Bacteriophages of sourdough lactobacilli: isolation, characterisation and lysogeny.

Many Italian sweet and salted baked products are still made through sourdough technology that requires the use of mixed cultures of lactobacilli and yeast. Until now phage infections did not cause serious problems just because sourdough processes are still carried out in a batch system with mixtures that contain several strains, often belonging to different bacterial species and not all sensitive to a given phage type. However, the risk of a phage infection increases when a continuous system or selected starter culture is employed like it’s going to happen for industrial bakeries that would shorten the fermentative processes. Several studies about bacteriophages of sourdough lactobacilli have been carried out. The viral particles were induced from different lactobacilli strains or isolated directly from sourdoughs through a technique proposed by our group. Each phage was investigated on the basis of morphology, host range, plaque formation, structural proteins and genome characteristics. Phages isolated from sourdoughs (FE5-B1, Z63-B1, Z63-B2, BU77-B1, and EV3) were found to be active on Lb. fermentum, Lb. brevis and Lb. sanfrancisciensis strains. All the phages were ascribed to the B1 morphotype or Siphoviridae family with the exception of FE5-B1 that belonged to A1 morphotype or Myoviridae family. FE5-B1 and Z63-B1 were isolated from the same sourdough sample but they showed different host specificity as well as differences in adsorption and one-step growth kinetics. The other 3 viral particles were isolated from 3 different sourdoughs. Z63-B2 was shown to be very similar to Z63-B1 and had a partial homology with BU77-B1. EV3 is the first bacteriophage active on the species Lb. sanfrancisciensis that has ever been isolated and it showed morphological and structural characteristics similar to other viral particles isolated from sourdoughs. The temperate phages (017, 064, 0209, 0Bu130, 0Fe129, FEM) induced from different Lb. fermentum strains belonged to Siphoviridae family. 4 out of 6 were ascribed to B1 group showing isometric heads, while the remaining 2 (064 and 0209) were ascribed to B3 group since they showed prolate heads, a morphotype that has been rarely isolated in phages of lactobacilli. Data on host ranges showed two separate clusters in which phages shared the ability to multiply on a common group of strains; because of the immunity, none of them caused the lysis of the respective harbouring strain. SDS-PAGE protein profiles, restriction analysis of DNA and Southern blot hybridisation revealed a high degree of homology between two pairs of phages. Virulent bacteriophages Z63-B2 and BU77-B1 were compared to the temperate ones and some similarities were found, both in host range and in DNA-DNA hybridisation analysis. As concern phage FEM it was highlighted a new induction mechanism: in fact, FEM is induced from the lysogenic Lb. fermentum Bu77 strain only when the supernatant of Lb. fermentum strain FE3 is present.