Identification and characterization of Serratia nematophila and Acinetobacter guillouiae from putrid-skin disease lesions in farmed Chinese spiny frog (Quasipaa spinosa)

ABSTRACT Serratia and Acinetobacter are recognized as significant opportunistic pathogens affecting aquatic animals and humans. However, their infections in amphibians are poorly documented, and their pathogenicity to the Chinese spiny frog (Quasipaa spinosa) remains unexplored. This study investigated an outbreak of putrid-skin disease among Q. spinosa on a farm in Lishui City, Zhejiang Province, China. Two pathogenic strains, GL-XJ7 and GL-XJ11, were isolated from disease lesions and identified as Serratia nematophila and Acinetobacter guillouiae, respectively, through morphological, physiological, biochemical, and molecular analyses including 16S rDNA sequencing and phylogenetic tree construction. Experimental infections revealed median lethal concentrations (LD50) over 72 h of 3.98×107 CFU/mL for S. nematophila strain GL-XJ7 and 3.16 × 106 CFU/mL for A. guillouiae strain GL-XJ11. Infected frogs exhibited symptoms consistent with natural infections, including reduced vitality, skin shedding, and ulceration. Histopathological examinations demonstrated that both strains induced hepatocellular damage, nuclear alterations, muscular atrophy, myofibrillar degeneration, and intestinal necrosis. Antibiotic susceptibility tests showed that S. nematophila strain GL-XJ7 was highly susceptible to ceftazidime and gentamicin, while A. guillouiae strain GL-XJ11 exhibited high susceptibilities to tetracycline, ceftazidime, and gentamicin. Both strains demonstrated resistance to penicillin, ampicillin, bacitracin, and clindamycin. This study provides the first description of natural S. nematophila strain GL-XJ7 and A. guillouiae strain GL-XJ11 infections and their pathogenesis in Q. spinosa, highlighting potential risks to other animals and human health. These findings establish a theoretical foundation for the clinical management and prevention of putrid-skin disease in artificially bred frogs.IMPORTANCEFrogs are among the most widely distributed amphibians globally. The Chinese spiny frog (Quasipaa spinosa) is a unique amphibious species endemic to China and holds significant economic value in aquaculture. Effective disease prevention and control are crucial for the sustainable development of frog breeding industries and the conservation of genetic resources. In this study, we investigated a putrid-skin disease outbreak at a frog farm in Lishui City, Zhejiang Province, Eastern China. We isolated and identified the causative pathogenic bacteria and analyzed their pathogenicity through artificial infection experiments and histopathological examinations. This research provides the first data on the pathogenic characteristics of Serratia nematophila and Acinetobacter guillouiae in Q. spinosa. Furthermore, we assessed the antibiotic susceptibility of these two pathogens, revealing their multidrug resistance. Our findings offer a scientific foundation for the accurate diagnosis and control of putrid-skin disease in frogs, contributing to the preservation of Q. spinosa genetic resources.