Rapid identification of gram-negative bacilli from blood cultures

Stevens and Parish developed a rapid method for identifying gram-negative bacilli directly from blood cultures using the Quantum II bacterial identification system combined with differential centrifugation. They tested 90 blood culture specimens from three culture types: conventional media, radiometric (Bactec), and Septi-Chek systems. A differential centrifugation technique was used to prepare bacterial pellets from positive blood cultures, which were then inoculated into Quantum II cartridges for 4-5 hour identification. Results were confirmed using the API 20E method on isolated colonies 24 hours later. Of 90 organisms tested, 83 identifications matched between both systems. The seven discrepancies were resolved through conventional tests, revealing four misidentifications by Quantum II and three by API. The method successfully identified 15 of 17 non-fermenting gram-negative bacilli. The authors concluded this procedure reliably provides presumptive identification 24 hours earlier than conventional methods, with minimal additional laboratory requirements, enabling clinicians to obtain organism identification and presumptive antimicrobial susceptibility information on the same day blood cultures test positive.

Key findings

• A differential centrifugation technique successfully prepared suitable bacterial inocula from positive blood cultures for rapid identification systems
• Quantum II BID provided presumptive identification of gram-negative bacilli within 4-5 hours, approximately 24 hours faster than conventional methods
• The Quantum II system correctly identified 83 of 90 gram-negative bacilli, with high concordance to API 20E confirmation testing
• The combination of Quantum II BID with radiometric (Bactec) blood culture detection provided the optimal rapid identification system
• The method requires only standard microbiology laboratory equipment and minimal extension of routine procedures