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Lectin-like bacteriocins are antibacterial proteins constituted of two structurally similar monocot mannose-binding lectin (MMBL) domains, and are followed by a short C-terminal extension. These so-called LlpA bacteriocins have been characterized in strains of Pseudomonas, Xanthomonas and Burkholderia, and display highly specific intra-genus activity [1]. In contrast to typical Gram-negative bacteriocins (such as Escherichia coli colicins or Pseudomonas aeruginosa S-type pyocins), LlpAs do not require a cognate immunity protein [1,2]. These antibacterials are retrieved in soil-dwelling and plant-associated isolates, but are not widely distributed in these populations. Recently it was demonstrated that only one of the MMBL modules of LlpAs displays sugar-binding properties. Interestingly, LlpAs seem to prefer D-rhamnose over D-mannose. The latter property enables the use of D-rhamnose-containing lipopolysaccharide as a cellular receptor, presumably for initial attachment of the bacteriocin to the outer membrane of a target cell. In contrast, the other MMBL domain of the LlpAs serves a role as a target strain specificity determinant. Presence of both lectin modules is mandatory to obtain a fully active antibacterial molecule [3,4], but the actual killing mechanism of lectin-like bacteriocins remains unknown. We identified two highly-divergent groups of lectin-like bacteriocin genes in recently sequenced Pseudomonas aeruginosa genomes. The encoded proteins show only borderline sequence homology to previously identified LlpAs. Recombinant His-tagged proteins, purified via affinity chromatography, were tested against a large panel of Pseudomonas isolates and other Gram-negative bacteria. Antagonistic interactions were detected solely against P. aeruginosa strains. In addition to environmental isolates, these L pyocins also targeted multidrug-resistant clinical isolates, with minimum inhibitory concentrations down to the nanomolar range. Spectrum overlap between close homologues belonging to one of the L pyocin groups was only minimal. This is in contrast to the spectral redundancy that was observed for LlpAs in other Pseudomonas species. Residues differing between these close homologues, potentially responsible for the spectrum differences observed, were mapped on the L pyocin 3D-structure [4], and appeared to mainly cluster in the domain lacking D-rhamnose binding capacity. We found no correlation between susceptibility to L pyocins and phylogenetic relatedness of the P. aeruginosa isolates, suggesting that a highly variable characteristic is responsible for the spectrum differences observed. Out of 15 O serotypes tested, 13 contained L pyocin susceptible strains, excluding the possibility that the highly variable and immunogenic O serotype antigen of the LPS coating would represent a susceptibility-discriminating factor [5]. Keywords: bacteriocin; lectin; O-specific antigen; Pseudomonas aeruginosa References [1] Ghequire M, De Mot R. (2014) Ribosomally encoded antibacterial proteins and peptides from Pseudomonas. FEMS Microbiology Reviews 38:523-568. [2] Cascales E, Buchanan S, Duché D, Kleanthous C, Lloubès R, Postle K, Riley M, Slatin S, Cavard S. (2007) Colicin biology. Microbiology and Molecular Biology Reviews 71: 158-229. [3] Ghequire M, Garcia-Pino A, Lebbe E, Spaepen S, Loris R, De Mot R. (2013) Structural determinants for activity and specificity of the bacterial toxin LlpA. PLoS Pathogens 9: e1003199. [4] McCaughey L, Grinter R, Josts I, Roszak A, Waløen K, Cogdell R, Milner J, Evans T, Kelly S, Tucker N, Byron O, Smith B, Walker D. (2014) Lectin-like bacteriocins from Pseudomonas spp. utilise D-rhamnose containing lipopolysaccharide as a cellular receptor. PLoS Pathogens 10: e1003898. [5] Ghequire M, Dingemans J, Pirnay JP, De Vos D, De Mot R. (2014) O serotype-independent susceptibility of Pseudomonas aeruginosa to lectin-like pyocins. Microbiology Open [in press]