Abstract

Probiotic treatments in aquaculture are applied to prevent infection and improve overall fish health.¹ Disease prevention and treatment are essential for reducing economic loss. Edwardsiella ictaluri, a problematic bacterial pathogen, has caused widespread mortality in the tropical fish industry over the past decade affecting many species, including zebrafish (Danio rerio), cultured for their importance in the aquarium and laboratory trades.^2,3

The objective of this study was to investigate the use of commercial probiotics as preventative treatments for the pathogenic bacteria, E. ictaluri in zebrafish. The study was a two-part investigation. First, we aimed to evaluate the effect of probiotic treatments on the gastrointestinal microbiome of zebrafish following pond treatments with either commercial probiotic A, commercial probiotic B, or an untreated control group. Three replicate ponds for each treatment at two participating aquaculture farms were included in the study. Second, we measured the presence and relative abundance of E. ictaluri on zebrafish skin and in water samples to assess the potential impact of probiotic treatments on E. ictaluri presence. Fecal and skin samples were collected from male and female fish at larval and adult stages. Water was also collected at each timepoint from each pond. Length, girth, and weight measurements were recorded for juvenile and adult/harvest stages.

Histological analysis of intestinal tissue shows no difference among treatments. Fish size was highly variable between ponds and no significant difference between treatments was detected. The results from the fecal microbiome Bray-Curtis beta diversity analysis show that fish treated with probiotic B were significantly different in diversity compared to those fish treated with probiotic A and to untreated control fish. Probiotic A did not significantly affect the microbial diversity in the zebrafish gastrointestinal system. Sex was not a significant factor in fecal microbial diversity. In contrast, a significant difference in microbial diversity was shown between zebrafish feces and pond water. Probiotic B increased the presence of the genera Aeromonas and Pseudomonas in the gastrointestinal microbiome but decreased relative levels of the genera Cetobacterium. The genera Edwardsiella was only detected in the feces of three individual fish. However, E. ictaluri was detected on the skin of adult fish in all experimental groups. The fewest number of E. ictaluri detections were in the probiotic A group and the greatest number of detections were in the control group in both farms. Predictive tools will be used to determine the functional role of the bacteria identified in the fecal microbiome. These findings are expected to inform on the use of probiotics as a preventative treatment of E. ictaluri in ornamental fish aquaculture.

Acknowledgements:

We thank the USDA NIFA Southern Regional Aquaculture Center for funding. EBR is supported by the National Science Foundation Graduate Research Fellowship (Program Grant No. 2019285699). Any opinions, conclusions or recommendations expressed in this work are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Literature Cited:

1.  Van Doan H, Hoseinifar SH, Ringø E, Ángeles Esteban M, Dadar M, Dawood MA, Faggio C, 2020. Host-associated probiotics: a key factor in sustainable aquaculture. Rev. Fish. Sci. Aquac. 28:16–42.

2.  Aristizabal-Henao JJ, Ahmadireskety A, Griffin EK, Da Silva BF, & Bowden JA. 2020. Lipidomics and Environmental Toxicology: Recent Trends. Curr Opin Environ Sci Health.

3.  Hawke JP, Kent M, Rogge M, Baumgartner W, Wiles J, Shelley J, Savolainen LC, Wagner R, Murray K, Peterson TS. 2013. Edwardsiellosis Caused by Edwardsiella ictaluri in Laboratory Populations of Zebrafish Danio rerio. J. Aquat. Anim. Health 25:171–183.