Genomic characterisation of a novel species of Erysipelothrix associated with mortalities among endangered seabirds

Infectious diseases threaten endangered species, particularly in small isolated populations. Seabird populations on the remote Amsterdam Island in the Indian Ocean have been in decline for the past three decades, with avian cholera caused by Pasteurella multocida proposed as the primary driver. However, Erysipelothrix spp. has also been sporadically detected from albatrosses on Amsterdam Island and may be contributing to some of the observed mortality. In this study, we genomically characterised 16 Erysipelothrix spp. isolates obtained from three Indian yellow-nosed albatross chick carcasses in 2019. Two isolates were sequenced using both Illumina short-read and MinION long-read approaches, which - following hybrid assembly - resulted in closed circular genomes. Mapping of Illumina reads from the remaining isolates to one of these new reference genomes revealed that all 16 isolates were closely related, with a maximum of 13 nucleotide differences distinguishing any pair of isolates. The nucleotide diversity of isolates obtained from the same or different carcasses was similar, suggesting all three chicks were likely infected from a common source. These genomes were compared with a global collection of genomes from E. rhusiopathiae and other species from the same genus. The isolates from albatrosses were phylogenetically distinct, sharing a most recent common ancestor with E. rhusiopathiae. Based on phylogenomic analysis and standard thresholds for average nucleotide identity and digital DNA-DNA hybridisation, these isolates represent a novel Erysipelothrix species, for which we propose the name Erysipelothrix amsterdamensis sp. nov. The type strain is E. amsterdamensis A18Y020dT. The implications of this bacterium for albatross conservation will require further study.