Due to phosphate and carboxylic moieties in LPS, the Gram—negative bacterial cell surface is negatively charged. Divalent cations such as Mg2+ cross-link LPS, thus stabilizing OM structure. The OM becomes more permeable to hydrophobic compounds when the cations are removed using chelating agents such as EDTA. In addition to LPS and phospholipids, the OM contains various proteins which have Various functions including lipoprotein (Figure 2.6). Active transport of nutrients takes place across the cytoplasmic membrane. Nutrients therefore need to cross the OM, and for this the OM has porins and receptors. Porins are hydrophilic channels formed by three peptides spanning the OM, allowing the diffusion of solutes of molecular weightless than 600 Da. Escherichia colt‘ has three porins, OmpC, OmpF and PhoE - OmpC and OmpF being non-specific. OmpF is slightly bigger than OmpC, the synthesis of which is stimulated