As aquaculture continues to expand globally, diseases caused by Vibrio species are becoming increasingly prevalent. Vibriosis encompasses a range of infections, which can lead to symptoms such as skin lesions, hemorrhaging, and high mortality rates in fish and shellfish, especially in high-density farming systems, resulting in significant economic losses. Simultaneously, the extensive use of antibiotics has fostered the emergence of antibiotic-resistant bacteria, exacerbated disease outbreaks, and complicated control measures. Phage therapy, which leverages bacteriophages as natural antibacterial agents, offers a promising eco-friendly alternative to the antibiotics used in aquaculture. This study aimed to evaluate the potential of two vibriophages for phage therapy in aquaculture. Two virulent vibriophages, vB_VcaP_R24D and vB_VcaP_R25D, were isolated from aquaculture wastewater from seafood markets using Vibrio campbellii LMG 11216T as the host strain. The two vibriophages were identified based on their morphology, infection dynamics, host range, genomic features, lytic activity, and environmental stability. Both phages belong to the podovirus morphotype and exhibit a lytic life cycle characterised by a short latent period (< 10 min). Genomic analyses confirmed the absence of lysogenic genes, virulence factors, and antibiotic-resistance genes, thereby ensuring genetic safety. Additionally, both phages demonstrated high stability over a broad range of temperatures (4–45 °C) and pH (3–10). Lytic curve analyses further indicated a robust lytic efficiency during the logarithmic growth phase of the vibriophages. These biological and genomic characteristics highlight the potential of vB_VcaP_R24D and vB_VcaP_R25D as effective biocontrol agents for mitigating vibriosis in aquaculture. Although this study demonstrates their narrow host range, the possibility of phage infection in other untested hosts cannot be entirely excluded. Furthermore, the findings offer valuable insights for future research on phage-host interactions and the development of phage cocktails to improve disease management in aquaculture systems.

Bacterial diseases caused by Vibrio spp. are prevalent in aquaculture and can lead to high mortality rates among aquatic species and significant economic losses. With the increasing emergence of multidrug-resistant Vibrio strains, phage therapy is being explored as a potential alternative to antibiotics for biocontrol of infectious diseases. Here, a new lytic phage named vB_VhaS_R21Y (R21Y) was isolated against Vibrio harveyi BVH1 obtained from seawater from a scallop-farming area in Rongcheng, China. Its morphology, infection cycle, lytic profile, phage stability, and genetic features were characterized. Transmission electronic microscopy indicated that R21Y is siphovirus-like, comprising an icosahedral head (diameter 73.31 ± 2.09 nm) and long noncontractile tail (205.55 ± 0.75 nm). In a one-step growth experiment, R21Y had a 40-min latent period and a burst size of 35 phage particles per infected cell. R21Y was highly species-specific in the host range test and was relatively stable at pH 4–10 and 4–55 °C. Genomic analysis showed that R21Y is a double-stranded DNA virus with a genome size of 82,795 bp and GC content of 47.48%. Its high tolerance and lytic activity indicated that R21Y may be a candidate for phage therapy in controlling vibriosis in aquacultural systems.

Vibrio alginolyticus causes substantial economic losses in the aquaculture industry. With the rise of multidrug-resistant Vibrio strains, phages present a promising solution. Here, a novel lytic Vibrio phage, vB_ValC_RH2G (RH2G), that efficiently infects the pathogenic strain V. alginolyticus ATCC 17749^T, was isolated from mixed wastewater from an aquatic market in Xiamen, China. Transmission electron microscopy revealed that RH2G has the morphology of Siphoviruses, featuring an icosahedral head (73 ± 2 nm diameter) and long noncontractile tail (142 ± 4 nm). A one-step growth experiment showed that RH2G had a short latent period (10 min) and a burst size of 48 phage particles per infected cell. Additionally, RH2G was highly species-specific and was relatively stable at 4–55 °C and pH 4–10. A genomic analysis showed that RH2G has a 116,749 bp double-stranded DNA genome with 43.76% GC content. The intergenomic similarity between the genome sequence of RH2G and other phages recorded in the GenBank database was below 38.8%, suggesting that RH2G represents a new genus. RH2G did not exhibit any virulence or resistance genes. Its rapid lysis capacity, lytic activity, environmental resilience, and genetic safety suggested that RH2G may be a safe candidate for phage therapy in combatting vibriosis in aquaculture settings.

Bacteria belonging to the Roseobacter clade are key players in marine ecosystems, contributing significantly to carbon and sulfur cycles. Marine viruses, particularly those targeting Roseobacter, play crucial roles in regulating microbial communities and biogeochemical processes. Despite their importance, phages infecting organisms of the Roseobacter clade remain poorly understood. In this study, a novel roseophage, vB_DshS-R26L (R26L), infecting Dinoroseobacter shibae DFL12^T, was isolated and characterized in terms of physiological and genomic properties. R26L has siphovirus morphology with an elongated head and a long, non-flexible tail. The phage has a narrow host range and demonstrates a long infection cycle with a latent period of 3.5 h and a burst size of 22 plaque-forming units (PFU cell^− 1). R26L possesses a circular, double-stranded DNA genome of 79,534 bp with a G + C content of 62.6%, encoding a total of 116 open reading frames. Notably, seven auxiliary metabolic genes (AMGs), including those related to phosphate metabolism and queuosine biosynthesis, were identified. Phylogenetic and taxonomic analyses revealed that R26L represents a new genus, with its highest intergenomic similarities being 54.7% to another roseophage (R5C). By elucidating the unique characteristics of R26L, this study highlights the complexity of phage infections and the genomic diversity of roseophages, offering valuable insights into the ecological significance of Roseobacter–phage interactions in marine environments.

Erythrobacter, an aerobic anoxygenic photoheterotrophic bacterial genus, plays a vital role in carbon and energy cycling in marine environments. However, their phage predators remain poorly understood, with only two strains previously reported. This study isolated and characterized a novel Erythrobacter phage, vB_EauS-R34L1 (R34L1), and its sub-strain vB_EauS-R34L2 (R34L2), from coastal seawater. Both phages exhibit long-tailed, icosahedral morphologies and relatively narrow but slightly different host ranges. One-step growth curve analysis revealed a 160-min latent period and burst sizes of 81 and 91 PFU/cell for R34L1 and R34L2, respectively. Genomic analysis showed that the phages possess dsDNA genomes of 56,415 bp (R34L1) and 54,924 bp (R34L2), with G + C contents of 61.60 and 61.19%, respectively. Both phages harbor a suite of unique genes, including GapR and GH19, which are crucial for host interaction and ecological functionality. Blastn analysis indicated a 99.73% genome similarity between them. Taxonomic and phylogenetic analyses positioned them in a novel viral genus cluster, Eausmariqdvirus, under the family Casjensviridae, indicating a distant evolutionary relationship with known phages. Metagenomic queries suggested that R34L1- and R34L2-like phages are exclusively abundant in temperate and tropical epipelagic zones. This study expands our understanding of Erythrobacter phages and provides insights into their ecological roles in marine ecosystems.