Novel endolysin from Streptococcus iniae-Specific Prophage Selectively Inhibits Target Bacteria

Streptococcus iniae is a gram-positive, spherical- or ovoid-shaped, facultative anaerobic bacterium and is one of the major species causing streptococcosis, resulting in economic losses in aquaculture. Endolysins, peptidoglycan hydrolases produced by bacteriophages, are emerging as replacements for antibiotics due to their specific lytic activity against pathogens. This study aimed to develop a novel endolysin, SinLys1930, that specifically targets and kills S. iniae. The molecular and structural characteristics of SinLys1930 were predicted based on bioinformatic approaches. The lytic activity of SinLys1930 was evaluated against S. iniae KCTC 3657 under various conditions, including different dosages, pH levels, temperatures, NaCl concentrations, and metal ions, to identify the optimal conditions, and its effectiveness was further tested in sterilized seawater. The conserved domain analysis revealed that SinLys1930 possesses two enzymatically active domains (NlpC/P60 and glucosaminidase superfamilies) with two cell wall-binding domains (CW-7 superfamily) positioned between them. The lytic activity of SinLys1930 was highest at pH 6.0 to 6.5 and temperatures between 16 and 37°C, and it was maintained even under high NaCl concentration. SinLys1930 reduced the optical density of S. iniae in sterilized seawater by approximately 60% after incubation for 1 h. Therefore, SinLys1930 could potentially serve as an alternative to antibiotics for preventing streptococcosis caused by S. iniae in the aquaculture industry.