Aerococcus christensenii: an emerging pathogen associated with infections and bacteremia in pregnancy-genomic insights and pathogenicity evaluation.

Aerococcus christensenii (A. christensenii) is a symbiotic bacterium that primarily colonizes the vagina. Infections caused by A. christensenii are rare but can also pose a significant health threat. In this study, two rare cases of A. christensenii bacteremia in pregnant women complicated with chorioamnionitis were investigated; and two strains KSW23 and KWL24, which were isolated from blood samples, were analyzed for their genomic characteristics and pathogenic potential. Whole-genome sequencing revealed that the genome sizes of KSW23 and KWL24 were approximately 1.6 Mb, and predicted multiple genes associated with pathogenicity (tuf, eno, plr/gapA, galU, galE, groEL, gndA, sugC, lplA1, mgtB, clpC, clpP, and lmb), antibiotic resistance (ermB and tet(M)), and mobile genetic elements (plasmid replicon repUS43 and transposon Tn6009). Correspondingly, these strains showed multidrug resistance to Macrolides, Lincosamides, and Tetracyclines. Pangenome analysis revealed close evolutionary relationships and significant genomic conservation between these two strains and the previously isolated strains, especially with respect to genes related to pathogenicity and antibiotic resistance. Notably, a mouse bacteremia model confirmed the pathogenicity and virulence of A. christensenii strains KSW23 and KWL24, which induced bacteremia and mortality, but not as strongly as Staphylococcus aureus (S. aureus) strain ATCC25923. Additionally, A. christensenii exhibited a robust survival ability in human blood comparable to those observed in S. aureus strain ATCC25923. To our knowledge, this study is the first genomic research on A. christensenii, and confirms the species' bloodstream invasive capacity and pathogenicity based on genomic studies and experimental validation. These findings underscore its role as a pathogen in the ascending genital tract in the obstetric population.