Isolation and characterization ofLactococcus garvieaefrom rainbow trout, Onchorhyncus mykiss, from California, USA

Abstract

Lactococcus garvieae is an emergent bacterial pathogen of salmonid fish in North America that causes acute infections particularly at water temperatures above 15°C. During 2020, L. garvieae was detected in rainbow trout, Onchorhyncus mykiss, cultured in Southern California and the Eastern Sierras. Infected fish exhibited high mortalities and nonspecific clinical signs of lethargy, erratic swimming, dark skin pigmentation, and exophthalmia. Macroscopic changes included external and internal hemorrhages, mainly in the eyes, liver, coelomic fat, intestine, and brain. Histological examination revealed splenitis, branchitis, panophthalmitis, hepatitis, enteritis, and coelomitis, with variable degrees of tissue damage among evaluated fish. Pure colonies of L. garvieae were isolated from infected trout and specific PCR primers for L. garvieae confirmed the preliminary diagnosis. Multilocus sequence analysis showed that the strains recovered from diseased trout represent a novel genetic group. Isolates were able to form biofilms within 24 h that increased their resistance to disinfection by hydrogen peroxide. Laboratory challenge methods for inducing lactococcosis in steelhead trout, O. mykiss, were evaluated by intracoelomic injection with serial dilutions of L. garvieae. The median lethal dose 21 days post challenge was ∼20 colony‐forming units/fish. Experimentally infected trout presented similar clinical signs, gross changes, and microscopic lesions as those with natural disease, fulfilling Koch's postulates and demonstrating the high virulence of the recovered strains.

Development of a quantitative polymerase chain reaction assay for detection of the aetiological agents of piscine lactococcosis

“…Haemorrhages are commonly observed in the brain, swim bladder and intestine (Shahin et al, 2021). In addition to the high mortalities associated with lactococcosis, the disease results in reduced growth, increased labour costs and treatment expenditures.…”

“…Piscine lactococcosis, is an emergent bacterial disease that has been associated with significant economic losses in both farmed and wild fish stocks in various geographical locations, including America, Africa, Europe, Asia and Oceania (Balta & Balta, 2019; Bwalya et al., 2021; Meyburgh et al., 2017; Shahi et al., 2017; Shahin et al., 2021). Lactococcus garvieae represents one of the most important pathogens in rainbow trout ( Oncorhynchus mykiss ) aquaculture (Vendrell et al., 2006).…”

“…Internally, fish have congestion and enlargement of the kidney, liver and spleen accompanied by cardiac pallor and hepatic mottling. Haemorrhages are commonly observed in the brain, swim bladder and intestine (Shahin et al., 2021). In addition to the high mortalities associated with lactococcosis, the disease results in reduced growth, increased labour costs and treatment expenditures.…”

“…samples screened for the target Lactococcus species using the developed qPCR assay included fish posterior kidneys, bird faecal droppings and water samples randomly collected from four rainbow trout facilities in Southern California and the Eastern Sierras during an outbreak of lactococcosis in 2020(Shahin et al, 2021).The diagnostic performance of the developed qPCR was initially compared to bacterial isolation from naturally infected rainbow trout posterior kidneys during an outbreak of lactococcosis as the 'gold standard' diagnostic method for piscine lactococcosis. Thirtythree farmed rainbow trout were collected from three out of four affected facilities (facility 1: n = 3, facility 2: n = 15 and facility 3: n = 15 samples).…”

Comparative Transcriptomic Immune Responses of Mullet (Mugil cephalus) Infected by Planktonic and Biofilm Lactococcus Garvieae

“…Infected fish present with bleeding, ascites, meningitis, and other symptoms ( Chen et al., 2002 ; Vendrell et al., 2006 ; Evans et al., 2009 ). L. garvieae outbreaks have also been found on many farms, with mortality rates as high as 75%, untreatable with antibiotics ( Eldar et al., 1999 ; Shahin et al., 2021 ). The current study finds that the biofilm produced by L. garvieae prevents antibiotics from killing the bacteria ( Dahdouh et al., 2020 ).…”