The energy source for the uptake of substrates through the outer membrane receptors is provided by the Ton complex from an electrochemical potential gradient across the inner membrane. The Ton complex consists of three proteins, ExbB, ExbD and TonB. ExbD has a peptidoglycan-binding domain (Garcia-Herrero et al., 2007; Wojdyla et al., 2015) and also interacts with TonB through their periplasmic domains for function (Ollis et al., 2009; Gresock et al., 2015). Substrates include siderophore, vitamin B12, haem, maltodextrin, sucrose, nickel, cobalt, colicin, etc. (Faraldo-Gómez and Sansom, 2003; Noinaj et al., 2010; Krewulak and Vogel, 2011; Schauer et al., 2008). With the help of periplasmic binding proteins, the substrates are then transported into the cell by specific ABC transporters using the energy provided by ATP hydrolysis. There are many TonB-dependent outer membrane receptors (FhuA, FepA, FecA, BtuB, HemR etc.), periplasmic-binding proteins (FhuD, FepB, FecB, BtuF, HemT etc.), and ABC transporters (FhuBC, FepCDG, FecCDE, BtuCD, HemUV etc.) (Faraldo-Gómez and Sansom, 2003; Noinaj et al., 2010; Krewulak and Vogel, 2011; Schauer et al., 2008). Some substrates use other transporters to pass through the inner membrane (Schauer et al., 2008). IM, inner membrane; PG, peptidoglycan layer; OM, outer membrane.