Bacterial Microbiome Of Dusky Kob Argyrosomus Japonicus Eggs And Rearing Water And The Bacteriostatic Effect Of Selected Disinfectants

The dusky kob Argyrosomus japonicus is an emerging aquaculture species in South Africa. To support industry growth, existing hatcheries frequently sell or donate excess eggs to neighbouring farms, raising concern over the potential transfer of infectious disease agents. Chemical disinfectants are frequently used to prevent transmission of diseases, but their application has shown large variations between finfish species and very little is known about the efficacy and impact of disinfectants on dusky kob eggs or larvae. A thorough understanding of the bacterial communities associated with specific fish species can inform the need for specific chemical agents and guide a more targeted approach for disease control and treatment. Therefore, this study used next generation sequencing (NGS, 16S rDNA gene) and downstream bioinformatics analysis to describe the bacterial microbiome associated with the eggs and rearing water of dusky kob. Four chemical disinfectants [chloramine-T (C-T), povo-iodine (P-I), hydrogen peroxide (H2O2) & formaldehyde (CH2O)], applied at different concentrations and times, were then tested for their ability to disinfect eggs and seawater. The impact of each disinfectant on early larval development was also assessed. A total of 11,605 individual bacterial operational transcriptional units (OTUs) were identified from the dusky kob egg (7352 OTUs) and hatchery water (4253 OTUs) samples. Non-metric multidimensional scaling (NMDS) analysis revealed distinct differences in the composition of the microbiota associated with the egg and seawater samples. A moderate level of bacterial diversity was observed in all samples, with the seawater samples dominated by Rhodobacteraceae (65%) and Flavobacteriaceae (14%); whereas egg samples were dominated by Pseudoalteromonadaceae (49%), Vibrionaceae (27%) and Colwelliaceae (8%). A classical univariate statistical comparison identified 6 differentially abundant OTUs at phylum level, 12 at family level and 19 at genus level; with members of the family Vibrionaceae found to be significantly (P = 0.0011) more abundant on dusky kob eggs samples. All tested chemical disinfectants were capable of reducing the bacterial load in seawater in a dose dependent manner, with the number of culturable bacteria on eggs decreasing significantly following exposure to P-I and CH2O for five minutes (43% and 88% reduction in CFUs, respectively), compared with C-T and H2O2 (3% & 10%). Similarly, P-I and CH2O significantly decreased the culturability of Vibrio species on eggs (79% & 75%), compared with C-T and H2O2 (12% & 14%). None of the disinfectants appeared to have any adverse impact on larval growth and morphology.