Spacer Acquisition Rates Determine the Immunological Diversity of the Type II CRISPR-Cas Immune Response

Assessing spacer acquisition rates in E. coli type I-E CRISPR arrays

CRISPR/Cas is an adaptive defense mechanism protecting prokaryotes from viruses and other potentially harmful genetic elements. Through an adaptation process, short "spacer" sequences, captured from these elements and incorporated into a CRISPR array, provide target specificity for the immune response. CRISPR arrays and array expansion are also central to many emerging biotechnologies. The rates at which spacers integrate into native arrays within bacterial populations have not been quantified. Here, we measure naïve spacer acquisition rates in Escherichia coli Type I-E CRISPR, identify factors that affect these rates, and model this process fundamental to CRISPR/Cas defense. Prolonged Cas1-Cas2 expression produced fewer new spacers per cell on average than predicted by the model. Subsequent experiments revealed that this was due to a mean fitness reduction linked to array-expanded populations. In addition, the expression of heterologous non-homologous end-joining DNA-repair genes was found to augment spacer acquisition rates, translating to enhanced phage infection defense. Together, these results demonstrate the impact of intracellular factors that modulate spacer acquisition and identify an intrinsic fitness effect associated with array-expanded populations.

CRISPR-Cas spacer acquisition is a rare event in human gut microbiome

Host-parasite relationships drive the evolution of both parties. In microbe-phage dynamics, CRISPR functions as an adaptive defense mechanism, updating immunity via spacer acquisition. Here, we investigated these interactions within the human gut microbiome, uncovering low frequencies of spacer acquisition at an average rate of one spacer every ∼2.9 point mutations using isolates' whole genomes and ∼2.7 years using metagenome time series. We identified a highly prevalent CRISPR array in Bifidobacterium longum spreading via horizontal gene transfer (HGT), with six spacers found in various genomic regions in 15 persons from the United States and Europe. These spacers, targeting two prominent Bifidobacterium phages, comprised 76% of spacer occurrence of all spacers targeting these phages in all B. longum populations. This result suggests that HGT of an entire CRISPR-Cas system introduced three times more spacers than local CRISPR-Cas acquisition in B. longum. Overall, our findings identified key ecological and evolutionary factors in prokaryote adaptive immunity.

A novel genus of Pectobacterium bacteriophages display broad host range by targeting several species of Danish soft rot isolates

The bacterial diseases black leg and soft rot in potatoes cause heavy losses of potatoes worldwide. Bacteria within the genus Pectobacteriaceae are the causative agents of black leg and soft rot. The use of antibiotics in agriculture is heavily regulated and no other effective treatment currently exists, but bacteriophages (phages) have shown promise as potential biocontrol agents. In this study we isolated soft rot bacteria from potato tubers and plant tissue displaying soft rot or black leg symptoms collected in Danish fields. We then used the isolated bacterial strains as hosts for phage isolation. Using organic waste, we isolated phages targeting different species within Pectobacterium. Here we focus on seven of these phages representing a new genus primarily targeting P. brasiliense; phage Ymer, Amona, Sabo, Abuela, Koroua, Taid and Pappous. TEM image of phage Ymer showed siphovirus morphotype, and the proposed Ymer genus belongs to the class Caudoviricetes, with double-stranded DNA genomes varying from 39 kb to 43 kb. In silico host range prediction using a CRISPR-Cas spacer database suggested both P. brasiliense, P. polaris and P. versatile as natural hosts for phages within the proposed Ymer genus. A following host range experiment, using 47 bacterial isolates from Danish tubers and plants symptomatic with soft rot or black leg disease verified the in silico host range prediction, as the genus as a group were able to infect all three Pectobacterium species. Phages did, however, primarily target P. brasiliense isolates and displayed differences in host range even within the species level. Two of the phages were able to infect two or more Pectobacterium species. Despite no nucleotide similarity with any phages in the NCBI database, the proposed Ymer genus did share some similarity at the protein level, as well as gene synteny, with currently known phages. None of the phages encoded integrases or other genes typically associated with lysogeny. Similarly, no virulence factors nor antimicrobial resistance genes were found, and combined with their ability to infect several soft rot-causing Pectobacterium species from Danish fields, demonstrates their potential as biocontrol agents against soft rot and black leg diseases in potatoes.

My host's enemy is my enemy: plasmids carrying CRISPR-Cas as a defence against phages

Bacteria are infected by mobile genetic elements like plasmids and virulent phages, and those infections significantly impact bacterial ecology and evolution. Recent discoveries reveal that some plasmids carry anti-phage immune systems like CRISPR-Cas, suggesting that plasmids may participate in the coevolutionary arms race between virulent phages and bacteria. Intuitively, this seems reasonable as virulent phages kill the plasmid's obligate host. However, the efficiency of CRISPR-Cas systems carried by plasmids can be expected to be lower than those carried by the chromosome due to continuous segregation loss, creating susceptible cells for phage amplification. To evaluate the anti-phage protection efficiency of CRISPR-Cas on plasmids, we develop a stochastic model describing the dynamics of a virulent phage infection against which a conjugative plasmid defends using CRISPR-Cas. We show that CRISPR-Cas on plasmids provides robust protection, except in limited parameter sets. In these cases, high segregation loss favours phage outbreaks by generating a population of defenceless cells on which the phage can evolve and escape CRISPR-Cas immunity. We show that the phage's ability to exploit segregation loss depends strongly on the evolvability of both CRISPR-Cas and the phage itself.

Rarely acquired type II-A CRISPR-Cas spacers mediate anti-viral immunity through the targeting of a non-canonical PAM sequence

The Streptococcus pyogenes type II-A CRISPR-Cas systems provides adaptive immunity through the acquisition of short DNA sequences from invading viral genomes, called spacers. Spacers are transcribed into short RNA guides that match regions of the viral genome followed by a conserved NGG DNA motif, known as the PAM. These RNA guides, in turn, are used by the Cas9 nuclease to find and destroy complementary DNA targets within the viral genome. While most of the spacers present in bacterial populations that survive phage infection target protospacers flanked by NGG sequences, there is a small fraction that target non-canonical PAMs. Whether these spacers originate through accidental acquisition of phage sequences and/or provide efficient defense is unknown. Here we found that many of them match phage target regions flanked by an NAGG PAM. Despite being scarcely present in bacterial populations, NAGG spacers provide substantial immunity in vivo and generate RNA guides that support robust DNA cleavage by Cas9 in vitro; with both activities comparable to spacers that target sequences followed by the canonical AGG PAM. In contrast, acquisition experiments showed that NAGG spacers are acquired at very low frequencies. We therefore conclude that discrimination against these sequences occurs during immunization of the host. Our results reveal unexpected differences in PAM recognition during the spacer acquisition and targeting stages of the type II-A CRISPR-Cas immune response.

CRISPR arrays as high-resolution markers to track microbial transmission during influenza infection

BACKGROUND

Disruption of the microbial community in the respiratory tract due to infections, like influenza, could impact transmission of bacterial pathogens. Using samples from a household study, we determined whether metagenomic-type analyses of the microbiome provide the resolution necessary to track transmission of airway bacteria. Microbiome studies have shown that the microbial community across various body sites tends to be more similar between individuals who cohabit in the same household than between individuals from different households. We tested whether there was increased sharing of bacteria from the airways within households with influenza infections as compared to control households with no influenza.

RESULTS

We obtained 221 respiratory samples that were collected from 54 individuals at 4 to 5 time points across 10 households, with and without influenza infection, in Managua, Nicaragua. From these samples, we generated metagenomic (whole genome shotgun sequencing) datasets to profile microbial taxonomy. Overall, specific bacteria and phages were differentially abundant between influenza positive households and control (no influenza infection) households, with bacteria like Rothia, and phages like Staphylococcus P68virus that were significantly enriched in the influenza-positive households. We identified CRISPR spacers detected in the metagenomic sequence reads and used these to track bacteria transmission within and across households. We observed a clear sharing of bacterial commensals and pathobionts, such as Rothia, Neisseria, and Prevotella, within and between households. However, due to the relatively small number of households in our study, we could not determine if there was a correlation between increased bacterial transmission and influenza infection.

CONCLUSION

We observed that airway microbial composition differences across households were associated with what appeared to be different susceptibility to influenza infection. We also demonstrate that CRISPR spacers from the whole microbial community can be used as markers to study bacterial transmission between individuals. Although additional evidence is needed to study transmission of specific bacterial strains, we observed sharing of respiratory commensals and pathobionts within and across households. Video Abstract.

Unveil the Secret of the Bacteria and Phage Arms Race

Bacteria have developed different mechanisms to defend against phages, such as preventing phages from being adsorbed on the surface of host bacteria; through the superinfection exclusion (Sie) block of phage's nucleic acid injection; by restricting modification (R-M) systems, CRISPR-Cas, aborting infection (Abi) and other defense systems to interfere with the replication of phage genes in the host; through the quorum sensing (QS) enhancement of phage's resistant effect. At the same time, phages have also evolved a variety of counter-defense strategies, such as degrading extracellular polymeric substances (EPS) that mask receptors or recognize new receptors, thereby regaining the ability to adsorb host cells; modifying its own genes to prevent the R-M systems from recognizing phage genes or evolving proteins that can inhibit the R-M complex; through the gene mutation itself, building nucleus-like compartments or evolving anti-CRISPR (Acr) proteins to resist CRISPR-Cas systems; and by producing antirepressors or blocking the combination of autoinducers (AIs) and its receptors to suppress the QS. The arms race between bacteria and phages is conducive to the coevolution between bacteria and phages. This review details bacterial anti-phage strategies and anti-defense strategies of phages and will provide basic theoretical support for phage therapy while deeply understanding the interaction mechanism between bacteria and phages.

Epidemiological and evolutionary consequences of different types of CRISPR-Cas systems

Bacteria have adaptive immunity against viruses (phages) in the form of CRISPR-Cas immune systems. Currently, 6 types of CRISPR-Cas systems are known and the molecular study of three of these has revealed important molecular differences. It is unknown if and how these molecular differences change the outcome of phage infection and the evolutionary pressure the CRISPR-Cas systems faces. To determine the importance of these molecular differences, we model a phage outbreak entering a population defending exclusively with a type I/II or a type III CRISPR-Cas system. We show that for type III CRISPR-Cas systems, rapid phage extinction is driven by the probability to acquire at least one resistance spacer. However, for type I/II CRISPR-Cas systems, rapid phage extinction is characterized by an a threshold-like behaviour: any acquisition probability below this threshold leads to phage survival whereas any acquisition probability above it, results in phage extinction. We also show that in the absence of autoimmunity, high acquisition rates evolve. However, when CRISPR-Cas systems are prone to autoimmunity, intermediate levels of acquisition are optimal during a phage outbreak. As we predict an optimal probability of spacer acquisition 2 factors of magnitude above the one that has been measured, we discuss the origin of such a discrepancy. Finally, we show that in a biologically relevant parameter range, a type III CRISPR-Cas system can outcompete a type I/II CRISPR-Cas system with a slightly higher probability of acquisition.

CRISPR-Cas systems are adaptive immune systems that use a complex 3-step molecular mechanism to defend prokaryotes against phages. Viral infections of populations defending themselves with CRISPR-Cas can result in rapid phage extinction or in medium-term phage maintenance. To investigate what controls the fate of the phage population, we use mathematical modeling of type I/II and type III CRISPR-Cas systems, and show that two parameters control the epidemiological short-term outcome: the type of CRISPR-Cas systems and CRISPR-Cas probability of resistance acquisition. Furthermore, the latter impacts host fitness. From this, we derive that 1) for both types, CRISPR-Cas acquisition probability is a key predictor of the efficiency and of the cost of a CRISPR-Cas system, 2) during an outbreak, there is an optimal probability of resistance acquisition balancing the cost of autoimmunity and immune efficiency and 3) type I/II CRISPR-Cas systems are likely to evolve higher acquisition probability than type III.

Cleavage of viral DNA by restriction endonucleases stimulates the type II CRISPR-Cas immune response

Prokaryotic organisms have developed multiple defense systems against phages; however, little is known about whether and how these interact with each other. Here, we studied the connection between two of the most prominent prokaryotic immune systems: restriction-modification and CRISPR. While both systems employ enzymes that cleave a specific DNA sequence of the invader, CRISPR nucleases are programmed with phage-derived spacer sequences, which are integrated into the CRISPR locus upon infection. We found that restriction endonucleases provide a short-term defense, which is rapidly overcome through methylation of the phage genome. In a small fraction of the cells, however, restriction results in the acquisition of spacer sequences from the cleavage site, which mediates a robust type II-A CRISPR-Cas immune response against the methylated phage. This mechanism is reminiscent of eukaryotic immunity in which the innate response offers a first temporary line of defense and also activates a second and more robust adaptive response.

Immune lag is a major cost of prokaryotic adaptive immunity during viral outbreaks

Clustered regularly interspaced short palindromic repeat (CRISPR)-Cas adaptive immune systems enable bacteria and archaea to efficiently respond to viral pathogens by creating a genomic record of previous encounters. These systems are broadly distributed across prokaryotic taxa, yet are surprisingly absent in a majority of organisms, suggesting that the benefits of adaptive immunity frequently do not outweigh the costs. Here, combining experiments and models, we show that a delayed immune response which allows viruses to transiently redirect cellular resources to reproduction, which we call ‘immune lag’, is extremely costly during viral outbreaks, even to completely immune hosts. Critically, the costs of lag are only revealed by examining the early, transient dynamics of a host–virus system occurring immediately after viral challenge. Lag is a basic parameter of microbial defence, relevant to all intracellular, post-infection antiviral defence systems, that has to-date been largely ignored by theoretical and experimental treatments of host-phage systems.

Viral recombination systems limit CRISPR-Cas targeting through the generation of escape mutations

CRISPR-Cas systems provide immunity to bacteria by programing Cas nucleases with RNA guides that recognize and cleave infecting viral genomes. Bacteria and their viruses each encode recombination systems that could repair the cleaved viral DNA. However, it is unknown whether and how these systems can affect CRISPR immunity. Bacteriophage λ uses the Red system (gam-exo-bet) to promote recombination between related phages. Here, we show that λ Red also mediates evasion of CRISPR-Cas targeting. Gam inhibits the host E. coli RecBCD recombination system, allowing recombination and repair of the cleaved DNA by phage Exo-Beta, which promotes the generation of mutations within the CRISPR target sequence. Red recombination is strikingly more efficient than the host's RecBCD-RecA in the production of large numbers of phages that escape CRISPR targeting. These results reveal a role for Red-like systems in the protection of bacteriophages against sequence-specific nucleases, which may facilitate their spread across viral genomes.

Coevolution between bacterial CRISPR-Cas systems and their bacteriophages

CRISPR-Cas systems provide bacteria and archaea with adaptive, heritable immunity against their viruses (bacteriophages and phages) and other parasitic genetic elements. CRISPR-Cas systems are highly diverse, and we are only beginning to understand their relative importance in phage defense. In this review, we will discuss when and why CRISPR-Cas immunity against phages evolves, and how this, in turn, selects for the evolution of immune evasion by phages. Finally, we will discuss our current understanding of if, and when, we observe coevolution between CRISPR-Cas systems and phages, and how this may be influenced by the mechanism of CRISPR-Cas immunity.

Survival Strategies of Streptococcus pyogenes in Response to Phage Infection

Bacteriophages exert strong evolutionary pressure on their microbial hosts. In their lytic lifecycle, complete bacterial subpopulations are utilized as hosts for bacteriophage replication. However, during their lysogenic lifecycle, bacteriophages can integrate into the host chromosome and alter the host’s genomic make-up, possibly resulting in evolutionary important adjustments. Not surprisingly, bacteria have evolved sophisticated immune systems to protect against phage infection. Streptococcus pyogenes isolates are frequently lysogenic and their prophages have been shown to be major contributors to the virulence of this pathogen. Most S. pyogenes phage research has focused on genomic prophages in relation to virulence, but little is known about the defensive arsenal of S. pyogenes against lytic phage infection. Here, we characterized Phage A1, an S. pyogenes bacteriophage, and investigated several mechanisms that S. pyogenes utilizes to protect itself against phage predation. We show that Phage A1 belongs to the Siphoviridae family and contains a circular double-stranded DNA genome that follows a modular organization described for other streptococcal phages. After infection, the Phage A1 genome can be detected in isolated S. pyogenes survivor strains, which enables the survival of the bacterial host and Phage A1 resistance. Furthermore, we demonstrate that the type II-A CRISPR-Cas system of S. pyogenes acquires new spacers upon phage infection, which are increasingly detectable in the absence of a capsule. Lastly, we show that S. pyogenes produces membrane vesicles that bind to phages, thereby limiting the pool of phages available for infection. Altogether, this work provides novel insight into survival strategies employed by S. pyogenes to combat phage predation.

It is unclear how important CRISPR-Cas systems are for protecting natural populations of bacteria against infections by mobile genetic elements

Articles on CRISPR commonly open with some variant of the phrase "these short palindromic repeats and their associated endonucleases (Cas) are an adaptive immune system that exists to protect bacteria and archaea from viruses and infections with other mobile genetic elements." There is an abundance of genomic data consistent with the hypothesis that CRISPR plays this role in natural populations of bacteria and archaea, and experimental demonstrations with a few species of bacteria and their phage and plasmids show that CRISPR-Cas systems can play this role in vitro. Not at all clear are the ubiquity, magnitude, and nature of the contribution of CRISPR-Cas systems to the ecology and evolution of natural populations of microbes and the strength of selection mediated by different types of phage and plasmids to the evolution and maintenance of CRISPR-Cas systems. In this perspective, with the aid of heuristic mathematical-computer simulation models, we explore the a priori conditions under which exposure to lytic and temperate phage and conjugative plasmids will select for and maintain CRISPR-Cas systems in populations of bacteria and archaea. We review the existing literature addressing these ecological and evolutionary questions and highlight the experimental and other evidence needed to fully understand the conditions responsible for the evolution and maintenance of CRISPR-Cas systems and the contribution of these systems to the ecology and evolution of bacteria, archaea, and the mobile genetic elements that infect them.

Avoidance of Self during CRISPR Immunization

The battle between microbes and their viruses is ancient and ongoing. Clustered regularly interspaced short palindromic repeat (CRISPR) immunity, the first and, to date, only form of adaptive immunity found in prokaryotes, represents a flexible mechanism to recall past infections while also adapting to a changing pathogenic environment. Critical to the role of CRISPR as an adaptive immune mechanism is its capacity for self versus non-self recognition when acquiring novel immune memories. Yet, CRISPR systems vary widely in both how and to what degree they can distinguish foreign from self-derived genetic material. We document known and hypothesized mechanisms that bias the acquisition of immune memory towards non-self targets. We demonstrate that diversity is the rule, with many widespread but no universal mechanisms for self versus non-self recognition.

Co-evolution within structured bacterial communities results in multiple expansion of CRISPR loci and enhanced immunity

Type II CRISPR-Cas systems provide immunity against phages and plasmids that infect bacteria through the insertion of a short sequence from the invader's genome, known as the 'spacer', into the CRISPR locus. Spacers are transcribed into guide RNAs that direct the Cas9 nuclease to its target on the invader. In liquid cultures, most bacteria acquire a single spacer. Multiple spacer integration is a rare event which significance for immunity is poorly understood. Here, we found that when phage infections occur on solid media, a high proportion of the surviving colonies display complex morphologies that contain cells with multiple spacers. This is the result of the viral-host co-evolution, in which the immunity provided by the initial acquired spacer is easily overcome by escaper phages. Our results reveal the versatility of CRISPR-Cas immunity, which can respond with both single or multiple spacer acquisition schemes to solve challenges presented by different environments.

Cas9 Cleavage of Viral Genomes Primes the Acquisition of New Immunological Memories

Type II CRISPR-Cas systems defend prokaryotes from bacteriophage infection through the acquisition of short viral DNA sequences known as spacers, which are transcribed into short RNA guides to specify the targets of the Cas9 nuclease. To counter the potentially devastating propagation of escaper phages with mutations in the target sequences, the host population acquires many different spacers. Whether and how pre-existing spacers in type II systems affect the acquisition of new ones is unknown. Here, we demonstrate that previously acquired spacers promote additional spacer acquisition from the vicinity of the target DNA site cleaved by Cas9. Therefore, CRISPR immune cells acquire additional spacers at the same time as they destroy the infecting virus. This anticipates the rise of escapers or related viruses that could escape targeting by the first spacer acquired. Our results thus reveal Cas9's role in the generation of immunological memories.

(Ph)ighting Phages: How Bacteria Resist Their Parasites

Bacteria are under constant attack from bacteriophages (phages), bacterial parasites that are the most abundant biological entity on earth. To resist phage infection, bacteria have evolved an impressive arsenal of anti-phage systems. Recent advances have significantly broadened and deepened our understanding of how bacteria battle phages, spearheaded by new systems like CRISPR-Cas. This review aims to summarize bacterial anti-phage mechanisms, with an emphasis on the most recent developments in the field.