Molecular characterization of a new abortive infection system (AbiU) from Lactococcus lactis LL51-1

Correlation of Pseudomonas aeruginosa Phage Resistance with the Numbers and Types of Antiphage Systems

Phage therapeutics offer a potentially powerful approach for combating multidrug-resistant bacterial infections. However, to be effective, phage therapy must overcome existing and developing phage resistance. While phage cocktails can reduce this risk by targeting multiple receptors in a single therapeutic, bacteria have mechanisms of resistance beyond receptor modification. A rapidly growing body of knowledge describes a broad and varied arsenal of antiphage systems encoded by bacteria to counter phage infection. We sought to understand the types and frequencies of antiphage systems present in a highly diverse panel of Pseudomonas aeruginosa clinical isolates utilized to characterize novel antibacterials. Using the web-server tool PADLOC (prokaryotic antiviral defense locator), putative antiphage systems were identified in these P. aeruginosa clinical isolates based on sequence homology to a validated and curated catalog of known defense systems. Coupling this host bacterium sequence analysis with host range data for 70 phages, we observed a correlation between existing phage resistance and the presence of higher numbers of antiphage systems in bacterial genomes. We were also able to identify antiphage systems that were more prevalent in highly phage-resistant P. aeruginosa strains, suggesting their importance in conferring resistance.

The Battle between Bacteria and Bacteriophages: A Conundrum to Their Immune System

Bacteria and their predators, bacteriophages, or phages are continuously engaged in an arms race for their survival using various defense strategies. Several studies indicated that the bacterial immune arsenal towards phage is quite diverse and uses different components of the host machinery. Most studied antiphage systems are associated with phages, whose genomic matter is double-stranded-DNA. These defense mechanisms are mainly related to either the host or phage-derived proteins and other associated structures and biomolecules. Some of these strategies include DNA restriction-modification (R-M), spontaneous mutations, blocking of phage receptors, production of competitive inhibitors and extracellular matrix which prevent the entry of phage DNA into the host cytoplasm, assembly interference, abortive infection, toxin-antitoxin systems, bacterial retrons, and secondary metabolite-based replication interference. On the contrary, phages develop anti-phage resistance defense mechanisms in consortium with each of these bacterial phage resistance strategies with small fitness cost. These mechanisms allow phages to undergo their replication safely inside their bacterial host's cytoplasm and be able to produce viable, competent, and immunologically endured progeny virions for the next generation. In this review, we highlight the major bacterial defense systems developed against their predators and some of the phage counterstrategies and suggest potential research directions.

Reconstructing horizontal gene flow network to understand prokaryotic evolution

Horizontal gene transfer (HGT) is a major source of phenotypic innovation and a mechanism of niche adaptation in prokaryotes. Quantification of HGT is critical to decipher its myriad roles in microbial evolution and adaptation. Advances in genome sequencing and bioinformatics have augmented our ability to understand the microbial world, particularly the direct or indirect influence of HGT on diverse life forms. Methods for detecting HGT can be classified into phylogenetic-based and parametric or composition-based approaches. Here, we exploited the complementary strengths of both the approaches to construct a high confidence horizontal gene flow network. Our network is unique in its ability to detect the transfer of native genes of a genome to genomes from other taxa, thus establishing donor and recipient organisms (taxa), rather than through a post hoc analysis as is the practice with several other approaches. The scale-free horizontal gene flow network presented here provides new insights into modes of transfer for the exchange of genetic information and also illuminates differential gene flow across phyla.

Reconstructing horizontal gene flow network to understand prokaryotic evolution

Horizontal gene transfer (HGT) is a major source of phenotypic innovation and a mechanism of niche adaptation in prokaryotes. Quantification of HGT is critical to decipher its myriad roles in microbial evolution and adaptation. Advances in genome sequencing and bioinformatics have augmented our ability to understand the microbial world, particularly the direct or indirect influence of HGT on diverse life forms. Methods for detecting HGT can be classified into phylogenetic-based and parametric or composition-based approaches. Here, we exploited the complementary strengths of both the approaches to construct a high confidence horizontal gene flow network. Our network is unique in its ability to detect the transfer of native genes of a genome to genomes from other taxa, thus establishing donor and recipient organisms (taxa), rather than through a post hoc analysis as is the practice with several other approaches. The scale-free horizontal gene flow network presented here provides new insights into modes of transfer for the exchange of genetic information and also illuminates differential gene flow across phyla.

PADLOC: a web server for the identification of antiviral defence systems in microbial genomes

Most bacteria and archaea possess multiple antiviral defence systems that protect against infection by phages, archaeal viruses and mobile genetic elements. Our understanding of the diversity of defence systems has increased greatly in the last few years, and many more systems likely await discovery. To identify defence-related genes, we recently developed the Prokaryotic Antiviral Defence LOCator (PADLOC) bioinformatics tool. To increase the accessibility of PADLOC, we describe here the PADLOC web server (freely available at https://padloc.otago.ac.nz), allowing users to analyse whole genomes, metagenomic contigs, plasmids, phages and archaeal viruses. The web server includes a more than 5-fold increase in defence system types detected (since the first release) and expanded functionality enabling detection of CRISPR arrays and retron ncRNAs. Here, we provide user information such as input options, description of the multiple outputs, limitations and considerations for interpretation of the results, and guidance for subsequent analyses. The PADLOC web server also houses a precomputed database of the defence systems in > 230,000 RefSeq genomes. These data reveal two taxa, Campylobacterota and Spriochaetota, with unusual defence system diversity and abundance. Overall, the PADLOC web server provides a convenient and accessible resource for the detection of antiviral defence systems.

Interaction of phages, bacteria, and the human immune system: Evolutionary changes in phage therapy

Phages and bacteria are known to undergo dynamic and co-evolutionary arms race interactions in order to survive. Recent advances from in vitro and in vivo studies have improved our understanding of the complex interactions between phages, bacteria, and the human immune system. This insight is essential for the development of phage therapy to battle the growing problems of antibiotic resistance. It is also pivotal to prevent the development of phage-resistance during the implementation of phage therapy in the clinic. In this review, we discuss recent progress of the interactions between phages, bacteria, and the human immune system and its clinical application for phage therapy. Proper phage therapy design will ideally produce large burst sizes, short latent periods, broad host ranges, and a low tendency to select resistance.

RNA damage in biological conflicts and the diversity of responding RNA repair systems

RNA is targeted in biological conflicts by enzymatic toxins or effectors. A vast diversity of systems which repair or ‘heal’ this damage has only recently become apparent. Here, we summarize the known effectors, their modes of action, and RNA targets before surveying the diverse systems which counter this damage from a comparative genomics viewpoint. RNA-repair systems show a modular organization with extensive shuffling and displacement of the constituent domains; however, a general ‘syntax’ is strongly maintained whereby systems typically contain: a RNA ligase (either ATP-grasp or RtcB superfamilies), nucleotidyltransferases, enzymes modifying RNA-termini for ligation (phosphatases and kinases) or protection (methylases), and scaffold or cofactor proteins. We highlight poorly-understood or previously-uncharacterized repair systems and components, e.g. potential scaffolding cofactors (Rot/TROVE and SPFH/Band-7 modules) with their respective cognate non-coding RNAs (YRNAs and a novel tRNA-like molecule) and a novel nucleotidyltransferase associating with diverse ligases. These systems have been extensively disseminated by lateral transfer between distant prokaryotic and microbial eukaryotic lineages consistent with intense inter-organismal conflict. Components have also often been ‘institutionalized’ for non-conflict roles, e.g. in RNA-splicing and in RNAi systems (e.g. in kinetoplastids) which combine a distinct family of RNA-acting prim-pol domains with DICER-like proteins.

The Lactococcus lactis plasmidome: much learnt, yet still lots to discover

Lactococcus lactis is used extensively worldwide for the production of a variety of fermented dairy products. The ability of L. lactis to successfully grow and acidify milk has long been known to be reliant on a number of plasmid-encoded traits. The recent availability of low-cost, high-quality genome sequencing, and the quest for novel, technologically desirable characteristics, such as novel flavour development and increased stress tolerance, has led to a steady increase in the number of available lactococcal plasmid sequences. We will review both well-known and very recent discoveries regarding plasmid-encoded traits of biotechnological significance. The acquired lactococcal plasmid sequence information has in recent years progressed our understanding of the origin of lactococcal dairy starter cultures. Salient points on the acquisition and evolution of lactococcal plasmids will be discussed in this review, as well as prospects of finding novel plasmid-encoded functions.

A widespread bacteriophage abortive infection system functions through a Type IV toxin-antitoxin mechanism

Bacterial abortive infection (Abi) systems are 'altruistic' cell death systems that are activated by phage infection and limit viral replication, thereby providing protection to the bacterial population. Here, we have used a novel approach of screening Abi systems as a tool to identify and characterize toxin-antitoxin (TA)-acting Abi systems. We show that AbiE systems are encoded by bicistronic operons and function via a non-interacting (Type IV) bacteriostatic TA mechanism. The abiE operon was negatively autoregulated by the antitoxin, AbiEi, a member of a widespread family of putative transcriptional regulators. AbiEi has an N-terminal winged-helix-turn-helix domain that is required for repression of abiE transcription, and an uncharacterized bi-functional C-terminal domain, which is necessary for transcriptional repression and sufficient for toxin neutralization. The cognate toxin, AbiEii, is a predicted nucleotidyltransferase (NTase) and member of the DNA polymerase β family. AbiEii specifically bound GTP, and mutations in conserved NTase motifs (I-III) and a newly identified motif (IV), abolished GTP binding and subsequent toxicity. The AbiE systems can provide phage resistance and enable stabilization of mobile genetic elements, such as plasmids. Our study reveals molecular insights into the regulation and function of the widespread bi-functional AbiE Abi-TA systems and the biochemical properties of both toxin and antitoxin proteins.

SP10 infectivity is aborted after bacteriophage SP10 infection induces nonA transcription on the prophage SPβ region of the Bacillus subtilis genome

Bacteria have developed various strategies for phage resistance. Infection with phage induces the transcription of part of the phage resistance gene, but the regulatory mechanisms of such transcription remain largely unknown. The phage resistance gene nonA is located on the SPβ prophage region of the Bacillus subtilis Marburg strain genome. The nonA transcript was detected at the late stage of SP10 infection but is undetectable in noninfected cells. The nonA transcript was detected after the induction of the sigma factor Orf199-Orf200 (σ(Orf199-200)), when sigma factors encoded in the SP10 genome were expressed from a xylose-inducible plasmid. Thus, the SP10 sigma factor is an activator of a set of SP10 genes and nonA. The nonA gene encodes a 72-amino-acid protein with a transmembrane motif and has no significant homology with any protein in any database. NonA overexpression halted cell growth and reduced the efficiency of B. subtilis colony formation and respiration activity. In addition, SP10 virion protein synthesis was inhibited in the nonA(+) strain, and SP10 virion particles were scarce in it. These results indicate that NonA is a novel protein that can abort SP10 infection, and its transcription was regulated by SP10 sigma factor.

Structure and activity of AbiQ, a lactococcal endoribonuclease belonging to the type III toxin-antitoxin system

AbiQ is a phage resistance mechanism found on a native plasmid of Lactococcus lactis that abort virulent phage infections. In this study, we experimentally demonstrate that AbiQ belongs to the recently described type III toxin-antitoxin systems. When overexpressed, the AbiQ protein (ABIQ) is toxic and causes bacterial death in a bacteriostatic manner. Northern and Western blot experiments revealed that the abiQ gene is transcribed and translated constitutively, and its expression is not activated by a phage product. ABIQ is an endoribonuclease that specifically cleaves its cognate antitoxin RNA molecule in vivo. The crystal structure of ABIQ was solved and site-directed mutagenesis identified key amino acids for its anti-phage and/or its RNase function. The AbiQ system is the first lactococcal abortive infection system characterized to date at a structural level.

Bacteriophages in food fermentations: new frontiers in a continuous arms race

Phage contamination represents an important risk to any process requiring bacterial growth, particularly in the biotechnology and food industries. The presence of unwanted phages may lead to manufacturing delays, lower quality product, or, in the worst cases, total production loss. Thus, constant phage monitoring and stringent application of the appropriate control measures are indispensable. In fact, a systematic preventive approach to phage contamination [phage analysis and critical control points (PACCP)] should be put in place. In this review, sources of phage contamination and novel phage detection methods are described, with an emphasis on bacterial viruses that infect lactic acid bacteria used in food fermentations. Recent discoveries related to antiphage systems that are changing our views on phage-host interactions are highlighted. Finally, future directions are also discussed.

Lactococcal abortive infection protein AbiV interacts directly with the phage protein SaV and prevents translation of phage proteins

AbiV is an abortive infection protein that inhibits the lytic cycle of several virulent phages infecting Lactococcus lactis, while a mutation in the phage gene sav confers insensitivity to AbiV. In this study, we have further characterized the effects of the bacterial AbiV and its interaction with the phage p2 protein SaV. First, we showed that during phage infection of lactococcal AbiV(+) cells, AbiV rapidly inhibited protein synthesis. Among early phage transcripts, sav gene transcription was slightly inhibited while the SaV protein could not be detected. Analyses of other phage p2 mRNAs and proteins suggested that AbiV blocks the activation of late gene transcription, probably by a general inhibition of translation. Using size exclusion chromatography coupled with on-line static light scattering and refractometry, as well as fluorescence quenching experiments, we also demonstrated that both AbiV and SaV formed homodimers and that they strongly and specifically interact with each other to form a stable protein complex.

Bacteriophage resistance mechanisms

Phages are now acknowledged as the most abundant microorganisms on the planet and are also possibly the most diversified. This diversity is mostly driven by their dynamic adaptation when facing selective pressure such as phage resistance mechanisms, which are widespread in bacterial hosts. When infecting bacterial cells, phages face a range of antiviral mechanisms, and they have evolved multiple tactics to avoid, circumvent or subvert these mechanisms in order to thrive in most environments. In this Review, we highlight the most important antiviral mechanisms of bacteria as well as the counter-attacks used by phages to evade these systems.

Identification and characterization of the phage gene sav, involved in sensitivity to the lactococcal abortive infection mechanism AbiV

Lactococcus lactis phage mutants that are insensitive to the recently characterized abortive infection mechanism AbiV were isolated and analyzed in an effort to elucidate factors involved in the sensitivity to AbiV. Whole-genome sequencing of the phage mutants p2.1 and p2.2 revealed mutations in an orf that is transcribed early, indicating that this orf was responsible for AbiV sensitivity. Sequencing of the homologous regions in the genomes of other AbiV-insensitive mutants derived from p2 and six other lactococcal wild-type phages revealed point mutations in the homologous orf sequences. The orf was named sav (for sensitivity to AbiV), and the encoded polypeptide was named SaV. The purification of a His-tagged SaV polypeptide by gel filtration suggested that the polypeptide formed a dimer in its native form. The overexpression of SaV in L. lactis and Escherichia coli led to a rapid toxic effect. Conserved, evolutionarily related regions in SaV polypeptides of different phage groups are likely to be responsible for the AbiV-sensitive phenotype and the toxicity.

AbiV, a novel antiphage abortive infection mechanism on the chromosome of Lactococcus lactis subsp. cremoris MG1363

Insertional mutagenesis with pGhost9::ISS1 resulted in independent insertions in a 350-bp region of the chromosome of Lactococcus lactis subsp. cremoris MG1363 that conferred phage resistance to the integrants. The orientation and location of the insertions suggested that the phage resistance phenotype was caused by a chromosomal gene turned on by a promoter from the inserted construct. Reverse transcription-PCR analysis confirmed that there were higher levels of transcription of a downstream open reading frame (ORF) in the phage-resistant integrants than in the phage-sensitive strain L. lactis MG1363. This gene was also found to confer phage resistance to L. lactis MG1363 when it was cloned into an expression vector. A subsequent frameshift mutation in the ORF completely eliminated the phage resistance phenotype, confirming that the ORF was necessary for phage resistance. This ORF provided resistance against virulent lactococcal phages belonging to the 936 and c2 species with an efficiency of plaquing of 10(-4), but it did not protect against members of the P335 species. A high level of expression of the ORF did not affect the cellular growth rate. Assays for phage adsorption, DNA ejection, restriction/modification activity, plaque size, phage DNA replication, and cell survival showed that the ORF encoded an abortive infection (Abi) mechanism. Sequence analysis revealed a deduced protein consisting of 201 amino acids which, in its native state, probably forms a dimer in the cytosol. Similarity searches revealed no homology to other phage resistance mechanisms, and thus, this novel Abi mechanism was designated AbiV. The mode of action of AbiV is unknown, but the activity of AbiV prevented cleavage of the replicated phage DNA of 936-like phages.

Exclusion of glucosyl-hydroxymethylcytosine DNA containing bacteriophages is overcome by the injected protein inhibitor IPI*

The Escherichia coli isolate CT596 excludes infection by the Myoviridae T4 ip1(-) phage that lacks the encapsidated IPI* protein normally injected into the host with the phage DNA. Screening of a CT596 genomic library identified adjacent genes responsible for this exclusion, gmrS (942 bp) and gmrD (708 bp) that are encoded by a cryptic prophage DNA. The two genes are necessary and sufficient to confer upon a host the ability to exclude infection by T4 ip1(-) phage and other glucosyl-hydroxymethylcytosine (glc-HMC) Tevens lacking the ip1 gene, yet allow infection by phages with non-glucoslyated cytosine (C) DNA that lack the ip1 gene. A plasmid expressing the ip1 gene product, IPI*, allows growth of Tevens lacking ip1 on E. coli strains carrying the cloned gmrS/gmrD genes. Members of the Teven family carry a diverse and, in some cases, expanded set of ip1 locus genes. In vivo analysis suggests a family of gmr genes that specifically target sugar-HMC modified DNA have evolved to exclude Teven phages, and these exclusion genes have in turn been countered by a family of injected exclusion inhibitors that likely help determine the host range of different glc-HMC phages.

Involvement of the LlaKR2I methylase in expression of the AbiR bacteriophage defense system in Lactococcus lactis subsp. lactis biovar diacetylactis KR2

The native lactococcal plasmid, pKR223, from Lactococcus lactis subsp. lactis biovar diacetylactis KR2 encodes two distinct bacteriophage-resistant mechanisms, the LlaKR2I restriction and modification (R/M) system and the abortive infection (Abi) mechanism, AbiR, that impedes bacteriophage DNA replication. This study completed the characterization of AbiR, revealing that it is the first Abi system to be encoded by three genes, abiRa, abiRb, and abiRc, arranged in an operon and that it requires the methylase gene from the LlaKR2I R/M system. An analysis of deletion and insertion clones demonstrated that the AbiR operon was toxic in L. lactis without the presence of the LlaKR2I methylase, which is required to protect L. lactis from AbiR toxicity. The novelty of the AbiR system resides in its original gene organization and the unusual protective role of the LlaKR2I methylase. Interestingly, the AbiR genetic determinants are flanked by two IS982 elements generating a likely transposable AbiR composite. This observation not only substantiated the novel function of the LlaKR2I methylase in the AbiR system but also illustrated the evolution of the LlaKR2I methylase toward a new and separate cellular function. This unique structure of both the LlaKR2I R/M system and the AbiR system may have contributed to the evolution of the LlaKR2I methylase toward a novel role comparable to that of the cell cycle-regulated methylases that include Dam and CcrM methylases. This new role for the LlaKR2I methylase offers a unique snapshot into the evolution of the cell cycle-regulated methylases from an existing R/M system.

Plasmids of lactococci – genetic accessories or genetic necessities?

Lactococci are one of the most exploited microorganisms used in the manufacture of food. These intensively used cultures are generally characterized by having a rich plasmid complement. It could be argued that it is the plasmid complement of commercially utilized cultures that gives them their technical superiority and individuality. Consequently, it is timely to reflect on the desirable characteristics encoded on lactococcal plasmids. It is argued that plasmids play a key role in the evolution of modern starter strains and are a lot more than just selfish replicosomes but more essential necessities of intensively used commercial starters. Moreover, the study of plasmid biology provides a genetic blueprint that has proved essential for the generation of molecular tools for the genetic improvement of Lactococcus lactis.

Resistance of corynebacterial strains to infection and lysis by corynephage BFK 20

AIMS

Defence mechanisms of the corynebacterial strains against corynephage BFK 20, which causes lysis of Brevibacterium flavum CCM 251.

METHODS AND RESULTS

We tested adsorption of the phage BFK 20 to the corynebacterial cell surface. We observed strong adsorption ranging from ca 79 to 93% on the cells of B. flavum ATCC strains, but only ca 76% for B. flavum CCM 251. Minor adsorption for Brevibacterium lactofermentum BLOB (ca 13%) and no adsorption for Corynebacterium glutamicum RM3 were determined. BFK 20 infection had no significant effect on growth and viability of C. glutamicum and B. lactofermentum, but significantly influenced growth and viability of B. flavum ATCC 21127, 21128 and 21474. Cell growth stopped in short time after infection but with no lysis. Brevibacterium flavum CCM 251 cell growth was arrested too and lysis occurred. The Southern hybridization confirmed the presence of significant amount of BFK 20 DNA in samples from B. flavum CCM 251 and B. flavum ATCC strains after BFK 20 infection. Only weak hybridization signal was detected for DNA from infected cells of B. lactofermentum BLOB and no signal for C. glutamicum RM3.

CONCLUSIONS

Based on the above results we suggest presence of a mechanism leading to abortive infection in B. flavum ATCC 21127, 21128 and 21474. In B. lactofermentum BLOB and C. glutamicum RM3 the adsorption barrier is more likely.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study increases the knowledge on defence mechanisms of corynebacteria against bacteriophages.

Expression and site-directed mutagenesis of the lactococcal abortive phage infection protein AbiK

Abortive infection mechanisms of Lactococcus lactis form a heterogeneous group of phage resistance systems that act after early phage gene expression. One of these systems, AbiK, aborts infection of the three most prevalent lactococcal phage groups of the dairy industry. In this study, it is demonstrated that the antiphage activity depends on the level of expression of the abiK gene and on the presence of a reverse transcriptase (RT) motif in AbiK. The abiK gene was shown to be part of an operon that includes two additional open reading frames, with one of these encoding a phage-related transcriptional repressor named Orf4. Expression of AbiK is driven by two promoters, PabiK and Porf3, the latter being repressed by Orf4 in vivo. Binding of the purified Orf4 to the Porf3 promoter was demonstrated in vitro by gel retardation assays. The N-terminal half of the deduced AbiK protein possesses an RT motif that was modified by site-directed mutagenesis. Conservative mutations in key positions resulted in the complete loss of the resistance phenotype. These data suggest that an RT activity might be involved in the phage resistance activity of AbiK. A model for the mode of action of AbiK is proposed.

Alterations in phage-typing patterns in vancomycin-intermediate Staphylococcus aureus

The ability of phage-typing and SmaI chromosomal RFLPs to conclude appropriate strain relatedness between a collection of 12 well-characterized in vitro-selected vancomycin-intermediate Staphylococcus aureus (VISA) strains and their seven vancomycin-susceptible parent strains is reported. Generally, no SmaI RFLP alterations were observed in VISA strains when they were compared with their respective parent strains, and clonal relationships between isogenic strains were clearly evident. Unlike the SmaI RFLP results, parent strains and VISA derivatives generally did not share similar phage-typing profiles. Depending on the phage set investigated, some VISA strains even became untypable by this method. Loss of phage infectivity is probably due to cell wall (phage receptor) alterations that are expressed by the VISA strains investigated. Collectively, these findings indicate that inappropriate relationships between VISA and vancomycin-susceptible parents might be drawn if only phage-typing and antibiotic susceptibility are utilized to determine epidemiological relationships.

Characterization of the two-component abortive phage infection mechanism AbiT from Lactococcus lactis

During the production of fermented dairy products, virulent bacteriophages infecting Lactococcus lactis can delay or stop the milk acidification process. A solution to this biological problem consists of introducing natural phage barriers into the strains used by the dairy industry. One such hurdle is called abortive infection (Abi) and causes premature cell death with no or little phage progeny. Here, we describe the isolation and characterization of a novel Abi mechanism encoded by plasmid pED1 from L. lactis. The system is composed of two constitutively cotranscribed genes encoding putative proteins of 127 and 213 amino acids, named AbiTi and AbiTii, respectively. Site-directed mutagenesis indicated that a hydrophobic region at the C-terminal extremity of AbiTi is essential to the antiphage phenotype. The AbiT system is effective against phages of the 936 and P335 species (efficiency of plaquing between 10(-5) and 10(-7)) and causes a 20-fold reduction in the efficiency to form centers of infection as well as a 10- to 12-fold reduction in the burst size. Its efficacy could be improved by raising the plasmid copy number, but changing the intrinsic ratio of AbiTi and AbiTii did not greatly affect the antiphage activity. The monitoring of the intracellular phage infection process by DNA replication, gene expression, and electron microscopy as well as the study of phage mutants by genome mapping indicated that AbiT is likely to act at a later stage of the phage lytic cycle.

Effectiveness of the lactococcal abortive infection systems AbiA, AbiE, AbiF and AbiG against P335 type phages

Four lactococcal abortive infection mechanisms were introduced into strains which were sensitive hosts for P335 type phages and plaque assay experiments performed to assess their effect on five lactococcal bacteriophages from this family. Results indicate that AbiA inhibits all five P335 phages tested, while AbiG affects phiP335 itself and phiQ30 but not the other P335 species phages. AbiA was shown to retard phage Q30 DNA replication as previously reported for other phages. It was also demonstrated that AbiG, previously shown to act at a point after DNA replication in the cases of c2 type and 936 type phages, acts at the level of, or prior to phage Q30 DNA replication. AbiE and AbiF had no effect on the P335 type phages examined.