I have 2 test tubes labeled “Strep E” and “Strep E Buffer”

The “Strep E” test tube contains “Staphylococcus Epidermidis and Lysozyme”

The “Strep E Buffer” test tube contains “Staphylococcus Epidermidis and a Buffercontrol”

I understand how Lysozyme breaks down peptidoglycan in the “Strep E” test tube

So my question is

Why is the “Strep E Buffer” test tube that contains no Lysozyme useful in this experiment?

How can it be used as a comparison?

Thanks