Mytilus galloprovincialis as a Natural Reservoir of Vibrio harveyi: Insights from GFP-Tagged Strain Tracking

Vibrios are widespread in marine environments, and their persistence is often linked to natural reservoirs such as filter-feeding bivalves. This study investigated the capacity of the Mediterranean mussel, <i>Mytilus galloprovincialis</i>, to act as a reservoir of <i>Vibrio harveyi</i> using a GFP-tagged strain in controlled experiments. Mussels (shell length 4-6 cm) were exposed to <i>V. harveyi</i> gfp in estuarine and seawater at 12 °C and 20 °C over six days. Bacterial accumulation in gills, digestive gland, and gonads, as well as in feces and pseudofeces, was quantified, and the immune response following microbial challenge was assessed by histopathological analysis. Mussels actively removed <i>V. harveyi</i> from the water, but not completely. Vibrios were rapidly accumulated in organs, with the highest densities in the digestive gland (up to 10⁷-10⁸ CFU g^-1), and substantial bacterial loads detected in biodeposits (1.55-3.77 × 10⁷ CFU g^-1). Salinity had a greater effect than temperature on bacterial accumulation, with consistently higher counts in seawater assays. Concurrently with bacterial accumulation, mussels activated their immune system, as evidenced by the detection of granulocytomas and hemocytic infiltrations. Overall, these results demonstrate that <i>M. galloprovincialis</i> accumulates <i>V. harveyi</i> in tissues and biodeposits, serving as a natural reservoir for this bacterium.