Download the PowerPoint

PowerPoint Contents

Figure 1: Uninoculated Casein Plates/Milk AgarMilk agar contains skim milk (lactose and casein), peptone, and agar. Many organisms can grow on this medium. This medium is used to detect the production of proteases/caseases that digest casein to soluble peptides. This results in a clear zone. Soluble peptides can then be absorbed by the cell. Casein is responsible for the white color of milk. When digested by exoenzymes, the white agar turns clear and colorless. Bacterial pigments can be seen distinctly on this agar. (Tasha Sturm, Cabrillo College, Aptos, CA)

Figure 2: Casein Plates/Milk Agar inoculatedCasein Plates/Milk Agar inoculated with (A) Pseudomonas aeruginosa, where casein hydrolysis is indicated by a zone of clearing around the growing colony (green color masking clearing in agar is the diffusable bacterial pigment pyocyanin); (B) Serratia marcescens, where casein hydrolysis is indicated by a zone of clearing around the growing colony (red pigment of bacterium is due to prodigiosin production); (C) Escherichia coli, no casein hydrolysis. (Tasha Sturm, Cabrillo College, Aptos, CA)

Figure 3: Close-up view of Pseudomonas aeruginosa growing on Casein MediaClose-up view of Pseudomonas aeruginosa growing on Casein Media showing casein hydrolysis indicated by a zone of clearing around the growing colony. (Tasha Sturm, Cabrillo College, Aptos, CA)

Figure 4: Close-up view of Serratia marcescens growing on Casein MediaClose-up view of Serratia marcescens growing on Casein Media showing casein hydrolysis indicated by a zone of clearing around the growing colony. (Tasha Sturm, Cabrillo College, Aptos, CA)

Figure 5: Close-up view of E. coli with no casein hydrolysisClose-up view of E. coli with no casein hydrolysis. (Tasha Sturm, Cabrillo College, Aptos, CA)

Contact Information

ASM Education, education@asmusa.org