Targeting LPS biosynthesis and transport in gram-negative bacteria in the era of multi-drug resistance

Abstract

Gram-negative bacteria pose a major threat to human health in an era fraught with multi-drug resistant bacterial infections. Despite extensive drug discovery campaigns over the past decades, no new antibiotic target class effective against gram-negative bacteria has become available to patients since the advent of the carbapenems in 1985. Antibiotic discovery efforts against gram-negative bacteria have been hampered by limited intracellular accumulation of xenobiotics, in large part due to the impermeable cell envelope comprising lipopolysaccharide (LPS) in the outer leaflet of the outer membrane, as well as a panoply of efflux pumps. The biosynthesis and transport of LPS are essential to the viability and virulence of most gram-negative bacteria. Thus, both LPS biosynthesis and transport are attractive pathways to target therapeutically. In this review, we summarize the LPS biosynthesis and transport pathways and discuss efforts to find small molecule inhibitors against targets within these pathways.

Keywords: Antibiotics; Cell envelope; Drug resistance; Gram-negative; Lipopolysaccharide.

Fig. 1.

(A) The cell envelope of gram-negative bacteria is depicted, including the LPS-laden outer leaflet of the outer membrane, inner membrane, and intervening periplasm containing peptidoglycan. (B) LPS is represented with its three components: lipid A, core oligosaccharide, and o-antigen.

Fig. 2.

The Raetz pathway of lipid A biosynthesis in E. coli K12. The first six enzymes – LpxA, LpxC, LpxD, LpxH/G/I, LpxB and LpxK – are essential and together catalyze the diacylation of uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), subsequent condensation of two molecules, and phosphorylation yielding lipid IV A. Three functional orthologs carry out the pyrophosphate cleavage of UDP-DAGn to form Lipid X: LpxH in β-proteobacteria and γ-proteobacteria; LpxI in α-proteobacteria; and LpxG in Chlamydiae. The remaining three enzymes – KdtA, LpxL and LpxM – are non-essential and responsible for the further glycosylation and acylation into Kdo₂-lipid A, the final product to which core oligosaccharide and O-antigen are added.

Fig. 3.

Summary of small molecule inhibitors of LPS biosynthesis. Representative chemical structures of inhibitors from each compound class are shown.

Fig. 4.

The PEZ model for LPS transport. Kdo2-lipid A is synthesized and modified by the addition of core oligosaccharide to yield lipooligosaccharide (LOS), which is subsequently flipped across the inner membrane by the ABC transporter MsbA. Once anchored to the outer leaflet of the inner membrane, the biosynthesis of LPS is completed by the addition of O-antigen polysaccharides to LOS. After the conversion of LOS to LPS in the periplasmic compartment, LPS molecules are extracted from the outer leaflet of the inner membrane by the LptABCDEFG transporter and loaded into the periplasmic bridge formed by the head-to-tail assembly of the β-jellyroll domains of LptF, LptC, LptA and LptD. As processive rounds of ATP hydrolysis insert more LPS molecules into the periplasmic bridge, LPS molecules are pushed sequentially toward the outer membrane like candy in a PEZ dispenser. Once near the outer membrane, LPS molecules finally cross the outer membrane through the LptDE protein complex, a transmembrane heterodimer of LptD and LptE proteins.

Fig. 5.

Summary of small molecule and peptidomimetic inhibitors of LPS transport. Representative chemical structures of inhibitors from each compound class are shown.