Species identification, antibiotic resistance, and virulence in complex clinical isolates from South Korea.

This study aimed to identify the species of complex (ECC) isolates and compare the genotype, antibiotic resistance, and virulence among them. A total of 183 ECC isolates were collected from patients in eight hospitals in South Korea. Based on partial sequences of and phylogenetic analysis, all ECC isolates were identified as nine species and six subspecies. was the predominant species (47.0%), followed by , , , and . Multilocus sequence typing analysis revealed that dissemination was not limited to a few clones, but subsp. , subsp. , and formed large clonal complexes. Antibiotic resistance rates were different between the ECC species. In particular, , , , and isolates were highly resistant to colistin, whereas most and isolates were susceptible to colistin. Virulence was evaluated through serum bactericidal assay and the larvae infection model. Consistency in the results between the serum resistance and the larvae infection assay was observed. Serum bactericidal assay showed that , , and were significantly more virulent than and . In this study, we identified the predominant ECC species in South Korea and observed the differences in antibiotic resistance and virulence between the species. Our findings suggest that correct species identification, as well as continuous monitoring is crucial in clinical settings.