A method for measuring bacterial resistance to metals employing epifluorescent microscopy

Abstract

A direct viable counting method has been developed which can be used to measure resistance of bacteria to metal (DVCMR bio-assay). Results obtained using DVCMR was compared with classical cultural methods and proven superior. The direct viable counting method was modified by addition of Hepes buffer and heavy metals at increaing concentrations and used as a bio-assay to evaluate the metal resistance patterns for pure cultures of bacteria. The percent resistance (% R) plotted against metal concentration used in the bio- assay proved to be a sensitive measurement of the level of resistance of the strain. Regression analysis was used to determine the concentration of metal resulting in % R = 50, which permitted differentiation of resistance from sensitivity to the metal. Evaluation of test strains resistant to arsenic or manganese showed a parabolic curve, whereas sensitive strains demonstrated a logarithmic curve fit. The DVCMR bio-assay method to determine % R was approximately twice as sensitive for detecting metal-resistant strains and 23 times for metal-sensitive strains, compared with cultural methods. It is concluded that the DVCMR bio-assay may be better suited for use in microbial biogeochemistry, i.e., contamination monitoring and mineral prospecting, than existing cultural methods.