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Abstract “Jumbo phages” are tailed phages with genome sizes >200 kbp and physical dimensions reaching up to 0.45 μm. Although jumbo phages represent only a small fraction of the isolated phages to date, metagenomic surveys have shown that they are broadly distributed in a wide range of environments. In this study, we surveyed metagenomic data from aquatic systems and identified 25 genomes from a heretofore-undescribed lineage of jumbo phages with genomes reaching up to 307 kbp. We refer to these phages as “moraphages”, from the Gaelic word ‘mór’, for large. Moraphages represent a diverse lineage with inter-genome average amino acid identity (AAI) ranging from 39 to 95%, and our pan-genomic analysis identified only 26 viral orthologous groups (VOGs) found in at least 80% of the genomes. Our phylogenomic analysis suggests that moraphages are distant relatives of a recently described lineage of huge phages from marine sediment. Moraphages lack much of the genetic machinery found in other lineages of large phages, but they have a range of genes that may be used to take over host cellular machinery and subvert host defenses, such as glutamine synthetases, antitoxin genes, and chaperones. The predicted hosts of most moraphages are members of the phylumBacteroidota, and some encode homologs of the chaperones DnaK and DnaJ that bear evidence of recent gene transfer from members of the orderFlavobacteriales. Our work sheds light on the emerging diversity of large phages that are found across the biosphere. 

Vibrio parahaemolyticus(VP) is a bacterial pathogen found in brackish and marine water that infects many marine organisms, such as oysters and shrimp. Consumption of raw or undercooked seafood contaminated withV. parahaemolyticusis a primary cause of seafood-borne gastroenteritis in humans. Due to increasing ocean temperatures,V. parahaemolyticuscontamination of oyster beds in the United States has spread up the east and west coasts to the northern-most states. Promising new research is exploring the isolation of bacteriophages againstV. parahaemolyticuswith a long-term goal to possibly decontaminate oyster beds, thereby expanding the harvest season and allowing for safer consumption of seafood. In this study, store-bought oysters harvested from the Chesapeake Bay in Virginia were used to isolate four bacteriophages with activity against a specificV. parahaemolyticusstrain. A standard double agar overlay plaque assay was used to identify phage activity. After phage isolation, the genomes were sequenced, and transmission electron microscopy (TEM) was performed to visualize the virions. The genomes and TEM images revealed four distinct phages. Three of the phages are distinct isolates that exhibit podovirus-like morphology with short tails and genome sizes of approximately 43 kbp. One phage has siphovirus-like morphology and is a mid-sized tailed phage with a genome size of 80 kbp. Although spot tests performed with the oyster homogenates on up to 10 differentV. parahaemolyticusstrains recovered activity across a wide range of hosts, plaque assays with the isolated phages showed limited host range. Future work will be necessary to determine the viability of using the bacteriophages for elimination ofV. parahaemolyticusin harvested oysters, treatment of aquaculture seed and spat, and/or the environment.