AUTHOR=Katharios Pantelis , Kalatzis Panos G. , Kokkari Constantina , Pavlidis Michail , Wang Qiyao 

TITLE=Characterization of a Highly Virulent Edwardsiella anguillarum Strain Isolated From Greek Aquaculture, and a Spontaneously Induced Prophage Therein

JOURNAL=Frontiers in Microbiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00141

DOI=10.3389/fmicb.2019.00141

ISSN=1664-302X

ABSTRACT=<p><italic>Edwardsiella</italic>-associated outbreaks are increasingly reported on both marine and freshwater aquaculture setups, accounting for severe financial and biomass losses. <italic>E. tarda, E. ictaluri</italic>, and <italic>E. hoshinae</italic> have been the traditional causative agents of edwardsiellosis in aquaculture, however, intensive studies due to the significance of the disease have just recently revealed two more species, <italic>E. piscicida</italic> and <italic>E. anguillarum</italic>. Whole genome sequencing that was conducted on the strain EA011113, isolated from farmed <italic>Diplodus puntazzo</italic> after an edwardsiellosis outbreak in Greece, confirmed it as a new clinical strain of <italic>E. anguillarum</italic>. Extensive phylogenetic analysis showed that this Greek strain is closely related to an Israeli <italic>E. piscicida-</italic>like clinical strain, isolated from diseased groupers, <italic>Epinephelus aeneus</italic> and <italic>E. marginatus</italic> in Red Sea. Bioinformatic analyses of <italic>E. anguillarum</italic> strain EA011113 unveiled a wide repertoire of potential virulence factors, the effect of which was corroborated by the mortalities that the strain induced in adult zebrafish, <italic>Danio rerio</italic>, under different levels of infection intensity (LD<sub>50</sub> after 48 h: 1.85 × 10<sup>4</sup> cfu/fish). This strain was non-motile and according to electron microscopy lacked flagella, a fact that is not typical for <italic>E. anguillarum</italic>. Comparative genomic analysis revealed a deletion of 36 nt found in the flagellar biosynthetic gene (<italic>FlhB</italic>) that could explain that trait. Further <italic>in silico</italic> analysis revealed an intact prophage that was integrated in the bacterial genome. Following spontaneous induction, the phage was isolated, purified, characterized and independently sequenced, confirming its viability as a free, inducible virion as well. Separate genomic analysis of the prophage implies a plausible case of lysogenic conversion. Focusing on edwardsiellosis as a rapidly emerging aquaculture disease on a global scale, this work offers some insight into the virulence, fitness, and potential lysogenic conversion of a of a newly described, yet highly pathogenic, strain of <italic>E. anguillarum</italic>.</p>