Multidrug-Resistant Providencia rettgeri: An Emerging Pathogen in Farmed Siamese Crocodiles (Crocodylus siamensis) in Hainan, China.

The Siamese crocodile (Crocodylus siamensis) farming industry in China, particularly in Hainan Province, has grown substantially, yet it faces increasing threats from infectious diseases. Providencia spp., especially P. rettgeri, are increasingly reported in reptilian disease outbreaks, but their epidemiological and pathogenic profiles in farmed C. siamensis remain poorly understood. In this study, necropsy and histopathological examination of 18 deceased juvenile Siamese crocodiles from three farms in Hainan Province, China, revealed severe multiorgan lesions indicative of septicemia. Sixty-five Providencia isolates were recovered and assigned to four species-level clusters (P. rettgeri, P. stuartii, P. hangzhouensis, and P. alcalifaciens) via 16S rDNA and fusA phylogenetic analyses. Most isolates exhibited strong motility but weak biofilm-forming ability. Crucially, all 65 isolates were multidrug-resistant and carried diverse antimicrobial resistance genes (ARGs), with 74% harboring class 1 integrons. Virulence-associated genes (VGs) were widespread. Zebrafish challenge assays confirmed that all five representative isolates were pathogenic, with P. rettgeri strains PL-425 and PL-688 exhibiting the highest virulence (100% mortality within 24-30 h). Whole-genome sequencing of PL-425 identified multiple virulence loci related to motility, capsule, adhesion, and toxins. These findings identify multidrug-resistant P. rettgeri as a dominant and highly virulent pathogen in farmed Siamese crocodiles, highlighting its emergence as a significant threat to the industry and underscoring the urgent need for integrated surveillance and antimicrobial stewardship.