Conjugative type IVb pilus recognizes lipopolysaccharide of recipient cells to initiate PAPI-1 pathogenicity island transfer in Pseudomonas aeruginosa

Toan Phuoc Hong,Joseph S Lam,Paolo Struffi,Stephen Lory,Olivier Jousson,Michelle Q Carter,Stefano Casonato,Youai Hao

doi:10.1186/s12866-017-0943-4

Toan Phuoc Hong, Joseph S Lam + Show 6 more

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https://doi.org/10.1186/s12866-017-0943-4

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Abstract

BackgroundPseudomonas aeruginosa pathogenicity island 1 (PAPI-1) is one of the largest genomic islands of this important opportunistic human pathogen. Previous studies have shown that PAPI-1 encodes several putative virulence factors, including a major regulator of biofilm formation and antibiotic-resistance traits. PAPI-1 is horizontally transferable into recipient strains lacking this island via conjugation mediated by the specialized type IV pilus. The PAPI-1 encodes a cluster of ten genes associated with the synthesis and assembly of the type IV pilus. The PAPI-1 acquisition mechanism is currently not well understood.ResultsIn this study, we performed a series of conjugation experiments and identified determinants of PAPI-1 acquisition by analyzing transfer efficiency between the donor and a series of mutant recipient strains. Our data show that common polysaccharide antigen (CPA) lipopolysaccharide (LPS), a homopolymer of D-rhamnose, is required for initiating PAPI-1 transfer, suggesting that this structure acts as a receptor for conjugative type IV pilus in recipient strains. These results were substantiated by experimental evidence from PAPI-1 transfer assay experiments, in which outer membrane or LPS preparations from well-defined LPS mutants were added to the transfer mix to assess the role of P. aeruginosa LPS in PAPI-1 transfer and in vitro binding experiments between pilin fusion protein GST-pilV2’ and immobilized LPS molecules were performed. Our data also showed that P. aeruginosa strains that had already acquired a copy of PAPI-1 were unable to import additional copies of the island, and that such strains produced proportionally lower amounts of CPA LPS compared to the strains lacking PAPI-1.ConclusionsThese results suggest that a PAPI-1 exclusion mechanism exists in P. aeruginosa that might serve to regulate the avoidance of uncontrolled expansions of the bacterial genome.

Highlights

Pseudomonas aeruginosa pathogenicity island 1 (PAPI-1) is one of the largest genomic islands of this important opportunistic human pathogen
PAO1 derivatives defective in common polysaccharide antigen (CPA) LPS biosynthesis are deficient in the acquisition of PAPI-1 It has been known that PAPI-1 island is transferable from one to another P.aeruginosa strain lacking it by conjugation mechanism via type IVb pilus
Since in conjugal plasmid system the donor pilus is known to interact with specific components of LPS on the recipient membrane to initiate the transfer [29], we examined the effects of using various PAO1 mutants for LPS biosynthesis as recipients on PAPI-1 transfer assay (Fig. 1)

Summary

Introduction

Pseudomonas aeruginosa pathogenicity island 1 (PAPI-1) is one of the largest genomic islands of this important opportunistic human pathogen. PAPI-1 is horizontally transferable into recipient strains lacking this island via conjugation mediated by the specialized type IV pilus. Horizontal gene transfer (HGT) mediated by microorganisms is a major evolutionary mechanism for the acquisition of new functionalities. A number of GIs appear to belong to the group of ancient ICEs that became fixed in the bacterial chromosome due to degeneration of their conjugative elements by deletion of integration sites or mutations in genes encoding transfer functions [9]. The best-characterized ICEs to date contain specific features associated with conjugative plasmids and bacteriophages; can be transferred horizontally following recognition of the recipient cell by the donor utilizing a conjugative mechanism that, in many instances, is associated with the type IV protein secretion pathway [10]. The recipient cell is recognized by the pilus structure that is part of the type IV secretion apparatus of the donor [11]

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