Bacteriostatic water is the default diluent for reconstituting lyophilised peptides in a laboratory setting, and it is routinely confused with sterile water for injection. They are not the same material and they do not behave the same way.
What is actually in it
Bacteriostatic water is sterile water containing approximately 0.9% benzyl alcohol as a bacteriostatic preservative. That preservative is the entire difference.
“Bacteriostatic” means it inhibits bacterial growth. It does not mean bactericidal — it does not sterilise a contaminated solution. What it does is prevent organisms introduced during a needle entry from proliferating in the vial afterwards.
The practical consequence
| Diluent | Preservative | Practical use |
|---|---|---|
| Sterile water | None | Single-entry; discard after first use |
| Bacteriostatic water | 0.9% benzyl alcohol | Multi-entry over a working period |
| Sterile saline | None (isotonic) | Single-entry; different osmolarity |
If a reconstituted vial will be entered more than once — which is nearly always the case with a 10 mg peptide vial — a preserved diluent is what makes that workable.
When bacteriostatic water is the wrong choice
- Benzyl alcohol incompatibility. A small number of peptides show reduced stability in the presence of benzyl alcohol. Where a supplier or the literature specifies an alternative diluent, follow that.
- Assays sensitive to the preservative. Cell-based work in particular can be affected by benzyl alcohol at the concentrations involved. Where that is a concern, sterile water with single-use aliquoting is the cleaner approach.
Choosing a volume
Vial size determines how much flexibility you have. A 10 mL vial covers several reconstitutions; a 3 mL vial suits a single working session with less material left to age.
Since concentration is simply vial mass divided by diluent volume, deciding the volume is deciding the concentration — and it cannot be undone afterwards. Work out the concentration you want before drawing anything.
Storage
Store bacteriostatic water at controlled room temperature. Once entered, most references treat the vial as having a limited working life — commonly around 28 days — after which it should be discarded regardless of remaining volume. Label the vial with the date of first entry.
Aegis Bio supplies bacteriostatic water in 3 mL and 10 mL vials, individually and in ten-vial packs. See the reconstitution and dilution guide for the concentration arithmetic.