LAUSR

Abstract A new bacteriophage, Str-PAP-1, which infects Streptococcus parauberis, was isolated and characterized. Str-PAP-1, classified as a member of the family Siphoviridae, has a double-stranded DNA genome with 35.3% GC content and 36,595 base pairs containing putative open reading frames. A BLAST search revealed that bacteriophage Str-PAP-1 bears no similarity to any previously reported bacteriophages. In a susceptibility test, 35 of the 55 (63.6%) S. parauberis field isolates tested, which were collected from streptococcosis-afflicted olive flounder (Paralichthys olivaceus) from the year 2010 to 2013 in Jeju-do province, Korea, were susceptible to the bacteriophage Str-PAP-1. This suggests that the bacteriophage Str-PAP-1 can be used to treat or prevent streptococcosis caused by S. parauberis. In addition, the effects of dietary supplementation with bacteriophage Str-PAP-1 at varying doses (T1: 1 × 105 pfu/g, T2: 1.5 × 105 pfu/g and T3: 2 × 105 pfu/g) on fish growth performance, the incidence of S. parauberis infections, and fish mortality, were evaluated. Feeding fish with the bacteriophage Str-PAP-1 improved growth (length: 9.3%, 5.4%, and 9.7%; weight: 27.1%, 21.2%, and 27.5% by T1, T2, and T3 doses, respectively), and reduced both the detection rate of S. parauberis and mortality (relative percent survival (RPS): 52.3%, 51.6%, and 53.1%, respectively) in all test groups. Thus, bacteriophage Str-PAP-1 is an environmentally friendly agent that can prevent and treat streptococcosis caused by S. parauberis. Statement of relevance S. parauberis is a major pathogenic bacterium that causes streptococcosis in farmed fish and often leads to mass mortality of farmed fish. In particular, streptococcosis caused by S. parauberis has continually increased in fish farming in Korea. Although various antibiotics have been used for the treatment of streptococcosis caused by S. parauberis, the efficacy of antibiotic-based treatment has continually decreased due to increased antibiotic resistance. The development of a new in-feed antibacterial agent that is safe, effective, and environmentally friendly is necessary. This study reports a bacteriophage that has potential as an in-feed alternative to antibiotics.