This study provides a novel attempt to put forward, in general terms and explanations, the toxicological threshold for Hg(II) detoxification. Quantitative analysis of mercuric ion toxicity to a mercury-resistant strain Pseudomonas aeruginosa PU21 and identification of the threshold phenomena are included. It is revealed that rate of cellular viability loss depends upon the ratio of Hg2+ concentration to cell population, (or "multiplicity of toxicity" - MOT). The threshold of unacclimated cells (or "non-growth cells" PU21u) and hyperresistant strain (PU21r) occurs at MOT of 1.6 × 1010 ∼ 3.2 × 1010 and 1.7 × 1011 ∼ 3.4 × 1011 molecules Hg2+/cfu, respectively. The threshold of PU21r increase approx. 10-fold compared with unacclimated resistant strain PU21u. This indicated that sequential treatment of culture with selection pressures (i.e. mercury-containing media) led to over 10-fold increase in mercury resistance. Thus, this quantitative evaluation of toxicity threshold among resistant populations can be used as a design criterion for long-term Hg2+ detoxification bioprocesses.
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