Functional impact of a type VI secretion system on RsmE-mediated spatial structure formation in Pseudomonas fluorescens Pf0-1

Bacteria naturally form densely structured populations wherein both space and nutrients become locally limited. In densely populated Pseudomonas fluorescens Pf0-1 colonies, spatiogenetic patches with drastically reduced cellular density naturally emerge through spontaneous mutations in rsmE. RsmE is a posttranscriptional regulator that binds to specific mRNAs, including those that consequently repress the production of extracellular secretions. We have previously shown that {Delta}rsmE produces an extracellular polysaccharide (EPS) and biosurfactant that collectively function to define the spatiogenetic structure and locally outcompete the WT. Here, we identify additional RsmE-regulated secretions through a combination of RNA-sequencing and LC-MS/MS. In particular, a type VI secretion system (T6SS) was exclusively produced by {Delta}rsmE. Confocal microscopy imaging of WT and genetically modified {Delta}rsmE cocultures showed that the T6SS kills WT cells that invade the low- density spatiogenetic structure. However, its impact on competition with the WT was both quantifiably and spatially limited in the absence of the normally co-produced EPS and biosurfactant, largely due to the consequently altered spatial structure. These results highlight the importance of understanding both the individual and collective spatial functions of extracellular secretions, especially in the ecological context of densely structured microbial populations. IMPORTANCEIn crowded wild type (WT) Pseudomonas fluorescens Pf0-1 colonies, rsmE mutants spontaneously emerge by forming a low cellular density spatial structure that is devoid of WT cells. RsmE functions primarily to repress the production of various extracellular secretions, including an extracellular polysaccharide and biosurfactant, that collectively form the unique spatial structure. Here, we describe an RsmE-regulated type VI secretion system that kills the invading WT cells and protect the low-density structure. Importantly, individual RsmE-regulated secretions carry out unique functions, but their efficacies largely depend on other co-regulated products. Extracellular secretions that are mechanistically similar to those described here are co- produced across diverse biofilm-forming and virulent bacterial species, where they also likely play complex molecular and ecological roles.