The β-barrel Assembly Machinery (BAM) complex inserts proteins into the outer membrane of Gram-negative bacteria, ensuring proper folding. The Escherichia coli BAM complex comprises the outer membrane protein BamA and the lipoproteins BamB through BamE. The structures of the BAM complex have been solved in multiple states, revealing that the structure of BamC is diverse, with some domains being disordered in certain structures. In addition, in vivo analysis has demonstrated that BamC is exposed on the cell surface. These suggest that BamC is a dynamically associated subunit of the BAM complex. However, the state of association of BamC within the BAM complex in vivo is not fully captured. In this study, we aimed to capture every state of BamC topology in vivo.We prepared polyclonal antibodies against BamC and then performed a dot-blot analysis, which assesses surface-exposed proteins by antibody decoration onto E. coli cells fixed on a nitrocellulose membrane. We also employed in situ photo-crosslinking to capture protein proximity using UV-crosslinking with the unnatural amino acid pBPA. Dot-blot analysis with our BamC antibody demonstrated that BamC is exposed on the cell surface. Analysis of various growth stages of E. coli revealed that BamC is significantly exposed on the cell surface during the log phase but not the stationary phase. In situ photo-crosslinking, introducing pBPA into BamA, showed that BamC binds to both the periplasmic domain and the extracellular region of BamA. Interestingly, the interaction between BamC and the surface-exposed region of BamA was disrupted by detergent solubilization, but not the periplasmic domain of BamA. These results suggest that BamC is exposed on the cell surface during the log phase, which requires the most efficient protein assembly. Since structural analysis often prepares membrane proteins by detergent solubilization from the stationary phase, may not capture the cell surface-exposed form of BamC.