Bacteriophages (phages,
bacteria-specific viruses) have biotechnological
and therapeutic potential. To apply phages as pure or heterogeneous
mixtures, it is essential to have a robust mechanism for transport
and storage, with different phages having very different stability
profiles across storage conditions. For many biologics, cryopreservation
is employed for long-term storage and cryoprotectants are essential
to mitigate cold-induced damage. Here, we report that poly(ethylene
glycol) can be used to protect phages from cold damage, functioning
at just 10 mg·mL
–1
(∼1 wt %) and outperforms
glycerol in many cases, which is a currently used cryoprotectant.
Protection is afforded at both −20 and −80 °C,
the two most common temperatures for frozen storage in laboratory
settings. Crucially, the concentration of the polymer required leads
to frozen solutions at −20 °C, unlike 50% glycerol (which
results in liquid solutions). Post-thaw recoveries close to 100% plaque-forming
units were achieved even after 2 weeks of storage with this method
and kill assays against their bacterial host confirmed the lytic function
of the phages. Initial experiments with other hydrophilic polymers
also showed cryoprotection, but at this stage, the exact mechanism
of this protection cannot be concluded but does show that water-soluble
polymers offer an alternative tool for phage storage. Ice recrystallization
inhibiting polymers (poly(vinyl alcohol)) were found to provide no
additional protection, in contrast to their ability to protect proteins
and microorganisms which are damaged by recrystallization. PEG’s
low cost, solubility, well-established low toxicity/immunogenicity,
and that it is fit for human consumption at the concentrations used
make it ideal to help translate new approaches for phage therapy.