Virulence-associated factors as targets for phage infection

Bacterial pathogens can infect a wide range of hosts and pose a threat to public and animal health as well as to agriculture. The emergence of antibiotic-resistant strains has increased this risk by making the treatment of bacterial infections even more challenging. Pathogenic bacteria thrive in various ecological niches, but they can also be specifically targeted and killed by bacteriophages (phages). Lytic phages are now investigated and even used, in some cases, as alternatives or complements to antibiotics for preventing or treating bacterial infections (phage therapy). As such, it is key to identify factors responsible for phage specificity and efficiency. Here, we review recent advances in virulence-associated factors that are targeted by phages. We highlight components of the bacterial cell surface, effector systems, and motility structures exploited by phages and the effects of phages on cell aggregation and communication. We also look at the fitness trade-off of phage resistance.

Building pyramids against the evolutionary emergence of pathogens

Mutations allowing pathogens to escape host immunity promote the spread of infectious diseases in heterogeneous host populations and can lead to major epidemics. Understanding the conditions that slow down this evolution is key for the development of durable control strategies against pathogens. Here, we use theory and experiments to compare the efficacy of three strategies for the deployment of resistance: (i) a mixing strategy where the host population contains two single-resistant genotypes, (ii) a pyramiding strategy where the host carries a double-resistant genotype, (iii) a combining strategy where the host population is a mix of a single-resistant genotype and a double-resistant genotype. First, we use evolutionary epidemiology theory to clarify the interplay between demographic stochasticity and evolutionary dynamics to show that the pyramiding strategy always yields lower probability of evolutionary emergence. Second, we test experimentally these predictions with the introduction of bacteriophages into bacterial populations where we manipulated the diversity and the depth of immunity using a Clustered Regularly Interspaced Short Palindromic Repeats-CRISPR associated (CRISPR-Cas) system. These biological assays confirm that pyramiding multiple defences into the same host genotype and avoiding combination with single-defence genotypes is a robust way to reduce pathogen evolutionary emergence. The experimental validation of these theoretical recommendations has practical implications in various areas, including for the optimal deployment of resistance varieties in agriculture and for the design of durable vaccination strategies.

Complete genome of Escherichia phages Carena and JoYop

Escherichia phages Carena and JoYop were isolated from water samples in Abidjan (Cote d'Ivoire). Their genomes comprise 39,283 and 169,193 bp, encoding 44 and 246 predicted genes, respectively. Carena shares 93.4% nucleotide identity with Escherichia podophage CarlSpitteler (Berlinvirus), and JoYop shows 95.6% identity with Escherichia myophage ADUt (Tequatrovirus).

Fermentation Practices Select for Thermostable Endolysins in Phages

Endolysins are produced by (bacterio)phages and play a crucial role in degrading the bacterial cell wall and the subsequent release of new phage progeny. These lytic enzymes exhibit a remarkable diversity, often occurring in a multimodular form that combines different catalytic and cell wall-binding domains, even in phages infecting the same species. Yet, our current understanding lacks insight into how environmental factors and ecological niches may have influenced the evolution of these enzymes. In this study, we focused on phages infecting Streptococcus thermophilus, as this bacterial species has a well-defined and narrow ecological niche, namely, dairy fermentation. Among the endolysins found in phages targeting this species, we observed limited diversity, with a singular structural type dominating in most of identified S. thermophilus phages. Within this prevailing endolysin type, we discovered a novel and highly conserved calcium-binding motif. This motif proved to be crucial for the stability and activity of the enzyme at elevated temperatures. Ultimately, we demonstrated its positive selection within the host's environmental conditions, particularly under the temperature profiles encountered in the production of yogurt, mozzarella, and hard cheeses that rely on S. thermophilus.

Diversity of Salmonella enterica phages isolated from chicken farms in Kenya

IMPORTANCE

Non-typhoidal Salmonella enterica infections are one of the leading causes of diarrhoeal diseases that spread to humans from animal sources such as poultry. Hence, keeping poultry farms free of Salmonella is essential for consumer safety and for a better yield of animal products. However, the emergence of antibiotic resistance due to over usage has sped up the search for alternative biocontrol methods such as the use of bacteriophages. Isolation and characterization of novel bacteriophages are key to adapt phage-based biocontrol applications. Here, we isolated and characterized Salmonella phages from samples collected at chicken farms and slaughterhouses in Kenya. The genomic characterization of these phage isolates revealed that they belong to four ICTV (International Committee on Taxonomy of Viruses) phage genera. All these phages are lytic and possibly suitable for biocontrol applications because no lysogenic genes or virulence factors were found in their genomes. Hence, we recommend further studies on these phages for their applications in Salmonella biocontrol.

Escherichia coli CRISPR arrays from early life fecal samples preferentially target prophages

CRISPR-Cas systems are defense mechanisms against phages and other nucleic acids that invade bacteria and archaea. In Escherichia coli, it is generally accepted that CRISPR-Cas systems are inactive in laboratory conditions due to a transcriptional repressor. In natural isolates, it has been shown that CRISPR arrays remain stable over the years and that most spacer targets (protospacers) remain unknown. Here, we re-examine CRISPR arrays in natural E. coli isolates and investigate viral and bacterial genomes for spacer targets using a bioinformatics approach coupled to a unique biological dataset. We first sequenced the CRISPR1 array of 1769 E. coli isolates from the fecal samples of 639 children obtained during their first year of life. We built a network with edges between isolates that reflect the number of shared spacers. The isolates grouped into 34 modules. A search for matching spacers in bacterial genomes showed that E. coli spacers almost exclusively target prophages. While we found instances of self-targeting spacers, those involving a prophage and a spacer within the same bacterial genome were rare. The extensive search for matching spacers also expanded the library of known E. coli protospacers to 60%. Altogether, these results favor the concept that E. coli's CRISPR-Cas is an antiprophage system and highlight the importance of reconsidering the criteria use to deem CRISPR-Cas systems active.

The infant gut virome is associated with preschool asthma risk independently of bacteria

Bacteriophage (also known as phage) communities that inhabit the gut have a major effect on the structure and functioning of bacterial populations, but their roles and association with health and disease in early life remain unknown. Here, we analyze the gut virome of 647 children aged 1 year from the Copenhagen Prospective Studies on Asthma in Childhood2010 (COPSAC2010) mother-child cohort, all deeply phenotyped from birth and with longitudinally assessed asthma diagnoses. Specific temperate gut phage taxa were found to be associated with later development of asthma. In particular, the joint abundances of 19 caudoviral families were found to significantly contribute to this association. Combining the asthma-associated virome and bacteriome signatures had additive effects on asthma risk, implying an independent virome-asthma association. Moreover, the virome-associated asthma risk was modulated by the host TLR9 rs187084 gene variant, suggesting a direct interaction between phages and the host immune system. Further studies will elucidate whether phages, alongside bacteria and host genetics, can be used as preclinical biomarkers for asthma.

Dairy phages escape CRISPR defence of Streptococcus thermophilus via the anti-CRISPR AcrIIA3

Bacterial community collapse due to phage infection is a major risk in cheese making processes. As virulent phages are ubiquitous and diverse in milk fermentation factories, the use of phage-resistant lactic acid bacteria (LAB) is essential to obtain high-quality fermented dairy products. The LAB species Streptococcus thermophilus contains two type II-A CRISPR-Cas systems (CRISPR1 and CRISPR3) that can effectively protect against phage infection. However, virulent streptococcal phages carrying anti-CRISPR proteins (ACR) that block the activity of CRISPR-Cas systems have emerged in yogurt and cheese environments. For example, phages carrying AcrIIA5 can impede both CRISPR1 and CRISPR3 systems, while AcrIIA6 stops only CRISPR1. Here, we explore the activity and diversity of a third streptococcal phage anti-CRISPR protein, namely AcrIIA3. We were able to demonstrate that AcrIIA3 is efficiently active against the CRISPR3-Cas system of S. thermophilus. We used AlphaFold2 to infer the structure of AcrIIA3 and we predicted that this new family of functional ACR in virulent streptococcal phages has a new α-helical fold, with no previously identified structural homologs. Because ACR proteins are being explored as modulators in genome editing applications, we also tested AcrIIA3 against SpCas9. We found that AcrIIA3 could block SpCas9 in bacteria but not in human cells. Understanding the diversity and functioning of anti-defence mechanisms will be of importance in the design of long-term stable starter cultures.

MQM1, a bacteriophage infecting strains of Aeromonas salmonicida subspecies salmonicida carrying Prophage 3

Aeromonas salmonicida subsp. salmonicida is a Gam-negative bacterium responsible for furunculosis in fish. Because this aquatic bacterial pathogen has a rich reservoir of antibiotic-resistant genes, it is essential to investigate antibacterial alternatives, including the use of phages. Yet, we have previously demonstrated the inefficiency of a phage cocktail designed against A. salmonicida subsp. salmonicida strains due to a phage resistance phenotype associated to a prophage, namely Prophage 3. To bypass this resistance, one of the solutions is to isolate novel phages capable of infecting Prophage 3-bearing strains. Here we report on the isolation and characterization of the new virulent phage vB_AsaP_MQM1 (or MQM1), which is highly specific to A. salmonicida subsp. salmonicida strains. Phage MQM1 inhibited the growth of 01-B516, a strain carrying Prophage 3, including when combined to the previous phage cocktail. MQM1 infected 26 out of the 30 (87%) Prophage 3-bearing strains tested. Its linear dsDNA genome contains 63,343 bp, with a GC content of 50.2%. MQM1 genome can encode 88 proteins and 8 tRNAs, while no integrase or transposase-encoding genes were found. This podophage has an icosahedral capsid and a non-contractile short tail. We suggest that MQM1 may be a good addition to future phage cocktails against furunculosis to resolve the Prophage 3-resistance issue.

The never-ending battle between lactic acid bacteria and their phages

Over the past few decades, the interest in lactic acid bacteria (LAB) has been steadily growing. This is mainly due to their industrial use, their health benefits as probiotic bacteria and their ecological importance in host-related microbiota. Phage infection represents a significant risk for the production and industrial use of LAB. This created the need to study the various means of defense put in place by LAB to resist their viral enemies, as well as the countermeasures evolved by phages to overcome these defenses. In this review, we discuss defense systems that LAB employ to resist phage infections. We also describe how phages counter these mechanisms through diverse and sophisticated strategies. Furthermore, we discuss the way phage-host interactions shape each other's evolution. The recent discovery of numerous novel defense systems in other bacteria promises a new dawn for phage research in LAB.

Longitudinal Study of Lactococcus Phages in a Canadian Cheese Factory

The presence of virulent phages is closely monitored during cheese manufacturing, as these bacterial viruses can significantly slow down the milk fermentation process and lead to low-quality cheeses. From 2001 to 2020, whey samples from cheddar cheese production in a Canadian factory were monitored for the presence of virulent phages capable of infecting proprietary strains of Lactococcus cremoris and Lactococcus lactis used in starter cultures. Phages were successfully isolated from 932 whey samples using standard plaque assays and several industrial Lactococcus strains as hosts. A multiplex PCR assay assigned 97% of these phage isolates to the Skunavirus genus, 2% to the P335 group, and 1% to the Ceduovirus genus. DNA restriction profiles and a multilocus sequence typing (MLST) scheme distinguished at least 241 unique lactococcal phages from these isolates. While most phages were isolated only once, 93 of them (out of 241, 39%) were isolated multiple times. Phage GL7 was isolated 132 times from 2006 to 2020, demonstrating that phages can persist in a cheese factory for long periods of time. Phylogenetic analysis of MLST sequences showed that phages could be clustered based on their bacterial hosts rather than their year of isolation. Host range analysis showed that Skunavirus phages exhibited a very narrow host range, whereas some Ceduovirus and P335 phages had a broader host range. Overall, the host range information was useful in improving the starter culture rotation by identifying phage-unrelated strains and helped mitigating the risk of fermentation failure due to virulent phages. IMPORTANCE Although lactococcal phages have been observed in cheese production settings for almost a century, few longitudinal studies have been performed. This 20-year study describes the close monitoring of dairy lactococcal phages in a cheddar cheese factory. Routine monitoring was conducted by factory staff, and when whey samples were found to inhibit industrial starter cultures under laboratory conditions, they were sent to an academic research laboratory for phage isolation and characterization. This led to a collection of at least 241 unique lactococcal phages, which were characterized through PCR typing and MLST profiling. Phages of the Skunavirus genus were by far the most dominant. Most phages lysed a small subset of the Lactococcus strains. These findings guided the industrial partner in adapting the starter culture schedule by using phage-unrelated strains in starter cultures and removing some strains from the starter rotation. This phage control strategy could be adapted for other large-scale bacterial fermentation processes.

The impact of Brevibacterium aurantiacum virulent phages on the production of smear surface-ripened cheeses

Phages are ubiquitous and are particularly abundant in environments where their bacterial hosts thrive, such as those in the cheese industry. Although it is well documented that phages infect lactic acid bacteria, their impact has been notably overlooked on cheese ripening strains, such as Brevibacterium aurantiacum. Here, we aimed to study the impact of B. aurantiacum phages on the production of smear-ripened cheeses. We used model cheeses in industrial settings to monitor the development of the color of the cheese rind as well as of its microbial composition in presence or absence of virulent B. aurantiacum phages. Our results showed that the presence of B. aurantiacum phages significantly slowed down the development of the orange rind color in the model cheeses. In the final days of cheese ripening, phages were also detected in the control curds. By analyzing a hypervariable region of B. aurantiacum phage genomes, we detected phages with tandem repeat patterns that were different from those used in the phage-inoculated cheeses. Our results highlight the risks of using a phage-sensitive strain in smear-ripened cheese production. This is the first study to report on the impact of B. aurantiacum phages on smear-ripened cheeses.

Insight into the Binding Mechanisms of Quartz-Selective Peptides: Toward Greener Flotation Processes

Mining practices, chiefly froth flotation, are being critically reassessed to replace their use of biohazardous chemical reagents in favor of biofriendly alternatives as a path toward green processes. In this regard, this study aimed at evaluating the interactions of peptides, as potential floatation collectors, with quartz using phage display and molecular dynamics (MD) simulations. Quartz-selective peptide sequences were initially identified by phage display at pH = 9 and further modeled by a robust simulation scheme combining classical MD, replica exchange MD, and steered MD calculations. Our residue-specific analyses of the peptides revealed that positively charged arginine and lysine residues were favorably attracted by the quartz surface at basic pH. The negatively charged residues at pH 9 (i.e., aspartic acid and glutamic acid) further showed affinity toward the quartz surface through electrostatic interactions with the positively charged surface-bound Na⁺ ions. The best-binding heptapeptide combinations, however, contained both positively and negatively charged residues in their composition. The flexibility of peptide chains was also shown to directly affect the adsorption behavior of the peptide. While attractive intrapeptide interactions were dominated by a weak peptide-quartz binding, the repulsive self-interactions in the peptides improved the binding propensity to the quartz surface. Our results showed that MD simulations are fully capable of revealing mechanistic details of peptide adsorption to inorganic surfaces and are an invaluable tool to accelerate the rational design of peptide sequences for mineral processing applications.

Expanding known viral diversity in the healthy infant gut

The gut microbiome is shaped through infancy and impacts the maturation of the immune system, thus protecting against chronic disease later in life. Phages, or viruses that infect bacteria, modulate bacterial growth by lysis and lysogeny, with the latter being especially prominent in the infant gut. Viral metagenomes (viromes) are difficult to analyse because they span uncharted viral diversity, lacking marker genes and standardized detection methods. Here we systematically resolved the viral diversity in faecal viromes from 647 1-year-olds belonging to Copenhagen Prospective Studies on Asthma in Childhood 2010, an unselected Danish cohort of healthy mother-child pairs. By assembly and curation we uncovered 10,000 viral species from 248 virus family-level clades (VFCs). Most (232 VFCs) were previously unknown, belonging to the Caudoviricetes viral class. Hosts were determined for 79% of phage using clustered regularly interspaced short palindromic repeat spacers within bacterial metagenomes from the same children. Typical Bacteroides-infecting crAssphages were outnumbered by undescribed phage families infecting Clostridiales and Bifidobacterium. Phage lifestyles were conserved at the viral family level, with 33 virulent and 118 temperate phage families. Virulent phages were more abundant, while temperate ones were more prevalent and diverse. Together, the viral families found in this study expand existing phage taxonomy and provide a resource aiding future infant gut virome research.

Gender-Responsive Design of Bacteriophage Products to Enhance Adoption by Chicken Keepers in Kenya

Women and men keeping chickens in Kenya aspire to have a source of income, feed their families healthy food, and grow their businesses. Managing animal diseases and minimizing input costs enable their success. This study uses qualitative methods to recommend design opportunities for a veterinary product under development in Kenya that contains bacteriophages (phages) that target pathogenic Salmonella strains responsible for fowl typhoid, salmonellosis, and pullorum in chickens and foodborne illness in people. Our findings revealed the interplay between gender and two production systems: free-range and semi-intensive. Chicken keepers in both systems could benefit from phages combined with the orally administered Newcastle disease vaccine, one of the most commonly used preventive veterinary interventions, or phages as a treatment for fowl typhoid. Oral administration is less labor intensive, with greater benefits for women who have less control over family labor and reported doing more care tasks themselves. Men in free-range systems usually pay for veterinary inputs. In semi-intensive production systems, a phage-based product used prophylactically could be an alternative to expensive, intramuscular fowl typhoid vaccines. Keeping layers was common for women in semi-intensive systems, as they are more economically impacted by reduced laying caused by bacterial diseases. Awareness of zoonoses was low, but men and women were concerned about the negative health effects of drug residues in meat and eggs. Therefore, highlighting the lack of a withdrawal period for a phage product may appeal to customers. Antibiotics are used to both treat and prevent diseases, and phage products will need to do both to compete in the Kenyan market. These findings guide the ongoing design of a phage-based product with the goal of introducing a new veterinary product that meets the diverse needs of chicken keepers in Africa and serves as an alternative or complement to antibiotics.

CRISPR-Cas provides limited phage immunity to a prevalent gut bacterium in gnotobiotic mice

Many bacteria and archaea harbor the adaptive CRISPR-Cas system, which stores small nucleotide fragments from previous invasions of nucleic acids via viruses or plasmids. This molecular archive blocks further invaders carrying identical or similar nucleotide sequences. However, few of these systems have been confirmed experimentally to be active in gut bacteria. Here, we demonstrate experimentally that the type I-C CRISPR-Cas system of the prevalent gut bacterium Eggerthella lenta can specifically target and cleave foreign DNA in vitro by using a plasmid transformation assay. We also show that the CRISPR-Cas system acquires new immunities (spacers) from the genome of a virulent E. lenta phage using traditional phage assays in vitro but also in vivo using gnotobiotic (GB) mice. Both high phage titer and an increased number of spacer acquisition events were observed when E. lenta was exposed to a low multiplicity of infection in vitro, and three phage genes were found to contain protospacer hotspots. Fewer new spacer acquisitions were detected in vivo than in vitro. Longitudinal analysis of phage-bacteria interactions showed sustained coexistence in the gut of GB mice, with phage abundance being approximately one log higher than the bacteria. Our findings show that while the type I-C CRISPR-Cas system is active in vitro and in vivo, a highly virulent phage in vitro was still able to co-exist with its bacterial host in vivo. Taken altogether, our results suggest that the CRISPR-Cas defense system of E. lenta provides only partial immunity in the gut.

Detection of nucleotide modifications in bacteria and bacteriophages: Strengths and limitations of current technologies and software

RNA and DNA modifications occur in eukaryotes and prokaryotes, as well as in their viruses, and serve a wide range of functions, from gene regulation to nucleic acid protection. Although the first nucleotide modification was discovered almost 100 years ago, new and unusual modifications are still being described. Nucleotide modifications have also received more attention lately because of their increased significance, but also because new sequencing approaches have eased their detection. Chiefly, third generation sequencing platforms PacBio and Nanopore offer direct detection of modified bases by measuring deviations of the signals. These unusual modifications are especially prevalent in bacteriophage genomes, the viruses of bacteria, where they mostly appear to protect DNA against degradation from host nucleases. In this Opinion article, we highlight and discuss current approaches to detect nucleotide modifications, including hardwares and softwares, and look onward to future applications, especially for studying unusual, rare, or complex genome modifications in bacteriophages. The ability to distinguish between several types of nucleotide modifications may even shed new light on metagenomic studies.

Competition and coevolution drive the evolution and the diversification of CRISPR immunity

The diversity of resistance challenges the ability of pathogens to spread and to exploit host populations. Yet, how this host diversity evolves over time remains unclear because it depends on the interplay between intraspecific competition among host genotypes and coevolution with pathogens. Here we study experimentally the effect of coevolving phage populations on the diversification of bacterial CRISPR immunity across space and time. We demonstrate that the negative-frequency-dependent selection generated by coevolution is a powerful force that maintains host resistance diversity and selects for new resistance mutations in the host. We also find that host evolution is driven by asymmetries in competitive abilities among different host genotypes. Even if the fittest host genotypes are targeted preferentially by the evolving phages, they often escape extinctions through the acquisition of new CRISPR immunity. Together, these fluctuating selective pressures maintain diversity, but not by preserving the pre-existing host composition. Instead, we repeatedly observe the introduction of new resistance genotypes stemming from the fittest hosts in each population. These results highlight the importance of competition on the transient dynamics of host-pathogen coevolution.

Salmonella Enteritidis Bacteriophages Isolated from Kenyan Poultry Farms Demonstrate Time-Dependent Stability in Environments Mimicking the Chicken Gastrointestinal Tract

Multi-drug resistant (MDR) Salmonella enterica Enteritidis is one of the major causes of foodborne illnesses worldwide. This non-typhoidal Salmonella (NTS) serovar is mainly transmitted to humans through poultry products. Bacteriophages (phages) offer an alternative to antibiotics for reducing the incidence of MDR NTS in poultry farms. Phages that survive the harsh environment of the chicken gastrointestinal tract (cGIT), which have low pH, high temperatures, and several enzymes, may have a higher therapeutic or prophylactic potential. In this study, we analysed the stability of 10 different S. Enteritidis phages isolated from Kenyan poultry farms in different pH-adjusted media, incubation temperatures, as well as simulated gastric and intestinal fluids (SGF and SIF, respectively). Furthermore, their ability to persist in water sources available in Kenya, including river, borehole, rain and tap water, was assessed. All phages were relatively stable for 12 h at pHs ranging from 5 to 9 and at temperatures ranging from 25 °C to 42 °C. At pH 3, a loss in viral titre of up to three logs was observed after 3 h of incubation. In SGF, phages were stable for 20 min, after which they started losing infectivity. Phages were relatively stable in SIF for up to 2 h. The efficacy of phages to control Salmonella growth was highly reduced in pH 2- and pH 3-adjusted media and in SGF at pH 2.5, but less affected in SIF at pH 8. River water had the most significant detrimental effect on phages, while the other tested waters had a limited impact on the phages. Our data suggest that these phages may be administered to chickens through drinking water and may survive cGIT to prevent salmonellosis in poultry.

Phage endolysins are adapted to specific hosts and are evolutionarily dynamic

Endolysins are produced by (bacterio)phages to rapidly degrade the bacterial cell wall and release new viral particles. Despite sharing a common function, endolysins present in phages that infect a specific bacterial species can be highly diverse and vary in types, number, and organization of their catalytic and cell wall binding domains. While much is now known about the biochemistry of phage endolysins, far less is known about the implication of their diversity on phage–host adaptation and evolution. Using CRISPR-Cas9 genome editing, we could genetically exchange a subset of different endolysin genes into distinct lactococcal phage genomes. Regardless of the type and biochemical properties of these endolysins, fitness costs associated to their genetic exchange were marginal if both recipient and donor phages were infecting the same bacterial strain, but gradually increased when taking place between phage that infect different strains or bacterial species. From an evolutionary perspective, we observed that endolysins could be naturally exchanged by homologous recombination between phages coinfecting a same bacterial strain. Furthermore, phage endolysins could adapt to their new phage/host environment by acquiring adaptative mutations. These observations highlight the remarkable ability of phage lytic systems to recombine and adapt and, therefore, explain their large diversity and mosaicism. It also indicates that evolution should be considered to act on functional modules rather than on bacteriophages themselves. Furthermore, the extensive degree of evolvability observed for phage endolysins offers new perspectives for their engineering as antimicrobial agents.

Endolysins are produced by bacteriophages to degrade the host cell wall and release new particles, but the implications of their diversity on phage-host adaptation and evolution is unknown. This study uses CRISPR-Cas9 genome editing to reveal novel insights into bacteriophage endolysin diversity and phage-bacteria interactions as well as into endolysin adaptation towards a new bacterial host.

Phage Cocktail Development against Aeromonas salmonicida subsp. salmonicida Strains Is Compromised by a Prophage

Aquaculture is a rapidly growing food production sector. Fish farmers are experiencing increasing problems with antibiotic resistance when fighting against pathogenic bacteria such as Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis. Phage therapy may provide an alternative, but effective use must be determined. Here, we studied the inhibition of A. salmonicida subsp. salmonicida strains by five phages (HER98 [44RR2.8t.2], HER110 [65.2], SW69-9, L9-6 and Riv-10) used individually or as combinations of two to five phages. A particular combination of four phages (HER98 [44RR2.8t.2], SW69-9, Riv-10, and HER110 [65.2]) was found to be the most effective when used at an initial multiplicity of infection (MOI) of 1 against the A. salmonicida subsp. salmonicida strain 01-B526. The same phage cocktail is effective against other strains except those bearing a prophage (named Prophage 3), which is present in 2/3 of the strains from the province of Quebec. To confirm the impact of this prophage, we tested the effectiveness of the same cocktail on strains that were either cured or lysogenized with Prophage 3. While the parental strains were sensitive to the phage cocktail, the lysogenized ones were much less sensitive. These data indicate that the prophage content of A. salmonicida subsp. salmonicida can affect the efficacy of a cocktail of virulent phages for phage therapy purposes.

Analysis of viromes and microbiomes from pig fecal samples reveals that phages and prophages rarely carry antibiotic resistance genes

Understanding the transmission of antibiotic resistance genes (ARGs) is critical for human health. For this, it is necessary to identify which type of mobile genetic elements is able to spread them from animal reservoirs into human pathogens. Previous research suggests that in pig feces, ARGs may be encoded by bacteriophages. However, convincing proof for phage-encoded ARGs in pig viromes is still lacking, because of bacterial DNA contaminating issues. We collected 14 pig fecal samples and performed deep sequencing on both highly purified viral fractions and total microbiota, in order to investigate phage and prophage-encoded ARGs. We show that ARGs are absent from the genomes of active, virion-forming phages (below 0.02% of viral contigs from viromes), but present in three prophages, representing 0.02% of the viral contigs identified in the microbial dataset. However, the corresponding phages were not detected in the viromes, and their genetic maps suggest they might be defective. We conclude that among pig fecal samples, phages and prophages rarely carry ARG. Furthermore, our dataset allows for the first time a comprehensive view of the interplay between prophages and viral particles, and uncovers two large clades, inoviruses and Oengus-like phages.

Zebrafish: a big fish in the study of the gut microbiota

The importance of the gut microbiota in host health is now well established, but the underlying mechanisms remain poorly understood. Among the animal models used to investigate microbiota-host interactions, the zebrafish (Danio renio) is gaining attention. Several factors contribute to the recent interest in this model, including its low cost, the ability to assess large cohorts, the possibility to obtain germ-free larvae from non-axenic parents, and the availability of optical methodologies to probe the transparent larvae and adults from various genetic lines. We review recent findings on the zebrafish gut microbiota and its modulation by exogenous microbes, nutrition, and environmental factors. We also highlight the potential of this model for assessing the impact of the gut microbiota on brain development.

Induction and Elimination of Prophages Using CRISPR Interference

Prophages are widely spread among bacterial genomes, and they can have positive or negative effects on their hosts. A key aspect in the study of prophages is the discovery of their induction signals. Prophage induction can occur by inactivating a phage transcriptional repressor, which is responsible for maintaining the lysogenic state. This repressor can be inactivated through the bacterial SOS response. However, the induction signals for numerous prophages do not involve the SOS system, and therefore significant efforts are needed to identify these conditions. Similarly, curing bacterial strains of inducible prophages is a tedious process, requiring the screening of several colonies. Here, we investigated whether transcriptional silencing of a prophage repressor using CRISPR interference (CRISPRi) would lead to prophage induction. Using Escherichia coli phages λ and P2 as models, we demonstrated the efficiency of CRISPRi for prophage induction and for curing lysogenic strains of their prophages.

Streamlining CRISPR spacer-based bacterial host predictions to decipher the viral dark matter

Thousands of new phages have recently been discovered thanks to viral metagenomics. These phages are extremely diverse and their genome sequences often do not resemble any known phages. To appreciate their ecological impact, it is important to determine their bacterial hosts. CRISPR spacers can be used to predict hosts of unknown phages, as spacers represent biological records of past phage–bacteria interactions. However, no guidelines have been established to standardize host prediction based on CRISPR spacers. Additionally, there are no tools that use spacers to perform host predictions on large viral datasets. Here, we developed a set of tools that includes all the necessary steps for predicting the hosts of uncharacterized phages. We created a database of >11 million spacers and a program to execute host predictions on large viral datasets. Our host prediction approach uses biological criteria inspired by how CRISPR–Cas naturally work as adaptive immune systems, which make the results easy to interpret. We evaluated the performance using 9484 phages with known hosts and obtained a recall of 49% and a precision of 69%. We also found that this host prediction method yielded higher performance for phages that infect gut-associated bacteria, suggesting it is well suited for gut-virome characterization.

Genomic diversity and CRISPR-Cas systems in the cyanobacterium Nostoc in the High Arctic

Nostoc (Nostocales, Cyanobacteria) has a global distribution in the Polar Regions. However, the genomic diversity of Nostoc is little known and there are no genomes available for polar Nostoc. Here we carried out the first genomic analysis of the Nostoc commune morphotype with a recent sample from the High Arctic and a herbarium specimen collected during the British Arctic Expedition (1875-76). Comparisons of the polar genomes with 26 present-day non-polar members of the Nostocales family highlighted that there are pronounced genetic variations among Nostoc strains and species. Osmoprotection and other stress genes were found in all Nostoc strains, but the two Arctic strains had markedly higher numbers of biosynthetic gene clusters for uncharacterised non-ribosomal peptide synthetases, suggesting a high diversity of secondary metabolites. Since viral-host interactions contribute to microbial diversity, we analysed the CRISPR-Cas systems in the Arctic and two temperate Nostoc species. There were a large number of unique repeat-spacer arrays in each genome, indicating diverse histories of viral attack. All Nostoc strains had a subtype I-D system, but the polar specimens also showed evidence of a subtype I-B system that has not been previously reported in cyanobacteria, suggesting diverse cyanobacteria-virus interactions in the Arctic.

Functional Study of the Type II-A CRISPR-Cas System of Streptococcus agalactiae Hypervirulent Strains

Nearly all strains of Streptococcus agalactiae, the leading cause of invasive infections in neonates, encode a type II-A clustered regularly interspaced short palindromic repeats (CRISPR)-Cas system. Interestingly, S. agalactiae strains belonging to the hypervirulent Sequence Type 17 (ST17) contain significantly fewer spacers in their CRISPR locus than other lineages, which could be the result of a less functional CRISPR-Cas system. Here, we revealed one large deletion in the ST17 cas promoter region and we evaluated its impact on the transcription of cas genes as well as the functionalities of the CRISPR-Cas system. We demonstrated that Cas9 interference is functional and that the CRISPR-Cas system of ST17 strains can still acquire new spacers, despite the absence of a regular cas promoter. We demonstrated that a promoter sequence upstream of srn036, a small RNA partially overlapping the antisense tracrRNA, is responsible for the ST17 CRISPR-Cas adaptation and interference activities.

Ectopic Spacer Acquisition in Streptococcus thermophilus CRISPR3 Array

Streptococcus thermophilus relies heavily on two type II-A CRISPR-Cas systems, CRISPR1 and CRISPR3, to resist siphophage infections. One hallmark of these systems is the integration of a new spacer at the 5' end of the CRISPR arrays following phage infection. However, we have previously shown that ectopic acquisition of spacers can occur within the CRISPR1 array. Here, we present evidence of the acquisition of new spacers within the array of CRISPR3 of S. thermophilus. The analysis of randomly selected bacteriophage-insensitive mutants of the strain Uy01 obtained after phage infection, as well as the comparison with other S. thermophilus strains with similar CRISPR3 content, showed that a specific spacer within the array could be responsible for misguiding the adaptation complex. These results also indicate that while the vast majority of new spacers are added at the 5' end of the CRISPR array, ectopic spacer acquisition is a common feature of both CRISPR1 and CRISPR3 systems in S. thermophilus, and it can still provide phage resistance. Ectopic spacer acquisition also appears to have occurred naturally in some strains of Streptococcus pyogenes, suggesting that it is a general phenomenon, at least in type II-A systems.

The endless battle between phages and CRISPR-Cas systems in Streptococcus thermophilus

This review describes the contribution of basic research on phage-bacteria interactions to the understanding of CRISPR-Cas systems and their various applications. It focuses on the natural function of CRISPR-Cas systems as adaptive defense mechanisms against mobile genetic elements such as bacteriophage genomes and plasmids. Some of the advances in the characterization of the type II-A CRISPR-Cas system of Streptococcus thermophilus and Streptococcus pyogenes led to the development of the CRISPR-Cas9 genome-editing technology. We mostly discuss the 3 stages of the CRISPR-Cas system in S. thermophilus, namely the adaptation stage, which is unique to this resistance mechanism; the CRISPR RNA biogenesis; and the DNA-cutting activity in the interference stage to protect bacteria against phages. Finally, we look into applications of CRISPR-Cas in microbiology, including overcoming limitations in genome editing.

Delivery of CRISPR-Cas systems using phage-based vectors

Antimicrobial resistance has spread quickly on a worldwide scale, reducing therapeutic options for bacterial infections. CRISPR-Cas is an adaptive immune system found in many prokaryotes that can be designed to target bacterial genomes, leading to cell death. Repurposing the CRISPR-Cas system as a therapeutic strategy offers an attractive way to overcome antimicrobial resistance. However, this strategy requires efficient vectors for the CRISPR-Cas system to reach the bacterial genomes. Engineered phages offer an attractive option as cargo delivery vectors. In this review, we discuss the production of phage-based vectors and the relevance of using repurposed CRISPR-Cas systems as antimicrobials. We also discuss recent progress in phage engineering that can potentially overcome the limitations and increase the efficiency of CRISPR-Cas delivery.

Detection of preQ0 deazaguanine modifications in bacteriophage CAjan DNA using Nanopore sequencing reveals same hypermodification at two distinct DNA motifs

In the constant evolutionary battle against mobile genetic elements (MGEs), bacteria have developed several defense mechanisms, some of which target the incoming, foreign nucleic acids e.g. restriction-modification (R-M) or CRISPR-Cas systems. Some of these MGEs, including bacteriophages, have in turn evolved different strategies to evade these hurdles. It was recently shown that the siphophage CAjan and 180 other viruses use 7-deazaguanine modifications in their DNA to evade bacterial R-M systems. Among others, phage CAjan genome contains a gene coding for a DNA-modifying homolog of a tRNA-deazapurine modification enzyme, together with four 7-cyano-7-deazaguanine synthesis genes. Using the CRISPR-Cas9 genome editing tool combined with the Nanopore Sequencing (ONT) we showed that the 7-deazaguanine modification in the CAjan genome is dependent on phage-encoded genes. The modification is also site-specific and is found mainly in two separate DNA sequence contexts: GA and GGC. Homology modeling of the modifying enzyme DpdA provides insight into its probable DNA binding surface and general mode of DNA recognition.

Structural Insights into Lactococcal Siphophage p2 Baseplate Activation Mechanism

Virulent phages infecting L. lactis, an industry-relevant bacterium, pose a significant risk to the quality of the fermented milk products. Phages of the Skunavirus genus are by far the most isolated lactococcal phages in the cheese environments and phage p2 is the model siphophage for this viral genus. The baseplate of phage p2, which is used to recognize its host, was previously shown to display two conformations by X-ray crystallography, a rested state and an activated state ready to bind to the host. The baseplate became only activated and opened in the presence of Ca²⁺. However, such an activated state was not previously observed in the virion. Here, using nanobodies binding to the baseplate, we report on the negative staining electron microscopy structure of the activated form of the baseplate directly observed in the p2 virion, that is compatible with the activated baseplate crystal structure. Analyses of this new structure also established the presence of a second distal tail (Dit) hexamer as a component of the baseplate, the topology of which differs largely from the first one. We also observed an uncoupling between the baseplate activation and the tail tip protein (Tal) opening, suggesting an infection mechanism more complex than previously expected.

Source Tracking Based on Core Genome SNV and CRISPR Typing of Salmonella enterica Serovar Heidelberg Isolates Involved in Foodborne Outbreaks in Québec, 2012

Whole-genome sequencing (WGS) is the method of choice for bacterial subtyping and it is rapidly replacing the more traditional methods such as pulsed-field gel electrophoresis (PFGE). Here we used the high-resolution core genome single nucleotide variant (cgSNV) typing method to characterize clinical and food from Salmonella enterica serovar Heidelberg isolates in the context of source attribution. Additionally, clustered regularly interspaced short palindromic repeats (CRISPR) analysis was included to further support this method. Our results revealed that cgSNV was highly discriminatory and separated the outbreak isolates into distinct clusters (0-4 SNVs). CRISPR analysis was also able to distinguish outbreak strains from epidemiologically unrelated isolates. Specifically, our data clearly demonstrated the strength of these two methods to determine the probable source(s) of a 2012 epidemiologically characterized outbreak of S. Heidelberg. Using molecular cut-off of 0-10 SNVs, the cgSNV analysis of 246 clinical and food isolates of S. Heidelberg collected in Québec, in the same year of the outbreak event, revealed that retail and abattoir chicken isolates likely represent an important source of human infection to S. Heidelberg. Interestingly, the isolates genetically related by cgSNV also harbored the same CRISPR as outbreak isolates and clusters. This indicates that CRISPR profiles can be useful as a complementary approach to determine source attribution in foodborne outbreaks. Use of the genomic analysis also allowed to identify a large number of cases that were missed by PFGE, indicating that most outbreaks are probably underestimated. Although epidemiological information must still support WGS-based results, cgSNV method is a highly discriminatory method for the resolution of outbreak events and the attribution of these events to their respective sources. CRISPR typing can serve as a complimentary tool to this analysis during source tracking.

DNA tandem repeats contribute to the genetic diversity of Brevibacterium aurantiacum phages

This report presents the characterization of the first virulent phages infecting Brevibacterium aurantiacum, a bacterial species used during the manufacture of surface-ripened cheeses. These phages were also responsible for flavour and colour defects in surface-ripened cheeses. Sixteen phages (out of 62 isolates) were selected for genome sequencing and comparative analyses. These cos-type phages with a long non-contractile tail currently belong to the Siphoviridae family (Caudovirales order). Their genome sizes vary from 35,637 to 36,825 bp and, similar to their host, have a high GC content (~61%). Genes encoding for an immunity repressor, an excisionase and a truncated integrase were found, suggesting that these virulent phages may be derived from a prophage. Their genomic organization is highly conserved, with most of the diversity coming from the presence of long (198 bp) DNA tandem repeats (TRs) within an open reading frame coding for a protein of unknown function. We categorized these phages into seven genomic groups according to their number of TR, which ranged from two to eight. Moreover, we showed that TRs are widespread in phage genomes, found in more than 85% of the genomes available in public databases.

The EcoChip 2: An Autonomous Sensor Platform for Multimodal Bio-environmental Monitoring of the Northern Habitat

This paper presents the EcoChip 2, an autonomous multimodal bio-environmental sensor platform for the monitoring of microorganisms in the northern habitat. The EcoChip 2 prototype includes an array of 96-wells for the continuous monitoring of microbiological growth through a multichannel electrochemical impedance analyzer circuit. In addition, the platform includes luminosity, humidity, temperature sensors and monitoring. The developed electronic board uses an ultra-low-power microcontroller unit, a custom power management unit, a low-power wireless ISM-2.45 GHz transceiver, and a flash memory to accumulate and store the sensor data over extended monitoring periods. When a wireless base station is placed within the transmission range of the EcoChip 2, an embedded low-power wireless transceiver transmits the 96-wells impedance data and the other sensor data stored in the flash memory to the user interface. We present the measured performance of the prototype, along with laboratory test results of bacterial growth measurements inside the 96 wells in parallel. We show that the EcoChip 2 can successfully measure the impedances associated with bacterial growth over several hours using an excitation frequency of 2 kHz with power consumption of 114.6 mW under operating mode.

A short overview of the CRISPR-Cas adaptation stage

CRISPR research began over 30 years ago with the incidental discovery of an unusual nucleotide arrangement in the Escherichia coli genome. It took 20 years to find the main function of CRISPR-Cas systems as an adaptive defence mechanism against invading nucleic acids, and our knowledge of their biology has steadily increased ever since. In parallel, the number of applications derived from CRISPR-Cas systems has risen spectacularly. The CRISPR-based genome editing tool is arguably the most exciting application in both basic and applied research. Lately, CRISPR-Cas research has partially shifted to the least understood aspect of its biology: the ability of CRISPR-Cas systems to acquire new immunities during the so-called adaptation step. To date, the most efficient natural system to readily acquire new spacers is the type II-A system of the gram-positive dairy bacterium Streptococcus thermophilus. The discovery of additional systems able to acquire new spacers will hopefully draw more attention to this step of CRISPR-Cas biology. This review focuses on the breakthroughs that have helped to unravel the adaptation phase and on questions that remain to be answered.

Comparative genomic analysis of 142 bacteriophages infecting Salmonella enterica subsp. enterica

BACKGROUND

Bacteriophages are bacterial parasites and are considered the most abundant and diverse biological entities on the planet. Previously we identified 154 prophages from 151 serovars of Salmonella enterica subsp. enterica. A detailed analysis of Salmonella prophage genomics is required given the influence of phages on their bacterial hosts and should provide a broader understanding of Salmonella biology and virulence and contribute to the practical applications of phages as vectors and antibacterial agents.

RESULTS

Here we provide a comparative analysis of the full genome sequences of 142 prophages of Salmonella enterica subsp. enterica which is the full complement of the prophages that could be retrieved from public databases. We discovered extensive variation in genome sizes (ranging from 6.4 to 358.7 kb) and guanine plus cytosine (GC) content (ranging from 35.5 to 65.4%) and observed a linear correlation between the genome size and the number of open reading frames (ORFs). We used three approaches to compare the phage genomes. The NUCmer/MUMmer genome alignment tool was used to evaluate linkages and correlations based on nucleotide identity between genomes. Multiple sequence alignment was performed to calculate genome average nucleotide identity using the Kalgin program. Finally, genome synteny was explored using dot plot analysis. We found that 90 phage genome sequences grouped into 17 distinct clusters while the remaining 52 genomes showed no close relationships with the other phage genomes and are identified as singletons. We generated genome maps using nucleotide and amino acid sequences which allowed protein-coding genes to be sorted into phamilies (phams) using the Phamerator software. Out of 5796 total assigned phamilies, one phamily was observed to be dominant and was found in 49 prophages, or 34.5% of the 142 phages in our collection. A majority of the phamilies, 4330 out of 5796 (74.7%), occurred in just one prophage underscoring the high degree of diversity among Salmonella bacteriophages.

CONCLUSIONS

Based on nucleotide and amino acid sequences, a high diversity was found among Salmonella bacteriophages which validate the use of prophage sequence analysis as a highly discriminatory subtyping tool for Salmonella. Thorough understanding of the conservation and variation of prophage genomic characteristics will facilitate their rational design and use as tools for bacterial strain construction, vector development and as anti-bacterial agents.

Variability in the durability of CRISPR-Cas immunity

The durability of host resistance is challenged by the ability of pathogens to escape the defence of their hosts. Understanding the variability in the durability of host resistance is of paramount importance for designing more effective control strategies against infectious diseases. Here, we study the durability of various clustered regularly interspaced short palindromic repeats-Cas (CRISPR-Cas) alleles of the bacteria Streptococcus thermophilus against lytic phages. We found substantial variability in durability among different resistant bacteria. Since the escape of the phage is driven by a mutation in the phage sequence targeted by CRISPR-Cas, we explored the fitness costs associated with these escape mutations. We found that, on average, escape mutations decrease the fitness of the phage. Yet, the magnitude of this fitness cost does not predict the durability of CRISPR-Cas immunity. We contend that this variability in the durability of resistance may be because of variations in phage mutation rate or in the proportion of lethal mutations across the phage genome. These results have important implications on the coevolutionary dynamics between bacteria and phages and for the optimal deployment of resistance strategies against pathogens and pests. Understanding the durability of CRISPR-Cas immunity may also help develop more effective gene-drive strategies based on CRISPR-Cas9 technology. This article is part of a discussion meeting issue 'The ecology and evolution of prokaryotic CRISPR-Cas adaptive immune systems'.

Phage diversity, genomics and phylogeny

Recent advances in viral metagenomics have enabled the rapid discovery of an unprecedented catalogue of phages in numerous environments, from the human gut to the deep ocean. Although these advances have expanded our understanding of phage genomic diversity, they also revealed that we have only scratched the surface in the discovery of novel viruses. Yet, despite the remarkable diversity of phages at the nucleotide sequence level, the structural proteins that form viral particles show strong similarities and conservation. Phages are uniquely interconnected from an evolutionary perspective and undergo multiple events of genetic exchange in response to the selective pressure of their hosts, which drives their diversity. In this Review, we explore phage diversity at the structural, genomic and community levels as well as the complex evolutionary relationships between phages, moulded by the mosaicity of their genomes.

Virulent coliphages in 1-year-old children fecal samples are fewer, but more infectious than temperate coliphages

Bacteriophages constitute an important part of the human gut microbiota, but their impact on this community is largely unknown. Here, we cultivate temperate phages produced by 900 E. coli strains isolated from 648 fecal samples from 1-year-old children and obtain coliphages directly from the viral fraction of the same fecal samples. We find that 63% of strains hosted phages, while 24% of the viromes contain phages targeting E. coli. 150 of these phages, half recovered from strain supernatants, half from virome (73% temperate and 27% virulent) were tested for their host range on 75 E. coli strains isolated from the same cohort. Temperate phages barely infected the gut strains, whereas virulent phages killed up to 68% of them. We conclude that in fecal samples from children, temperate coliphages dominate, while virulent ones have greater infectivity and broader host range, likely playing a role in gut microbiota dynamics.

The impact of bacteriophages in the human gut microbiome remains poorly understood. Here, the authors characterize coliphages isolated from a large cohort of 1-year-old infants and show that temperate coliphages dominate, while virulent ones have greater infectivity and broader host range.

Versatile and robust genome editing with Streptococcus thermophilus CRISPR1-Cas9

Targeting definite genomic locations using CRISPR-Cas systems requires a set of enzymes with unique protospacer adjacent motif (PAM) compatibilities. To expand this repertoire, we engineered nucleases, cytosine base editors, and adenine base editors from the archetypal Streptococcus thermophilus CRISPR1-Cas9 (St1Cas9) system. We found that St1Cas9 strain variants enable targeting to five distinct A-rich PAMs and provide a structural basis for their specificities. The small size of this ortholog enables expression of the holoenzyme from a single adeno-associated viral vector for in vivo editing applications. Delivery of St1Cas9 to the neonatal liver efficiently rewired metabolic pathways, leading to phenotypic rescue in a mouse model of hereditary tyrosinemia. These robust enzymes expand and complement current editing platforms available for tailoring mammalian genomes.

7-Deazaguanine modifications protect phage DNA from host restriction systems

Genome modifications are central components of the continuous arms race between viruses and their hosts. The archaeosine base (G+), which was thought to be found only in archaeal tRNAs, was recently detected in genomic DNA of Enterobacteria phage 9g and was proposed to protect phage DNA from a wide variety of restriction enzymes. In this study, we identify three additional 2'-deoxy-7-deazaguanine modifications, which are all intermediates of the same pathway, in viruses: 2'-deoxy-7-amido-7-deazaguanine (dADG), 2'-deoxy-7-cyano-7-deazaguanine (dPreQ0) and 2'-deoxy-7- aminomethyl-7-deazaguanine (dPreQ1). We identify 180 phages or archaeal viruses that encode at least one of the enzymes of this pathway with an overrepresentation (60%) of viruses potentially infecting pathogenic microbial hosts. Genetic studies with the Escherichia phage CAjan show that DpdA is essential to insert the 7-deazaguanine base in phage genomic DNA and that 2'-deoxy-7-deazaguanine modifications protect phage DNA from host restriction enzymes.

A mutation in the methionine aminopeptidase gene provides phage resistance in Streptococcus thermophilus

Streptococcus thermophilus is a lactic acid bacterium widely used by the dairy industry for the manufacture of yogurt and specialty cheeses. It is also a Gram-positive bacterial model to study phage-host interactions. CRISPR-Cas systems are one of the most prevalent phage resistance mechanisms in S. thermophilus. Little information is available about other host factors involved in phage replication in this food-grade streptococcal species. We used the model strain S. thermophilus SMQ-301 and its virulent phage DT1, harboring the anti-CRISPR protein AcrIIA6, to show that a host gene coding for a methionine aminopeptidase (metAP) is necessary for phage DT1 to complete its lytic cycle. A single mutation in metAP provides S. thermophilus SMQ-301 with strong resistance against phage DT1. The mutation impedes a late step of the lytic cycle since phage adsorption, DNA replication, and protein expression were not affected. When the mutated strain was complemented with the wild-type version of the gene, the phage sensitivity phenotype was restored. When this mutation was introduced into other S. thermophilus strains it provided resistance against cos-type (Sfi21dt1virus genus) phages but replication of pac-type (Sfi11virus genus) phages was not affected. The mutation in the gene coding for the MetAP induces amino acid change in a catalytic domain conserved across many bacterial species. Introducing the same mutation in Streptococcus mutans also provided a phage resistance phenotype, suggesting the wide-ranging importance of the host methionine aminopeptidase in phage replication.

Diversity and Host Specificity Revealed by Biological Characterization and Whole Genome Sequencing of Bacteriophages Infecting Salmonella enterica

Phages infecting members of the opportunistic human pathogen, Salmonella enterica, are widespread in natural environments and offer a potential source of agents that could be used for controlling populations of this bacterium; yet, relatively little is known about these phages. Here we describe the isolation and characterization of 45 phages of Salmonella enterica from disparate geographic locations within British Columbia, Canada. Host-range profiling revealed host-specific patterns of susceptibility and resistance, with several phages identified that have a broad-host range (i.e., able to lyse >40% of bacterial hosts tested). One phage in particular, SE13, is able to lyse 51 out of the 61 Salmonella strains tested. Comparative genomic analyses also revealed an abundance of sequence diversity in the sequenced phages. Alignment of the genomes grouped the phages into 12 clusters with three singletons. Phages within certain clusters exhibited extraordinarily high genome homology (>98% nucleotide identity), yet between clusters, genomes exhibited a span of diversity (<50% nucleotide identity). Alignment of the major capsid protein also supported the clustering pattern observed with alignment of the whole genomes. We further observed associations between genomic relatedness and the site of isolation, as well as genetic elements related to DNA metabolism and host virulence. Our data support the knowledge framework for phage diversity and phage-host interactions that are required for developing phage-based applications for various sectors, including biocontrol, detection and typing.

Mobilome of Brevibacterium aurantiacum Sheds Light on Its Genetic Diversity and Its Adaptation to Smear-Ripened Cheeses

Brevibacterium aurantiacum is an actinobacterium that confers key organoleptic properties to washed-rind cheeses during the ripening process. Although this industrially relevant species has been gaining an increasing attention in the past years, its genome plasticity is still understudied due to the unavailability of complete genomic sequences. To add insights on the mobilome of this group, we sequenced the complete genomes of five dairy Brevibacterium strains and one non-dairy strain using PacBio RSII. We performed phylogenetic and pan-genome analyses, including comparisons with other publicly available Brevibacterium genomic sequences. Our phylogenetic analysis revealed that these five dairy strains, previously identified as Brevibacterium linens, belong instead to the B. aurantiacum species. A high number of transposases and integrases were observed in the Brevibacterium spp. strains. In addition, we identified 14 and 12 new insertion sequences (IS) in B. aurantiacum and B. linens genomes, respectively. Several stretches of homologous DNA sequences were also found between B. aurantiacum and other cheese rind actinobacteria, suggesting horizontal gene transfer (HGT). A HGT region from an iRon Uptake/Siderophore Transport Island (RUSTI) and an iron uptake composite transposon were found in five B. aurantiacum genomes. These findings suggest that low iron availability in milk is a driving force in the adaptation of this bacterial species to this niche. Moreover, the exchange of iron uptake systems suggests cooperative evolution between cheese rind actinobacteria. We also demonstrated that the integrative and conjugative element BreLI (Brevibacterium Lanthipeptide Island) can excise from B. aurantiacum SMQ-1417 chromosome. Our comparative genomic analysis suggests that mobile genetic elements played an important role into the adaptation of B. aurantiacum to cheese ecosystems.

Beyond the A-layer: adsorption of lipopolysaccharides and characterization of bacteriophage-insensitive mutants of Aeromonas salmonicida subsp. salmonicida

Aeromonas salmonicida subsp. salmonicida is a fish pathogen that causes furunculosis. Antibiotherapy used to treat furunculosis in fish has led to resistance. Virulent phages are increasingly seen as alternatives or complementary treatments against furunculosis in aquaculture environments. For phage therapy to be successful, it is essential to study the natural mechanisms of phage resistance in A. salmonicida subsp. salmonicida. Here, we generated bacteriophage-insensitive mutants (BIMs) of A. salmonicida subsp. salmonicida, using a myophage with broad host range and characterized them. Phage plaques were different depending on whether the A-layer surface array protein was expressed or not. The genome analysis of the BIMs helped to identify mutations in genes involved in the biogenesis of lipopolysaccharides (LPS) and on an uncharacterized gene (ASA_1998). The characterization of the LPS profile and gene complementation assays identified LPS as a phage receptor and confirmed the involvement of the uncharacterized protein ASA_1998 in phage infection. In addition, we confirmed that the presence of an A-layer at the bacterial surface could act as protection against phages. This study brings new elements into our understanding of the phage adsorption to A. salmonicida subsp. salmonicida cells.