Interactions between bacterial and fungal cells shape many polymicrobial communities. Bacteria elaborate diverse strategies to interact and compete with other organisms, including the deployment of protein secretion systems. The type VI secretion system (T6SS) delivers toxic effector proteins into host eukaryotic cells and competitor bacterial cells, but, surprisingly, T6SS-delivered effectors targeting fungal cells have not been reported. Here we show that the ‘antibacterial’ T6SS of Serratia marcescens can act against fungal cells, including pathogenic Candida species, and identify the previously undescribed effector proteins responsible. These antifungal effectors, Tfe1 and Tfe2, have distinct impacts on the target cell, but both can ultimately cause fungal cell death. ‘In competition’ proteomics analysis revealed that T6SS-mediated delivery of Tfe2 disrupts nutrient uptake and amino acid metabolism in fungal cells, and leads to the induction of autophagy. Intoxication by Tfe1, in contrast, causes a loss of plasma membrane potential. Our findings extend the repertoire of the T6SS and suggest that antifungal T6SSs represent widespread and important determinants of the outcome of bacterial–fungal interactions.
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This work was supported by the Wellcome Trust (Senior Research Fellowship in Basic Biomedical Science to S.J.C., 104556; 097377, J.Q.; 101873 and 200208, N.A.R.G.), the MRC (MR/K000111X/1, S.J.C.; MC_UU_12016/5, M.T.), and the BBSRC (BB/K016393/1 and BB/P020119/1, J.Q.). We thank M. Fritsch, M. López Martín and B. Hollmann for help with strain construction; G. Eitzen for construction of pGED1; D. MacCallum for the gift of Candida glabrata ATCC2001; J. Morschhäuser for the gift of pNIM1; G. Milne (Microscopy and Histology facility, University of Aberdeen) for assistance with TEM; and P. Taylor, G. Mariano, M. Porter, L. Monlezun and C. Rickman for advice and technical assistance.
The authors declare no competing interests.
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Supplementary Figures 1–18, Supplementary Tables 1–4, Supplementary References.
Proteomics data for all S. marcescens proteins quantified by label-free quantitative mass spectrometry analysis of the S. marcescens cellular proteome.
Proteomics data for all C. albicans and S. marcescens proteins quantified by TMT-labelling mass spectrometry analysis following co-culture (in competition proteomics experiment).
Full clustering of ANOVA-positive C. albicans proteins from the in competition proteomics experiment.
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Trunk, K., Peltier, J., Liu, YC. et al. The type VI secretion system deploys antifungal effectors against microbial competitors. Nat Microbiol 3, 920–931 (2018). https://doi.org/10.1038/s41564-018-0191-x
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