Growing Staphylococcus aureus in Synthetic Cystic Fibrosis Medium Promotes Colonization in a Murine Pneumonia Model

Staphylococcus aureus is a leading cause of bacterial infections worldwide and can lead to diseases such as osteomyelitis, skin, and lung infections in humans. Murine mouse models have been essential to the study of S. aureus virulence but are not without their limitations. In murine pneumonia models, colonization of the lungs by S. aureus are generally not well maintained. To increase the level and duration of S. aureus respiratory colonization, various methods have been employed including embedding the bacteria in agar beads and suppressing the mouses immune system. These modifications have improved colonization, but they do not accurately represent clinical infections of diseases such as cystic fibrosis (CF). We hypothesize that culturing S. aureus in a different media such as Synthetic CF Medium 2 (SCFM2) would increase colonization compared to growing the bacteria on standard rich media agar plates. We observed that culturing 6 different S. aureus strains in SCFM2 led to either a neutral or increased level of lung colonization compared to agar plates. For one strain, WU1, culturing in SCFM2 improved colonization in the oropharynx compared to agar plates and led to a sustained long-term infection in the lungs. Finally, when cultured in SCFM2 compared to agar plates, infection with WU1 led to increased inflammation in both the left and right lung lobe. Overall, we have shown that culturing S. aureus in different conditions prior to infection impacts colonization and host response. IMPORTANCEStaphylococcus aureus is a bacterial pathogen that can infect multiple anatomical sites in humans. To study S. aureus virulence, murine mouse models have been an essential tool. However, it has been difficult to establish and maintain these infections. To improve S. aureus pneumonia murine models, changes to the bacterial dose and mice have been utilized but limits its accuracy to represent clinical infections. In this study, we compared how culturing S. aureus in two different conditions prior to infection impacts colonization. We found that the effects of growing S. aureus in these different media on acute infections is strain specific. Following up with one of these strains, we showed that Synthetic Cystic Fibrosis Medium 2 (SCFM2) increases S. aureus colonization in the oropharynx and leads to a sustain long-term infection in the lungs. Additionally, we showed how culturing S. aureus in these conditions impacts the host response.