Bacteriological, molecular, and pathological studies on the Gram‐positive bacteria Aerococcus viridans and Enterococcus faecalis and their effects on Oreochromis niloticus in Egyptian fish farms

Abstract

In this field study, two private Oreochromis niloticus fish farms in El Fayoum and El Sharkia governorates showed signs of septicaemia, with a mortality of 20%–25% between autumn 2018 and summer 2019. Postmortem examination of affected fish revealed loose scales, ulceration in the dorsal part of the head, petechial haemorrhages on the body surfaces, unilateral exophthalmia, and corneal opacity together with congested blood vessels of nearly all fleshy portions of the body fins. Internal examination of the viscera showed congestion of the blood vessels in the kidneys and brain, a dark enlarged spleen, and a liver with a marbled appearance. Fish samples (representing 10% of the fish showing clinical signs) were collected and examined for histopathological and bacteriological examination. Aerococcus viridans was isolated from El Fayoum and El Sharkia governorates at rates of 18.18% and 28.89%, respectively, throughout the year, whereas Enterococcus faecalis was isolated at rates of 81.82% and 71.11%, respectively; seasonal prevalence rate of infection were recorded. Antibiotic sensitivity testing showed that the two microorganisms were resistant to ampicillin, oxolinic acid, nalidixic acid, amikacin, and gentamicin, but they were sensitive to amoxicillin and nitrofurantoin. E. faecalis exhibited resistance to erythromycin and oxytetracycline, whereas A. viridans was sensitive to these agents. To study the pathogenicity of such microorganisms in this fish species, experimental infection and reisolation of both bacterial isolates in O. niloticus was performed to confirm the field isolation. The pathological lesions in different organs of the infected fish were fully recorded.

Evaluation of Argulus indicus on striped snakehead (Channa striata) in Towuti Lake, South Sulawesi, Indonesia

“…For example, in 2018 and 2019 disease caused by infection with A. viridans caused extensive financial losses in the Egyptian tilapia aquaculture sector with morbidity and mortality rates of 20-25%. Research has shown that mortality in fish purposively infected with A. viridans can be as high as 91% by 6 days after injection (Elgohary et al 2021).…”

Ergasilus extensusand bacterial co‐infection in flathead grey mullet,Mugil cephalus(Linnaeus, 1758), are associated with pathological changes and immunological gene expression alterations

“…extensus ectoparasites. Cells, especially hepatocytes from different fish, typically evoke various tissue responses against stresses, including those from bacterial infections (Elgohary et al, 2020).…”

“…Similarly, some Enterococcus sp. have been isolated from both healthy and infected fish species, causing streptococcallike infections in Yellowtail (Seriola quinqueradiata) in Japan, tilapia (Oreochromis niloticus) in Egypt, catfish (Clarias batrachus) in Bangladesh and common carp (Cyprinus carpio) in Thailand (Elgohary et al, 2020;Nieto et al, 1995;Petersen & Dalsgaard, 2003;Plumb & Hanson, 2010;Rahman et al, 2017). (He et al, 2015;Kaewngernsong et al, 2019).…”

Bacterial diseases of tilapia, their zoonotic potential and risk of antimicrobial resistance

“…The diseased fish showed congested gills and abdomen, a swollen gall bladder and a severe diffusion in the liver. A. viridans infections have been subsequently reported in Indonesian, 278 and in Egyptian tilapia farms, 279,280 always in combination with other bacteria. In Indonesia, the bacterium was isolated in a screening of water from a tilapia pond and in faeces of tilapia, and was identified by biochemistry.…”

“…In Indonesia, the bacterium was isolated in a screening of water from a tilapia pond and in faeces of tilapia, and was identified by biochemistry 278 . In Egypt, the bacterium was isolated as one of 17 in a multibacterial infection of wild caught tilapia from the Nile River, 279 and it was isolated from diseased tilapia from two tilapia farms, in combination with Enterococcus faecalis , 280 and the A . viridans were identified by molecular methods, like 16SrRNA typing.…”