Use of bacteriophage vB_Pd_PDCC-1 as biological control agent of Photobacterium damselae subsp. damselae during hatching of longfin yellowtail (Seriola rivoliana) eggs

AIMS

This study describes the effect of phage therapy on hatching of longfin yellowtail (Seriola rivoliana) eggs challenged with Photobacterium damselae subsp. damselae.

METHODS AND RESULTS

A lytic phage (vB_Pd_PDCC-1) against P. damselae subsp. damselae was isolated and characterized. The use of phage vB_Pd_PDCC-1 increased the hatching rate of eggs, and reduced presumptive Vibrio species to non-detectable numbers, even in non-disinfected eggs. High-throughput 16S rRNA gene sequencing analysis revealed that phage vB_Pd_PDCC-1 caused significant changes in the composition and structure of the associated microbiota, allowing that members (e.g. those belonging to the family Vibrionaceae) of the class Gammaproteobacteria to be displaced by members of the class Alphaproteobacteria.

CONCLUSIONS

To the best of our knowledge, this represents the first study evaluating phage therapy to control potential negative effects of P. damselae subsp. damselae during hatching of longfin yellowtail eggs.

SIGNIFICANCE AND IMPACT OF THE STUDY

The Seriola genus includes several important commercial fish species due to its rapid growth and easy adaptability to confinement conditions. However, bacterial infections (especially those caused by Vibrio and Photobacterium species) are among the main limiting factors for the intensification of marine fish aquaculture, particularly during early development stages. Therefore, the use of phages, which are natural killers of bacteria, represents a promising strategy to reduce the mortality of farmed organisms caused by pathogenic bacteria.

Silver nanoparticles are lethal to the ciliate model Tetrahymena and safe to the pike silverside Chirostoma estor

Ciliate ectoparasites are one of the most important groups of pathogens in fish culture, and the traditional treatments are sometimes harmful to the fish and the environment. Thus, the search for novel compounds that are effective at low concentrations and safe for fish are necessary to optimise treatments in aquaculture. The antiprotozoal capacity of silver nanoparticles (AgNPs) against the ciliate Tetrahymena has been documented; however, their toxicity may vary with the synthesis methodology and nanoparticle size. The objectives of this study were a) to evaluate the acute toxicity in vitro of two AgNPs (Argovit™ and UTSA) on Tetrahymena sp., a biological model for ciliated ectoparasites of fish and b) to test the safety of lethal and higher doses of UTSA AgNPs for ciliates on the fish C. estor. Light microscopy and scanning electron microscopy (SEM) were used to determine whether AgNPs affected the structure of the cell surface of Tetrahymena. The mortality, histopathological alterations and metagenomics of the fish were used to determine the major effects of UTSA AgNPs. In Tetrahymena, the median lethal concentration (LC₅₀) for Argovit™ was 2501 ± 1717 ng/L at 15 min and 796 ± 510 ng/L at 60 min, while the LC₅₀ for UTSA AgNPs was 4 ± 2 and 1 ± 0.6 ng/L at 15 min and 60 min, respectively. A concentration of 3300 ng/L Argovit™ and 10.6 ng/L UTSA AgNPs for 15 and 60 min, respectively, was 100% effective against Tetrahymena. After 60 min of exposure to 0.25 and 0.50 ng/L UTSA AgNPs, the number of cilia significantly reduced, there were small holes on the cell surface, and the cellular membrane was ruptured. In fish exposed to lethal (10.6 ng/L) and higher (31.8 and 95.4 ng/L) doses of UTSA, the AgNPs did not affect fish survival after 96 h, and there were no signs of histopathological damage or gut microbial changes. This study is the first report on microscopic and ultrastructural changes in Tetrahymena after exposure to significantly low concentrations of UTSA AgNPs with antiprotozoal efficacy without evidence of harmful effects on fish. These results provide the basis for further studies of both pet aquarium and commercial fish that may validate these findings at a larger experimental scale, taking into account AgNPs bioaccumulation, safety for human consumption and environmental impact.