Compatibility of antimicrobial preservatives with therapeutic bacteriophages of the genera Pbunavirus and Kayvirus

Implementing bacteriophages into dosage forms is a significant step for the practical application of phage therapy. While designing a dosage form, bacteriophages as active ingredients may be exposed to excipients, guaranteeing microbial quality. However, only a few antimicrobial preservatives have been studied regarding their interaction with bacteriophages during long-term storage. Here, the stability of the staphylococcal Kayvirus and pseudomonal Pbunavirus with twelve commonly used preservatives was monitored for thirteen weeks to assess the risk of destabilisation of phage suspensions by excipients. The effectiveness of preservatives on the test bacteria, yeast and mould was determined using a microdilution method and the phage lytic activity by plaque enumeration. The antimicrobial activity of preservatives with bacteriophages was confirmed, except benzalkonium chloride and chlorhexidine digluconate, which showed precipitation and were classified as incompatible. A complete loss of phage potency in both tested phages occurred with diazolidinyl urea and in Kayvirus with benzalkonium chloride. For both phages, a slight decrease in titer, by one order of magnitude, was observed with m-cresol, sodium propionate, sodium benzoate, and phenylethyl alcohol. For Kayvirus, thimerosal, parabens, and mono propylene glycol and for Pbunavirus, phenoxyethanol also met the criteria. The decrease by two or more orders was determined for the remaining cases. This study helps select antimicrobial preservatives for optimizing dosage formulations with the therapeutically applicable bacteriophages.