In Vitro Assay for Quantifying Clumping of Staphylococcus aureus

Staphylococcus aureus interacts with fibrinogen in plasma to form macroscopic clumps of cells. A simple and rapid slide agglutination test using rabbit plasma has been employed in clinical labs to distinguish S. aureus from most coagulase-negative Staphylococci. The method described here is a quantitative clumping assay in which S. aureus cells are mixed with either plasma or purified fibrinogen, and clumps are allowed to sediment out of solution. Clearing of the overlying solution is monitored over time by measuring the optical density at 600 nm and comparing these values to the initial turbidity. This simple assay can be used to study regulation and expression of various cell wall-anchored adhesins.