Western CEDAR

Potential bacterial pathogens of P. ochraceus stars suffering from sea star wasting disease and the role of quorum sensing activity in disease development

Sea star wasting disease (SSWD) affects asteroids across an unprecedented spatial, temporal, and taxonomic range. The symptoms begin with limb twisting and turgor loss; next, their tissues begin to decay, leading to fragmentation and death. Our population surveys of the sea star, Pisaster ochraceus, in Birch Bay, WA indicate that SSWD still affects P. ochraceus stars in the wild and increases in severity from early to late summer. These animals are keystone species; therefore, SSWD could lead to severe ecological changes if populations continue to be diminished by reduced fitness and mortalities. Previous studies suggest that SSWD is likely caused by a microbial agent, but so far, a pathogen has not been conclusively identified. We first sought to characterize potential bacterial pathogens. We isolated bacteria with tissue-degrading potential from diseased and healthy P. ochraceus at two different times from the same population. These bacterial isolates were identified to the genus level via 16S rRNA gene sequencing. Our results indicated that although the class Gammaproteobacteria dominated our isolates from both healthy and sick animals, the community structure differed at the genus level. While the different bacterial community structure of sick sea stars might in part represent an influx of saprophytic bacteria, the community structure also differed between unaffected tissues of diseased stars and healthy stars. Living, healthy tissues and unaffected tissues should not attract bacteria whose lifestyle is mainly saprophytic. Rather, the microbiome changes are likely selected for at the level of the entire animal rather than just within lesions (decaying tissues). In addition to being present, a potential pathogen must express virulence genes, which are often regulated by quorum sensing. To explore the role of quorum sensing signals in SSWD, we extracted tissue homogenates of both diseased and healthy sea stars with acidified ethyl acetate. We then used an Agrobacterium tumefaciens bioreporter bearing PT7-traR fused to lacZ. This bioreporter combines a strong promoter with a promiscuous LuxR homolog able to detect AHLs of different chain lengths, saturation levels, and oxidation states. This bioreporter, therefore, can detect crude differences in the amount of total AHLs from our samples without having identified these signals. We found that extracts from healthy and diseased stars differed in how strongly they activated lacZ in our reporter strain. Altogether, these results suggest that the community structure of sea star associated bacteria changes when animals develop SSWD. Intriguingly, these results also suggest that quorum sensing is associated with the functional shift in sea star-associated bacterial communities towards pathogenicity.