In the laboratory, a lyophilised (freeze-dried) peptide has to be dissolved before any work can begin. Two liquids come up most often: bacteriostatic water and sterile water. At first glance they look the same, since both are clear and sterile. The difference is a single added ingredient. That ingredient, however, decides how long an opened vial can be used and which experiments the liquid is suitable for.
What bacteriostatic water is
The United States Pharmacopeia (USP) defines bacteriostatic water (Bacteriostatic Water for Injection) as sterile, pharmacopoeial-grade water that contains one or more suitable antimicrobial agents. The preservative is usually benzyl alcohol. The Hospira label, for example, describes sterile, nonpyrogenic water with 0.9% benzyl alcohol (9 mg/mL), or 1.1% in one of its presentations. According to the same label, the pH is about 5.7 (range 4.5–7.0).
According to the official label, bacteriostatic water is intended solely for diluting or dissolving other preparations. In other words, it is a diluent, not a product in its own right. The pharmacopoeia also states explicitly that due regard must be given to the compatibility of the preservative with the substance being dissolved. We will come back to this.
The word "bacteriostatic" means that a substance inhibits the growth and multiplication of bacteria. It does not mean that it reliably kills them. Substances that kill bacteria are called bactericidal. For multi-dose vials, the US Centers for Disease Control and Prevention (CDC) notes that the preservative helps limit bacterial growth, but has no effect on viruses and does not fully protect against contamination. Bacteriostatic water is therefore no substitute for aseptic technique.
What sterile water is
The European Pharmacopoeia monograph Water for injections (0169) describes two forms of this water. Water "in bulk" serves as a vehicle in the manufacture of medicines. Sterilised water (sterilised water for injections) is intended for dissolving or diluting other preparations. This is also summarised in the European Medicines Agency (EMA) guideline on the quality of water for pharmaceutical use. The US Pharmacopeia has an equivalent product called Sterile Water for Injection.
What matters is what sterile water does not contain. The Hospira label states that it contains no bacteriostat, no antimicrobial agent and no added buffer, and that it is supplied only in single-dose containers. The manufacturer's instruction is clear: do not reuse single-dose containers and discard any unused portion.
The reason is simple. Once a vial is opened or punctured, microorganisms can get in. Without a preservative, nothing slows their growth. Sterile water is "cleaner" in the sense that it contains nothing but water. Once opened, however, it is treated as single-use.
Why a preservative: multi-dose containers
Bacteriostatic water is supplied in multi-dose containers from which, according to the label, repeated withdrawals may be made. That is exactly why it contains a preservative. The US Pharmacopeia specifies glass containers of no more than 30 mL for it.
Even a preservative does not mean unlimited use after opening. In US healthcare settings and pharmacies, multi-dose containers are handled according to the recommendation of chapter USP <797>, which the CDC also summarises: once opened (for example, once the stopper has been punctured), the vial is dated and discarded within 28 days at most, unless the manufacturer specifies a different, shorter or longer, period for the opened vial. According to the CDC, single-dose vials typically lack a preservative.
We mention this as an example of general practice for multi-dose containers, not as an instruction. In the laboratory, the manufacturer's information on the label of the specific product and your laboratory's internal procedures (SOPs) always take precedence.
When benzyl alcohol can be a problem in the lab
Benzyl alcohol is not an inert additive. In some situations it can affect the dissolved substance or the outcome of an experiment.
Proteins and aggregation. Preservatives in multi-dose protein formulations are known to be able to promote protein aggregation (clumping). A 2014 study compared four common preservatives using interferon alpha-2a. All of them induced aggregation, and benzyl alcohol ranked third, after m-cresol and phenol. Another study looked at a lyophilised protein (interleukin-1 receptor antagonist) that the researchers reconstituted either in water or in a solution containing 0.9% benzyl alcohol. More aggregates formed with benzyl alcohol. The amount depended on how well the protein had kept its structure during lyophilisation.
These studies concern proteins, not short peptides, and their results cannot simply be transferred. The behaviour of each substance depends on its sequence. It therefore cannot be assumed that benzyl alcohol is compatible with every substance.
Cell cultures and sensitive assays. Benzyl alcohol acts on cell membranes. In experiments with cultured endothelial cells, it increased membrane fluidity in a dose-dependent manner, and at 50 mmol/L it significantly increased the permeability of the cell monolayer. For comparison, 0.9% benzyl alcohol corresponds to roughly 83 mmol/L. After dilution into culture medium the concentration is, of course, lower. For sensitive cell-based assays, however, it is worth considering a preservative-free solvent, or at least including a solvent-only control.
Solubility. Not every peptide dissolves well in water, with or without a preservative. Peptide suppliers such as GenScript and Bachem describe a general approach in their guides:
- test solubility first on a small amount of the substance,
- for short peptides, try sterile water first,
- dissolve basic peptides (overall positive charge) in a small amount of dilute acetic acid, then dilute,
- dissolve acidic peptides (overall negative charge) in a small amount of dilute ammonia; GenScript advises against this for peptides containing cysteine,
- dissolve hydrophobic and neutral peptides in a small amount of an organic solvent, typically DMSO, and only then dilute with water or buffer.
Bachem also points out that high concentrations of organic solvents are incompatible with biological systems such as cells. According to Bachem, there is no universal protocol for all peptides. The choice of solvent is therefore guided by the experimental protocol and by solubility data, where the supplier provides them in a technical data sheet or a certificate of analysis.
Dissolving a lyophilised peptide: general principles
The following points describe general laboratory practice. Amounts and concentrations are always set by the protocol of the specific experiment.
- Let the vial reach room temperature. Peptides tend to be hygroscopic. Bachem therefore recommends allowing the closed container to reach ambient temperature, ideally in a desiccator, before opening it. Absorbed moisture reduces the peptide content and may decrease its stability.
- Work aseptically. Wipe the stoppers of both the peptide vial and the solvent vial with an alcohol swab and let them dry. Use sterile single-use equipment.
- Add the solvent slowly down the inside wall of the vial, not as a direct stream onto the lyophilised cake.
- Swirl gently, do not shake. Mechanical stress, such as vigorous shaking, is among the described factors that can induce protein aggregation. If some material stays undissolved, Bachem mentions a few minutes in an ultrasonic bath.
- Check the appearance. The solution should be clear, without turbidity or visible particles. Turbidity may indicate incomplete dissolution or aggregation. In that case it is better to go back to the supplier's solubility data than to carry on.
- Label the vial with the name of the substance, the solvent used and the date of preparation.
Storing opened bacteriostatic water and prepared solutions
Bacteriostatic water. The Hospira label states storage at 20–25 °C. The bacteriostatic water we offer is stored at room temperature and protected from light. The information on the label of the specific product always applies. After first opening, it is good practice to write the date on the vial.
The label explicitly states that aseptic technique must be used for single or multiple entry and withdrawal. In practice, this means wiping the stopper with an alcohol swab before each withdrawal and letting it dry, using new sterile equipment and never returning anything to the vial. As a general rule, contents that are cloudy or contain visible particles are not used.
Prepared peptide solutions. Bachem recommends not storing peptides in solution long term, not even in sterile solution. Where this cannot be avoided, it advises dividing the solution into smaller portions (aliquots). According to Bachem, peptides containing asparagine, glutamine, methionine, cysteine or tryptophan have a shorter shelf life. Specific storage conditions are always set by the supplier of the substance. For Arion Bios products, the instructions given with the product apply: store at 2–8 °C, protect from light, do not freeze and keep in the original closed packaging.
Comparison: bacteriostatic water vs sterile water
| Property | Bacteriostatic water | Sterile water |
|---|---|---|
| Composition | Sterile pharmacopoeial-grade water with a preservative, typically 0.9% benzyl alcohol | Sterile pharmacopoeial-grade water with no added substances |
| Preservative | Yes – inhibits bacterial growth, does not reliably kill bacteria, no effect on viruses | No |
| Container | Multi-dose, allows repeated withdrawals | Single-dose |
| After opening | Repeated withdrawals under aseptic conditions; the period is set by the manufacturer (general practice for multi-dose containers: 28 days unless the manufacturer states otherwise) | Any unused portion is discarded |
| Typical laboratory use | Dissolving substances that are compatible with benzyl alcohol, when solvent is withdrawn repeatedly from one vial | Experiments where a preservative would interfere (sensitive cell cultures and assays, aggregation-prone substances) and one-off preparation of a solution |
| Limitations | Benzyl alcohol can promote protein aggregation, acts on cell membranes and may affect some assays | No protection against microbial growth once opened |
Bacteriostatic water at Arion Peptides
We offer bacteriostatic water as a laboratory accessory for lyophilised research peptides: bacteriostatic water. Like all our products, it is intended exclusively for laboratory and research use. Certificates of analysis for Arion Bios peptide batches are available on the certificates of analysis page.
FAQ
Is bacteriostatic water the same as sterile water?
No. The base is the same: sterile, pharmacopoeial-grade water. Bacteriostatic water also contains a preservative, usually 0.9% benzyl alcohol, and comes in a multi-dose container. Sterile water contains no additives and, once opened, is intended for single use.
Does bacteriostatic water kill bacteria?
Not reliably. The preservative inhibits the growth and multiplication of bacteria, but it is not a disinfectant. According to the CDC, it has no effect on viruses and does not fully protect against contamination. Aseptic technique is therefore still required.
How long can an opened vial of bacteriostatic water be used?
The manufacturer's information on the label is decisive. In healthcare, a period of 28 days from first opening generally applies to multi-dose containers under USP <797>, unless the manufacturer states otherwise. Preservative-free sterile water is single-dose, and any remainder is discarded after opening.
When should sterile water or another solvent be chosen?
Where benzyl alcohol could affect the result, for example with sensitive cell cultures and assays or with aggregation-prone substances. The solubility of the substance itself may also call for a different solvent, such as dilute acetic acid for basic peptides or DMSO for hydrophobic ones. The experimental protocol and the supplier's data are decisive.
Sources
- Hospira, Inc. (Pfizer), “Bacteriostatic Water for Injection, USP”, prescribing information: labeling.pfizer.com
- Hospira, Inc. (Pfizer), “Sterile Water for Injection, USP”, prescribing information: labeling.pfizer.com
- United States Pharmacopeia, “Bacteriostatic Water for Injection”, monograph, USP 38–NF 33, 2015: drugfuture.com
- European Medicines Agency, “Guideline on the quality of water for pharmaceutical use”, EMA/CHMP/CVMP/QWP/496873/2018, 2020 (referring to Ph. Eur. monograph 0169 Water for injections): ema.europa.eu
- Centers for Disease Control and Prevention (CDC), “Preventing Unsafe Injection Practices”, summary of USP <797> recommendations for multi-dose and single-dose vials: cdc.gov
- Bis RL, Mallela KMG, “Antimicrobial preservatives induce aggregation of interferon alpha-2a: the order in which preservatives induce protein aggregation is independent of the protein”, International Journal of Pharmaceutics, 2014;472:356–361. DOI: 10.1016/j.ijpharm.2014.06.044
- Roy S, Jung R, Kerwin BA, Randolph TW, Carpenter JF, “Effects of benzyl alcohol on aggregation of recombinant human interleukin-1-receptor antagonist in reconstituted lyophilized formulations”, Journal of Pharmaceutical Sciences, 2005;94(2):382–396. DOI: 10.1002/jps.20258
- Mahler HC, Friess W, Grauschopf U, Kiese S, “Protein aggregation: pathways, induction factors and analysis”, Journal of Pharmaceutical Sciences, 2009;98(9):2909–2934. DOI: 10.1002/jps.21566
- Alvarado A, Butterfield DA, Hennig B, “Disruption of endothelial barrier function: relationship to fluidity of membrane extracellular lamella”, International Journal of Biochemistry, 1994;26(4):575–581. DOI: 10.1016/0020-711x(94)90016-7
- GenScript, “Peptide Solubility Guidelines”, technical guide: genscript.com
- Bachem, “Handling and Storage Guidelines for Peptides”, Knowledge Center: bachem.com