BACKGROUND

Staphylococcus aureus infections remain a major complication of orthopaedic surgery. Although serum C-reactive protein is useful for diagnosis, there are no specific tests for host immunity that can assess a patient's risk for serious infection. On the basis of the identification of glucosaminidase as a potentially protective antigen in animal models, we tested the hypotheses that anti-glucosaminidase IgG (immunoglobulin G) levels vary in sera of mice and orthopaedic patients with Staphylococcus aureus infections and that physical and neutralizing titers correlate.

METHODS

In vitro ELISAs (enzyme-linked immunosorbent assays) were developed to quantify binding (physical) and enzyme-neutralizing (functional) anti-glucosaminidase IgG titers. The assays were validated with use of sera from naive, Staphylococcus aureus-challenged, and glucosaminidase-immunized mice. The physical, functional, and isotype titers of anti-glucosaminidase IgG were measured in sera from twenty-four patients with a confirmed Staphylococcus aureus infection following orthopaedic surgery and in sera from twenty noninfected patients. The specificity of the anti-glucosaminidase assay was evaluated by means of linear regression and receiver-operator characteristic curve analysis.

RESULTS

In mice, the analytic range of the physical titer assay for anti-glucosaminidase IgG was determined to be 1 ng/mL to 1 μg/mL, and physical titers correlated with functional titers (p < 0.002). Although all patients had measurable anti-glucosaminidase IgG, the physical titers in the infected patients were significantly higher by a factor of two compared with those in the healthy controls (p = 0.015). The physical titers were significantly correlated with the functional titers (p < 0.0001). Receiver-operator characteristic curve analysis demonstrated a diagnostic specificity of 0.72 (p = 0.014) for the assay. The anti-glucosaminidase titer in almost every patient was dominated by the IgG1 isotype.

CONCLUSIONS

Humoral immunity against glucosaminidase varied in mammals with Staphylococcus aureus osteomyelitis. Anti-glucosaminidase titers in sera were a potential biomarker of infection and have the potential to assess the quality of host immunity against Staphylococcus aureus.

CLINICAL RELEVANCE

Staphylococcus aureus infections can be challenging to diagnose, and there is no diagnostic test for host immunity. We demonstrated a cost-effective assay for determining the anti-glucosaminidase titer, which can be readily combined with conventional serology to improve diagnosis and to assess host immunity against Staphylococcus aureus.