Abstract
The bacterium Pseudomonas aeruginosa (PA) and the yeast Candida albicans (CA) are pathogens that cohabit the mucosa of the respiratory tracts of animals and humans. Their virulence is largely determined by chemical communication driven by quorum sensing systems (QS), and the cross perception of their quorum sensing molecules (QSM) can modulate the prevalence of one microorganism over the other. Aiming to investigate whether some of the protein components dissolved in the mucus layering the respiratory mucosa might interfere with virulence and cross-communication of these, and eventually other microorganisms, ligand binding assays were carried out to test the scavenging potential of the bovine and porcine forms of the Lipocalin odorant binding protein (OBP) for several QSMs (farnesol, and acylhomoserine lactones), and for pyocyanin, a toxin produced by PA. In addition, the direct antimicrobial activity of the OBPs was tested by time kill assay (TKA) against CA, PA and other bacteria and yeasts. The positivity of all the ligand binding assays and the antimicrobial activity determined for CA, and for some of the other microorganisms tested, let hypothesize that vertebrate OBPs might behave as humoral components of innate immunity, active against pathogenic bacteria and fungi. In addition, TKAs with mutants of bovine OBP with structural properties different from those of the native form, and with OBP forms tagged with histidines at the amino terminal, provided information about the mechanisms responsible of their antimicrobial activity and suggested possible applications of the OBPs as alternative or co-adjuvants to antibiotic therapeutic treatments.
Highlights
In the present study we investigated whether vertebrate odorant binding protein (OBP) might exhibit antimicrobial activity against Candida albicans, Pseudomonas aeruginosa and other pathogenic microorganisms
The research, which was focused on the bovine and porcine forms of OBP, was divided in two steps: i) characterization of the binding capabilities of OBPs toward small diffusible molecules produced by Candida albicans (CA) and Pseudomonas aeruginosa (PA), i.e. farnesol, acyl-homoserine lactones (AHLs) and pyocyanin, able to influence their pathogenicity as well as their ecological relationships; ii) time kill assays to highlight the direct potential antimicrobial activities of the protein against different pathogenic yeasts and bacteria
This condition, on one side allows to detect the interactions between the OBPs and compounds whose affinities are lower than those of the typical ligands of the protein, and on the other it simulates the physiological environment of the mucus layering the epithelia of the respiratory tract, where the OBP is dissolved at millimolar levels
Summary
The Gram-negative bacterium Pseudomonas aeruginosa (PA) and the yeast Candida albicans (CA), which are commonly found in mixed infections in numerous animal species and in humans, are examples of pathogenic microorganisms that can be fatal in both. Each QS is characterized by a signal compound, named quorum sensing molecule (QSM), that is produced by a synthase (I-protein), released into the environment and recognized by a receptor (R-protein) of other microorganisms of the same species present in the same niche [4]. Due to the partial positivity of these tests, the contribution of the tertiary and quaternary structures on the efficacy of the antimicrobial activity of the proteins against CA was evaluated using the two functional deswapped monomeric mutants of bovine OBP, namely M3-bOBP (with the alpha helix freely movable in solution) [32] and GCC-bOBP (with a reconstructed monomeric Lipocalin folding) [33] characterized by the same ligand binding specificity and affinity with respect to the native dimeric form. With the aim of evaluating the possibility of a biotechnological applications of these proteins as antimicrobials, TKA tests were performed with both bovine and porcine OBPs tagged with six histidine residues at the amino terminal [23], since the histidine content is considered a crucial element to provide histatines, natural peptides dissolved in saliva, with antifungal properties against CA [34]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
