Multilocus sequence typing, antimicrobial susceptibility, and phage discovery of motile Aeromonas species from the pond-cultured goldfish Carassius auratus on two farms in Japan

Motile Aeromonas spp., including Aeromonas hydrophila, A. caviae, and A. veronii biovar sobria can cause motile aeromonad disease and pose serious threats to the aquaculture industry. Motile aeromonads showing high minimum inhibitory concentrations (MICs) to widely used various antibiotics in global aquaculture production have been reported. Several studies have shown that A. hydrophila phages are effective in treating infected fish. However, in Japan, epidemiological reports and phage studies of motile Aeromonas spp. isolated from fish are scarce. In this work, we visited two goldfish farms in Japan between 2020 and 2021, and obtained 33 motile Aeromonas isolates from Carassius auratus exhibiting symptoms of motile aeromonad disease. Multilocus sequence typing analysis revealed that the isolated motile aeromonads belonged to a wide variety of sequence types (STs) including new STs. The ST2007 accounted for 50% (6/12) of A. hydrophila isolates, and each was isolated at different timepoints and farms. The isolated bacteria exhibited high MICs for oxolinic acid, nitrofural, sulfamerazine, and oxytetracycline; 13 isolates (39%) which presented with MICs above commercially used concentrations of oxolinic acid, nitrofural, and sulfamerazine in Japan and were classified as non-wild types for oxytetracycline. The subsequently isolated phages showed high specificity for the motile aeromonads isolates.