Limitations of Murine Models for Assessment of Antibody-Mediated Therapies or Vaccine Candidates against Staphylococcus epidermidis Bloodstream Infection.

Staphylococcus epidermidis is normally a commensal colonizer of human skin and mucus membranes, but, due to its ability to form biofilms on indwelling medical devices, it has emerged as a leading cause of nosocomial infections. Bacteremia or bloodstream infection is a frequent and costly complication resulting from biofilm fouling of medical devices. Our goal was to develop a murine model of S. epidermidis infection to identify potential vaccine targets for the prevention of S. epidermidis bacteremia. However, assessing the contribution of adaptive immunity to protection against S. epidermidis challenge was complicated by a highly efficacious innate immune response in mice. Naive mice rapidly cleared S. epidermidis infections from blood and solid organs, even when the animals were immunocompromised. Cyclophosphamide-mediated leukopenia reduced the size of the bacterial challenge dose required to cause lethality but did not impair clearance after a nonlethal challenge. Nonspecific innate immune stimulation, such as treatment with a Toll-like receptor 4 (TLR4) agonist, enhanced bacterial clearance. TLR2 signaling was confirmed to accelerate the clearance of S. epidermidis bacteremia, but TLR2(-/-)mice could still resolve a bloodstream infection. Furthermore, TLR2 signaling played no role in the clearance of bacteria from the spleen. In conclusion, these data suggest that S. epidermidis bloodstream infection is cleared in a highly efficient manner that is mediated by both TLR2-dependent and -independent innate immune mechanisms. The inability to establish a persistent infection in mice, even in immunocompromised animals, rendered these murine models unsuitable for meaningful assessment of antibody-mediated therapies or vaccine candidates.

Antibodies to PhnD inhibit staphylococcal biofilms.

Biofilm formation on central lines or peripheral catheters is a serious threat to patient well-being. Contaminated vascular devices can act as a nidus for bloodstream infection and systemic pathogen dissemination. Staphylococcal biofilms are the most common cause of central-line-associated bloodstream infections, and antibiotic resistance makes them difficult to treat. As an alternative to antibiotic intervention, we sought to identify anti-staphylococcal biofilm targets for the development of a vaccine or antibody prophylactic. A screening strategy was devised using a microfluidic system to test antibody-mediated biofilm inhibition under biologically relevant conditions of shear flow. Affinity-purified polyclonal antibodies to target antigen PhnD inhibited both Staphylococcus epidermidis and S. aureus biofilms. PhnD-specific antibodies blocked biofilm development at the initial attachment and aggregation stages, and deletion of phnD inhibited normal biofilm formation. We further adapted our microfluidic biofilm system to monitor the interaction of human neutrophils with staphylococcal biofilms and demonstrated that PhnD-specific antibodies also serve as opsonins to enhance neutrophil binding, motility, and biofilm engulfment. These data support the identification of PhnD as a lead target for biofilm intervention strategies performed either by vaccination or through passive administration of antibodies.