The effect of cadmium on starved heterotrophic bacteria isolated from marine waters

Abstract

The effect of Cd2+ on two starved marine Gram-negative bacteria, S14 and DW1, was examined. Viability, total numbers, the inhibition of DNA and protein synthesis, uptake of leucine and thymidine and oxygen consumption were measured. The cells were exposed to Cd2+ during the initial phase of starvation in nutrient-free artificial sea water. Short-term starved cells tolerated Cd2+ at concentrations as high as 1.4/ μM. The exposure of DW1 and S14 cells to 70 μM and 140 μM, respectively, immediately prevented the initial increase in number, typical of the starvation survival response of these cells. The viability was tested both with a direct quantification of the proportion of respiring cells by using 2-( p -iodophenyl)-3( p -nitrophenyl)-5 phenyltetrazolium chloride (INT), and the proportion of colony-forming units grown on agar plates. Treatment with Cd2+ caused a loss in the capacity to form colonies on agar plates rather than a loss in viability. The proportion of respiring cells always exceeded the proportion of colony-forming units. Initial Cd2+ exposure gave a rapid inhibition of the incorporation of [3H]thymidine into DNA, while oxygen consumption, protein synthesis and uptake activities were less affected. More than 1 h of Cd2+ incubation, however, caused a pronounced inhibition of all the metabolic parameters examined. Cadmium-exposed cells recovered the ability to proliferate when incubated in Cd2+-free nutrient broth. 24 h Cd2+-exposed cells increased the respiration rate immediately when incubated in Cd2+- free broth but proliferation and [3H]thymidine incorporation were not seen prior to a lag period of 7–8 h. Starved, previously Cd2+-exposed cells which were incubated in a Cd2+-free starvation regimen gradually regained their colony-forming capacity. The increase in the number of colonyforming units in the starvation regimen occurred without a concomitant increase in cell numbers.