Bacillus phage phi18-2 is a novel temperate virus with an unintegrated genome present in the cytoplasm of lysogenic cells as a linear phage-plasmid

Bacillus subtilis is a Gram-positive bacterium that is widely used in fermentation and in the pharmaceutical industry. Phage contamination occasionally occurs in various fermentation processes and causes significant economic loss. Here, we report the isolation and characterization of a temperate B. subtilis phage, termed phi18-2, from spore powder manufactured in a fermentation plant. Transmission electron microscopy showed that phi18-2 has a symmetrical polyhedral head and a long noncontractile tail. Receptor analysis showed that phi18-2 recognizes wall teichoic acid (WTA) for infection. The phage virions have a linear double-stranded DNA genome of 64,467 bp with identical direct repeat sequences of 309 bp at each end of the genome. In lysogenic cells, the phage genome was found to be present in the cytoplasm without integration into the host cell chromosome, and possibly as a linear phage-plasmid with unmodified ends. Our data may provide some insight into the molecular basis of the unique lysogenic cycle of phage phi18-2.