Review: Towards antibacterial strategies: studies on the mechanisms of interaction between antibacterial peptides and model membranes

Lipopolysaccharides (LPSs) play a dual role as inflammation-inducing and as membrane-forming molecules. The former role attracts significantly more attention from scientists, possibly because it is more closely related to sepsis and septic shock. This review aims to focus the reader's attention to the other role, the function of LPS as the major constituent of the outer layer of the outer membrane of Gram-negative bacteria, in particular those of enterobacterial strains. In this function, LPS is a necessary component of the cell envelope and guarantees survival of the bacterial organism. At the same time, it represents the first target for attacking molecules which may either be synthesized by the host's innate or adaptive immune system or administered to the human body. The interaction of these molecules with the outer membrane may not only directly cause the death of the bacterial organism, but may also lead to the release of LPS into the circulation. Here, we review membrane model systems and their application for the study of molecular mechanisms of interaction of peptides such as those of the human complement system, the bactericidal/permeability-increasing protein (BPI), cationic antibacterial peptide 18 kDa (CAP18) as an example of cathelicidins, defensins, and polymyxin B (PMB). Emphasis is on electrical measurements with a reconstitution system of the lipid matrix of the outer membrane which was established in the authors' laboratory as a planar asymmetric bilayer with one leaflet being composed solely of LPS and the other of the natural phospholipid mixture. The main conclusion, which can be drawn from these investigations, is that LPS and in general its negative charges are the dominant determinants for specific peptide—membrane interactions. However, the detailed mechanisms of interaction, which finally lead to bacterial killing, may involve further steps and differ for different antibacterial peptides.

Protective ability of subcellular extracts from Salmonella Enteritidis and from a rough isogenic mutant against salmonellosis in mice

We evaluated the efficacy of surface components enriched hot saline extracts (HE) from parental and two isogenic rough mutant strains of Salmonella Enteritidis as subcellular vaccine candidates. By a randomized mutagenesis approach from a clinical isolate of S. Enteritidis there were selected two rough mutants defective in LPS synthesis (R1 and R2 mutants). The mutations mapped to the wcaI gene and gmd gene, respectively, of the O-antigen gene cluster involved in O-antigen synthesis. BALB/c mice received intraperitoneally one single dose of 30 microg of HE from parental and mutant strains, and the protection against a lethal infection with S. Enteritidis was determined. In contrast to the wild type extract, immunization with rough extracts did not induce any distress symptoms in the mice. HE extract from wild type and R1 strains induced the highest immunogenic response with respect IFN-gamma eliciting splenic cells, in contrast with HE-R2. These results correlated with the obtained levels of protection. Thus, at day 63 post-infection, HE from parental strain rendered an 80% level of protection; HE-R1 conferred a 60% level of protection, whereas HE-R2 did not protect the mice. Any of the antigenic extracts elicited systemic IgG1 and IgG2a responses, although these antibodies did not, however, correlate with protection. These results put forward the importance of cellular immune response mediated by IFN-gamma in protection against salmonellosis. The significantly different protective capacity between HE extracts from both rough mutants suggest that other factors independent of the O-chain, like outer membrane proteins and fimbrial antigens, may be involved in protection. In summary, the HE is a good candidate acellular extract for evaluation of its protective ability against salmonellosis following vaccination in poultry.

pagP is required for resistance to antibody-mediated complement lysis during Bordetella bronchiseptica respiratory infection

To efficiently colonize and persist in the lower respiratory tract, bacteria must survive multiple host immune mechanisms. Bordetella bronchiseptica is a gram-negative respiratory pathogen that naturally infects mice and persists in the lower respiratory tract for up to 49 days postinoculation. In this work, we examined the effect of mutation of the pagP gene on the persistence of B. bronchiseptica in the lower respiratory tract of mice. The pagP gene encodes a palmitoyl transferase that is responsible for the addition of a palmitoyl group to the lipid A region of B. bronchiseptica lipopolysaccharide. Data presented here confirm that a B. bronchiseptica deltapagP mutant demonstrates defective persistence in the lower respiratory tract of wild-type mice. We hypothesized that the defective persistence of the B. bronchiseptica deltapagP mutant was due to an increased susceptibility of this mutant to a host immune response. In vivo data indicate that both B cells and the complement component C3 are required for the reduced bacterial numbers of the deltapagP mutant on day 14 postinoculation. In addition, an in vitro complement killing assay demonstrated that B. bronchiseptica exhibits pagP-dependent resistance to antibody-mediated complement killing at low concentrations of immune serum. Taken together, these results suggest that pagP is required for B. bronchiseptica to resist antibody-mediated complement lysis during respiratory infection.

Pore formation and function of phosphoporin PhoE of Escherichia coli are determined by the core sugar moiety of lipopolysaccharide

The lipid matrix of the outer membrane of Gram-negative bacteria is an asymmetric bilayer composed of a phospholipid inner leaflet and a lipopolysaccharide outer leaflet. Incorporated into this lipid matrix are, among other macromolecules, the porins, which have a sieve-like function for the transport or exclusion of hydrophilic substances. It is known that a reduced amount of porins is found in the outer membrane of rough mutants as compared with wild-type bacteria. This observation was discussed to be caused by a reduced number of insertion sites in the former. We performed electrical measurements on reconstituted planar bilayers composed of lipopolysaccharide on one side and a phospholipid mixture on the other side using lipopolysaccharide from various rough mutant strains of Salmonella enterica serovar Minnesota. We found that pore formation by PhoE trimers that were added to the phospholipid side of the bilayers increased with the increasing length of the lipopolysaccharide core sugar moiety. These results allow us to conclude that the length of the sugar moiety of lipopolysaccharide is the parameter governing pore formation and that no particular insertion sites are required. Furthermore, we found that the voltage gating of the porin channels is strongly dependent on the composition of the lipid matrix.