Isolation and characterization of the lytic Aeromonas phage HJ05,a new member of Caudovirales

Abstract

Abstract Phages are proposed for treating and preventing bacteria diseases largely due to their narrow spectrum of activity and capacity to multiply at the site of infection. Aeromonas hydrophila, a gram-negative and facultatively anaerobic bacterium, is widely distributed in aquatic environments causing diseases opportunistically in almost all freshwater fish displaying various symptoms including Aeromonas septicemia and ulcerative infections. In the present study, a virulent phage of A. hydrophila, designated as HJ05, from natural water was isolated and characterized. The morphological structure of HJ05 was observed by transmission electron microscopy, which indicated that the head of phage HJ05 was icosahedral with a diameter of about 77 nm, and the length of the straight and shrinkable tail could reach 193 nm. Analysis of the biological characteristics of HJ05 revealed the optimal multiplicity of infection (MOI) of 0.1, the latent period of phage was 10 min, the burst period was 40 min, and the burst size was 120 PFU/cell. The titer decreases rapidly from 45–50 ℃ indicating HJ05’s sensitivity to temperature, and optimum pH range for HJ05 replication was widely distributed from pH 5 to 11. Genome analysis of HJ05 revealed a linear doubled-stranded DNA genome of 79124 bp with a G + C content of 58.63%. Comparative genomic analysis revealed that the HJ05 genome has little similarity to any of the currently sequenced Aeromonas phage genomes, indicating that HJ05 does not belong to any of the known genera. Among 94 putative open reading frames identified in the genome, only 45 ORFs encoded proteins displaying significant homology to the known functional proteins in NCBI database. Phylogenetic relatedness analysis of two ORFs (ORF30 encoding head morphogenetic protein and ORF74 encoding DNA polymerase) did indicate HJ05 formed a sister clade with Rhizobium phages. According to the classification criteria of the International Committee on Taxonomy of Viruses, HJ05 was identified to order Caudovirales. This study suggested that HJ05 might represent a novel member of the family Caudovirales, and the lytic activity of HJ05 against A. hydrophila predicted its potential role in phage therapy toward the control of bacterial infections in aquaculture.