Genome-wide fitness profiling reveals flagellar rotation as an energetic liability during anaerobic maintenance in Pseudomonas aeruginosa

Abstract

Bacteria in nature and disease frequently spend time in non-growing states, yet the genes that allow cells to survive without growing remain poorly understood. Using Pseudomonas aeruginosa strain PA14 as a model system to study non-growth powered by anaerobic phenazine cycling, we employed randomly barcoded transposon-insertion sequencing (RB-TnSeq) to identify the genes required for this maintenance state using a platform that sustains anaerobic survival via continuous phenazine reoxidation. We found 167 genes whose disruption altered survival, including genes involved in transcription, translation, protein quality control, and cell envelope maintenance. Comparing our results to published TnSeq data from other forms of energy-limited growth arrest (conditions where electron acceptor or carbon availability constrains energy conservation below that required for cell growth/division) in PA14 revealed that while a subset of genes are fitness determinants across distinct growth-arrested states, most are condition-specific. Notably, genes involved in flagellar regulation and assembly were broadly detrimental to survival under this maintenance condition. Leveraging a high-throughput electrochemical system that allows for quantitative and mechanistic dissection of the phenazine cycling-dependent maintenance state, we found that flagellar abundance influences cells’ survival, metabolic rate, and ATP levels. Moreover, removing the flagellar stator proteins MotAC, which are required for flagellar rotation but not assembly, reversed these defects, indicating that the energetic cost of flagella comes from their rotation rather than construction under these conditions. These results show that in a low-powered maintenance state, limiting energy-dissipating processes, such as proton-motive force loss through flagellar rotation, supports cell survival. Importance The genetic landscape governing non-growth survival under energy limitation, and the cellular processes that help or hinder it, remains largely uncharacterized. What does a cell need to do, and what must it avoid, to survive when growth-arrested? Answering this question is clinically important because non-growing bacteria are often antibiotic tolerant. Here, we used a genome-wide screen of Pseudomonas aeruginosa strain PA14, an opportunistic pathogen, to identify fitness determinants of viability during a non-growth state powered by anaerobic phenazine-cycling, a state mimicking the anoxic cores of biofilms. Our findings suggest that growth-arrested cells live at a bioenergetic knife’s edge, where an energy-dissipating process, like a rotating flagellum, can tip the balance between vitality and death, providing a possible bioenergetic rationale for the known repression of flagellar expression in mature biofilm cores.