Characterizing potential bacterial pathogens of Pisaster ochraceus sea stars with wasting disease

In 2013 sea star wasting disease (SSWD) caused an epizootic in over 20 species of asteroids along the west coast of North America. To see if SSWD was still affecting wild populations, we surveyed populations of the sea star, Pisaster ochraceus, in Birch Bay, WA. Our surveys indicated that advanced symptoms (lesion formation) increased 73% from July to September in 2017. To understand the role of bacteria in SSWD, we isolated bacteria with tissue-degrading potential from epidermal tissues of P. ochraceus animals in Birch Bay, WA. Next, we identified these isolates via 16S rRNA gene sequencing. Our results indicated that the community structure differed between SSWD-affected and healthy stars. In addition, we found that many of the Vibrio species isolated primarily from diseased sea stars lyse red blood cells, avidly degrade gelatin, and are related to species that cause disease in other marine animals. Other researchers have suggested that SSWD is caused by a microbial agent smaller than most bacterial cells (0.2 microns). Although none of the bacteria we isolated were smaller than 0.2 microns, we theorized that their signaling molecules could transmit disease. Quorum sensing signals (QSSs) are bacterially-produced, autoinducing molecules that regulate gene expression in a population-dependent manner within bacterial populations. We hypothesized that quorum sensing signals from diseased animals could facilitate development of SSWD on healthy individuals by triggering expression of virulence genes in opportunistic bacterial pathogens creating a state of dysbiosis. We tested bacterial isolates and tissue extracts from diseased and healthy sea stars for the presence of a subset of QSSs, acyl homoserine lactones (AHLs). We found that organic extracts from healthy and diseased stars differed in how strongly they activated a QSS bioreporter, and that multiple of the bacterial isolates associating with diseased stars produced AHLs. Thus, quorum sensing might contribute to transmission of SSWD.