Hello Kong, I am not familiar with the expression "inner standard protein" as you used it. But if it is what I think it is, that is, the most abundant and highly conserved protein in Staphylococcus aureus, then it should be the Asp23 protein. There are about 25,000 molecules of this protein per bacterial cell. Attempt at deletion of its gene triggered a cell envelop stress response. See the attached for details.

Thank you for your answer. Sorry I did not make myself clear. The real propose for me is to figure out which protein expression level can maintain the same during the growth of s.a. So that I can use this protein as an inner standard to adjust the loading amount when doing westernblot.