Ornithinibacillus liuyingii sp. nov.: a novel multidrug-resistant and potentially pathogenic bacterium isolated from Apodemus chevrieri lung tissue.

Ornithinibacillus liuyingii sp. nov. was proposed, with type strain 27F3T (= GDMCC 1.5626T = JCM 38003T) isolated from the lung of Apodemus chevrieri collected on the Guizhou Plateau, China. Phylogenetic distinctiveness from its closest relative, Ornithinibacillus scapharcae TW25T, was confirmed by 98.5% 16S rRNA gene similarity, 76.98% average nucleotide identity, and 21.9% digital DNA–DNA hybridization value. Phenotypically, strain 27F3T was characterized as a motile, rod-shaped bacterium exhibiting a Gram-stain-negative reaction, an atypical feature within the otherwise Gram-positive genus Ornithinibacillus. The strain formed greyish-light purple colonies and displayed a distinct antimicrobial resistance profile, including resistance to cefepime, polymyxin E, and nalidixic acid. Chemotaxonomic analysis revealed iso-C15:0, anteiso-C15:0, and iso-C17:0 as major fatty acids and menaquinone MK-7 as the predominant respiratory quinone. Genomic analysis identified genes associated with sporulation and oxidative stress defense, and numerous homologs of virulence-associated factors were detected. However, 16S rRNA gene amplicon sequencing of 90 additional rodent lungs did not detect close relatives (max identity 95.8%), suggesting that O. liuyingii should not be a common colonizer. This study describes the Ornithinibacillus species isolated from a mammalian respiratory tract and highlights its atypical Gram-stain reaction, which may contributing to our understanding into the diversity of the genus. The discovery may underscore the hidden complexity within wildlife-associated microbiota and emphasizes the importance of pathogen surveillance in a One Health context.