Diversity of sea bass pathogen strains highlights challenge in Mediterranean aquaculture

A new study analyzing three strains of the bacterium Vibrio harveyi found among farmed sea bass in the Mediterranean has underlined the diversity of the pathogen, which poses a severe threat to the region’s aquaculture.

V. harveyi is one of the major causes of vibriosis. Researchers from the Croatian Veterinary Institute and the University of Valencia sampled fish from the Adriatic Sea showing clinical signs of disease, including uncoordinated swimming, corneal opacity and hemorrhages at key sites.

They went on to characterize three strains of the bacterium — the first isolated from sea bass to have their genome sequenced. The work saw six new virulence genes identified, while the strains were also found to have numerous differences and signs of antibiotic resistance.

Pathogen’s differing characteristics highlighted

V. harveyi can cause very high mortality, slow larval metamorphosis, retarded growth and body malformations. Strains of the bacterium are diverse, with some causing high mortalities while others are non-virulent.

This study marked the first attempts to compare the biochemical, physiological and genetic characteristics of serologically different V. harveyi strains originating from the Mediterranean, the researchers said. The comparison included two strains belonging to identified V. harveyi serotypes (A and tentatively “B”) and a strain serologically different from those. Serotype A is the most common serotype found in European sea bass.

Despite the isolates coming from the same species in a small geographic area, differences were detected at every level of comparison, they said.

Antimicrobial susceptibility was measured in the study, with the three strains tested against 13 different drugs. Resistance to ampicillin and novobiocin was seen, they explained, while predicted genes linked to antimicrobial resistance did not match with in vitro susceptibility testing.

Still much to learn

The researchers warned that there is some ambiguity between V. harveyi strains, meaning that caution is needed in their identification. They also noted that given the strains studied were isolated in different environmental conditions, that could influence the expression of the virulence and resistance genes.

Further research should be done to isolate and serotype more strains, particularly from other parts of the Mediterranean, to see if any characteristics are shared by specific serotypes, they added.

 

You can read the full article in Nature.