Method of differentiating microbial colonies in an image

A method, comprising:

• using an imaging device to produce a first image of a thin film culture device, the culture device having a front side having a transparent film cover sheet and a back side having a translucent substrate;
• wherein the first image is produced while providing illumination to the front side of the device;
• wherein the culture device comprises first and second indicator compounds;
• wherein the first indicator compound is converted by a microorganism to a first product having a first color;
• wherein the second indicator compound is converted by a microorganism to a water-diffusible second product that forms a second color;
• using the imaging device to produce a second image of the thin film culture device, wherein the second image is produced while providing illumination to the back side of the device;
• analyzing the first and second images to identify microorganism colonies in each image;
• analyzing the first image to calculate a first value of a size parameter for a colony at a particular location in the culture device;
• analyzing the second image to calculate a second value of the size parameter at the particular location in the culture device;
• and
• comparing the first value to the second value.