At first the bacteria grow densely outwards. However, the agar has been deliberately made so that there is an insufficient concentration of nutrients for the bacteria to feed as rapidly as they grow.
Can you relate the nutrients in the agar to a quantity in the metal deposition experiment?
Do you see a similarity to the fingers formed by the bacteria and the fingers you observed in the liquid fingering experiment? Watch this movie as we scan the finger of a bacterial colony under the microscope.
What is your image of how the bacteria are behaving? Run the movie to see.
Many of these bacteria experiments are done using Bacillus subtilis or Escherichia coli.