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     In the MetSTICK heavy metal toxicity test we use immobilized test bacteria to determine the heavy metal toxicity of solids such as sediments, soils and, possibly, biosolids. As for MetPAD and MetPLATE the test is specifically sensitive to heavy metals. This is probably the first in situ toxicity test for heavy metals in solids.

   The test bacteria (also used in MetPAD/MetPLATE) are solidly immobilized on a plastic slide. 

Plastic slide with immobilized bacteria

   The slides are attached to a wooden stick which is inserted in the sediment to the desired depth and then allowed to remain in the sediment for a period of 4 hours. One can attach several slides to the wooden stick to determine the heavy metal toxicity in both water and sediment simultaneously. After the 4-hr time period, the wooden stick is removed from the sediment and brought back to a laboratory for enzyme assay and determination of percent inhibition.

   Briefly, enzyme (beta-galactosidase) activity is assayed directly on the plastic slide. 0.3 ml of enzyme substrate, methyl umbelliferryl galactopyranoside (MUGA) is added on top of the test bacteria and the slide is incubated at 35C for 0.5 to 1 hr. Subsequently, 0.1 or 0.2 ml of the liquid is added into a well of a 96-well black microplate.  The fluorescence is measured, using a fluorometer microplate reader. To calculate the % enzyme inhibition, we compare the fluorescence of the slide AFTER exposure to the sediment and BEFORE exposure to the sediment (i.e., each slide serve as it own negative control). One can also use a non toxic sandy sediment as a negative control. MetSTICK can also be used as a qualitative test by using a long-range UV lamp and visually compare the fluorescence of the slide AFTER exposure to the sediment and BEFORE exposure to the sediment.

   More information is given in MetSTICK Instructions.

   The figure below shows a fluorescent spot for the negative control and less fluorescence for a toxic sample.

   Negative control      Toxic sample

    The MetSTICK plastic slides can be stored in a refrigerator for a few months without any significant loss of activity.

   They can also be conveniently shipped in a padded envelope across continents without significant loss of activity.

      Pipettor and tips

     Incubator (35C)

     Microplate fluorometer with filters: 360nm for exitation and 485 nm for emission