Abstract
Large-scale fish farming faces many environmental stresses, which affect their immune systems, growth performance, and physiological homeostasis, resulting in increase in their susceptibility to infections. Some of the most common bacterial infections of cobia fish (Rachycentron canadum) include streptococcosis, vibriosis, furunculosis and mycobacteriosis, and pastelleurosis. Probiotics could be helpful in reducing or limiting the incidence of severe disease infections or outbreaks. Therefore, the present study aimed to isolate the indigenous bacterial species from healthy cobia fish and then selected 3 strains, including Bacillus sp. RCS1 (MW560712), Pantoea agglomerans RCS2 (MW560713), and Bacillus cereus RCS3 (MW560714) from the gut of juvenile’s cobia having advantageous assets or positive characteristics. Their analysis indicated the presence of similar biochemical profiles and all could effectively utilize carbon sources. The biosafety assessment did not show any pathological symptoms after 10 days of injecting the fish with isolated bacteria. The results showed that all the isolated bacteria in the present study had low auto-aggregation capacity within the first 3 h of incubation. The isolated bacteria showed strong tolerance when exposed to a range of pH. Although asymmetrically, a slow rise in the growth of isolated bacteria was observed within the pH range of 1–8 for RC1, 1–7 for RC2, and 1–6 for RC3. The antagonistic effects of isolated bacterial strains on the development of pathogens, including Vibrio alginolyticus, Vibrio harveyi, Streptococcus iniae, and Streptococcus agalactiae, were investigated using Luria-Bertani (LB) agar plates. All the isolated bacteria exhibited inhibitory effects against the pathogens, including V. alginolyticus, V. harveyi, S. iniae, and S. agalactiae. These isolated bacteria were characterized with a wide range of antagonistic activities, non-hemolytic activities, high survivability after heat-treatments and safety confidence, and antibiotic susceptibility. Generally, the characteristics displayed by these strains indicated that they could be used as potential probiotics in the aquaculture industry.
Highlights
Aquaculture is a reliable production source for sustenance and nourishment of the global population’s increasing demand for animal and fish proteins
RCS1, Pantoea agglomerans RCS2, and Bacillus cereus RCS3, which were selected as microorganisms to be investigated in this study (Table 1)
The gene sequencing analysis of 16S rRNA gene indicated that the isolated bacteria were Bacillus sp., Bacillus cereus strain, and Pantoea agglomerans strain
Summary
Aquaculture is a reliable production source for sustenance and nourishment of the global population’s increasing demand for animal and fish proteins. Cobia (Rachycentron canadum) is appreciated for its characteristic features, which make it an important culture fish It has a fast growth rate and inadequate availability in the wild, along with high market values due to its excellent flesh quality. These characteristics have inspired its production in aquaculture all over Asia, especially in China and other parts of the globe as well (Holt et al, 2007; Benetti et al, 2008, 2010; Zhang et al, 2013; Chen et al, 2016; Huang et al, 2020; Xie et al, 2021). The opportunistic species from Photobacterium genus causes a disease pastelleurosis that has been identified as a significant emerging problem for the aquaculture of cobia throughout its production cycle (Lopez et al, 2002; Liu et al, 2003; Rajan et al, 2003; Liao et al, 2004; Chen and Hsu, 2005)
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