Identification of pathogenic bacteria in blood cultures: comparison between conventional and PCR methods

Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, Acinetobacter baumanii, and Klebsiella pneumoniae were found to be the most prevalent bacteremia-causing bacteria in a survey in a medical center. A PCR method for identification of these five most common pathogens in blood cultures was developed. A unique sequence was chosen for each pathogen and used for primer design. Sixty-one blood samples (from hospitalized patients) in which bacterial growth was detected were processed in parallel by conventional microbiological methods and by the PCR method. The results obtained by PCR were identical to those obtained by conventional methods in 93.4% of the cases. PCR failed to identify bacteria which were found conventionally in only 6.6% of the cases (mostly bacteria not included in the PCR cassette). Another group of eighty-eight blood samples from patients were processed immediately upon their arrival at the laboratory by taking aliquots for the PCR method. The blood sample bottles were processed in parallel by conventional methods. In 78.4% of the cases the results of both methods were identical. In 12.5% of the cases, PCR afforded identification of bacteria but conventional methods showed no bacteria in the sample. On the other hand, PCR afforded 9.1% negative results while conventional methods identified bacteria not included in the PCR cassette. It is concluded that the molecular method appears to be a specific and precise method for identifying pathogenic bacteria in blood samples.