Streptococcus mitis Strains Causing Severe Clinical Disease in Cancer Patients

The genetically diverse viridans group streptococci (VGS) are increasingly recognized as the cause of a variety of human diseases. We used a recently developed multilocus sequence analysis scheme to define the species. of 118 unique VGS strains causing bacteremia in patients with cancer; Streptococcus mitts (68 patients) and S. oralis (22 patients) were the most frequently identified strains. Compared with patients infected with non S. mitis strains, patients infected with S. mitts strains were more likely to have moderate or severe clinical disease (e.g., VGS shock syndrome). Combined with the sequence data, whole-genome analyses showed that S. mitts strains may more precisely be considered as >= 2 species. Furthermore, we found that multiple S. mitis strains induced disease in neutropenic mice in a dose-dependent fashion. Our data define the prominent clinical effect of the group of organisms currently classified as S. mills and lay the groundwork for increased understanding of this understudied pathogen.