Characterization and genome sequence of marine Alteromonas gracilis phage PB15 isolated from the Yellow Sea, China

A novel marine Alteromonas gracilis siphovirus, phage PB₁₅, was isolated from the surface water of the Yellow Sea in August 2015. It has a head diameter of 58 ± 5 nm head and a contractile tail approximately 105 ± 10 nm in length, and overall the morphology suggests that PB₁₅ belongs to the family Siphoviridae. PB₁₅ phage is stable at over the temperature range 0-60^oC. The best MOI of these phage was 0.1 and infectivity decreased above 60^oC. The results suggest that phage is stable at pH value ranging between 3.0 and 11.0. Chloroform test shows that PB₁₅ is not a lipid-containing phage. A one-step growth curve with a strain of A. gracilis gave a latent period of 16 minutes and rise period of 24 minutes and burst size of 60 PFU/cell. Genomic analysis of PB₁₅ reveals a genome size of 37,333bp with 45.52% G+C content, and 61 ORFs. ORF sequences accounted for 30.36% of the genome sequence. There is no obvious similarity between PB₁₅ and other known phages by genomic comparison using the BLASTN tool in the NCBI database.