A subcellular biochemical model for T6SS dynamics reveals winning competitive strategies

Abstract

The Type VI secretion system (T6SS) is a broadly distributed interbacterial weapon that can be used to eliminate competing bacterial populations. Although unarmed target populations are typically used to study T6SS function, bacteria most likely encounter other T6SS-armed competitors in nature. The outcome of such battles is not well understood, neither is the connection between the outcomes with the subcellular details of the T6SS. Here, we incorporated new biological data derived from natural competitors of Vibrio fischeri light organ symbionts to build a biochemical model for T6SS function at the single cell level. The model accounts for activation of structure formation, structure assembly, and deployment. By developing an integrated agent-based model (IABM) that incorporates strain-specific T6SS parameters, we replicated outcomes of biological competitions, validating our approach. We used the IABM to isolate and manipulate strain-specific physiological differences between competitors, in a way that is not possible using biological samples, to identify winning strategies for T6SS-armed populations. We found that a tipping point exists where the cost of building more T6SS weapons outweighs their protective ability. Furthermore, we found that competitions between a T6SS-armed population and a unarmed target had different outcomes dependent on the geometry of the battlefield: target cells survived at the edges of a range expansion scenario where unlimited territory could be claimed, while competitions within a confined space, much like the light organ crypts where natural V. fischeri compete, resulted in the rapid elimination of the unarmed competitor.

TheVibrio fischeritype VI secretion system incurs a fitness cost under host-like conditions

“…Future work exploring the extent to which the T6SS2 weapon is maintained or decommissioned in stationary phase cells will greatly inform our understanding of the fitness cost and how such regulatory strategies may impact competitive outcomes. Despite these unknowns, recent work that integrates subcellular T6SS dynamics into an agent-based model for V. fischeri probed the impact of the cost of this weapon in silico (11). Lin and Smith et al competed strains that both wield T6SS weapons in an integrated agent-based model where the per cell sheath number and rate of sheath synthesis incurred a growth penalty.…”

“…Despite these unknowns, recent work that integrates subcellular T6SS dynamics into an agent-based model for V. fischeri probed the impact of the cost of this weapon in silico (11). Lin and Smith et al .…”

“…The copyright holder for this preprint this version posted March 7, 2023. ; https://doi.org/10.1101/2023.03.07.529561 doi: bioRxiv preprint structures per cell (11), where cells must build three new T6SS structures after cell division and all six structures fire at a rate of 12 secretion events hr -1 structure -1 (2). For stationary phase cells where growth and death rates are balanced, we assume no net T6SS structure synthesis and only calculate the estimated cost of T6SS firing for six structures cell -1 .…”

“…Briefly, calculations included the energy required to build all protein subunits at a cost of 4-5 ATP equivalents per peptide bond (23), as well as ATP-dependent ClpV cost estimates (2). We assumed each cell contains six T6SSs per cell (11), and each daughter cell must synthesize three new structures during active growth, while stationary cells only incur the cost of firing the T6SS (Fig 2). To calculate the percent of total ATP use, we divided the ATP cost estimates for T6SS2 cell -1 by the total ATPs used cell -1 over the same time scale using values reported in Deng et al, (24), which calculated ATPs used cell -1 sec -1 for E. coli growth in exponential (6.4 million ATPs cell -1 sec -1 ) and stationary phase (200,000 -800,000 ATPs cell -1 sec -1 ), and found that the cellular ATP consumption rate increased with increasing growth rate.…”

“…Moreover, several protein subunits are secreted with each “firing” and a ClpV ATPase is required to disassemble the contracted sheath, adding to the cost of each secretion event (2). Because cells have a limited amount of ATP to hydrolyze, and this ATP pool must be divided into biomass production (including T6SSs) and cellular maintenance (including T6SS firing) (Fig 1B), it has been predicted that T6SS use may be energetically costly (2, 5, 6, 7, 8, 9, 10, 11). Yet the cost of the T6SS has remained largely theoretical, with only one reported observation of a conditional fitness cost in Campylobacter jejuni (12).…”