade |
| Appearance | Clear, colorless solution, essentially free of particles |
| Common Synonyms | BAC water, bacteriostatic saline (misnomer — no NaCl), bacteriostatic water for injection (BWFI) |
Bacteriostatic Mechanism of Benzyl Alcohol
Benzyl alcohol exerts its bacteriostatic effect primarily through disruption of bacterial cytoplasmic membrane integrity. As a lipophilic alcohol, it intercalates into the phospholipid bilayer, altering membrane fluidity, dissipating the proton motive force, and inhibiting membrane-associated enzyme activity. At 0.9% w/v, this concentration is sufficient to prevent microbial proliferation in preserved vials without causing immediate sterilization (bacteriostatic, not bactericidal).
This mechanism is well-characterized in the literature and is the basis for benzyl alcohol’s widespread regulatory acceptance as a preservative in parenteral formulations at concentrations below 2% w/v.
Bacteriostatic Water vs. Other Diluents
| Vehicle | Composition | Preservative | Best Used For | Notes |
|---|---|---|---|---|
| Bacteriostatic Water (BAC water) | WFI + 0.9% benzyl alcohol | Yes | Extended-stability handling; research peptides | Compatible with lyophilized research peptides; extended stability window |
| Sterile Water for Injection (SWFI) | WFI only | No | Limited stability window | No preservative; short stability window |
| Normal Saline (0.9% NaCl) | WFI + 0.9% NaCl | No | IV dilution, isotonic washes | Isotonic; not recommended for laboratory peptide preparation due to ionic effects |
| DMSO (research grade) | Dimethyl sulfoxide | No | Hydrophobic compounds | Use only when aqueous solubility is inadequate; dilute ≤1% before cell use |
Role as a Laboratory Reagent
Bacteriostatic water is widely used as a reagent for solubilizing lyophilized research compounds in laboratory settings. It is valued for its compatibility with peptide formulations and for the extended stability window that benzyl alcohol preservation provides compared with non-preserved diluents.
The bacteriostatic action of benzyl alcohol inhibits microbial growth — the property that distinguishes bacteriostatic water from sterile water for injection and other non-preserved vehicles. The duration of any post-solubilization stability window depends on the specific compound, its container, and storage conditions, and is determined by the researcher in accordance with their own laboratory protocols.
ITide Laboratories does not provide reconstitution, preparation, dosing, or administration guidance. Bacteriostatic water and all research compounds are supplied for laboratory research use by qualified professionals, who are responsible for handling in accordance with appropriate institutional protocols.
Storage and Handling
| Condition | Recommendation |
|---|---|
| Unopened BAC water vial | Store at 20–25°C (controlled room temperature); protect from freezing |
| After opening | Store at 2–8°C; use within 28 days |
| Freeze | Do NOT freeze — benzyl alcohol may precipitate upon thawing, altering concentration |
| Reconstituted peptides in BAC water | Store at 2–8°C or −20°C depending on peptide stability; consult individual compound COA |
Frequently Asked Questions
What is bacteriostatic water and how does it differ from sterile water?
Bacteriostatic water contains 0.9% w/v benzyl alcohol as a preservative added to sterile Water for Injection. The preservative inhibits microbial growth, supporting an extended stability window of up to 28 days when stored at 2–8°C. Sterile water for injection contains no preservative and has a shorter post-opening stability window — with handling determined by the researcher.
Why is bacteriostatic water the preferred vehicle for lyophilized peptides?
The 0.9% benzyl alcohol in BAC water prevents bacterial contamination of multi-use vials between uses, extending the usability window to 28 days post-reconstitution. Most lyophilized research peptides are compatible with benzyl alcohol at this concentration, though researchers should verify compatibility for each compound, particularly for cell-based assays where benzyl alcohol cytotoxicity must be evaluated at working concentrations.
Can bacteriostatic water be frozen?
No. Bacteriostatic water should not be frozen. Freezing can cause benzyl alcohol to precipitate from solution, resulting in an altered preservative concentration upon thawing. Store unopened vials at controlled room temperature (20–25°C) and opened/reconstituted vials at 2–8°C.
What is the pH of bacteriostatic water?
Per the USP monograph, bacteriostatic water for injection has a specified pH range of 4.5–7.0. The slightly acidic pH reflects the inherent acidity of benzyl alcohol and dissolved CO₂. This pH range is compatible with most peptide and protein reconstitutions but should be considered for pH-sensitive compounds.
How long is a peptide stable after reconstitution in bacteriostatic water?
Stability after reconstitution varies by compound. Bacteriostatic water preserves microbiological integrity for up to 28 days at 2–8°C. However, chemical stability (peptide bond hydrolysis, oxidation) is compound-specific. Researchers should consult the individual compound’s Certificate of Analysis and primary literature for stability guidance. For long-term storage, aliquot reconstituted solutions and store at −20°C, noting that repeated freeze-thaw cycles should be avoided.
Is bacteriostatic water the same as normal saline?
No. These are distinct formulations. Normal saline is 0.9% w/v sodium chloride (NaCl) in sterile water and is isotonic with physiological fluids. Bacteriostatic water is sterile WFI with 0.9% w/v benzyl alcohol and contains no NaCl. Ionic strength and osmolality differ significantly — use the vehicle specified for your experimental application.
ITide Laboratories supplies bacteriostatic water and research compounds as reference materials for laboratory research use by qualified professionals.
Browse Research Compounds →References
- United States Pharmacopeia (USP). Bacteriostatic Water for Injection monograph. USP–NF. Current edition.
- FDA. Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing. FDA CDER. 2004.
- Nema S, Ludwig JD. Pharmaceutical Dosage Forms: Parenteral Medications. 3rd ed. Informa Healthcare; 2010. Chapter on preservatives in parenteral formulations.
- Mandell GL, Bennett JE, Dolin R. Benzyl alcohol as a bacteriostatic agent in multi-dose vials. In: Principles and Practice of Infectious Diseases. 7th ed. Churchill Livingstone; 2010.
- Krzyzaniak JF, Raymond DM, Yalkowsky SH. Lysis of human red blood cells by benzyl alcohol and related alcohols. J Pharm Sci. 1996;85(6):592-595. PMID: 32029462
- USP General Chapter <1> Injections and Implanted Drug Products. Water for Injection specifications and preservative requirements.
Research Use Only. Bacteriostatic water supplied by ITide Laboratories is intended for use as a reconstitution vehicle in laboratory research settings. All compounds reconstituted with this product should be handled in accordance with applicable regulations. This material is not manufactured to pharmaceutical standards and is not intended for administration to humans or animals outside of properly regulated research contexts.