Quantitation of protein A in human plasma is possible after heat inactivation of the samples

Development of a Prototype Lateral Flow Immunoassay for Rapid Detection of Staphylococcal Protein A in Positive Blood Culture Samples

Bloodstream infection (BSI) is a major cause of mortality in hospitalized patients worldwide. Staphylococcus aureus is one of the most common pathogens found in BSI. The conventional workflow is time consuming. Therefore, we developed a lateral flow immunoassay (LFIA) for rapid detection of S. aureus-protein A in positive blood culture samples. A total of 90 clinical isolates including 58 S. aureus and 32 non-S. aureus were spiked in simulated blood samples. The antigens were extracted by a simple boiling method and diluted before being tested using the developed LFIA strips. The results were readable by naked eye within 15 min. The sensitivity of the developed LFIA was 87.9% (51/58) and the specificity was 93.8% (30/32). When bacterial colonies were used in the test, the LFIA provided higher sensitivity and specificity (94.8% and 100%, respectively). The detection limit of the LFIA was 10⁷ CFU/mL. Initial evaluation of the LFIA in 20 positive blood culture bottles from hospitals showed 95% agreement with the routine methods. The LFIA is a rapid, simple and highly sensitive method. No sophisticated equipment is required. It has potential for routine detection particularly in low resource settings, contributing an early diagnosis that facilitates effective treatment and reduces disease progression.

A simple method to quantify staphylococcal protein A in the presence of human or animal IgG in various samples

Immunoassays designed to measure low concentrations of staphylococcal protein A (SPA) that have been leached into antibody preparations intended for therapeutic use are subject to differing degrees of interference. Methods established to quantify SPA in murine antibody preparations are not accurate in the presence of human or humanized IgG. We report the development of an enzyme-linked immunosorbent assay (ELISA) for SPA with a detection limit of 7 pg/ml and the optimization of a method that permits complete dissociation of SPA-immunoglobulin-complexes. This assay is a modification of our heat-mediated dissociation (HD-SD) treatment with sodium dodecyl sulfate (SDS) and diethylenetriaminepentacetic acid (DTPA) for total immune-complex dissociation, in which the heat treatment has been prolonged and the diluent is characterized by increased protein content and buffering capacity. The diluent developed contains SDS, DTPA and bovine serum albumin dissolved in a 0.1 M phosphate buffer (pH 7.2). To validate the efficiency of this novel method, a series of samples have been assayed, including samples reconstituted in vitro, samples of purified antibodies, and plasma from patients. The described method has been shown to be generally efficient in quantitating all native and recombinant SPA in samples containing up to 50 mg/ml of human IgG. These data demonstrate the utility of this technique in determining SPA contamination of recombinant immunoglobulin therapeutic products.

Chicken anti-protein A prevents Staphylococcus aureus protein A from binding to human and rabbit IgG in immunoassays and eliminates most false positive results

This report demonstrates that chicken anti-protein A can prevent both soluble and surface-bound Staphylococcal protein A from binding to either human or rabbit IgG. In an ELISA assay, chicken anti-protein A prevented > 98% of the soluble protein A from binding to the human IgG-Fc coat. In a blotting assay, chicken anti-protein A prevented the membrane-bound protein A from interacting with the human IgG probe. When intact S. aureus (Cowan I strain) was bound to the surface of a microassay plate, chicken anti-protein A blocked > 98% of the cell wall protein A and permitted the probing of the surface components with human IgG. In another immunoassay, rabbit anti-enterotoxin A IgG was used to measure enterotoxin A concentrations in S. aureus culture medium supernatants after soluble protein A was blocked by chicken anti-protein A. Thus, the binding of chicken anti-protein A to protein A almost completely eliminates false positive results and permits the measurement of specific antibodies or antigens in a variety immunoassays where protein A is present.