Measurement of bacterial adhesion to metal surfaces with different chemical composition - evaluation of different methods

In the current study attention has been focused on the evaluation of different methods of measurement of bacterial adhesion to the metal surfaces with different chemical composition. The direct methods, which included determination of the number of bacterial cells using fluorescence microscopy and Colony Forming Units (CFU) on agar medium, and indirect methods using Alamar Blue (AB) and MTT assays, were evaluated. The chemical compositions of the metal surfaces included: copper, iron, chromium and nickel. Interaction effects of assay and metal compound have been specifically demonstrated in this study. It was found that metal ions reacted with components of the indirect colorimetric tests used in this study (AB and MTT assay). Consequently, those tests gave positive false results. In contrast to the indirect methods, direct counting methods such as microscopy techniques and CFU counting, were successfully applied for evaluation of bacterial adhesion to the metal surfaces. However, considering the limits for the surfaces of the examined samples for microscopy, the determination of the CFU was found to be the best method for testing the adhesion to metal surfaces. The method, combined with the appropriate detachment procedures allowed for a precise determination of the number of bacteria on the entire surface of the evaluated metal samples.