Module 10
Exoenzymes

This module uses various types of media to test for the presence of certain exoenzymes. This is the start of doing procedures that will indicate the biochemistry/physiology of the bacteria. This will be particularly useful when trying to identify bacteria.

Bacteria are unable to bring in particles for subsequent digestion, therefore, some bacteria produce "digestive" enzymes which are secreted into the surrounding environment in order to break material down enough to enter the bacteria. The molecules targeted are any of the four major molecules utilized by bacteria: carbohydrates, proteins, nucleic acids, lipids. By the utilization of specialized media, the presence of these enzymes is easily determined. In general, the molecules of interest are incorporated into the media along with some means of detecting its digestion.

Lipid digestion: lipase: spirit-blue agar: clearing of the blue color: due to a shift in pH as a result of the digestion.

DNA digestion: DNAse: DNA agar: clearing of the blue color: due to the digestion of the DNA and release of the dye.

Protein digestion: caseinase:Skim milk agar: clearing of white color: due to digestion of casein, a cheap milk protein.

Carbohydrate digestion: amylase:clear area after addition of iodine: due to digestion of starch leaving nothing to bind to iodine

Comments/tips:

Unless performing a streak for isolation, apply the bacteria in a single straight streak.
The Bacillus, due to its ability to move and cover the plate, should be applied as a small dot toward the edge of the plate.