Characterisation of two quorum sensing systems in the endophytic Serratia plymuthica strain G3: differential control of motility and biofilm formation according to life-style.

Xiaoguang Liu,Jinli Jia,Kexiang Gao,Stephen P Diggle,Roman Popat,Miguel Cámara,Jun Li,Catherine A Ortori

doi:10.1186/1471-2180-11-26

Xiaoguang Liu, Jinli Jia + Show 6 more

Open Access

https://doi.org/10.1186/1471-2180-11-26

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Abstract

BackgroundN-acylhomoserine lactone (AHL)-based quorum sensing (QS) systems have been described in many plant-associated Gram-negative bacteria to control certain beneficial phenotypic traits, such as production of biocontrol factors and plant growth promotion. However, the role of AHL-mediated signalling in the endophytic strains of plant-associated Serratia is still poorly understood. An endophytic Serratia sp. G3 with biocontrol potential and high levels of AHL signal production was isolated from the stems of wheat and the role of QS in this isolate was determined.ResultsStrain G3 classified as Serratia plymuthica based on 16S rRNA was subjected to phylogenetic analysis. Using primers to conserved sequences of luxIR homologues from the Serratia genus, splIR and spsIR from the chromosome of strain G3 were cloned and sequenced. AHL profiles from strain G3 and Escherichia coli DH5α expressing splI or spsI from recombinant plasmids were identified by liquid chromatography-tandem mass spectrometry. This revealed that the most abundant AHL signals produced by SplI in E. coli were N-3-oxo-hexanoylhomoserine lactone (3-oxo-C6-HSL), N-3-oxo-heptanoylhomoserine lactone (3-oxo-C7-HSL), N-3-hydroxy-hexanoylhomoserine lactone (3-hydroxy-C6-HSL), N-hexanoylhomoserine lactone (C6-HSL), and N-heptanoyl homoserine lactone (C7-HSL); whereas SpsI was primarily responsible for the synthesis of N-butyrylhomoserine lactone (C4-HSL) and N-pentanoylhomoserine lactone (C5-HSL). Furthermore, a quorum quenching analysis by heterologous expression of the Bacillus A24 AiiA lactonase in strain G3 enabled the identification of the AHL-regulated biocontrol-related traits. Depletion of AHLs with this lactonase resulted in altered adhesion and biofilm formation using a microtiter plate assay and flow cells coupled with confocal laser scanning microscopy respectively. This was different from the closely related S. plymuthica strains HRO-C48 and RVH1, where biofilm formation for both strains is AHL-independent. In addition, QS in G3 positively regulated antifungal activity, production of exoenzymes, but negatively regulated production of indol-3-acetic acid (IAA), which is in agreement with previous reports in strain HRO-C48. However, in contrast to HRO-C48, swimming motility was not controlled by AHL-mediated QS.ConclusionsThis is the first report of the characterisation of two AHL-based quorum sensing systems in the same isolate of the genus Serratia. Our results show that the QS network is involved in the global regulation of biocontrol-related traits in the endophytic strain G3. However, although free-living and endophytic S. plymuthica share some conservation on QS phenotypic regulation, the control of motility and biofilm formation seems to be strain-specific and possible linked to the life-style of this organism.

Highlights

N-acylhomoserine lactone (AHL)-based quorum sensing (QS) systems have been described in many plant-associated Gram-negative bacteria to control certain beneficial phenotypic traits, such as production of biocontrol factors and plant growth promotion
Phylogenetic classification of S. plymuthica G3 To classify phylogenetically the G3 strain isolated from wheat stems, the sequence from the 1474-bp fragment of 16S rDNA from this isolate we previously determined (EU344964) [23] was subjected to phylogenetic analysis with different 16S rDNA sequences from members of the genus Serratia and E. coli strain ATCC 25922 as the outgroup
It is worth noting that the atypical S. plymuthica RVH1 strain is unable to produce prodigiosin pigment when compared to the S. plymuthica DSM 4540 type strain, but a combined comparative analysis of 16S rRNA and gyrB sequences, DNADNA hybridization, and biochemical characteristics unequivocally identified this strain as S. plymuthica [7]

Summary

Introduction

N-acylhomoserine lactone (AHL)-based quorum sensing (QS) systems have been described in many plant-associated Gram-negative bacteria to control certain beneficial phenotypic traits, such as production of biocontrol factors and plant growth promotion. Endophytic bacteria reside within the living tissue of their host plants without substantively harming it [1] They can be beneficial to their host by promoting plant growth or acting as biocontrol agents [2,3]. Serratia plymuthica is ubiquitously distributed in nature, and most frequently associated with plants. This organism has been isolated from the rhizosphere and the phyllosphere of various plants, as an endophyte from the endorhiza of potato [4,5], or as a contaminant in a raw vegetable processing line [6,7]. Based on the international approved German directive (TRBA 466), it is nowadays classified within the risk group 1 by the DSMZ (German Collection of Micro-organisms and Cell Cultures), indicating that the species does not pose a threat to human health [5]

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