The Population and Evolutionary Dynamics of Phage and Bacteria with CRISPR–Mediated Immunity

• Type of Material: Article
• Authors: Bruce Levin, Emory University; Sylvain Moineau, Université Laval; Mary Bushman, Emory University; Rodolphe Barrangou, DuPont Nutrition and Health
• Language: English
• Date: 2013-03-14
• Publisher: Public Library of Science
• Publication Version: Final Published Version
• Copyright Statement: © 2013 Levin et al.
• License: Creative Commons Attribution 2.0 Generic
• Final Published Version (URL): http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1003312
• Title of Journal or Parent Work: PLoS Genetics
• ISSN: 1553-7390
• Volume: 9
• Issue: 3
• Start Page: e1003312
• End Page: e1003312
• Grant/Funding Information: Funding for this research was provided by a grant from the U.S. National Institutes of Health GM 091875 (BRL) and the Natural Sciences and Engineering Research Council (NSERC) of Canada (Discovery program) (SM).
• Research Categories: Biology, Microbiology; Biology, Genetics; Health Sciences, Immunology

Abstract

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), together with associated genes (cas), form the CRISPR–cas adaptive immune system, which can provide resistance to viruses and plasmids in bacteria and archaea. Here, we use mathematical models, population dynamic experiments, and DNA sequence analyses to investigate the host–phage interactions in a model CRISPR–cas system, Streptococcus thermophilus DGCC7710 and its virulent phage 2972. At the molecular level, the bacteriophage-immune mutant bacteria (BIMs) and CRISPR–escape mutant phage (CEMs) obtained in this study are consistent with those anticipated from an iterative model of this adaptive immune system: resistance by the addition of novel spacers and phage evasion of resistance by mutation in matching sequences or flanking motifs. While CRISPR BIMs were readily isolated and CEMs generated at high rates (frequencies in excess of 10−6), our population studies indicate that there is more to the dynamics of phage–host interactions and the establishment of a BIM–CEM arms race than predicted from existing assumptions about phage infection and CRISPR–cas immunity. Among the unanticipated observations are: (i) the invasion of phage into populations of BIMs resistant by the acquisition of one (but not two) spacers, (ii) the survival of sensitive bacteria despite the presence of high densities of phage, and (iii) the maintenance of phage-limited communities due to the failure of even two-spacer BIMs to become established in populations with wild-type bacteria and phage. We attribute (i) to incomplete resistance of single-spacer BIMs. Based on the results of additional modeling and experiments, we postulate that (ii) and (iii) can be attributed to the phage infection-associated production of enzymes or other compounds that induce phenotypic phage resistance in sensitive bacteria and kill resistant BIMs. We present evidence in support of these hypotheses and discuss the implications of these results for the ecology and (co)evolution of bacteria and phage.