Hitting with a BAM: Selective Killing by Lectin-Like Bacteriocins.

Maarten G. K. Ghequire,Toon Swings,René De Mot,Susan K. Buchanan,Jan Michiels,Marvin Whiteley,Stephen P. Diggle

doi:10.1128/mbio.02138-17

Abstract

ABSTRACTLectin-like bacteriocins (LlpAs) are secreted by proteobacteria and selectively kill strains of their own or related species, and they are composed of two B-lectin domains with divergent sequences. In Pseudomonas spp., initial binding of these antibacterial proteins to cells is mediated by the carboxy-terminal domain through d-rhamnose residues present in the common polysaccharide antigen of their lipopolysaccharide, whereas the amino-terminal domain accounts for strain selectivity of killing. Here, we show that spontaneous LlpA-resistant mutants carry mutations in one of three surface-exposed moieties of the essential β-barrel outer membrane protein insertase BamA, the core component of the BAM complex. Polymorphism of this loop in different Pseudomonas groups is linked to LlpA susceptibility, and targeted cells all share the same signature motif in this loop. Since heterologous expression of such a bamA gene confers LlpA susceptibility upon a resistant strain, BamA represents the primary bacteriocin selectivity determinant in pseudomonads. Contrary to modular bacteriocins that require uptake via the Tol or Ton system, parasitism of BamA as an LlpA receptor advocates a novel bacteriocin killing mechanism initiated by impairment of the BAM machinery.

Highlights

Lectin-like bacteriocins (LlpAs) are secreted by proteobacteria and selectively kill strains of their own or related species, and they are composed of two B-lectin domains with divergent sequences
Pseudomonas fluorescens Pf0-1 was chosen as an indicator. This strain is highly susceptible to recombinant LlpA1 from the biocontrol model strain Pseudomonas protegens Pf-5 [25, 26] and its genome has been completely sequenced [27], facilitating single nucleotide polymorphism (SNP) analysis
An extra gene involved in the fuzzy spreader phenotype and associated with a partially similar cluster was detected in P. syringae pv. actinidiae ICMP 18884 (Fig. S1) [29]. These findings suggest that the carbohydrate binding by LlpAs may involve accessory moieties in addition to D-rhamnose, and they hint at subtle differences in LlpA-LPS interactions that depend on the particular bacteriocin-target strain combination

Summary

Introduction

Lectin-like bacteriocins (LlpAs) are secreted by proteobacteria and selectively kill strains of their own or related species, and they are composed of two B-lectin domains with divergent sequences. We show that spontaneous LlpA-resistant mutants carry mutations in one of three surface-exposed moieties of the essential ␤-barrel outer membrane protein insertase BamA, the core component of the BAM complex Polymorphism of this loop in different Pseudomonas groups is linked to LlpA susceptibility, and targeted cells all share the same signature motif in this loop. A particular surface-exposed loop of BamA, critical for its function, serves as a key discriminant for cellular recognition, and polymorphisms in this loop determine whether a strain is susceptible or immune to a particular bacteriocin These findings suggest a novel mechanism of contact-dependent killing that does not require cellular uptake. Via the construction of engineered LlpA chimeras, strain selectivity of killing by LlpAs was attributed to the amino-terminal lectin domain [21]

Methods

Results

Conclusion