Gaining insights into novel pathogen hosts: Characterisation of Actinomyces weissii strains isolated from companion animals.

Actinomyces weissii was first isolated from the oral cavity of dogs in 2012. This study characterised the phenotypic and genotypic features of 11 additional strains obtained from diverse pathological lesions in companion animals. A multi-faceted approach was employed, combining culture techniques, biochemical profiling, MALDI-TOF MS, real-time PCR, sequencing of genetic markers, and whole-genome analysis. Despite minor phenotypic variation, MALDI-TOF MS and genotypic analyses consistently confirmed species identity and revealed intraspecies diversity. Whole-genome sequencing revealed four haemolysin family protein genes, and all isolates exhibited complete β-haemolysis. Pan-genome analysis defined a conserved core genome of 1559 genes, shared by all isolates and the reference strain, including haemolysin-related genes. Phylogenetic comparisons placed A. urogenitalis and A. trachealis as closest relatives. These findings broaden the host spectrum of A. weissii, with the first isolation from a cat, suggesting that this species may also occur in felines and merits consideration in veterinary diagnostics. Its presence in companion animals raises the possibility of zoonotic risk, and together with the detection of haemolysin family protein genes, underscores the need for further investigation to clarify its pathogenic significance and potential impact on veterinary practice and public health.