Bats possess unique physiological adaptations, including thermophysiological flexibility, which enable them to tolerate and harbor diverse pathogens without overt disease. Their ability to sustain powered flight and enter torpor or hibernation contributes to their immune resilience, making them important reservoirs for zoonotic pathogens such as viruses (rabies, Ebola, coronaviruses) and bacteria (Leptospira, Salmonella, Bartonella, Rickettsia). Bats' dense social roosting and migratory behaviors facilitate pathogen transmission, underscoring the need for effective surveillance. This study examined the blood microbiomes of five bat species from the Vespertilionidae and Rhinolophidae families inhabiting three caves in northern Bulgaria (Devetashka, Orlova, and Parnicite). Using pooled blood samples (n=120) and two bat hearts, DNA was extracted for microbiome analysis via shotgun metagenomic sequencing, alongside PCR-based detection of human pathogens. Bacterial phyla and genera distribution varied significantly among bat species, with prominent genera including Bacillus, Mycoplasma, Pseudomonas, and Sphingobacterium. Alpha diversity, measured by the Simpson index, revealed slightly higher microbial diversity in Miniopterus schreibersii and Myotis myotis, with no significant differences among cave populations. Principal Coordinates Analysis (PCoA) highlighted distinct microbial community compositions between bat species and cave locations. PCR results identified Mycobacterium in 26 of 34 blood samples and in 1 of 2 heart tissue samples, suggesting its prevalence in bat populations. No samples tested positive for Mycobacterium tuberculosis, Brucella spp., Coxiella burnetii, or Toxoplasma gondii, indicating the absence of these pathogens in the sampled individuals. The detection of atypical Mycobacteria and Brucella sp. in the bats' blood emphasizes its potential role as an opportunistic pathogen and the need for further investigation into its zoonotic significance. These findings highlight the diversity and zoonotic potential of bat-associated microbiomes, supporting the need for continued pathogen surveillance in bat populations. All metagenomic shotgun sequencing data were published at NCBI Sequence Read Archive under the bioproject number PRJNA1215671.
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