 Asymmetric pores are located between the adjacent trimeric building blocks.The size of the pores are sufficiently large to allow the exchange of metabolites between the organism and the external environment.Whereas the protein constituents of lipid-based barriers, such as bacterial outer membranes, can be rapidly modified in response to physiological or environmental stimuli, the large pore sizes of the S-layer composed of MSTP protein subunits are presumably required to allow passage of molecules across a protective barrier whose molecular features are difficult to modify.