This study aims to generate the first complete genome of Pantoea septica and provide a thorough genomic characterisation of this under-documented species. The study seeks to enhance understanding of P. septica, clarifying features relevant to opportunistic infection in vulnerable cohorts.

P. septica GABEPS69 was an opportunistic coloniser isolated from the saliva of a patient prescribed the antipsychotic clozapine, leading to a dysbiotic oral microbiome. A hybrid sequencing approach yielded a closed genome comprising a 4.1 Mb chromosome and six plasmids. Phenotypic susceptibility was determined by disk-diffusion and minimum inhibitory concentration (MIC) assays. Its chromosomal and plasmidic content was bioinformatically analysed alongside all canonical GenBank available P. septica genomes and the type strains of taxonomic neighbours Pantoea piersonii and "Pantoea latae", with focus on virulence-factors (VFs), antimicrobial-resistance-genes (ARGs), metal-resistance-genes (MRGs) and biosynthetic gene clusters.

GABEPS69 exhibited a narrow resistance spectrum, displaying resistance to the third-generation cephalosporin cefpodoxime. Plasmid pGABEPS69_1 harboured an aerobactin pathogenicity island homologue; a locus implicated in enhanced virulence, that was also identified across most other P. septica genomes and in the closely related human-pathogen Pantoea piersonii. A conserved chromosomal class-A β-lactamase homologue was also identified. Additionally, a universal presence of bioactive thiopeptide biosynthetic-gene-clusters was observed in P. septica genomes, suggesting a potential role in microbiome modulation.

This study presents a first complete genome of P. septica, revealing its genomic architecture, resistance, and virulence potential. Detailed plasmid analysis and comparative genomics enhance our understanding of the species clinical relevance and microbiome-modulating capacity. These findings motivate surveillance of transient oral microbiota in at-risk populations, including patients receiving clozapine.