Bartonella transmission and gut microbiome dynamics in Ceratophyllus sciurorum fleas and their edible dormouse hosts...

The genus Bartonella comprises facultative intraerythrocytic bacteria capable of causing long-lasting bacteremia in their natural hosts, with zoonotic potential across multiple species. Rodents serve as important reservoirs for a broad diversity of Bartonella spp., with blood-feeding arthropods, particularly fleas, mediating transmission. Despite their frequent association with humans, the role of edible dormice (Glis glis) and their fleas (Ceratophyllus sciurorum) in Bartonella ecology remains poorly understood.

We combined long-term ecological and epidemiological data with gut and body microbiome analyses of C. sciurorum to investigate the prevalence, diversity, host specificity, and transmission of Bartonella across flea life stages. The study was conducted over 6 years in a natural dormouse population. Bartonella detection and characterization were performed using multilocus PCR targeting gltA, rpoB, ftsZ, and ITS loci, bacterial cultivation on selective media, and long-read nanopore sequencing. Flea gut microbiomes were assessed in pooled and individual samples to determine the impact of Bartonella infection on microbial community structure. Transmission across flea life stages was evaluated by analyzing larvae, newly emerged adult fleas, and adults directly collected from dormice.

We observed a consistently high prevalence of Bartonella in both dormice and their fleas. Four species were identified: B. gliris and B. grahamii subsp. shimonis dominated, each represented by multiple genotypes, whereas B. washoensis and B. bilalgolemii were each detected in a single flea. Moreover, mixed infections of B. gliris and B. grahamii subsp. shimonis were frequent in both dormice and their fleas. Detection of Bartonella DNA in flea larvae and newly emerged adults indicates possible transstadial perpetuation. Flea gut microbiomes were highly variable but consistently dominated by Bartonella in infected fleas. In contrast, uninfected fleas exhibited more diverse communities, often enriched with Staphylococcus and other environmental or host-associated taxa. We, therefore, suggest that flea populations can maintain Bartonella over extended periods, even in the absence of continuous contact with vertebrate hosts.

The dormouse-flea-Bartonella system represents a valuable natural model for studying flea-borne zoonoses. The high prevalence and persistent bacteremia in dormice of different ages indicate their role as suitable reservoirs for at least two Bartonella species. Flea populations are capable of sustaining Bartonella over long periods, and detections in immature life stages suggest continuity of infection across flea generations. Therefore, C. sciurorum appears to play an important role in maintaining B. grahamii subsp. shimonis and B. gliris in nature. Infection is consistently associated with pronounced restructuring of the flea gut microbiome, highlighting the ecological and microbial dimensions of vector-borne pathogen transmission. These findings underscore the importance of considering vector-associated microbial communities when evaluating disease dynamics and zoonotic risk in natural host-vector systems.