Abstract
Microbes have evolved many strategies to adapt to changes in environmental conditions and population structures, including cooperation and competition. One apparently competitive mechanism is contact dependent growth inhibition (CDI). Identified in Escherichia coli, CDI is mediated by Two–Partner Secretion (TPS) pathway proteins, CdiA and CdiB. Upon cell contact, the toxic C-terminus of the TpsA family member CdiA, called the CdiA-CT, inhibits the growth of CDI− bacteria. CDI+ bacteria are protected from autoinhibition by an immunity protein, CdiI. Bioinformatic analyses indicate that CDI systems are widespread amongst α, β, and γ proteobacteria and that the CdiA-CTs and CdiI proteins are highly variable. CdiI proteins protect against CDI in an allele-specific manner. Here we identify predicted CDI system-encoding loci in species of Burkholderia, Ralstonia and Cupriavidus, named bcpAIOB, that are distinguished from previously-described CDI systems by gene order and the presence of a small ORF, bcpO, located 5′ to the gene encoding the TpsB family member. A requirement for bcpO in function of BcpA (the TpsA family member) was demonstrated, indicating that bcpAIOB define a novel class of TPS system. Using fluorescence microscopy and flow cytometry, we show that these genes are expressed in a probabilistic manner during culture of Burkholderia thailandensis in liquid medium. The bcpAIOB genes and extracellular DNA were required for autoaggregation and adherence to an abiotic surface, suggesting that CDI is required for biofilm formation, an activity not previously attributed to CDI. By contrast to what has been observed in E. coli, the B. thailandensis bcpAIOB genes only mediated interbacterial competition on a solid surface. Competition occurred in a defined spatiotemporal manner and was abrogated by allele-specific immunity. Our data indicate that the bcpAIOB genes encode distinct classes of CDI and TPS systems that appear to function in sociomicrobiological community development.
Highlights
Contact dependent growth inhibition (CDI) is a phenomenon discovered in Escherichia coli in which CDI+ bacteria inhibit the growth of CDI2 bacteria upon cell-to-cell contact
The genes encoding these systems are expressed in a stochastic manner such that only a few cells in the population produce the proteins at any given time when grown in broth. We show that these systems are required for aggregation on an abiotic surface, suggesting an important role for CDI in biofilm formation
We characterize the unique class of CDI systems produced by Burkholderia spp. We show that these systems compose a novel class of Two–Partner Secretion (TPS) system that requires a third protein for the large exoprotein to function, that expression of Burkholderia CDI proteinencoding genes is regulated in a probabilistic manner, that the gene products contribute to biofilm formation, and that CDImediated interbacterial competition in Burkholderia occurs on solid surfaces in a unique temporal and spatial pattern
Summary
Whether free in the environment or in close association with eukaryotic hosts, microbes typically share their living space with other viral, prokaryotic, and/or eukaryotic microorganisms. Survival under these conditions requires mechanisms for sensing, responding to, and cooperating or competing with neighboring organisms. Contact dependent growth inhibition (CDI) systems are protein toxin delivery mechanisms that appear to be involved in interbacterial competition [1]. TPS systems are widespread amongst Gram-negative bacteria. They export large exoproteins (TpsA family members such as CdiA) across the outer membrane using pore-forming b-barrel proteins (TpsB family members such as CdiB) [2,3]. Characterization of CDI in E. coli revealed an additional TpsA-
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