Wood Hemicellulose-Based Spray-Dried Microencapsulation of a Lytic Bacteriophage Preserves Phage Viability and Improves Control of the Bacterial Wilt Pathogen Ralstonia solanacearum

Abstract

Ralstonia solanacearum (RS) is a soil-borne phytopathogen responsible for bacterial wilt disease on a wide range of crops worldwide. Bacteriophage biocontrol is a promising sustainable RS management method. However, more work is needed to design methods to store, ship and apply phage that are effective, scalable and environmentally friendly. Here, we investigate the use of wood hemicellulose excipients—glucuronoxylans (GX) and galactoglucomannans (GGM) – to encapsulate phage PYO4, which can infect the pandemic RS strain UW551. Yield and preservation efficiencies of GX and GGM were compared to the conventional excipient maltodextrin (MD). Encapsulation via spray drying was carried out at two inlet/outlet temperatures, and the resulting powders were stored at room temperature or at 4°C. Phage titers were measured after spray drying, and then weekly for 25 weeks. GX yielded the highest titre of encapsulated phage and preserved phage survival effectively at 4°C. Phages encapsulated with MD had the highest stability at room temperature. GGM had poor results, with low survival after spray drying and low long-term stability at either temperature. In vitro experiments demonstrated that encapsulated phages inhibited RS as efficiently as unencapsulated phage. Phage encapsulated in GX and MD also reduced bacterial wilt symptoms on tomato. At low MOIs, phage encapsulated in GX and MD reduced symptoms more than unencapsulated phage, suggesting the excipients themselves could be affecting RS. We found that GX alone could inhibit RS growth in vitro and reduce disease progression in planta without phage. MD alone couldn't significantly reduce bacterial wilt symptoms or inhibit RS growth in vitro. Together, these results show that the encapsulation of phages in hemicelluloses has great promise for efficient biocontrol methods to combat plant pathogens. Not only are hemicelluloses effective in phage preservation, but also have potential to enhance the biocontrol efficacy of phages through their antimicrobial activities.