Cost and benefits of CRISPR spacer acquisition

TL;DR

This paper models how CRISPR spacer acquisition rates influence bacterial survival against phages, revealing optimal strategies depend on initial populations and auto-immunity costs, with some conditions ensuring protection without spacer acquisition.

Contribution

It introduces a simple coupled dynamics model to analyze how acquisition rates affect bacterial survival and auto-immunity risks, highlighting conditions for optimal immunity strategies.

Findings

Optimal acquisition rates depend on initial virus and bacteria populations.

Certain parameter combinations guarantee protection without spacer acquisition.

High auto-immunity costs limit spacer acquisition rates.

Abstract

CRISPR-Cas mediated immunity in bacteria allows bacterial populations to protect themselves against pathogens. However, it also exposes them to the dangers of auto-immunity by developing protection that targets its own genome. Using a simple model of the coupled dynamics of phage and bacterial populations, we explore how acquisition rates affect the survival rate of the bacterial colony. We find that the optimal strategy depends on the initial population sizes of both viruses and bacteria. Additionally, certain combinations of acquisition and dynamical rates and initial population sizes guarantee protection, due to a dynamical balance between the evolving population sizes, without relying on acquisition of viral spacers. Outside this regime, the high cost of auto-immunity limits the acquisition rate. We discuss these optimal survival strategies in terms of recent experiments.