Peribacillus suis sp. nov. Isolated From the Pig Louse Haematopinus suis Reveals Unexpected Pathogenic Potential in a...

The pig sucking louse Haematopinus suis is a major swine ectoparasite and vector of pathogens, yet its microbiome remains understudied. Here, we described the isolation of a previously unrecognized bacterial strain, P8-9^T, from the intestinal tract of H. suis. A comprehensive polyphasic analysis, including phenotypic characterization, phylogenomics, and whole-genome comparisons, placed this isolate within Peribacillus. Genome-based metrics confirmed its novelty, with average nucleotide identity and digital DNA-DNA hybridization values (<80% and <27%, respectively) falling far below accepted species delineation thresholds. We, therefore, proposed the designation Peribacillus suis sp. nov. Unexpectedly, the genome of P8-9^T encodes an extensive repertoire of 219 putative virulence-associated and antibiotic resistance genes; features atypical for a largely composed of environmental species considered saprophytic or beneficial. In vivo experiments revealed this pathogenic potential: intraperitoneal inoculation in mice resulted in all animals reaching predefined humane endpoints within 24 h, characterized by septicemia and widespread organ pathology, while oral exposure elicited splenomegaly, intestinal pathology, and a robust pro-inflammatory response. This work represents the first report of a Peribacillus species isolated from an ectoparasite and provides direct experimental evidence of virulence within a genus traditionally viewed as benign.