First fatal human bloodstream infection caused by Macrococcus caseolyticus subsp. caseolyticus in China: genomic insights into virulence and antimicrobial resistance.

BACKGROUND: Macrococcus caseolyticus is a Gram-positive bacterium mainly isolated from dairy products and animal skin and is usually associated with animal infections. M. caseolyticus comprises two subspecies, M. caseolyticus subsp. caseolyticus and M. caseolyticus subsp. hominis. Here, we report a fatal bloodstream infection caused by M. caseolyticus subsp. caseolyticus in China and characterize the clinical isolate by genomic and phenotypic analyses.

METHODS: Strain WH712 was isolated from the blood culture of a 91-year-old male patient and identified by matrix-assisted laser desorption ionization/time-of-flight mass spectrometry. Whole-genome sequencing was performed using the Illumina HiSeq platform. Antimicrobial susceptibility testing was conducted using the AutoMic-i600 system, and biofilm-forming capacity was evaluated using a 96-well microtiter plate assay.

RESULTS: WH712 was initially identified as M. caseolyticus by MALDI-TOF MS and confirmed as M. caseolyticus subsp. caseolyticus by whole-genome analysis. Homology-based genomic analysis identified putative homologs of bopD, ami, srtE, and pfbA, which have been associated in other bacteria with biofilm-related regulation, adhesion, and cell-surface functions. These findings suggest that WH712 may harbor genomic features relevant to host colonization and persistence. The isolate was phenotypically susceptible to most tested antibiotics but showed intermediate resistance to clindamycin, possibly associated with a vgaB homolog and efflux-related homologs, including macB and sdrM. Phenotypic testing demonstrated that WH712 had strong biofilm-forming capacity, comparable to that of Staphylococcus aureus ATCC 29213, which may have contributed to bacterial persistence and the poor clinical outcome.

CONCLUSIONS: This study reports a fatal human bloodstream infection associated with M. caseolyticus subsp. caseolyticus in China. Whole-genome analysis and phenotypic testing showed that this clinical isolate had a strong biofilm-forming phenotype, which may have contributed to bacterial persistence. However, the poor clinical outcome should be interpreted primarily in the context of the patient's advanced age, severe comorbidities, and impaired host defenses.