Quantitative detection of Staphylococcus aureus and Enterococcus faecalis DNA in blood to diagnose bacteremia in patients in the intensive care unit

Direct detection of bacterial DNA in blood offers a fast alternative to blood culture and is presumably unaffected by the prior use of antibiotics. We evaluated the performance of two real-time PCR assays for the quantitative detection of Staphylococcus aureus bacteremia and for Enterococcus faecalis bacteremia directly in blood samples, without prior cultivation. Whole-blood samples for PCR were obtained simultaneously with blood cultures from patients admitted to the intensive care unit of our hospital. After the extraction of DNA from 200 μl of blood, real-time PCR was performed for the specific detection and quantification of S. aureus and E. faecalis DNA. The sensitivity for bacteremia of the S. aureus PCR was 75% and that of the E. faecalis PCR was 73%, and both tests had high specificity values (93 and 96%, respectively). PCR amplification reactions were positive for S. aureus for 10 (7%) blood samples with negative blood cultures, and 7 (4%) PCR reactions were positive for E. faecalis. The majority of these PCR results were likely (50%) or possibly (42%) related to infection with the specific microorganism, based on clinical data and radiological and microbiological investigations. PCR results were concordant for 95% of paired whole-blood samples, and blood culture results were concordant for 97% of the paired samples. We conclude that the detection of S. aureus and E. faecalis DNA in blood by real-time PCR enables a rapid diagnosis of bacteremia and that a positive DNAemia is related to proven or possible infection with the specific microorganism in the majority of patients with negative blood cultures.