Lipid-mediated resistance of Gram-negative bacteria against various pore-forming antimicrobial peptides

Abstract

Lipopolysaccharides (LPSs) play a dual role as target and as effector molecules. The knowledge of the LPS-induced activation of human immune cells is increasing; however, surprisingly, much less effort seems to be directed towards the understanding of the mechanisms leading to the killing of the bacterial organisms, which eventually results in the release of LPS from the bacterial surface into the blood circulation. We demonstrate mechanisms of interaction of peptides of the innate immune system (e.g. defensins and cathelicidins) as well as of externally administered antibiotics (e.g. Polymyxin B) with Gram-negative bacteria. The main focus is directed on data derived from electrical measurements on a reconstitution system of the outer membrane as an asymmetric bilayer composed on one side of LPS and on the other of phospholipids. All these antimicrobial peptides (AMPs) are membrane-active and induce the permeabilization of the reconstituted membranes by the formation of lesions. We found that differences in the activity of the AMPs against various sensitive and resistant Gram-negative bacteria can be explained solely by variations in the chemical structure of LPS, e.g. in the composition of the sugar head group. A reduction of the net negative charge of LPS is responsible for a reduced interaction with the polycationic AMPs and thus for resistance. A most important side effect of positively charged AMPs is the neutralization of the negatively charged LPS released from the bacterial surface as a consequence of AMP-induced killing.

The Intestinal Archaea Methanosphaera stadtmanae and Methanobrevibacter smithii Activate Human Dendritic Cells

“…However, since methanoarchaea are considered to be commensals within the human intestine, it might also thinkable that they evolved mechanisms protecting themselves from human immune clearance. This would be in accordance with our recently published data on the susceptibility of methanoarchaea against numerous AMPs, in particular against LL32 [33] that is described as the shortest active unit of human LL37 [46].…”

Helicobacter pylori versus the Host: Remodeling of the Bacterial Outer Membrane Is Required for Survival in the Gastric Mucosa

“…H. pylori is highly resistant to the CAMP polymyxin B (PMB) with the minimal inhibitory concentration (MIC) of wild type strains ranging from 250–1000 µg/ml peptide [19]. PMB is an experimental substitute for CAMPs and commonly used to demonstrate CAMP resistance in laboratory settings because of a similar mechanism of action [27]. MICs were determined for all strains using PMB Etest strips.…”

Stress responses as determinants of antimicrobial resistance in Pseudomonas aeruginosa: multidrug efflux and more

“…Membrane-disrupting CAPs, including polymyxin B and proteins of innate immunity, also trigger stress responses whose outcome is a strengthening of membranes and, in the case of Gramnegative organisms, a modification of the lipopolysaccharide (LPS) to prevent CAP binding (Gunn 2001), LPS being the initial site of CAP interaction with cells (Gutsmann et al 2005). In P. aeruginosa, exposure of the cell to subinhibitory levels of CAPs, such as polymyxin B, colistin, and CAPs of innate immunity, promoted resistance to these agents by activating expression of the arnBCADTEF-ugd (a.k.a.…”