## 1 Introduction  
The most distinguishing feature of Gram-negative bacteria, such as _Escherichia coli_, is the presence of an asymmetric outer membrane (OM) [1]. This asymmetry is due to the characteristic location of lipopolysaccharide-saccharide (LPS) in the outer leaflet of the cell envelope, with phospholipids facing its inner leaflet [1, 2]. The maintenance of this OM asymmetry is essential to impart a permeability barrier, which prevents the entry of bulky toxic molecules, such as antibiotics and bile salts, inside the cells. Thus, with few exceptions, LPS is essential for bacterial viability and is one of the major virulence factors in pathogenic Gram-negative bacteria. LPS is a complex glycolipid and, in general, they share a common architecture and can be, for convenience, divided into three parts. The most conserved component is the hydrophobic membrane-anchored lipid A part, which constitutes the endotoxin principal. To the lipid A part is attached a core oligosaccharide, to which an oligosaccharide of variable length called the \(O\)-antigen is attached in bacteria with smooth LPS [3]. The biosynthesis of LPS at the genetic and biochemical levels is well established, although how LPS amounts are regulated and its final assembly in the OM is not well understood. This is particularly important because bacteria must maintain a strict balance of 1:0.15 between phospholipids and LPS, the two essential components of the cell envelope [4]. In _E. coli_, this is achieved by regulating the first committed step in LPS biosynthesis, catalyzed by the essential enzyme LpxC, because LPS and phospholipids use the same (\(R\))-3-hydroxymyristate as the common metabolic precursor [5]. However, the regulation of LpxC as we understand now turns out to be more complex than it was thought a few years ago, as it responds to multiple physiological cues and a regulated turnover by FtsH and HsIVU proteases (Figure 1) [5; 6]. In _E. coli_, the biosynthesis and transport of LPS require products of more than thirty genes and many of them are essential for bacterial viability [2]. Briefly, LPS biosynthesis occurs on the inner leaflet of the inner membrane and begins with the acylation of UDP-GlcNAc, catalyzed by LpxA. In _E. coli_, LpxA is selective for \(\beta\)-hydroxymristoyl acrylate protein [7]. This is followed by deacetylation of UDP-3-O-(acyl)-GlcNAcNc by the Zn\({}^{2+}\)-dependent LpxC enzyme [8]. As the equilibrium constant for LpxA-mediated oxidation is unfavorable, LpxC-catalyzed deactivation becomes the first committed step in lipid A biosynthesis [9]. Following the deactivation step, a second \(\beta\)-hydroxymristoyl chain is incorporated by _E. coli_ LpxD, leading to the synthesis of UDP-2,3-diacyl-GlcNAcN [10]. Three additional enzymes, LpxH, LpxB and LpxK, act successively to generate the intermediate lipid IV\({}_{A}\)[11]. This intermediate serves as an acceptor for the incorporation of two 3-deoxy-\(\alpha\)-D-_manno_-oct-2-ulosonic acid (Kdo) residues by WaaA, with CMP-Kdo as the donor, generating Kd\({}_{02}\)-lipid IV\({}_{A}\)[12]. In _E. coli_, up to the step of Kd\({}_{02}\)-lipid IV\({}_{A}\) biosynthesis, all enzymes are essential for bacterial viability, although strains synthesizing only a lipid IV\({}_{A}\) precursor or Kd\({}_{02}\)-lipid IV\({}_{A}\) can be constructed [13]. The incorporation of Kdo residues serves as a key intermediate to generate hexaacylated lipid A via sequential acylation by lauroyl (LpxL) and myristoyl (LpxM) transferases and further incorporation of sugar residues using specific glycosyltransferases to produce a core oligosaccharideschitz linked to the lipid A part [13; 14]. Strains synthesizing tetraacylated lipid A can only grow on minimal medium and exhibit an elevated cell envelope stress response [13; 14]. Interestingly, suppressors mapping to the _msbA_ gene encoding LPS flippase can allow the growth of strains devoid of LpxL, LpxM and LpxP on rich medium and at elevated temperatures.