Characterizing the advantages of diverse effector secretion in Pseudomonas aeruginosa

Abstract

In the environment, bacteria exist in complex communities with other bacterial species, where they compete for resources. Cells are frequently in close proximity to one another, and therefore, many bacteria have evolved pathways to intoxicate their neighbors directly. One such pathway is the type VI secretion system (T6SS) which is broadly distributed in Gram-negative bacterial species. The T6SS antagonizes nearby cells by directly injecting them with toxic antimicrobial proteins, termed effectors. Effectors target conserved, essential structures in the cell, and ultimately lead to cell death. The known biochemical activities of effectors are diverse, and T6SSs often encode multiple effectors. A single effector is enough to incapacitate a targeted cell, and therefore there is likely a benefit to the cell in deploying multiple toxins, however, this question has never been scientifically addressed. In this work, I investigate the advantages of multiple effector secretion and describe the toxic mechanism of an effector of unknown activity that synergizes with other T6SS effectors. I developed a technique to assess effector activity in parallel under a variety of environmental conditions and applied this technique to effectors of the Pseudomonas aeruginosa H1-T6SS. This approach revealed that T6SS effector potency is dependent on the external environment and on the activity of simultaneously secreted effectors. Furthermore, I discovered intoxication by T6SS effectors could be species-specific–highlighting the value of secreting more than one toxin against a broad phylogenetic range of bacterial species. Finally, I contributed to a deeper understanding of how these effectors are secreted by defining critical T6SS structural protein-protein interactions. In total, these data uncovered the significance of multiple effector secretion in overcoming the unpredictability of environmental conditions during bacterial competition.