Synergy by Perturbing the Gram-Negative Outer Membrane: Opening the Door for Gram-Positive Specific Antibiotics

Abstract

New approaches to target antibacterial agents toward Gram-negative bacteria are key, given the rise of antibiotic resistance. Since the discovery of polymyxin B nonapeptide as a potent Gram-negative outer membrane (OM)-permeabilizing synergist in the early 1980s, a vast amount of literature on such synergists has been published. This Review addresses a range of peptide-based and small organic compounds that disrupt the OM to elicit a synergistic effect with antibiotics that are otherwise inactive toward Gram-negative bacteria, with synergy defined as a fractional inhibitory concentration index (FICI) of <0.5. Another requirement for the inclusion of the synergists here covered is their potentiation of a specific set of clinically used antibiotics: erythromycin, rifampicin, novobiocin, or vancomycin. In addition, we have focused on those synergists with reported activity against Gram-negative members of the ESKAPE family of pathogens namely, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and/or Acinetobacter baumannii. In cases where the FICI values were not directly reported in the primary literature but could be calculated from the published data, we have done so, allowing for more direct comparison of potency with other synergists. We also address the hemolytic activity of the various OM-disrupting synergists reported in the literature, an effect that is often downplayed but is of key importance in assessing the selectivity of such compounds for Gram-negative bacteria.

Keywords: Gram-negative bacteria; Gram-positive antibiotics; outer membrane; permeabilization; potentiators; synergy.

Figure 1

(A) Schematic depiction of the OM disruption required for potentiation of Gram-positive specific antibiotics (created with BioRender.com). (B) Lipid A (from Escherichia coli K-12), the hydrophobic anchor of LPS.

Figure 2

Molecular structures of (A) polymyxin B (PMB), deacylpolymyxin B (DAPB), polymyxin B nonapeptide (PMBN), polymyxin B octapeptide (PMBO), and polymyxin B heptapeptide (PMBH) and (B) PMBN analogues SPR741, NAB739, and NAB7061.

Figure 3

Molecular structures of the dilipidated polymyxin analogues.

Figure 4

Lipopeptide and lipopeptidomimetic synergists. Representative structures of (A) dilipid ultrashort cationic lipopeptides (dUSCLs), (B) ultrashort tetrabasic lipopeptides (UTBLPs), (C) dilipid ultrashort tetrabasic peptidomimetics (dUSTBPs), and (D) oligo-acyl-lysyls (OAKs).

Figure 5

Overview of the synergistic steroids (A) squalamine, (B) squalamine mimic SM-7, (C) polycationic cholic acid ether-linked steroid synergists, (D) polycationic cholic acid ester-linked steroid synergists, and (E) steroid–peptide hybrids.

Figure 6

Representative structures of recently reported (A) bis-amidine synergists and (B) metformin.

Figure 7

Non-steroid small-molecule synergists: (A) synergists identified via HTS, (B) azaindole synergists, (C) d-LANA-14 based on a norspermidine core linked to two d-lysine residues and a central tetradecanoyl moiety, (D) joro spider toxin-inspired naphthylacetylspermine, (E) bisacyl-homospermines, (F) indole-3-acrylamidospermine conjugates, and (G) representation of 600 Da branched polyethylenimine (BPEI).

Figure 8

Plant-derived natural products reported to potentiate the activity of antibiotics against Gram-negative bacteria.

Figure 9

Synergists based on clinically used antibiotics: (A) tobramycin (TOB) conjugates, (B) nebramine (NEB) analogues, and (C) polybasic conjugated levofloxacin hybrids.

Figure 10

Chelating agents with demonstrated synergistic activity.