Vikash Kumar, Basanta Kumar Das, Suvra Roy, Pratyasha Bhowal, Arpita Roy, Timothy J. Bruce, Jorge Galindo-Villegas
Multidrug-resistant (MDR) bacterial pathogens represent an escalating challenge to sustainable aquaculture, particularly in high-value freshwater species such as Labeo rohita , a cornerstone of South Asian aquaculture. This study provides the first comprehensive integration of genomic, immunological, and microbiome analyses to characterize Proteus penneri as an emerging MDR pathogen associated with severe disease manifestations in L. rohita , including exophthalmia, ulceration, and hemorrhage. Robust identification through biochemical assays, 16S rRNA sequencing, and phylogenetic analysis confirms the clinical relevance of this isolate. Functional assays demonstrated pronounced virulence, evidenced by hemolysin activity, extensive histopathological damage, and dose-dependent mortality, underscoring its pathogenic capacity in vivo . The observed resistance to multiple frontline antibiotic classes, including tetracyclines, macrolides, and carbapenems, highlights a critical therapeutic limitation in aquaculture settings. Genomic analysis further revealed a diverse repertoire of antimicrobial resistance genes, virulence determinants (notably biofilm formation and secretion systems), and mobile genetic elements, suggesting a strong potential for persistence, adaptability, and horizontal gene transfer. Infection-associated gut microbiome disruption, marked by elevated MAR indices and enrichment of virulence-associated taxa, indicates that P. penneri not only exploits host tissues but also reshapes the microbial ecosystem in ways that may exacerbate disease severity and resistance dissemination. Concurrently, heightened serum cortisol, C3 , and Hsp70 levels, along with transcriptional upregulation of key immune and stress-related genes ( hsp70, nod, il6, sod, c3 , and myd88 ), reflect an intense pro-inflammatory and physiological stress response. In silico docking analyses implicating myd88 –lipopolysaccharide interactions provide mechanistic insight into potential immune-modulatory strategies employed by the pathogen. Collectively, these findings delineate a multifactorial basis for P. penneri virulence and MDR, emphasizing its significance as an emerging aquaculture pathogen. Future research should prioritize functional validation of key virulence and resistance genes, longitudinal surveillance to assess transmission dynamics and AMR spread, and experimental evaluation of alternative disease mitigation strategies, including probiotics, phage therapy, and immune-modulating interventions, to reduce antibiotic reliance and enhance fish health resilience in aquaculture systems.