Periodic Report Summary 1 - FLEAMICROBIOME (Effects of ecological factors on bacterial communities of fleas) The identification of potential ecological factors that affect the dynamics between hosts, arthropod vectors, and their microbes could have a marked impact on our ability to prevent and control vector-borne diseases. Recent studies have suggested that interactions among microbes within the individual vector/host may affect the density of a pathogen, and its transmission rate and virulence to the host. However, factors that affect the microbial composition of vectors or vertebrate hosts remain obscure. The goal of the first research period was to elucidate the effects of host age on the microbial diversity and composition of vectors. During field work, we have decided to broaden the scope of the study by investigating the effects of host age relative to other ecological factors on the symbiont composition of hosts, including different bacterial genotypes/species, fleas and ticks. We have collected fleas, ticks, and blood samples from tagged juveniles, non-reproductive adults, and reproductive females on two occasions, at the onset of the rodents’ reproduction season and at the end of summer, when all individuals have reached adulthood. The pyrosequencing analysis has revealed two most common genera in the blood and flea samples: Bartonella spp. and Mycoplasma, a lineage closely related to M. haemomuris. During the reproductive season, the probability of Mycoplasma infection and flea burden were higher for adults than for juvenile hosts whereas the probability of infection by Bartonella was higher for juvenile than for adults. However, host age was only a weak predictor of tick abundance and of the temporal variability in symbiont community composition. In contrast to the inter-genus diversity, intra-genus diversity of Bartonella bacteria was not affected by host age. The only association between host age, white blood cell measurements and symbionts was an increase in neutrophil to lymphocyte ratio in Mycoplasma-positive adults.The study adds at least three new insights to our current knowledge of age effects. It demonstrates that: (i) even age effects that are not associated with diet shifts or antibiotic can be significant relative to other ecological factors, but the relative role of host age in shaping symbiotic communities varies with respect to the scale (i.e. across host individuals versus time, inter- versus intra-genus diversity) and life history of the symbiont, (ii) age-related effects are likely to operate via different mechanisms, depending on the life history of the symbiont and its association with the host, and (iii) immunity may be only a minor mediator of the association between host age and symbiont composition. The results are applicable for wildlife management plans and for human disease risk assessments, as they provide the first comprehensive description of bacterial communities, including pathogens, in rodents, fleas and ticks in Israel and shed insights on when and where to direct managment efforts in natural communities of rodents. The current study has already contributed to my career development as it helped me to establish my research group as well as intra- and inter-national collaborations. It will also improve my publication record, as two manuscripts are already under review at high rank international journals.