The Response of Halophilic Bacteria to Heavy Metals

Abstract

The susceptibility of a great number of halophilic strains (145 extremely halophilic archaeobacteria and 308 moderate halophiles), including both culture collection strains and fresh isolates from widely differing geographical areas, to arsenate, cadmium, chromium, cobalt, copper, lead, mercury, nickel, silver and zinc ions, were surveyed by using an agar dilution method. The halobacterial culture collection strains showed different susceptibilities clustering into five groups. In contrast, the halococci collection strains exhibited similar responses to the majority of the heavy metals tested. The behaviour of the moderately halophilic collection strains was only similar with respect to six metal ions. Concerning the fresh isolates, the metal-susceptibility levels of the moderate halophiles were, in general, very heterogeneous among the four taxonomic groups as well as within the strains included in each group. All the halophilic strains were sensitive to mercury and silver and a great fraction of them were also sensitive to zinc. On the other hand, most of them were tolerant of lead and chromium. Different patterns of metal susceptibility were found for the rest of metals. A range of concentrations for defining metal-tolerance in halophilic bacteria for additional studies are proposed.