Data from: 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.