Determination of antibiotic susceptibility of bacteria by flow cytometric method

In this study, it was aimed to determine the antibiotic susceptibility of bacterial strains by using flow cytometry method by comparing them with current standardized methods. Eleven clinical isolates and 6 standard bacterial strains were included in the study. MIC values were determined by broth microdilution method (BMD), VITEK 2 (R) automated system and flow cytometric method (FCM). FCM was performed with the Accuri C6 flow cytometer. For all strains except P. aeuruginosa ATCC 27853 [BMD-FCM:r = 0.557(p = 0.048); VITEK 2-FCM:r = 0.529(p = 0.063)], E. faecalis ATCC 29212 [BMD-FCM:r = 0.393(p = 0.295); BMD-VITEK 2:r = 0.393(p = 0.295)], and vancomycin-resistant E. faecium clinical isolate [BMD-FCM:r = 0.452(p = 0.063)] r values were in the range of 0.802-0.969 for BMD-FCM (p < 0.001), 0.655-0.941 for BMD-VITEK 2 (p < 0.005) and 0.667-0.953 for FCM-VITEK 2 (p < 0.005). Correlation values of antibiotic susceptibility test results between three methods for Gram-negative bacteria were found as follows; r = 0.927(p < 0.001) for BMD-FCM, r = 0.851(p < 0.001) for BMD-VITEK 2, r = 0.807(p < 0.001) for VITEK 2-FCM. Correlation values were found as follows for Gram positive bacteria; r = 0.848(p < 0.001) for BMD-FCM, r = 0.877(p < 0.001) for BMD-VITEK 2, r = 0.800(p < 0.001) for VITEK 2-FCM. When all bacteria included in the study were evaluated as a total; it was r = 0.911(p < 0.001) for BMD-FCM, r = 0.888(p < 0.001) for BMD-VITEK 2, r = 0.835(p < 0.001) for VITEK 2-FCM. The methicillin resistance of the clinical methicillin resistant S. aureus isolate could not be detected by FCM. It was determined that there was a high level of correlation between methods. FCM shortens the duration of antibiotic susceptibility tests by 12-14 h and gives results within the same day. However, it has not been standardized to be widely used in microbiology laboratories and experienced personnel are needed for its implementation.

Automated summary

This study compared the flow cytometry method with current standardized methods for determining the antibiotic susceptibility of bacterial strains. The results showed a high level of correlation between the two methods. Flow cytometry provided faster results within the same day and shortened the duration of the antibiotic susceptibility tests.