Pathogenic Aeromonas hydrophila in Ornamental Neocaridina Shrimps: Biochemical Traits and Antibiotic Resistance.

Bacterial infections are increasingly recognised as a major threat to ornamental freshwater shrimp aquaculture. Diseased Neocaridina spp. displaying abnormal swimming, lethargy, hypoxia, cutaneous abrasions, dark-brown lesions, and occasional moulting were investigated. Pure cultures from moribund shrimp yielded Gram-negative, motile, catalase- and oxidase-positive rods producing hemolysis and resistant to vibriostatic agent O/129. Biochemical profiling (API 20E) identified all isolates (KRW1-KRW5) as Aeromonas hydrophila, clustering with A. hydrophila CECT 839 and A. bestiarum CECT 4227, while MALDI-TOF MS confirmed their highest similarity to A. hydrophila CECT 839T DSM. Virulence gene screening revealed the universal presence of aer, alongside variable combinations of ela, hlyA, act, alt, ast, and lip. Pathogenicity assays fulfilled Koch's postulates, linking isolates to 'black disease', with cumulative mortality reaching 100% by Day 4-5 postinfection for strains KRW1 and KRW2, whereas KRW4 was least virulent. Antimicrobial susceptibility testing revealed multidrug resistance, with MAR indices ranging from 0.130 (KRW4) to 0.478 (KRW5). These findings identify A. hydrophila as the etiological agent of black disease in Neocaridina shrimp, underscore its virulence heterogeneity, and highlight the alarming prevalence of antimicrobial resistance in strains circulating in ornamental aquaculture.