Survey of metal tolerance in moderately halophilic eubacteria

Abstract

The tolerance patterns, expressed as MICs, for 250 moderately halophilic eubacteria to 10 heavy metals were surveyed by using an agar dilution method. The moderate halophiles tested included 12 culture collection strains and fresh isolates representative of Deleya halophila (37 strains), Acinetobacter sp. (24 strains), Flavobacterium sp. (28 strains), and 149 moderately halophilic gram-positive cocci included in the genera Marinococcus, Sporosarcina, Micrococcus, and Staphylococcus. On the basis ofthe MICs, the collection strains showed, overall, similar responses to silver, cobalt, mercury, nickel, lead, and zinc. All were sensitive to silver, mercury, and zinc and tolerant of lead. The response to arsenate, cadmium, chromium, and copper was very heterogeneous. The metal susceptibility levels of the 238 freshly isolated strains were, in general, very heterogeneous among the four taxonomic groups as well as within the strains included in each group. The highest toxicities were found with mercury, silver, and zinc, while arsenate showed the lowest activity. All these strains were tolerant of nickel, lead, and chromium and sensitive to silver and mercury. Acinetobacter sp. strains were the most heavy-metal tolerant, with the mlijority of them showing tolerance of eight ditferent metal ions. In contrast, Flavobacterium sp. strains were the most metal sensitive. The inftuence of salinity and yeast extract concentrations of the culture medium on the toxicity of the heary metals tested for sorne representative strains was also studied. Lowering the salinity, in general, led to enhanced sensitivity to cadmium and, in sorne cases, to cobalt and copper. However, increasing the salinity resulted in only a slight decrease in the cadmium, copper, and nickel toxicities. Reduction in the yeast extract concentration resulted in an increased sensitivity to all metals, especially when this component was lowered to 0.01% (wt/vol). In contrast, a higher concentration only slightly lessened the toxicities of nickel and zinc.