Therapeutic Potential of Antimicrobial Peptide Scymicrosin_7-26 against the Emerging Pathogen Acinetobacter ursingii...

The Pacific white shrimp (Litopenaeus vannamei) aquaculture faces emerging threats from novel pathogens and escalating antibiotic resistance. This study successfully isolated and identified the pathogenic bacterium Acinetobacter ursingii strain 31C2 from diseased L. vannamei using an integrated approach combining microbiological, biochemical, and molecular techniques. The pathogenicity of this strain was confirmed in L. vannamei and marine medaka (Oryzias melastigma) infection models, exhibiting a strong dose-dependent mortality, with median lethal doses (LD₅₀) of 2.83 × 10⁴ CFU/g shrimp and 2.58 × 10⁶ CFU/fish, respectively. Infection caused severe hepatopancreatic necrosis (tubular deformation and epithelial vacuolation) and intestinal villi destruction. Antimicrobial susceptibility testing revealed that the 31C2 strain was resistant to tetracycline and azithromycin. To identify effective agents targeting this strain, the antimicrobial peptide Scymicrosin_7-26 (derived from Scylla paramamosain) was evaluated. The peptide had potent antibacterial activity against A. ursingii 31C2 in vitro (MIC: 3-6 µM). In vivo application significantly enhanced survival of L. vannamei and O. melastigma infected with 31C2 by 30% and 20%, respectively. Treatment drastically reduced bacterial loads in the hepatopancreas and intestine, restored tissue integrity, and modulated the immune response by suppressing the hyperactivation of the Toll and IMD pathways and their downstream transcription factors, dorsal and relish, while upregulating penaeidin3 and propo expression. This study identified A. ursingii as an emerging shrimp pathogen and validated Scymicrosin_7-26 as a promising antibiotic-free therapeutic for disease control in aquaculture.