Evaluation of a new protocol for rapid identification of Streptococcus pneumoniae in blood cultures using the modified bile solubility test: Gram staining is still standing.

Abstract

This study aimed to evaluate a new protocol of the bile solubility test performed directly on the blood from positive blood culture bottles to identify Streptococcus pneumoniae rapidly. Seventy-five positive blood cultures (PBC) showing Gram-positive cocci in pairs or chains on Gram stain, including 32 S. pneumoniae isolates and three reference American Type Culture Collection (ATCC) isolates were included to evaluate the performance of a modified bile solubility test (MBST). One milliliter of blood from the PBC bottle was mixed with 0.5 mL of 10% desoxycholate or a saline solution. Both suspensions were analyzed after 10 min of incubation through a Gram stain to detect solubilization. This technique was compared with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry identification, performed on PBC following extraction or on colonies after short or standard incubation, and the optochin susceptibility test on colonies. The capsular serotypes were determined for all S. pneumoniae, and the Belgian National Reference Center confirmed the identification. All 32 clinical isolates and the ATCC isolate of S. pneumoniae were solubilized on the desoxycholate-treated slides, while the other species tested remained visually unchanged on both, the test and control slides. The MBST test demonstrated a 100% sensitivity and specificity with a mean turnaround time (TAT) of just 39 min, making it 14 h and 56 min faster than the optochin susceptibility test. This rapid variant of the bile solubility test appears to be a reliable method to identify S. pneumoniae directly from positive blood culture bottles, with a TAT of 39 min. It is a cost-effective, easy-to-perform, and time-efficient technique. Negative results should be interpreted cautiously, as they may result from mixed infections with S. pneumoniae and other Gram-positive cocci.