Differential Media. Microbiologists use differential media when they want to differentiate among various kinds of microorganisms on an agar plate. For example,if a mixture of bacteria is inoculated into a blood- containing agar medium,some of the bacteria may produce enzymes that lyse the red blood cells,and others may not. Depending on the pattern of lysis surrounding each bacterial colony, one can distingguish between hemolytic and nonhemolytic bacteria from the same specimen. Thus a throat swab inoculated on a blood-agar plate will show whether streptococcus pyogenes is present. The cause of strep throat and scarlet fever,S. Pyogenes produces B hemolysis, or clear zones,around each colony.