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

Abstract

AbstractSpirobacillus cienkowskii(Spirobacillus,hereafter) is a widely distributed bacterial pathogen of zooplankton (Daphniasp.) that has significant impacts on zooplankton population dynamics. Despite its ecological importance, little is known aboutSpirobacillus’virulence and transmission biology. These knowledge gaps stymie our understanding ofDaphnia-Spirobacillusinteractions and the ecology of the newly described familySilvanigrellaceae,of whichSpirobacillusis the longest studied member. Here, we present our work to fill these gaps, experimentally. We found thatSpirobacillusis among the most virulent ofDaphniapathogens, killing its host within a week of exposure and reducing host fecundity. Our experiments further suggest thatSpirobacillusrequires host death to transmit horizontally i.e., it is an “obligate killer”, since it did not transmit horizontally among hosts unless the host died or was destroyed. In a series of further experiments characterizing the dynamics of horizontal transmission, we found thatSpirobacillus’per capita rate of transmission is low, does not change with host density or pathogen inoculum size, and decreases rapidly with time spent in the environment. Lastly, we complemented our experimental work with a field survey ofSpirobacillus’prevalence within ephippia, seed-like cases containingDaphniaoffspring. We foundSpirobacillusDNA in almost 40% of ephippia, suggesting thatSpirobacillusis vertically transmitted and/or can invade ephippia and use them to endure environmental stress, as their hosts do. Our work fills critical gaps in our understanding of this ecologically important pathogen’s biology.ImportanceSpirobacillus cienkowskiiis a bacterial pathogen of zooplankton, first described in the 19thCentury and recently placed in a new family of bacteria, theSilvanigrellaceae.Spirobacilluscauses large epidemics in lake zooplankton populations and increases the probability that zooplankton will be eaten by predators. However, little is known about howSpirobacillustransmits among hosts, to what extent it reduces host survival and reproduction (i.e., how virulent it is) and what role virulence plays inSpirobacillus’life cycle. Here, we experimentally quantifiedSpirobacillus’ virulence and showed thatSpirobacillusmust kill its host to transmit horizontally. We also found evidence thatSpirobacillusmay transmit vertically viaDaphnia’s seed-like egg sacks. Our work will help scientists to (i) understandSpirobacillusepidemics, (ii) useSpirobacillusas a model pathogen for the study of host-parasite interactions and (iii) better understand the unusual group of bacteria to whichSpirobacillusbelongs.