Rapid detection of low numbers of pathogens (101–102 cells mL1) in blood is necessary to detect bloodstream infection. Since the collected blood sample volumes usually range between 10 and 20mL, around 100–2000 bacterial cells are present in a whole blood sample. In the following experiments, we intend to demonstrate the feasibility of direct injection of nL-sample volumes [52] of the microbial suspensions containing clinically significant number of S. aureus cells per 1 mL, and their subsequent CZE separation and detection. First, a representative standard sample was prepared from the first five strains of S. aureus cultivated on MH agar or purified, blood-incubated cells listed in Table 1. The sample was injected into the capillary by syringe pump and separated as described in the Experimental section. The resultant electropherogram from the sample mixture composed of the MSSA and MRSA (1 107 cell mL1 each) and MSSA-B and MRSA-B (2 107 cell mL1 each) strains is presented in Fig. 2A. Only four narrow peaks were detected, i.e., the migration velocities of MSSA, MRSA, MSSA-B, and MRSA-B cells are characteristic for each group of the examined strains.
The dependence of peak areas of the examined strains on the number of injected cells is shown in Fig. 2Ai and Bi. A linear calibration was obtained for all test groups of MSSA or MRSA strains cultivated on MH agar or purified, blood-incubated staphylococci throughout the cell concentration range covered. Very good quantitative responses (coefficient of determination R2 = 0.99) were achieved. The peak areas of MSSA and MRSA (circle) or MSSA-B and MRSA-B (triangle) were not significantly different (full vs. empty circle or triangle, respectively). However, the peak areas of MSSA-B and MRSA-B were approximately twice higher than those of MSSA and MRSA, respectively. The dotted lines in Fig. 2Ai show the numbers of cells injected into the capillary by the syringe pump: 5 103 cells of MSSA and MRSA and 104 cells of MSSA-B and MRSA-B. The number of injected cells approximately corresponds to the peak areas in the electropherogram presented in Fig. 2A. We performed repetitive 100-nL injections of different microbial suspensions mentioned above by the syringe pump. RSDs of the migration time for all tested strains were below 1.8% whereas the peak area RSDs were within 5%.