Anti-CRISPR Phages Cooperate to Overcome CRISPR-Cas Immunity.

Some phages encode anti-CRISPR (acr) genes, which antagonize bacterial CRISPR-Cas immune systems by binding components of its machinery, but it is less clear how deployment of these acr genes impacts phage replication and epidemiology. Here, we demonstrate that bacteria with CRISPR-Cas resistance are still partially immune to Acr-encoding phage. As a consequence, Acr-phages often need to cooperate in order to overcome CRISPR resistance, with a first phage blocking the host CRISPR-Cas immune system to allow a second Acr-phage to successfully replicate. This cooperation leads to epidemiological tipping points in which the initial density of Acr-phage tips the balance from phage extinction to a phage epidemic. Furthermore, both higher levels of CRISPR-Cas immunity and weaker Acr activities shift the tipping points toward higher initial phage densities. Collectively, these data help elucidate how interactions between phage-encoded immune suppressors and the CRISPR systems they target shape bacteria-phage population dynamics.

Antibiotics that affect translation can antagonize phage infectivity by interfering with the deployment of counter-defenses.

It is becoming increasingly clear that antibiotics can both positively and negatively impact the infectivity of bacteriophages (phage), but the underlying mechanisms often remain unclear. Here we demonstrate that antibiotics that target the protein translation machinery can fundamentally alter the outcome of bacteria-phage interactions by interfering with the production of phage-encoded counter-defense proteins. Specifically, using Pseudomonas aeruginosa PA14 and phage DMS3vir as a model, we show that bacteria with Clustered Regularly Interspaced Short Palindromic Repeat, CRISPR associated (CRISPR-Cas) immune systems have elevated levels of immunity against phage that encode anti-CRISPR (acr) genes when translation inhibitors are present in the environment. CRISPR-Cas are highly prevalent defense systems that enable bacteria to detect and destroy phage genomes in a sequence-specific manner. In response, many phages encode acr genes that are expressed immediately following the infection to inhibit key steps of the CRISPR-Cas immune response. Our data show that while phage-carrying acr genes can amplify efficiently on bacteria with CRISPR-Cas immune systems in the absence of antibiotics, the presence of antibiotics that act on protein translation prevents phage amplification, while protecting bacteria from lysis.

The novel anti-phage system Shield co-opts an RmuC domain to mediate phage defense across Pseudomonas species.

Competitive bacteria-bacteriophage interactions have resulted in the evolution of a plethora of bacterial defense systems preventing phage propagation. In recent years, computational and bioinformatic approaches have underpinned the discovery of numerous novel bacterial defense systems. Anti-phage systems are frequently encoded together in genomic loci termed defense islands. Here we report the identification and characterisation of a novel anti-phage system, that we have termed Shield, which forms part of the Pseudomonas defensive arsenal. The Shield system comprises the core component ShdA, a membrane-bound protein harboring an RmuC domain. Heterologous production of ShdA alone is sufficient to mediate bacterial immunity against several phages. We demonstrate that Shield and ShdA confer population-level immunity and that they can also decrease transformation efficiency. We further show that ShdA homologues can degrade DNA in vitro and, when expressed in a heterologous host, can alter the organisation of the host chromosomal DNA. Use of comparative genomic approaches identified how Shield can be divided into four subtypes, three of which contain additional components that in some cases can negatively affect the activity of ShdA and/or provide additional lines of phage defense. Collectively, our results identify a new player within the Pseudomonas bacterial immunity arsenal that displays a novel mechanism of protection, and reveals a role for RmuC domains in phage defense.

Removal of AMR plasmids using a mobile, broad host-range CRISPR-Cas9 delivery tool.

Antimicrobial resistance (AMR) genes are widely disseminated on plasmids. Therefore, interventions aimed at blocking plasmid uptake and transfer may curb the spread of AMR. Previous studies have used CRISPR-Cas-based technology to remove plasmids encoding AMR genes from target bacteria, using either phage- or plasmid-based delivery vehicles that typically have narrow host ranges. To make this technology feasible for removal of AMR plasmids from multiple members of complex microbial communities, an efficient, broad host-range delivery vehicle is needed. We engineered the broad host-range IncP1-plasmid pKJK5 to encode cas9 programmed to target an AMR gene. We demonstrate that the resulting plasmid pKJK5::csg has the ability to block the uptake of AMR plasmids and to remove resident plasmids from Escherichia coli. Furthermore, due to its broad host range, pKJK5::csg successfully blocked AMR plasmid uptake in a range of environmental, pig- and human-associated coliform isolates, as well as in isolates of two species of Pseudomonas. This study firmly establishes pKJK5::csg as a promising broad host-range CRISPR-Cas9 delivery tool for AMR plasmid removal, which has the potential to be applied in complex microbial communities to remove AMR genes from a broad range of bacterial species.

Decolonizing drug-resistant E. coli with phage and probiotics: breaking the frequency-dependent dominance of residents.

Widespread antibiotic resistance in commensal bacteria creates a persistent challenge for human health. Resident drug-resistant microbes can prevent clinical interventions, colonize wounds post-surgery, pass resistance traits to pathogens or move to more harmful niches following routine interventions such as catheterization. Accelerating the removal of resistant bacteria or actively decolonizing particular lineages from hosts could therefore have a number of long-term benefits. However, removing resident bacteria via competition with probiotics, for example, poses a number of ecological challenges. Resident microbes are likely to have physiological and numerical advantages and competition based on bacteriocins or other secreted antagonists is expected to give advantages to the dominant partner, via positive frequency dependence. Since a narrow range of Escherichia coli genotypes (primarily those belonging to the clonal group ST131) cause a significant proportion of multidrug-resistant infections, this group presents a promising target for decolonization with bacteriophage, as narrow-host-range viral predation could lead to selective removal of particular genotypes. In this study we tested how a combination of an ST131-specific phage and competition from the well-known probiotic E. coli Nissle strain could displace E. coli ST131 under aerobic and anaerobic growth conditions in vitro. We showed that the addition of phage was able to break the frequency-dependent advantage of a numerically dominant ST131 isolate. Moreover, the addition of competing E. coli Nissle could improve the ability of phage to suppress ST131 by two orders of magnitude. Low-cost phage resistance evolved readily in these experiments and was not inhibited by the presence of a probiotic competitor. Nevertheless, combinations of phage and probiotic produced stable long-term suppression of ST131 over multiple transfers and under both aerobic and anaerobic growth conditions. Combinations of phage and probiotic therefore have real potential for accelerating the removal of drug-resistant commensal targets.

The diversity-generating benefits of a prokaryotic adaptive immune system.

Prokaryotic CRISPR-Cas adaptive immune systems insert spacers derived from viruses and other parasitic DNA elements into CRISPR loci to provide sequence-specific immunity. This frequently results in high within-population spacer diversity, but it is unclear if and why this is important. Here we show that, as a result of this spacer diversity, viruses can no longer evolve to overcome CRISPR-Cas by point mutation, which results in rapid virus extinction. This effect arises from synergy between spacer diversity and the high specificity of infection, which greatly increases overall population resistance. We propose that the resulting short-lived nature of CRISPR-dependent bacteria-virus coevolution has provided strong selection for the evolution of sophisticated virus-encoded anti-CRISPR mechanisms.

Compost spatial heterogeneity promotes evolutionary diversification of a bacterium.

Spatial resource heterogeneity is expected to be a key driver for the evolution of diversity. However, direct empirical support for this prediction is limited to studies carried out in simplified laboratory environments. Here, we investigate how altering spatial heterogeneity of potting compost-by the addition of water and mixing-affects the evolutionary diversification of a bacterial species, Pseudomonas fluorescens, that is naturally found in the environment. There was a greater propensity of resource specialists to evolve in the unmanipulated compost, while more generalist phenotypes dominated the compost-water mix. Genomic data were consistent with these phenotypic findings. Competition experiments strongly suggest these results are due to diversifying selection as a result of resource heterogeneity, as opposed to other covariables. Overall, our findings corroborate theoretical and in vitro findings, but in semi-natural, more realistic conditions.

Evolutionary emergence of infectious diseases in heterogeneous host populations.

The emergence and re-emergence of pathogens remains a major public health concern. Unfortunately, when and where pathogens will (re-)emerge is notoriously difficult to predict, as the erratic nature of those events is reinforced by the stochastic nature of pathogen evolution during the early phase of an epidemic. For instance, mutations allowing pathogens to escape host resistance may boost pathogen spread and promote emergence. Yet, the ecological factors that govern such evolutionary emergence remain elusive because of the lack of ecological realism of current theoretical frameworks and the difficulty of experimentally testing their predictions. Here, we develop a theoretical model to explore the effects of the heterogeneity of the host population on the probability of pathogen emergence, with or without pathogen evolution. We show that evolutionary emergence and the spread of escape mutations in the pathogen population is more likely to occur when the host population contains an intermediate proportion of resistant hosts. We also show that the probability of pathogen emergence rapidly declines with the diversity of resistance in the host population. Experimental tests using lytic bacteriophages infecting their bacterial hosts containing Clustered Regularly Interspaced Short Palindromic Repeat and CRISPR-associated (CRISPR-Cas) immune defenses confirm these theoretical predictions. These results suggest effective strategies for cross-species spillover and for the management of emerging infectious diseases.

Diversity in CRISPR-based immunity protects susceptible genotypes by restricting phage spread and evolution.

Diversity in host resistance often associates with reduced pathogen spread. This may result from ecological and evolutionary processes, likely with feedback between them. Theory and experiments on bacteria-phage interactions have shown that genetic diversity of the bacterial adaptive immune system can limit phage evolution to overcome resistance. Using the CRISPR-Cas bacterial immune system and lytic phage, we engineered a host-pathogen system where each bacterial host genotype could be infected by only one phage genotype. With this model system, we explored how CRISPR diversity impacts the spread of phage when they can overcome a resistance allele, how immune diversity affects the evolution of the phage to increase its host range and if there was feedback between these processes. We show that increasing CRISPR diversity benefits susceptible bacteria via a dilution effect, which limits the spread of the phage. We suggest that this ecological effect impacts the evolution of novel phage genotypes, which then feeds back into phage population dynamics.

Timely trigger of caterpillar zombie behaviour: temporal requirements for light in baculovirus-induced tree-top disease.

Host behavioural manipulation is a common strategy used by parasites to enhance their survival and/or transmission. Baculoviruses induce hyperactivity and tree-top disease (pre-death climbing behaviour) in their caterpillar hosts. However, little is known about the underlying mechanisms of this behavioural manipulation. A previous study showed that the baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) induced tree-top disease at 3 days post infection in third instar S. exigua larvae and that light plays a key role in triggering this behaviour. Here we investigated the temporal requirements for the presence of light to trigger this behaviour and found that light from above was needed between 43 and 50 h post infection to induce tree-top disease. Infected larvae that were not exposed to light from above in this period finally died at low positions. Exposure to light prior to this period did not affect the final positions where larvae died. Overall we conclude that light in a particular time frame is needed to trigger SeMNPV-induced tree-top disease in S. exigua larvae.

Host diversity limits the evolution of parasite local adaptation.

Specificity in the interactions between hosts and their parasites can lead to local adaptation. However, the degree of local adaptation is predicted to depend upon the diversity of resistance alleles within the host population; increasing host diversity should decrease mean parasite infectivity and hence reduce local adaptation. In this study, we empirically test this prediction using the highly specific interactions between bacteria with clustered regularly interspaced short palindromic repeats/CRISPR-associated (CRISPR/Cas) immunity and their bacteriophage. Bacteria acquire immunity to phage by incorporating a phage-derived spacer sequence into CRISPR loci on the host genome, and phage can escape the CRISPR-mediated immunity of a specific clone by mutating the targeted sequence. We found that high levels of CRISPR allele diversity that naturally evolve in host populations exposed to phage (because each bacterial clone captures a unique phage-derived sequence) prevents phage from becoming locally adapted. By manipulating the number of CRISPR alleles in the host population, we show that phage can become locally adapted to their bacterial hosts but only when CRISPR allele diversity is low.

Immigration of susceptible hosts triggers the evolution of alternative parasite defence strategies.

Migration of hosts and parasites can have a profound impact on host-parasite ecological and evolutionary interactions. Using the bacterium Pseudomonas aeruginosa UCBPP-PA14 and its phage DMS3vir, we here show that immigration of naive hosts into coevolving populations of hosts and parasites can influence the mechanistic basis underlying host defence evolution. Specifically, we found that at high levels of bacterial immigration, bacteria switched from clustered regularly interspaced short palindromic repeats (CRISPR-Cas) to surface modification-mediated defence. This effect emerges from an increase in the force of infection, which tips the balance from CRISPR to surface modification-based defence owing to the induced and fixed fitness costs associated with these mechanisms, respectively.

Baculovirus-induced tree-top disease: how extended is the role of egt as a gene for the extended phenotype?

Many parasites alter host behaviour to enhance their chance of transmission. Recently, the ecdysteroid UDP-glucosyl transferase (egt) gene from the baculovirus Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV) was identified to induce tree-top disease in L. dispar larvae. Infected gypsy moth larvae died at elevated positions (hence the term tree-top disease), which is thought to promote dissemination of the virus to lower foliage. It is, however, unknown whether egt has a conserved role among baculoviruses in inducing tree-top disease. Here, we studied tree-top disease induced by the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) in two different host insects, Trichoplusia ni and Spodoptera exigua, and we investigated the role of the viral egt gene therein. AcMNPV induced tree-top disease in both T. ni and S. exigua larvae, although in S. exigua a moulting-dependent effect was seen. Those S. exigua larvae undergoing a larval moult during the infection process died at elevated positions, while larvae that did not moult after infection died at low positions. For both T. ni and S. exigua, infection with a mutant AcMNPV lacking egt did not change the position where the larvae died. We conclude that egt has no highly conserved role in inducing tree-top disease in lepidopteran larvae. The conclusion that egt is a 'gene for an extended phenotype' is therefore not generally applicable for all baculovirus-host interactions. We hypothesize that in some baculovirus-host systems (including LdMNPV in L. dispar), an effect of egt on tree-top disease can be observed through indirect effects of egt on moulting-related climbing behaviour.

Insights in progressive myoclonus epilepsy: HSP70 promotes cystatin B polymerization.

Cystatin B (CSTB) is an anti-protease frequently mutated in progressive myoclonus epilepsy (EPM1), a devastating degenerative disease. This work shows that rat CSTB is an unstable protein that undergoes structural changes following the interaction with a chaperone, either prokaryotic or eukaryotic. Both the prokaryotic DnaK and eukaryotic HSP70 promote CSTB polymerization. Denaturated CSTB is polymerized by the chaperone alone. Native CSTB monomers are more stable than denatured monomers and require Cu(2+) for chaperone-dependent polymerization. Cu(2+) interacts with at least two conserved histidines, at positions 72 and 95 modifying the structure of native monomeric CSTB. Subsequently, CSTB becomes unstable and readily responds to the addition of DnaK or HSP70, generating polymers. This reaction depends strictly on the presence of this divalent metal ion and on the presence of one cysteine in the protein chain. The cysteine deletion mutant does not polymerize. We propose that Cu(2+) modifies the redox environment of the protein, allowing the oxidation of the cysteine residue of CSTB that triggers polymerization. These polymers are sensitive to reducing agents while polymers obtained from denatured CSTB monomers are DTT resistant. We propose that the Cu(2+)/HSP70 dependent polymers are physiological and functional in eukaryotic cells. Furthermore, while monomeric CSTB has anti-protease function, it seems likely that polymeric CSTB fulfils different function(s).

Baculovirus infection triggers a positive phototactic response in caterpillars to induce 'tree-top' disease.

Many parasites manipulate host behaviour to enhance parasite transmission and survival. A fascinating example is baculoviruses, which often induce death in caterpillar hosts at elevated positions ('tree-top' disease). To date, little is known about the underlying processes leading to this adaptive host manipulation. Here, we show that the baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) triggers a positive phototactic response in S. exigua larvae prior to death and causes the caterpillars to die at elevated positions. This light-dependent climbing behaviour is specific for infected larvae, as movement of uninfected caterpillars during larval development was light-independent. We hypothesize that upon infection, SeMNPV captures a host pathway involved in phototaxis and/or light perception to induce this remarkable behavioural change.

Hyperactivity and tree-top disease induced by the baculovirus AcMNPV in Spodoptera exigua larvae are governed by independent mechanisms.

Although many parasites are known to manipulate the behavior of their hosts, the mechanisms underlying such manipulations are largely unknown. Baculoviruses manipulate the behavior of caterpillar hosts by inducing hyperactivity and by inducing climbing behavior leading to death at elevated positions (tree-top disease or Wipfelkrankheit). Whether hyperactivity and tree-top disease are independent manipulative strategies of the virus is unclear. Recently, we demonstrated the involvement of the protein tyrosine phosphatase (ptp) gene of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) in the induction of hyperactivity in Spodoptera exigua larvae. Here we show that AcMNPV ptp is not required for tree-top disease, indicating that in S. exigua baculovirus-induced hyperactivity and tree-top disease are independently induced behaviors that are governed by distinct mechanisms.

CRISPR-Cas in Pseudomonas aeruginosa provides transient population-level immunity against high phage exposures.

The prokaryotic adaptive immune system, CRISPR-Cas (clustered regularly interspaced short palindromic repeats; CRISPR-associated), requires the acquisition of spacer sequences that target invading mobile genetic elements such as phages. Previous work has identified ecological variables that drive the evolution of CRISPR-based immunity of the model organism Pseudomonas aeruginosa PA14 against its phage DMS3vir, resulting in rapid phage extinction. However, it is unclear if and how stable such acquired immunity is within bacterial populations, and how this depends on the environment. Here, we examine the dynamics of CRISPR spacer acquisition and loss over a 30-day evolution experiment and identify conditions that tip the balance between long-term maintenance of immunity versus invasion of alternative resistance strategies that support phage persistence. Specifically, we find that both the initial phage dose and reinfection frequencies determine whether or not acquired CRISPR immunity is maintained in the long term, and whether or not phage can coexist with the bacteria. At the population genetics level, emergence and loss of CRISPR immunity are associated with high levels of spacer diversity that subsequently decline due to invasion of bacteria carrying pilus-associated mutations. Together, these results provide high resolution of the dynamics of CRISPR immunity acquisition and loss and demonstrate that the cumulative phage burden determines the effectiveness of CRISPR over ecologically relevant timeframes.

Multi-layered genome defences in bacteria.

Bacteria have evolved a variety of defence mechanisms to protect against mobile genetic elements, including restriction-modification systems and CRISPR-Cas. In recent years, dozens of previously unknown defence systems (DSs) have been discovered. Notably, diverse DSs often coexist within the same genome, and some co-occur at frequencies significantly higher than would be expected by chance, implying potential synergistic interactions. Recent studies have provided evidence of defence mechanisms that enhance or complement one another. Here, we review the interactions between DSs at the mechanistic, regulatory, ecological and evolutionary levels.