# Lipopolysaccharide: Recent Advances in Its Biosynthesis and Controlling Cell Envelope Homeostasis
###### Abstract
Recently, a slow, tight-binding LpxC inhibitor, LPC-233, with low picomolar affinity displaying good activity against a wide range of Gram-negative clinical isolates in vitro has been described, which holdssignificant promise for clinical use [49]. Research has also been directed toward finding inhibitors of other enzymes involved in LPS biosynthesis and transport, including LpxA, LpxD, LpxH, MsbA, LapC, and proteins of the Lpt complex [50; 51; 52; 53]. To this end, inhibitors of MsbA structure-based drug development have been analyzed. One series of MsbA inhibitors are quinoline-based molecules that prevent proper NBD closure to suppress ATP hydrolysis and substrate transport [54; 55]. Another series of MsbA inhibitors are tetrahydrobenzothiophene-based molecules that force a collapsed inward-facing state, thereby disrupting NBD-TMD communication and yielding higher basal ATPase activities [56; 57]. Another approach is to target Lpt proteins to prevent LPS transport to the OM. This is best illustrated by the characterization of the natural product thanatin and the synthetic peptide murepavadin, which have been shown to be effective against _E. coli_ and _Pseudomonas aeruginosa_, respectively. Thanatin inhibition of LPS transport has been proposed because of its binding to the N-terminal domain of LptA, which normally interacts with LptC [58]. Murepavadin is a peptidomimetic antibiotic that blocks the function of _P. aeruginosa_ lipopolysaccharidesaccharide transporter LptD [59]. However, the identification of mutations in _lpxL_ and _pmrB_ genes that confer resistance to murepavadin and colistin suggests a potentially additional mechanism of its target in _P. aeruginosa_[60]. As interest in the identification of new targets continues to grow, a new class of macrocyclic peptides targeting the Lpt system of _Acinetobacter baumannii_ has led to the identification of zosurabalpin, which has shown promise in clinical trials and has been found to inhibit the release of LPS from the LptFG cavity [61; 62]. (vii) Cellular response to LPS defects and structural alterations: Recent studies have shown that any gross structural alterations, inhibition of LPS synthesis or assembly or transport, or changes in PL levels trigger cellular responses similar to other envelope stress conditions [4]. _E. coli_ cell envelope stress response was initially studied as a response to defects in peripplasmic protein folding or imbalance in OMP composition, which is regulated by the extracytoplasmic function sigma factor RpoE and the two-component system CpxA/R [63; 64]. As OMP maturation defects also depend on LPS composition, LPS defects are also sensed by the RpoE and CpxA/R systems [65]. RpoE is an alternative sigma factor that is highly induced in the absence of LapB or when LapC is dysfunctional due to an imbalance in LPS vs. PL amounts [23; 39]. The RpoE counterpart (AlgU) in _P. aeruginosa_ was recently found to be induced upon treatment with murepavadin, indicating a role of the envelope stress response in bacterial tolerance to this antibiotic [66]. In _Salmonella enterica_ serovar Typhimurium, specific LPS defects (absence of LapB) or truncation of LapC have been shown to activate the Rcs two-component system [29]. In continuation of the above-mentioned discoveries in this Special Issue, six research articles were published that cover the structural diversity in LPS composition, techniques for LPS quantification in terms of biological activity, regulation of LPS by LapC and identification of new components that regulate LpxC, targeting OM asymmetry by antimicrobial peptides affecting PL migration, and structural alterations in the LPS core that impact the immune response in pathogenic bacteria. In the manuscript by [PERSON] et al., the authors first described the analysis of the complexity and diversity of LPS structures in selected well-studied bacterial species [67].