Isolation and characterization of novel soilborne lytic bacteriophages infecting Dickeya spp. biovar 3 ('D. solani').
Nine bacteriophages infecting Dickeya spp. biovar 3 ('Dickeya solani') were isolated from soil samples collected in different regions in Poland. The phages have a typical morphology of the members of the order Caudovirales, family Myoviridae, with a head diameter of c. 90-100 nm and tail length of c. 120-140 nm. In host range experiments, phage φD5 expressed the broadest host range, infecting members of all Dickeya spp., and phage φD7 showed the narrowest host range, infecting isolates of Dickeya dadantii and 'D. solani' only. None of the phages was able to infect Pectobacterium spp. isolates. All phages were prone to inactivation by pH 2, temperature of 85°C and by UV illumination for 10 min (50 mJ cm-2). Additionally, phages φD1, φD10 and φD11 were inactivated by 5 m NaCl and phage φD2 was inactivated by chloroform. Phages φD1, φD5, φD7 and φD10 were characterized for optimal multiplicity of infection and the rate of adsorption to the bacterial cells. The latent period was 30 min for φD1, 40 min for φD5, 20-30 min for φD7 and 40 min for φD10. The estimated burst size was c. 100 plaque-forming units per infected cell. The bacteriophages were able to completely stop the growth of 'D. solani' in vitro and to protect potato tuber tissue from maceration caused by the bacteria. The potential use of bacteriophages for the biocontrol of biovar 3 Dickeya spp. in potato is discussed.