Involvement of Enterococcus species in streptococcosis of Nile tilapia in Bangladesh
Introduction
Aquaculture has been profusely contributing with tremendous growth to cater to the demand of cheap animal protein source for human consumption in the world. Bangladesh is blessed to have rich and productive inland water resources and therefore, considered as one of the most vibrant regions with great potential for aquaculture development (Ghose, 2014; Shamsuzzaman et al., 2016). This country is the third largest producer of fish globally, after China and India (FAO, 2018). However, due to impacts of climate change and environmental pollution, the cultured fishes are inevitably susceptible to various types of microbial infections result in significant damages to total aquaculture production (Faruque and Kabir, 2016; Henriksson et al., 2018). It is estimated that fish diseases accounts for 15% of production losses every year (Foysal et al., 2019a). Nevertheless, bacteria is associated with most of the infections in cultured fish species in Bangladesh (Rahman et al., 2010).
Nile tilapia (Oreohromis niloticus) is one of the most economically important cultured fish species throughout the world including, Bangladesh. Inland culture of tilapia is rampant to meet-up the burgeoning demand for protein sources and also fetches competitive prices compared to other native fish species in the domestic market (Ahmed and Ahmed, 2009; Shamsuzzaman et al., 2017). The global market value of tilapia increased to the tune of 4000 million USD in 2010, compared to 184 million USD in 1984, and hence, the production has been boosted-up 30 times within a short span of eight years (1999–2007) (Osman et al., 2017). In Bangladesh, fish farmers produced around 381,215 metric tons of tilapia in 2017 (DoF, 2018). However, the incidence of streptococcosis disease recently emerges as a major threat leading to substantial commercial losses to tilapia growers (Foysal et al., 2019b). Although Streptococcus species is recognised as the major causative agent of streptococcosis, Lactococcus, Vagococcus and Enterococcus are also isolated from the diseased fish (El-Sayed and El-Gheit, 2005; Osman et al., 2017; Mishra et al., 2018).
Due to inadequate diagnostic facilities at fish farms, the majority of causative agents responsible for diseases remained uncharacterized, leading to the inappropriate treatment process that further increase the mortality of fish. Phenotypic plate-based characterization not always able to precisely classify the bacteria at the genus or species level, and therefore, often been criticised for its accuracy and preciseness (Franco-Duarte et al., 2019). Moreover, 16S rRNA gene sequence homology and specific PCR based identification revealed that various species including species of Streptococcus and Enterococcus faecalis have a crucial role in streptococcosis of tilapia (Li et al., 2014; Arumugam et al., 2017; Osman et al., 2017; Foysal et al., 2019b). Thus any previous studies have yet to identify the causative agents of streptococcosis in Nile tilapia precisely results in further augmentation of fish mortality.
Recent advancements in “omics” technologies, especially in next generation sequencing and downstream bioinformatics analysis have facilitated in more precise identification and in-depth analysis of microbial communities in a given sample or a particular environment. Additionally, the evolution of computational system and statistical packages make data analysis easier and comprehensive. Therefore, this study aimed to characterize the bacterial communities, causative agents and the pathogenicity of the bacteria associated with streptococcosis in Nile tilapia using metagenomics, phylogenetic analysis and in vivo infection challenge test.
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Ethics statement
The animal experiments were conducted with the permission (BSMRAU/IBGE/ERC/001) and supervision of the Ethical Review Committee of the Institute of Biotechnology and Genetic Engineering, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur, Bangladesh. At all the stages, fish were handled with care and compassion.
Collection of diseased fish and samples
The tilapia fish (N = 24) in this study were collected from eight different ponds during July 2018. Diseased fish (n = 12, average weight 266.3 ± 20.1 g) were collected
Alpha diversity measurements
After quality filtering, 16 samples generated 592,688 reads, which were classified into 13 phylum, 25 classes, 57 orders, 112 families and 178 genera. The rarefaction curve revealed that each sample was sequenced near to saturation and captured enough depth in terms of species diversity (Fig. 1A). The ANOVA measurement indicated significant (P < .05) differences in microbial communities between control and diseased fish gut and skin in terms of species richness, Shannon and Simpson indices. The
Discussion
Streptococcosis associated aquaculture production losses are enormous and have been gained significant attention in recent times especially in Bangladesh. Although the worldwide distribution and mass mortality have been recorded, the identification of causative agent for Streptococcosis has been poorly studied. Previous studies characterized Streptococcus agalactiae (Li et al., 2014), S. iniae (Foysal et al., 2019b), Enterococcus faecalis (El-Sayed and El-Gheit, 2005; Arumugam et al., 2017;
Funding
Bangladesh Academy of Sciences provided the major financial support for this work under the project “Identification of virulence genes involved in streptococcosis in tilapia and development of prevention measure against the disease” (project number BAS-USDA-PALS-FI-25).
Declaration of Competing Interest
Authors declare no conflict of interest.
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Authors contributed equally in this study.