How adaptive immunity constrains the composition and fate of large bacterial populations

TL;DR

This paper models how CRISPR-Cas immune systems, regulated by quorum sensing, influence bacterial population structure and stability, predicting a stable rank-abundance distribution of spacers and coexistence of multiple ecological states.

Contribution

It introduces a population-level model linking CRISPR spacer dynamics, quorum sensing regulation, and ecological stability, supported by empirical data.

Findings

Stable, time-invariant spacer rank-abundance distribution predicted by the model

Multiple stable ecological states with varying phage-to-bacterium ratios identified

Model validated with empirical spacer-tracking data

Abstract

Features of the CRISPR-Cas system, in which bacteria integrate small segments of phage genome (spacers) into their DNA to neutralize future attacks, suggest that its effect is not limited to individual bacteria but may control the fate and structure of whole populations. Emphasizing the population-level impact of the CRISPR-Cas system, recent experiments show that some bacteria regulate CRISPR-associated genes via the quorum sensing (QS) pathway. Here we present a model that shows that from the highly stochastic dynamics of individual spacers under QS control emerges a rank-abundance distribution of spacers that is time-invariant, a surprising prediction that we test with dynamic spacer-tracking data from literature. This distribution depends on the state of the competing phage-bacteria population, which due to QS-based regulation may coexist in multiple stable states that vary significantly in their phage-to-bacterium ratio, a widely used ecological measure to characterize microbial systems.