Identification of Staphylococcus aureus proteins recognized by the antibody-mediated immune response to a biofilm infection

Abstract

Staphylococcus aureus causes persistent, recurrent infections (e.g., osteomyelitis) by forming biofilms. To survey the antibody-mediated immune response and identify those proteins that are immunogenic in an S. aureus biofilm infection, the tibias of rabbits were infected with methicillin-resistant S. aureus to produce chronic osteomyelitis. Sera were collected prior to infection and at 14, 28, and 42 days postinfection. The sera were used to perform Western blot assays on total protein from biofilm grown in vitro and separated by two-dimensional gel electrophoresis. Those proteins recognized by host antibodies in the harvested sera were identified via matrix-assisted laser desorption ionization-time of flight analysis. Using protein from mechanically disrupted total and fractionated biofilm protein samples, we identified 26 and 22 immunogens, respectively. These included a cell surface-associated beta-lactamase, lipoprotein, lipase, autolysin, and an ABC transporter lipoprotein. Studies were also performed using microarray analyses and confirmed the biofilm-specific up-regulation of most of these genes. Therefore, although the biofilm antigens are recognized by the immune system, the biofilm infection can persist. However, these proteins, when delivered as vaccines, may be important in directing the immune system toward an early and effective antibody-mediated response to prevent chronic S. aureus infections. Previous works have identified S. aureus proteins that are immunogenic during acute infections, such as sepsis. However, this is the first work to identify these immunogens during chronic S. aureus biofilm infections and to simultaneously show the global relationship between the antigens expressed during an in vivo infection and the corresponding in vitro transcriptomic and proteomic gene expression levels.

FIG. 1.

In vitro biofilm reactor system. For a detailed description, please refer to Materials and Methods.

FIG. 2.

A representative 2DGE gel and Western blot of mechanically disrupted protein from a day 14 S. aureus biofilm showing 27 immunogenic protein spots (circled). pIs are indicated at the top.

FIG. 3.

Immunogenic proteins are located in all three cellular fractions of biofilm-associated S. aureus. Sera from a rabbit infected intratibially with S. aureus were utilized to visualize immunogens in the cell wall (A), membrane (B), and cytosolic (C) fractions. pIs are indicated at the top.

FIG. 4.

Variation in antibody-mediated response over time. Cell wall protein (500 μg) was resolved via 2DGE, and protein was transferred to nitrocellulose membranes. The membranes were then probed with sera from days 0, 14, 28, and 42 postinoculation.