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Single-tube mixed agglutination test for the detection of staphylococcal protein A
Abstract:
A simple, rapid mixed agglutination test using sheep erythrocytes (SRBC) sensitized with rabbit hemolysin and intact viable staphylococci is described for the detection of bound staphylococcal protein A. Soluble protein A was heat extracted from 50 clinical isolates as well as the Cowan I and Wood 46 strains of Staphylococcus aureus and titered by a hemagglutination test using sensitized SRBC and dilutions of soluble protein A. Protein A could be detected in all of these supernatants including that of S. aureus Wood 46, a strain generally considered to be protein A negative. These organisms were later retested by the mixed agglutination test and even those staphylococcal isolates expressing very low heat-extractable soluble protein A concentrations (1:2 titers) were positive, confirming the sensitivity of the test. In a screen of clinical isolates, only 4 of 235 (1.8%) coagulase-positive isolates were negative in the mixed agglutination test. Of 25 coagulase-negative isolates, none yielded a positive reaction.
Insights
A novel mixed agglutination test accurately detects staphylococcal protein A, even in low concentrations. This sensitive method identifies protein A in previously negative strains and clinical isolates.
Area of Science:
- Microbiology
- Immunology
Background:
- Staphylococcal protein A is a virulence factor.
- Accurate detection of protein A is crucial for understanding Staphylococcus aureus infections.
Purpose of the Study:
- To describe a simple, rapid mixed agglutination test for detecting bound staphylococcal protein A.
- To evaluate the sensitivity and applicability of this test on clinical isolates.
Main Methods:
- A mixed agglutination test was developed using sheep erythrocytes sensitized with rabbit hemolysin and intact viable staphylococci.
- Soluble protein A was heat-extracted from Staphylococcus aureus strains and clinical isolates.
- Extracted protein A was titered using a hemagglutination assay.
Main Results:
- The mixed agglutination test detected protein A in all tested strains, including S. aureus Wood 46, previously considered negative.
- The test identified isolates with very low soluble protein A concentrations (1:2 titers).
- In a screen of 235 coagulase-positive isolates, only 1.8% were negative; all 25 coagulase-negative isolates were also negative.
Conclusions:
- The described mixed agglutination test is a sensitive and effective method for detecting staphylococcal protein A.
- This assay can identify protein A in strains and isolates that may be missed by other methods.
- The test's simplicity and rapidity make it suitable for clinical diagnostics and research.