Genomic and Pathogenic Characterization of a Novel Capsule-Deficient Neonatal Meningitis-Associated Escherichia coli from Calves.

Neonatal meningitis-associated Escherichia coli (NMEC) is a formidable pathogen in veterinary medicine. The emergence of atypical, multidrug-resistant (MDR) variants complicates disease control. An Escherichia coli (E. coli) strain was isolated from the brain tissue of a deceased calf with acute meningitis. Comprehensive characterizations were performed, including whole-genome sequencing (WGS), multi-locus sequence typing (MLST), antimicrobial susceptibility testing (AST), murine pathogenicity assays, and RT-qPCR evaluation of neuroinflammatory cytokines. Results: The isolate (O18ab:H14) was identified as a capsule-deficient NMEC strain belonging to phylogroup A and sequence type ST1434. WGS showed that the genome size of this strain is 5.1 Mb, containing 73 strictly defined antimicrobial resistance genes and 202 virulence factors. These may be involved in the compensatory mechanism for capsule deficiency, and further functional verification is required. Phenotypically, it exhibited a robust MDR profile. In the murine model, the strain demonstrated high lethality, and induced severe multi-organ lesions characteristic of both meningitis and systemic sepsis. While intraperitoneal injection bypasses natural colonization routes, the brain-specific bacterial persistence and neuronal pathology imply neurotropic potential. Furthermore, RT-qPCR confirmed a severe neuroinflammatory response, marked by the significant upregulation of IL-1β, IL-6, and TNF-α in the infected brains. This study characterizes a novel, highly virulent, and MDR capsule-deficient NMEC/SEPEC hybrid strain. The findings emphasize the urgent need for continuous genomic surveillance of atypical E. coli pathotypes in livestock.