Zeta potential as a measure of the surface charge of mycobacterial cells

The surface charge of bacteria is closely related to their envelope structure and interactions with surfaces in natural environments. The aim of this study was to estimate the effect of experimental conditions on the zeta (zeta) potential of mycobacterial cells as a measure of their cell-surface charge. We observed that Mycobacterium smegmatis mc(2)155 cells at physiological conditions displayed a high and stable zeta potential (-42.9 +/- 5.9 mV) which increased from the late-exponential phase of growth and at pH levels of > 8.0. The optimal conditions for estimating the surface charge of mycobacteria using the zeta potential occurred when cells were harvested during the exponential growth phase (OD595 0.3-0.5) and then dispersed in solutions with pH levels of 7.0-10.0. These optimal conditions of zeta potential measurements were useful for differentiating between the virulent M. tuberculosis H37Rv strain and various non-virulent mycobacterial strains at pH 9.8. This study is the first to use zetametry to estimate the cell-surface charge of M. tuberculosis cells. We expect that the experimental conditions presented in this work will have further applications to estimate the cell-surface charge of other wild-type or genetically modified mycobacterial species and thereby further our understanding of the physicochemical interactions of mycobacteria with external surfaces in natural environments.