| Summary: | The moon jellyfish <i>Aurelia aurita</i> is associated with a highly diverse microbiota changing with provenance, tissue, and life stage. While the crucial relevance of bacteria to host fitness is well known, bacteriophages have often been neglected. Here, we aimed to isolate virulent phages targeting bacteria that are part of the <i>A. aurita</i>-associated microbiota. Four phages (<i>Pseudomonas</i> phage BSwM KMM1, <i>Citrobacter</i> phages BSwM KMM2–BSwM KMM4) were isolated from the Baltic Sea water column and characterized. Phages KMM2/3/4 infected representatives of <i>Citrobacter</i>, <i>Shigella</i>, and <i>Escherichia</i> (<i>Enterobacteriaceae</i>), whereas KMM1 showed a remarkably broad host range, infecting Gram-negative <i>Pseudomonas</i> as well as Gram-positive <i>Staphylococcus</i>. All phages showed an up to 99% adsorption to host cells within 5 min, short latent periods (around 30 min), large burst sizes (mean of 128 pfu/cell), and high efficiency of plating (EOP > 0.5), demonstrating decent virulence, efficiency, and infectivity. Transmission electron microscopy and viral genome analysis revealed that all phages are novel species and belong to the class of Caudoviricetes harboring a tail and linear double-stranded DNA (formerly known as <i>Siphovirus</i>-like (KMM3) and <i>Myovirus</i>-like (KMM1/2/4) bacteriophages) with genome sizes between 50 and 138 kbp. In the future, these isolates will allow manipulation of the <i>A. aurita</i>-associated microbiota and provide new insights into phage impact on the multicellular host.
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