D-072. Immune Response to Staphylococcus aureus Biofilm Infections

R. Prabhakara1, J. G. Leid2, J. W. Costerton3, M. E. Shirtliff1;
1Univ. of Maryland, Baltimore, MD, 2Northern Arizona Univ., Flagstaff, AZ, 3Univ. of Southern California, Los Angeles, CA.

Staphylococcus aureus is one of the most common etiological agents of nosocomial infections, including potentially life-threatening prosthetic implant infections and osteomyelitis. S. aureus possesses several virulence factors, including the ability to form a biofilm, which allow it to successfully evade the host immune response and cause chronic disease. The goal of this study is to further elucidate the range of host-pathogen interactions that take place during S. aureus biofilm infection. We hypothesize that S. aureus skews the host immune response away from a protective Th2 type response, in which opsonizing antibodies would normally prevent progression of an early biofilm to a mature biofilm infection that the immune system cannot clear. Instead, S. aureus promotes a robust but ineffective Th1 response. To test this hypothesis, we used a mouse tibial implant model of S. aureus infection, in which the left tibias of C57BL/6J mice were surgically implanted with stainless steel pins coated with methicillin-resistant S. aureus. Tibias from infected and uninfected control mice were removed at 14 and 28 days post-infection and homogenized. Supernatants from bone homogenates were used to determine CFUs/g bone and secretion levels of cytokines and chemokines, including IL-1β, IL-2, IL-4, IL-5, IL-6, IL-10, IL-12, IL-17, IFNγ, TNFα, KC, MIP-1α, MIP-1β, and GMCSF, using protein arrays. All infected mice survived the duration of these experiments and no sepsis developed. However, S. aureus was recovered from site of pin implantation in infected mice at both 14 days (4.4 x 106 CFU/g) and 28 days (5.2 x 106 CFU/g) after infection. The number of CFUs/g bone did not differ significantly in bones harvested at 14 and 28 days post-infection, demonstrating the chronic nature of the infection. Protein array data indicated that the tibias of infected mice contained significantly higher levels of the Th1-associated cytokines IL-2, IFNγ, and MIP-1β at day 14 post-infection. In summary, we have shown that chronic S. aureus biofilm-mediated infection elicits mainly Th1 type cytokines and results in insufficient bacterial clearance by the host.