NaOH-HCl neutralized urine preparation for direct testing of uropathogens by Bruker MS.

Background: We evaluated the analytical performance of a NaOH-HCl neutralization protocol for identifying uropathogens directly from urine samples by a Bruker MS system (Bruker Daltonics, German) and sought to establish a protocol for integrating the method with conventional screening.

Methods: Among all urine samples requested for Gram staining, UF-1000i, and urine cultures by physicians, we selected samples that were positive by both Gram staining and UF-1000i testing. Urine was prepared by neutralization using 0.01M NaOH-HCl, and the samples were processed by Bruker MS. For the low detection limit, one strain each of Escherichia coli and Enterococcus faecium was inoculated in sterile deionized water and sterile urine specimens at sequential dilutions.

Results: In a total of 1270 urine samples, 125 samples (9.8%) were positive by both Gram staining and UF-1000i. Of 94 samples showing a single morphotype on agar plates, 82 samples had colony counts ≥10  CFU/mL and most uropathogens (95.1%, 78 of 82) had UF-1000i counts ≥10  bacteria/mL. Among them, Bruker MS correctly identified 86.6% (71/82) of all isolates, 89.2% (66/74) of Gram-negative bacteria (GNB), and 62.5% (5/8) of Gram-positive cocci (GPC) with higher average scores for GPC (mean score 2.013) and GNB (mean score 2.110).

Conclusions: Bruker MS with urine preparation by NaOH-HCl neutralization provides a simple, cost-effective, and accurate method for identifying uropathogens directly from urine. Using Bruker MS when single morphotype Gram staining and the UF-1000i count as ≥10  bacteria/mL may improve the efficiency of bacterial identification in routine practice.