Morganella psychrotolerans as a possible opportunistic pathogen in rainbow trout (Oncorhynchus mykiss) fisheries

Abstract

Morganella psychrotolerans was isolated from various seafood items as a histamine producing bacterium, and has been implicated in incidents of histamine fish poisoning. The bacterium can be isolated from frozen seafood, such as vacuum-packed or cold-smoked tuna. In our study, bacteria forming a whitish colony, with rod-like shape and Gram-negative properties, were isolated from the liver and kidney of diseased rainbow trout. For the identification of bacteria, 7 housekeeping genes and 16S rRNA gene were analyzed, and M. psychrotolerans was identified. The 16S rRNA gene showed 99.63% similarity to M. psychrotolerans strain U2/3. Since the bacteria was not reported as a fish pathogen, artificial infection and histopathological analysis were performed for the verification. For pathogenicity measurement, a challenge trial was conducted; LD50 was calculated as 5 × 104 CFU/fish and the infected fish showed clinical signs of septicemia inducing mortality. For treatment of the disease, antibiotic susceptibility test was performed and it was resistant to several antibiotics such as tetracycline and erythromycin which were one of the most commonly used antibiotics in aquaculture. In our study, here we report M. psychrotolerans for the first time, as a rainbow trout pathogen in fisheries.