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A new serogroup of Vibrio harveyi in the Adriatic Sea – comparison of virulent vs non-virulent strains for sea bass.

Introduction: Vibrio harveyi is a highly diverse marine pathogen, with some strains causing severe losses in aquaculture, while others being non-virulent. During recent years disease outbreaks in the Mediterranean farmed fish due to V. harveyi are becoming more common, posing a serious threat to the industry. The exact mechanisms and interplay of different virulence factors are still largely unclear. During the summer months of past few years, several outbreaks of vibriosis in sea bass (Dicentrarchus labrax) characterized by haemorrhages, corneal opacity and uncoordinated swimming were recorded in several farms along the Croatian Adriatic coast. Samples of diseased fish were submitted for laboratory examination. Methodology: Samples of eyes, anterior kidneys, hearts and spleens were plated on both TSA supplemented with NaCl and Marine agar (MA) and cultured overnight at 25 oC. Pure colonies were obtained by streaking and restreaking on fresh MA plates. The non-virulent strain was isolated from the gut of healthy seabass. Genomic DNA was extracted from bacterial cultures using the Qiagen QIAamp DNA Mini QIAcube Kit. 16S rRNA gene was amplified using 27FYM and 1492R primers and sequenced to confirm the genus Vibrio, and toxR gene was amplified using toxRF1 and toxRR1 primers and sequenced to confirm the species V. harveyi. Slide agglutination test and ELISA were used to detect the main emerging pathogenic serogroup isolated from sea bass. Results: Several isolates were confirmed by searching the NCBI Nucleotide database using Megablast as V. harveyi. Serological assays showed that strain 94/17, isolated from the eye, belongs to the virulent Serovar A previously described in Spain. However, strain 99/17, isolated from the anterior kidney, seems to belong to a new virulent serovar of the species. The results of currently ongoing WGS of described isolates and the one isolated from healthy fish will be analyzed and presented. Conclusion: A new virulent serovar of V. harveyi has been isolated from diseased sea bass in the Adriatic Sea. Comparing the genome sequences of fish virulent and non-virulent strains as well as of virulent strains belonging to different serovars could shed some light on the cause of the differences in phenotype/pathogenicity.

Vibrio harveyi, sea bass, serogroup, Adriatic

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