Fig. 6. The LqqE1-triggered quorum quenching confers L. enzymogenes competitive advantages during co-culture with P. fluorescens.

A Fluorescent images showing the mCherry-labeled L. enzymogenes kills the AHL-deficient P. fluorescens labeled with GFP more efficiently when both species were co-cultured on agar plates with 1:60 rations. B Quantification of the GFP signal intensity in mixed colonies corresponding to (A). C Living cell numbers of P. fluorescens in mixed colonies corresponding to the panel A. Fluorescent images showing the lqqE1 expression P. fluorescens favored the killing efficacy by L. enzymogenes (D) during their co-culture on agar plants, and this is confirmed by quantifying the GFP signal intensity (E) in mixed colonies. Fluorescent images showing lqqE1 in-frame deletion in L. enzymogenes (ΔlqqE1) weakened the killing against P. fluorescens 2P24 (F) during their co-culture on agar plants. The chromosomal complement of lqqE1 at the ΔlqqE1 background restored this killing effect, while chromosomal complementation of lqqE1^E68A and lqqE1^S126A genes with mutations that affect its AHL-quenching function did not. G Quantification of the GFP signal intensity in mixed colonies corresponding to the panel F. In all panels, average data from three experiments was presented, ±SD. ***p < 0.0001, **p < 0.01, assessed by one-way ANOVA. H A proposed model showing the LqqE1-triggered interspecies quorum quenching during the L. enzymogenes-P. fluorescens competition occurring in soil microbiome. Via cell-to-cell contact, L. enzymogenes using T4ASS to kill the competitor, P. fluorescens. To protect the survival of the community members, P. fluorescens forms biofilms via producing the quorum-sensing signal AHL. As a counterattack, L. enzymogenes delivers a T4E protein LqqE1 into the cytoplasm of P. fluorescens. Upon the delivery, LqqE1 directly binds to the AHL synthase PcoI to interfere its recognition with SAM, one of the substrate required for AHL synthesis, by which the P. fluorescens quorum sensing is naturally quenched. Such an interspecies quorum quenching appears to prevent P. fluorescens to form community/biofilms, where the biofilms may act as a physical barrier to prevent the penetration of T4SS and/or delivery of toxic effector proteins from OH11, such as Le0908 presented in this study, enabling L. enzymogenes using T4ASS to more efficiently kill the free-living members of P. fluorescens, thereby gaining a competitive advantage.