LPS O-antigen polysaccharide length impacts outer membrane permeability of enteric gram-negative bacteria.

The gram-negative outer membrane (OM) forms the bacterial cell surface and acts as a barrier against antibiotic influx. In enteric species, the OM is covered by lipopolysaccharides (LPS) decorated with varying lengths of O-antigen (O-Ag) polysaccharide that protect bacteria against mammalian host defenses. Studies of lab-adapted Escherichia coli K-12 strains have proven instrumental in unraveling the essential processes of LPS synthesis, transport, and assembly into the OM. However, O-Ag synthesis was inactivated in K-12 strains during their lab adaption, and these cells produce a non-native, truncated LPS form. Surprisingly, we found that re-activating O-Ag synthesis in K-12 permeabilizes the OM to diverse antibiotics, causing susceptibility. The O-Ag that modifies LPS is directly responsible for the compromised OM barrier. Lengthening the O-Ag polysaccharide worsens antibiotic sensitivity, while shortening it, or removing it entirely, improves antibiotic resistance in both E. coli and the human pathogen, Shigella flexneri. Our data show that OM antibiotic barrier integrity is maintained by a balanced production of long and short LPS forms, and this balance is dysfunctional in model E. coli K-12 strains. Our findings reveal that long O-Ag polysaccharides are a double-edged sword: while well-recognized as critical for protection against external host assaults, their transport and assembly onto the surface comes at the inherent price of compromising the OM barrier. Hence, LPS production balances between competing needs in host defense and OM integrity. Moreover, we identify an inherent advantage for species that produce O-Ag-lacking lipooligosaccharide, rather than LPS. IMPORTANCE: The outer membrane (OM) of bacteria like Escherichia coli and Shigella flexneri forms a barrier that protects cells against antibiotics and immune effectors. The surface-exposed leaflet is filled by lipopolysaccharides (LPS) decorated with long "O-antigen" (O-Ag) polysaccharides. The benefit of covering the surface with O-Ag is well appreciated; these long polysaccharides shield against host assaults. Our study reveals a hidden cost to these long O-Ag polysaccharides: transporting and assembling LPS modified with O-Ag compromises integrity of the OM antibiotic barrier, rendering bacteria vulnerable to antibiotics. Cells must balance O-Ag across two parameters-protection from the host and preserving OM integrity. Our findings also present an inherent benefit to not producing O-Ag, a common feature among diverse bacterial pathogens.