[Identification of Staphylococcus aureus within 2 hours in blood culture broths]

Staphylococcus aureus identification is one of the priorities of the microbiological diagnosis of the staphylococcal infections. Current identification methods are carried out after a first step of colony isolation on agar media. We describe a fluorogenic method for S. aureus identification, which is directly applied to blood culture broths. This method uses a gel tube which allows an optimized microculture of the bacteria. 129 clinical samples of blood cultures (HEC) containing gram positive cocci in grapelike clusters (35 Vital bottles, 94 Bactec bottles), and 77 inoculated blood culture (HE) with collection strains of S. aureus are included in the study. Bacteria are concentrated and separated from other components of sample in the gel tube. Staphylococci are revealed during a microculture in the gel phase, by using a colorimetric substrate of their dehydrogenases. Then, staphylococci are recovered in an adapted culture medium containing human prothrombin and a fluorogenic substrate, which is specific for the staphylocoagulase. After 1 to 2 h incubation at 37 degrees C, a blue fluorescence shows the presence of S. aureus. Among the 40 HEC containing S. aureus the test is positive for 37 samples. For 3 cases, the test is not interpretable, due to non lysis red blood cells in the gel phase of the tube. No false positive result is observed for the HEC containing coagulase-negative staphylococci. Moreover, our method is positive with the 77 HE. 94.7% of tested samples (HEC and HE) show a fluorescence after only one hour and half. Sensibility and specificity are both 100%. We propose a rapid method for S. aureus identification directly applied to blood culture broths. This method saves 24 h, avoiding the isolation step on agar. Therefore, the treatment of staphylococcal infections and possible isolation measures could earlier set up.