Virulence and transmission biology of the widespread, ecologically important pathogen of zooplankton,Spirobacillus cienkowskii

Spirobacillus cienkowskii ( Spirobacillus, hereafter) is a widely distributed bacterial pathogen that has significant impacts on the population dynamics of zooplankton ( Daphnia spp. ), particularly in months when Daphnia are asexually reproducing. Yet little is known about Spirobacillus’ virulence, transmission mode and dynamics. As a result, we cannot explain the dynamics of Spirobacillus epidemics in nature or use Spirobacillus as a model pathogen, despite Daphnia’s tractability as a model-host. Here, we work to fill these knowledge gaps experimentally. We found that Spirobacillus is among the most virulent of Daphnia pathogens, killing its host within a week and reducing host fecundity. We further found that Spirobacillus did not transmit horizontally among hosts unless the host died or was destroyed (i.e., it is an “obligate killer”). In experiments aimed at quantifying the dynamics of horizontal transmission among asexually reproducing Daphnia , we demonstrated that Spirobacillus transmits poorly in the laboratory. In mesocosms, Spirobacillus failed to generate epidemics; in experiments wherein individual Daphnia were exposed, Spirobacillus’ transmission success was low. In the (limited) set of conditions we considered, Spirobacillus’ transmission success did not change with host density or pathogen dose and declined following environmental incubation. Lastly, we conducted a field survey of Spirobacillus’ prevalence within egg-cases (ephippia) made by sexually reproducing Daphnia . We found Spirobacillus DNA in ∼40% of ephippia, suggesting that, in addition to transmitting horizontally among asexually reproducing Daphnia , Spirobacillus may transmit vertically from sexually reproducing Daphnia . Our work fills critical gaps in the biology of Spirobacillus and illuminates new hypotheses vis-à-vis its life-history. Importance Spirobacillus cienkowskii is a bacterial pathogen of zooplankton, first described in the 19 th Century and recently placed in a new family of bacteria, the Silvanigrellaceae . Spirobacillus causes epidemics in lake zooplankton populations and increases the probability that zooplankton will be eaten by predators. However, little is known about how Spirobacillus transmits among hosts, its impact on host survival and reproduction (i.e., how virulent it is) in laboratory conditions and what role virulence plays in Spirobacillus’ life cycle. Here, we experimentally quantified Spirobacillus ’ virulence and showed that Spirobacillus must kill its host to transmit horizontally. We also found evidence that Spirobacillus may transmit vertically via Daphnia ’s seed-like egg cases. Our work will help scientists to (i) understand Spirobacillus epidemics, (ii) use Spirobacillus as a model pathogen for the study of host-parasite interactions and (iii) better understand the unusual group of bacteria to which Spirobacillus belongs.