Separating objects touching and overlapping with each other into single ones is very important for the counting and feature extracting.Most of the existing separating algorithms are based on two hypotheses:(1)the touching objects tend to form an acute angle on the contour where objects touched

and/or (2)there would always be a lower optical density at touching points.For colony image

the bacteria cells vary in sizes and touching manners.Many touching cells are short of above hypotheses.A new separating algorithm based on 8-connected contour tracking is proposed in the paper.The boundary of touching-cell was tracked and deleted layer by layer

and judgment was made by the tracking result to determine whether the touching-cell should be separated.The experiment results show the success ratio to separate bacteria touching-cell is 10% higher than the existing algorithms.