Resisting the Toxic Tide: Multi-Metal Resistance of Bacteria Originating from Contaminated Šibenik Bay Sediments

In this study, 74 bacterial isolates were obtained from sediments of Šibenik Bay, historically impacted by heavy-metal pollution. Isolates were tested for tolerance to cadmium (Cd), cobalt (Co) chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), tin (Sn) and zinc (Zn), both individually and in mixtures, and for their biofilm-forming ability. Toxicity followed the trend Hg > Sn > Zn/Cd/Cr > Co/Ni > Pb. Resistance traits were observed against all tested metals, with some isolates displaying multi-metal resistance to as many as seven metals, reflecting long-term selective pressure in the Bay. Bacillus species dominated the community (48 isolates across five clusters), confirming this genus as the principal group in metal-polluted sediments. Several less-explored genera including Ruegeria/ Cribrihabitans, Bhargavaea, Pseudoalteromonas, and Lysinibacillus/Sporosarcina also exhibited notable resistance traits, underscoring their potential as novel candidates for bioremediation. Eleven isolates from Bacillus/Mesobacillus/Cytobacillus, Bacillus/Peribacillus/Rossellomorea, Bacillus/Pseudoalkalibacillus/ Alkalibacillus, Lysinibacillus/Sporosarcina, and Ruegeria/Cribrihabitans clusters showed resistance and robust growth under metal mixtures. Among all isolates, 11, 32, 81, 82(Bacillus/Mesobacillus/ Cytobacillus and Bacillus/Peribacillus/Rossellomorea) combined broad multi-metal tolerance with strong biofilm formation, positioning them as candidates for site-specific, nature-based bioremediation of heavy-metal impacted coastal ecosystems such as Šibenik Bay.