Analysis of striped bass (Morone saxatilis) and white bass (M. chrysops) spleen transcriptome following Streptococcus iniae infection
Streptococcal disease results in major mortality events of both marine and freshwater fishes worldwide. Streptococcus iniae is among the prominent causative bacterial strains as it has been found to cause a higher incidence of mortality and act as a zoonotic pathogen. Here we examine the susceptibility of two important aquaculture species in the United States, striped bass (Morone saxatilis) and white bass (Morone chrysops), to S. iniae. A high incidence of mortality was observed in both species, although striped bass succumbed more rapidly than white bass. Spleen gene expression at three time points following infection was analyzed to further elucidate the mechanisms underlying these observations. The down-regulation of gene transcripts associated with pathogen detection (tlr1, tlr8, tlr9), antigen processing (cd74a), immune cell recruitment and migration (ccl44, ccr6b, ccr7), macrophage function (csf1ra), T-cell signaling and NF-kB activation (card11, fyna, tirap) was detected in both species. These findings potentially indicate impairment in these critical early immune system processes such that both species were ultimately highly susceptible to S. iniae infection despite the detected up-regulation of transcripts typically associated with effective immune response, such as cytokines (il1ß, il8, il12b2, il17rc, tnfb) and hepcidins (hamp, hamp2). The presented results collectively identify several candidate genes and associated pathways for further investigation to characterize the vulnerability of striped bass and white bass to S. iniae and that may be considered for selective breeding efforts, biotechnological intervention, and/or exploitation in the development of vaccines and alternative treatments. Overall design: Spleen tissue was sampled from 48 indiviudals at four timepoints prior to (t= 0 hr control) or following exposure to Streptococcus iniae  (t= 1 hr, 4 hr, and 24 hr post treatment) (n=24 striped bass, SB, and n=24 white bass, WB, total: 6 of each sampled at each of the four timepoints)