Abstract
Aquaculture is a burgeoning industry, requiring diversification into new farmed species, which are often at risk from infectious disease. We used a mesocosm technique to investigate the susceptibility of sharpsnout seabream (Diplodus puntazzo) larvae to potential environmental pathogens in seawater compared to control borehole water. Fish exposed to seawater succumbed to epitheliocystis from 21 days post hatching, causing mortality in a quarter of the hosts. The pathogen responsible was not chlamydial, as is often found in epitheliocystis, but a novel species of the γ-proteobacterial genus Endozoicomonas. Detailed characterisation of this pathogen within the infectious lesions using high resolution fluorescent and electron microscopy showed densely packed rod shaped bacteria. A draft genome sequence of this uncultured bacterium was obtained from preserved material. Comparison with the genome of the Endozoicomonas elysicola type strain shows that the genome of Ca. Endozoicomonas cretensis is undergoing decay through loss of functional genes and insertion sequence expansion, often indicative of adaptation to a new niche or restriction to an alternative lifestyle. These results demonstrate the advantage of mesocosm studies for investigating the effect of environmental bacteria on susceptible hosts and provide an important insight into the genome dynamics of a novel fish pathogen.
Highlights
Aquaculture is a burgeoning industry, requiring diversification into new farmed species, which are often at risk from infectious disease
We sought to develop a system to investigate epitheliocystis-causing bacteria in the eastern Mediterranean, especially important given the lack of methods for culture of epitheliocystis agents in vitro
The first epitheliocystis lesions were detected in the fins of larvae from the experimental tank at 21 dph (Fig. 1A,B), and first positive PCR signal for chlamydial signature sequence in DNA extracted from affected larvae
Summary
Aquaculture is a burgeoning industry, requiring diversification into new farmed species, which are often at risk from infectious disease. The pathogen responsible was not chlamydial, as is often found in epitheliocystis, but a novel species of the γ-proteobacterial genus Endozoicomonas. Endozoicomonas cretensis is undergoing decay through loss of functional genes and insertion sequence expansion, often indicative of adaptation to a new niche or restriction to an alternative lifestyle These results demonstrate the advantage of mesocosm studies for investigating the effect of environmental bacteria on susceptible hosts and provide an important insight into the genome dynamics of a novel fish pathogen. The aquaculture industry is constantly expanding to meet the needs of the global population and ever growing demand for quality protein Economic sustainability of this industry requires an increasing diversification of cultured fish species. A major threat to this type of rearing system is the introduction of pathogenic microorganisms from sea water leading to the emergence of novel diseases in potentially vulnerable hosts. Chlamydiae are not the only bacterial agents identified in association with epitheliocystis, as recent studies have shown that β - and γ -proteobacteria[21,22,23] (Seth-Smith, ISME J, in press) play a role
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