Enterococcus faecalis Enhances Expression and Activity of the Enterohemorrhagic Escherichia coli Type III Secretion System Elizabeth A. Cameron, a,b,c Vanessa Sperandio, a,b Gary M. Dunny c a Department of Microbiology, University of Texas Southwestern Medical Center, Dallas, Texas, USA b Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, Texas, USA c Department of Microbiology and Immunology, University of Minnesota, Minneapolis, Minnesota, USA ABSTRACT The gut microbiota can significantly impact invading pathogens and the disease they cause; however, many of the mechanisms that dictate commensal- pathogen interactions remain unclear. Enterohemorrhagic Escherichia coli (EHEC) is a potentially lethal human intestinal pathogen that uses microbiota-derived molecules as cues to efficiently regulate virulence factor expression. Here, we investigate the interaction between EHEC and Enterococcus faecalis, a common human gut com- mensal, and show that E. faecalis affects both expression and activity of the EHEC type III secretion system (T3SS) via two distinct mechanisms. First, in the presence of E. faecalis there is increased transcription of genes encoding the EHEC T3SS. This leads to increased effector translocation and ultimately greater numbers of pedestals formed on host cells. The same effect was observed with several strains of entero- cocci, suggesting that it is a general characteristic of this group. In a mechanism separate from E. faecalis-induced transcription of the T3SS, we report that an E. faecalis-secreted protease, GelE, cleaves a critical structural component of the EHEC T3SS, EspB. Our data suggest that this cleavage actually increases effector transloca- tion by the T3SS, supporting a model where EspB proteolysis promotes maximum T3SS activity. Finally, we report that treatment of EHEC with E. faecalis-conditioned cell-free medium is insufficient to induce increased T3SS expression, suggesting that this effect relies on cell contact between E. faecalis and EHEC. This work demon- strates a complex interaction between a human commensal and pathogen that im- pacts both expression and function of a critical virulence factor. IMPORTANCE This work reveals a complex and multifaceted interaction between a human gut commensal, Enterococcus faecalis, and a pathogen, enterohemorrhagic E. coli. We demonstrate that E. faecalis enhances expression of the enterohemorrhagic E. coli type III secretion system and that this effect likely depends on cell contact be- tween the commensal and the pathogen. Additionally, the GelE protease secreted by E. faecalis cleaves a critical structural component of the EHEC type III secretion system. In agreement with previous studies, we find that this cleavage actually in- creases effector protein delivery into host cells by the secretion system. This work demonstrates that commensal bacteria can significantly shape expression and activ- ity of pathogen virulence factors, which may ultimately shape the progression of dis- ease. KEYWORDS commensal pathogen interaction, gut microbiome, virulence regulation, EHEC, bacterial communication I nvading intestinal pathogens encounter a dense microbial community containing hundreds of different bacterial species, each with its own biochemical repertoire. The interactions between pathogens and members of the gut microbiota are complex, and both antagonistic and mutualistic relationships have been previously described (1). The Citation Cameron EA, Sperandio V, Dunny GM. 2019. Enterococcus faecalis enhances expression and activity of the enterohemorrhagic Escherichia coli type III secretion system. mBio 10:e02547-19. https:// doi.org/10.1128/mBio.02547-19. Editor Scott J. Hultgren, Washington University School of Medicine Copyright © 2019 Cameron et al. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license. Address correspondence to Gary M. Dunny, dunny001@umn.edu. This article is a direct contribution from Gary M. Dunny, a Fellow of the American Academy of Microbiology, who arranged for and secured reviews by Michael Gilmore, Harvard Medical School, and John Leong, Tufts University School of Medicine. Received 25 September 2019 Accepted 11 October 2019 Published RESEARCH ARTICLE Host-Microbe Biology November/December 2019 Volume 10 Issue 6 e02547-19 ® mbio.asm.org 1 19 November 2019 Downloaded from https://journals.asm.org/journal/mbio on 15 February 2022 by 3.81.219.114.