Effects of lyophilization on the stability of bacteriophages against different serogroups of Shiga toxin-producing Escherichia coli.

Lyophilization is commonly used to effectively preserve the stability of bacteriophages (phages) in long-term storage. However, information regarding the lyophilization of phages specific to Shiga toxin-producing Escherichia coli (STEC) strains is scarce. The objective of this study was to determine the effects of lyophilization with different cryoprotectants (sucrose and trehalose) and concentrations (0.1 M and 0.5 M) on the stability of seven lytic phages specific to STEC O157 and top 6 non-O157 strains during 6-month storage at -80 °C. The titers of lyophilized phages specific to STEC O26 (S1 O26) and STEC O121 (Pr121lvw) did not exhibit significant reduction after 6-month storage regardless of the use of cryoprotectants. Phages lytic against STEC O103 (Ro103C3lw) and STEC O145 (Ro145clw) with 0.1 M sucrose retained similar titers after lyophilization and frozen storage for 6 months (P > 0.05). Despite subtle differences, these results indicated that most of the selected phages had similar titer retention with the same cryoprotectants. Additionally, lytic activities of the phages against their primary hosts were not affected after lyophilization and 6-month frozen storage. Moreover, no detectable damage was observed on the lyophilized phage structures. These findings provide valuable insight into the use of lyophilization to preserve phages lytic against STEC strains.