Isolation and Genetic Characterization of Streptococcus iniae Virulence Factors in Adriatic Sturgeon (Acipenser naccarii)

Abstract

The first case of infection of Streptococcus iniae in Adriatic sturgeon (Acipenser naccarii) was recently reported in a raceway system located in Northern Italy. A second episode of infection in sturgeons with absence of mortality and evident clinical signs, was registered in November 2020 in the same farm and is reported in this study. Histopathological changes observed in infected organs are described. The strains isolated in the two episodes were compared using molecular analysis based on PCR, phylogeny and virulence factors analysis. Not all the major virulence factors were detected for the two strains; in particular the strains 78697, isolated in November, lacks cpsD, compared to the strains 64844, isolated in September. Moreover, genetic variations were reported for lctO and pmg genes. These findings let us hypothesize a different virulence of the strains in accordance with clinical findings related to the sturgeons.

Keywords: Acipenseridae; bacterial diseases; streptococcosis; virulence factors.

Figure 1

Liver: Mild (A), moderate (B), and severe (C) granulomatous chronic hepatitis cantered around the bile ducts and arterial vessels (asterisk, (B)). Granulomas consisted of a high number of macrophages, a moderate number of lymphocytes and plasma cells, scattered eosinophilic granular cells (inset (B)), and occasionally 80–100 µm, round, multinucleated cells with round peripherally-located nuclei with dispersed chromatin and granular eosinophilic cytoplasm (Langhans-type cells) (arrow, inset (C)). H.E. Bar (A–C) = 100 µm, inset 10 µm.

Figure 2

Spleen: Mild (A), moderate (B), and severe (C) granulomatous chronic splenitis mainly involving the red pulp (asterisk (C)), constituted of a high number of activated macrophages. H.E. Bar (A–C) = 100 µm.

Figure 3

Intestine. Mild (A) and severe (B,C) granulomatous chronic enteritis. (C): High magnification, Table 2. (B) showing a high number of activated macrophages located in the serosa and the outer muscular layer. H.E. Bar (A–C) = 100 µm.

Figure 4

Phylogenetic relationship based on 16S rRNA gene sequences of S. iniae isolated from sturgeon with other NCBI reported sequences of Streptococcus. The phylogenetic tree was constructed using MEGA 7 and the neighbor-joining method.