Live Feeds Used in the Larval Culture of Red Cusk Eel, Genypterus chilensis, Carry High Levels of Antimicrobial-Resistant Bacteria and Antibiotic-Resistance Genes (ARGs).

Luz Hurtado,Félix A. Godoy,Rodrigo Rojas,Claudio D. Miranda,Jaime Romero,Mark A. Añazco

doi:10.3390/ani10030505

Abstract

Simple SummaryThe culture of the marine fish red cusk eel Genypterus chilensis is currently considered a priority for Chilean aquaculture but low larval survival rates have prompted the need for the continuous use of antibiotics, mainly florfenicol. In this study, the role of live prey (rotifers and the brine shrimp Artemia franciscana) used to feed fish larvae as a source of antibacterial-resistant bacteria in a commercial culture of G. chilensis was investigated. Samples of live feeds were collected during the larval growth period and their bacterial contents were determined. High levels of potentially opportunistic pathogens, such as Vibrio spp., as well as florfenicol-resistant bacteria, were detected. Sixty-five florfenicol-resistant isolates were recovered from these cultures and identified as Vibrio (81.5%) and Pseudoalteromonas (15.4%), which exhibited a high incidence of co-resistance to the antibiotics streptomycin, oxytetracycline, co-trimoxazole, and kanamycin. The majority of them carried the florfenicol-resistance encoding genes floR and fexA. The high prevalence of antibiotic-resistant bacteria and the associated genetic elements in live feed administered to reared fish larvae requires the prompt implementation of efficient management strategies to prevent future therapy failures in fish larval cultures and the spread of antibiotic-resistant bacteria to associated aquatic environments.The culture of red cusk eel Genypterus chilensis is currently considered a priority for Chilean aquaculture but low larval survival rates have prompted the need for the continuous use of antibacterials. The main aim of this study was to evaluate the role of live feed as a source of antibacterial-resistant bacteria in a commercial culture of G. chilensis. Samples of rotifer and Artemia cultures used as live feed were collected during the larval growth period and culturable bacterial counts were performed using a spread plate method. Rotifer and Artemia cultures exhibited high levels of resistant bacteria (8.03 × 104 to 1.79 × 107 CFU/g and 1.47 × 106 to 3.50 × 108 CFU/g, respectively). Sixty-five florfenicol-resistant isolates were identified as Vibrio (81.5%) and Pseudoalteromonas (15.4%) using 16S rRNA gene sequence analysis. A high incidence of resistance to streptomycin (93.8%), oxytetracycline (89.2%), co-trimoxazole (84.6%), and kanamycin (73.8%) was exhibited by resistant isolates. A high proportion of isolates (76.9%) carried the florfenicol-resistance encoding genes floR and fexA, as well as plasmid DNA (75.0%). The high prevalence of multiresistant bacteria in live feed increases the incidence of the resistant microbiota in reared fish larvae, thus proper monitoring and management strategies for live feed cultures appear to be a priority for preventing future therapy failures in fish larval cultures.

Highlights

The current trend in Chilean marine aquaculture is toward the diversification of cultured aquatic species
According to the current hatchery protocols, rotifer cultures are not treated with antibiotics, in contrast to the Artemia cultures, which are routinely treated with florfenicol or other antibacterial agents, no treated rotifer mass cultures at the red cusk eel hatchery were available during the sampling period (Table 1)
Florfenicol-resistant bacterial counts were high in both live-feed cultures during the larval growth period, ranging from 104 to 107 colony forming units (CFU)/g of rotifer and from 106 to 108 CFU/g of Artemia (Table 1)

Summary

Introduction

The current trend in Chilean marine aquaculture is toward the diversification of cultured aquatic species. The priority is to develop the culture of native marine fish species, in particular, the red cusk eel (Genypterus chilensis), which is a Chilean native species of high gastronomic demand and market value. It has been identified as a good candidate for production on a commercial scale. Studies on the sanitary conditions during the culture of red cusk eel, including administered live feed during the larval stage are not available, despite the recognized importance of live feed as an important source of bacterial contaminants in marine fish larval cultures [7,8,9]

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