Rapid, direct identification of Staphylococcus aureus and Streptococcus pneumoniae from blood cultures using commercial immunologic kits and modified conventional tests

To develop safe and rapid methods for identification of Staphylococcus aureus and Streptococcus pneumoniae directly from positive blood culture bottles (BCB) (BACTEC, Johnston Laboratories), several commercial biochemical and immunological tests as well as modified conventional tests were evaluated. Preliminary studies demonstrated that both S. aureus and St. pneumoniae could be identified directly using only a small aliquot (100 μl) of the blood culture broth obtained via vent without need for centrifugation or other separation steps. A simple tube coagulase exhibited 98% sensitivity and 100% specificity for 32 S. aureus isolates and 157 blood cultures positive for coagulase-negative staphylococci when read at 2 hr. All systems employed for direct identification of St. pneumoniae exhibited excellent sensitivity and specificity using aliquots from blood culture broths, but Pneumoslide (BBL Microbiology Systems, Cockeysville, MD) was easiest to perform and interpret. The results of this study show that S. aureus and St. pneumoniae can be identified directly from blood culture broth aliquots using rapid methods that eliminate the need for centrifugation or use of needles and syringes.