Expression of antisense RNA targeted against Streptococcus thermophilus bacteriophages

Portable CRISPR-Cas9N System for Flexible Genome Engineering in Lactobacillus acidophilus, Lactobacillus gasseri, and Lactobacillus paracasei

Diverse Lactobacillus strains are widely used as probiotic cultures in the dairy and dietary supplement industries, and specific strains, such as Lactobacillus acidophilus NCFM, have been engineered for the development of biotherapeutics. To expand the Lactobacillus manipulation toolbox with enhanced efficiency and ease, we present here a CRISPR (clustered regularly interspaced palindromic repeats)-SpyCas9^D10A nickase (Cas9^N)-based system for programmable engineering of L. acidophilus NCFM, a model probiotic bacterium. Successful single-plasmid delivery system was achieved with the engineered pLbCas9^N vector harboring cas9 ^N under the regulation of a Lactobacillus promoter and a cloning region for a customized single guide RNA (sgRNA) and editing template. The functionality of the pLbCas9^N system was validated in NCFM with targeted chromosomal deletions ranging between 300 bp and 1.9 kb at various loci (rafE, lacS, and ltaS), yielding 35 to 100% mutant recovery rates. Genome analysis of the mutants confirmed precision and specificity of the pLbCas9^N system. To showcase the versatility of this system, we also inserted an mCherry fluorescent-protein gene downstream of the pgm gene to create a polycistronic transcript. The pLbCas9^N system was further deployed in other species to generate a concurrent single-base substitution and gene deletion in Lactobacillus gasseri ATCC 33323 and an in-frame gene deletion in Lactobacillus paracasei Lpc-37, highlighting the portability of the system in phylogenetically distant Lactobacillus species, where its targeting activity was not interfered with by endogenous CRISPR-Cas systems. Collectively, these editing outcomes illustrate the robustness and versatility of the pLbCas9^N system for genome manipulations in diverse lactobacilli and open new avenues for the engineering of health-promoting lactic acid bacteria.IMPORTANCE This work describes the development of a lactobacillus CRISPR-based editing system for genome manipulations in three Lactobacillus species belonging to the lactic acid bacteria (LAB), which are commonly known for their long history of use in food fermentations and as indigenous members of healthy microbiotas and for their emerging roles in human and animal commercial health-promoting applications. We exploited the established CRISPR-SpyCas9 nickase for flexible and precise genome editing applications in Lactobacillus acidophilus and further demonstrated the efficacy of this universal system in two distantly related Lactobacillus species. This versatile Cas9-based system facilitates genome engineering compared to conventional gene replacement systems and represents a valuable gene editing modality in species that do not possess native CRISPR-Cas systems. Overall, this portable tool contributes to expanding the genome editing toolbox of LAB for studying their health-promoting mechanisms and engineering of these beneficial microbes as next-generation vaccines and designer probiotics.

Development of a novel selection/counter-selection system for chromosomal gene integrations and deletions in lactic acid bacteria

Background

The underlying mechanisms by which probiotic lactic acid bacteria (LAB) enhance the health of the consumer have not been fully elucidated. Verification of probiotic modes of action can be achieved by using single- or multiple-gene knockout analyses of bacterial mutants in in vitro or in vivo models. We developed a novel system based on an inducible toxin counter-selection system, allowing for rapid and efficient isolation of LAB integration or deletion mutants. The Lactococcus lactis nisin A inducible promoter was used for expression of the Escherichia coli mazF toxin gene as counter-selectable marker.

Results

The flippase (FLP)/flippase recognition target (FRT) recombination system and an antisense RNA transcript were used to create markerless chromosomal gene integrations/deletions in LAB. Expression of NisR and NisK signalling proteins generated stable DNA integrations and deletions. Large sequences could be inserted or deleted in a series of steps, as demonstrated by insertion of the firefly bioluminescence gene and erythromycin resistance marker into the bacteriocin operons or adhesion genes of Lactobacillus plantarum 423 and Enterococcus mundtii ST4SA.

Conclusions

The system was useful in the construction of L. plantarum 423 and E. mundtii ST4SA bacteriocin and adhesion gene mutants. This provides the unique opportunity to study the role of specific probiotic LAB genes in complex environments using reverse genetics analysis. Although this work focuses on two probiotic LAB strains, L. plantarum 423 and E. mundtii ST4SA, the system developed could be adapted to most, if not all, LAB species.

Electronic supplementary material

The online version of this article (10.1186/s12867-019-0127-x) contains supplementary material, which is available to authorized users.

Generation of Bacteriophage-Insensitive Mutants of Streptococcus thermophilus via an Antisense RNA CRISPR-Cas Silencing Approach

Predation of starter lactic acid bacteria such as Streptococcus thermophilus by bacteriophages is a persistent and costly problem in the dairy industry. CRISPR-mediated bacteriophage insensitive mutants (BIMs), while straightforward to generate and verify, can quickly be overcome by mutant phages. The aim of this study was to develop a tool allowing the generation of derivatives of commercial S. thermophilus strains which are resistant to phage attack through a non-CRISPR-mediated mechanism, with the objective of generating BIMs exhibiting stable resistance against a range of isolated lytic S. thermophilus phages. To achieve this, standard BIM generation was complemented by the use of the wild-type (WT) strain which had been transformed with an antisense mRNA-generating plasmid (targeting a crucial CRISPR-associated [cas] gene) in order to facilitate the generation of non-CRISPR-mediated BIMs. Phage sensitivity assays suggest that non-CRISPR-mediated BIMs exhibit some advantages compared to CRISPR-mediated BIMs derived from the same strain.IMPORTANCE The outlined approach reveals the presence of a powerful host-imposed barrier for phage infection in S. thermophilus Considering the detrimental economic consequences of phage infection in the dairy processing environment, the developed methodology has widespread applications, particularly where other methods may not be practical or effective in obtaining robust, phage-tolerant S. thermophilus starter strains.

Mining Environmental Plasmids for Synthetic Biology Parts and Devices

The scientific and technical ambition of contemporary synthetic biology is the engineering of biological objects with a degree of predictability comparable to those made through electric and industrial manufacturing. To this end, biological parts with given specifications are sequence-edited, standardized, and combined into devices, which are assembled into complete systems. This goal, however, faces the customary context dependency of biological ingredients and their amenability to mutation. Biological orthogonality (i.e., the ability to run a function in a fashion minimally influenced by the host) is thus a desirable trait in any deeply engineered construct. Promiscuous conjugative plasmids found in environmental bacteria have evolved precisely to autonomously deploy their encoded activities in a variety of hosts, and thus they become excellent sources of basic building blocks for genetic and metabolic circuits. In this article we review a number of such reusable functions that originated in environmental plasmids and keep their properties and functional parameters in a variety of hosts. The properties encoded in the corresponding sequences include inter alia origins of replication, DNA transfer machineries, toxin-antitoxin systems, antibiotic selection markers, site-specific recombinases, effector-dependent transcriptional regulators (with their cognate promoters), and metabolic genes and operons. Several of these sequences have been standardized as BioBricks and/or as components of the SEVA (Standard European Vector Architecture) collection. Such formatting facilitates their physical composability, which is aimed at designing and deploying complex genetic constructs with new-to-nature properties.

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.

The CRISPR/Cas bacterial immune system cleaves bacteriophage and plasmid DNA

Bacteria and Archaea have developed several defence strategies against foreign nucleic acids such as viral genomes and plasmids. Among them, clustered regularly interspaced short palindromic repeats (CRISPR) loci together with cas (CRISPR-associated) genes form the CRISPR/Cas immune system, which involves partially palindromic repeats separated by short stretches of DNA called spacers, acquired from extrachromosomal elements. It was recently demonstrated that these variable loci can incorporate spacers from infecting bacteriophages and then provide immunity against subsequent bacteriophage infections in a sequence-specific manner. Here we show that the Streptococcus thermophilus CRISPR1/Cas system can also naturally acquire spacers from a self-replicating plasmid containing an antibiotic-resistance gene, leading to plasmid loss. Acquired spacers that match antibiotic-resistance genes provide a novel means to naturally select bacteria that cannot uptake and disseminate such genes. We also provide in vivo evidence that the CRISPR1/Cas system specifically cleaves plasmid and bacteriophage double-stranded DNA within the proto-spacer, at specific sites. Our data show that the CRISPR/Cas immune system is remarkably adapted to cleave invading DNA rapidly and has the potential for exploitation to generate safer microbial strains.

CRISPR analysis of bacteriophage-insensitive mutants (BIMs) of industrial Streptococcus thermophilus--implications for starter design

AIMS

An efficient approach for generation of bacteriophage-insensitive mutants (BIMs) of Streptococcus thermophilus starters was described in our laboratory [Mills et al. (2007) J Microbiol Methods70, 159-164]. The aim of this study was to analyse the phage resistance mechanism responsible for BIM formation.

METHODS AND RESULTS

Three clustered regularly interspaced short palindromic repeat (CRISPR) regions have been identified in Strep. thermophilus, and Strep. thermophilus can integrate novel spacers into these loci in response to phage attack. Characterization of three sets of BIMs indicated that two sets had altered CRISPR1 and/or CRISPR3 loci. A range of BIMs of yoghurt starter CSK938 were generated with the same phage in different phage challenge experiments, and each acquired unique spacer regions ranging between one and four new spacers in CRISPR1. In addition, the BIM that acquired only one new spacer in CRISPR1 also acquired an additional spacer in CRISPR3. A fourth BIM, generated with a different phage, had two spacers deleted from CRISPR1 but acquired two spacers in CRISPR3. Analysis of the Mozzarella starter CSK939 and its associated BIMs indicated that formation of second generation BIMs does not lead to increases in spacer number but to alterations in spacer regions. BIMs of an exopolysaccharide (EPS)-producing strain that lost the ability to produce EPS did not harbour an altered CRISPR, suggesting that phage sensitivity may be related to the EPS-producing phenotype.

CONCLUSIONS

Acquisition/deletion of new spacers in CRISPR loci in response to phage attack generates distinctly individual variants. It also demonstrates that other modifications may be responsible for the phage resistance of Strep. thermophilus BIMs.

SIGNIFICANCE AND IMPACT OF THE STUDY

Isolation of individual BIMs that have unique spacers towards the leader region of the CRISPR locus may be a very useful approach for rotation strategies with the same starter backbone. Upon phage infection, BIMs 'in reserve' can be slotted into the rotation scheme.

Identification of an operon and inducing peptide involved in the production of lactacin B by Lactobacillus acidophilus

AIM

To determine if a 9.5-kb region on the Lactobacillus acidophilus NCFM genome, encoded the genetic determinants for regulation and production of lactacin B, a class II bacteriocin.

METHODS

Transcriptional analysis was used to identify a 9.5-kb polycistronic region suspected of encoding the lab operon. The 12 putative open reading frames (LBA1803-LBA1791) were organized into three clusters: a production and regulation cluster encoding a putative two-component signal transduction system; an export cluster encoding a putative ABC transporter and a final cluster composed of three unknown proteins. Seven genes were typical of bacteriocins, encoding small, cationic peptides, each with an N-terminal double-glycine leader motif. Inactivation of a predicted ABC transporter completely abolished bacteriocin activity. When cloned and expressed together, LBA1803-LBA1800 resulted in markedly higher levels of lactacin B activity. The four peptides were chemically synthesized but exhibited no bacteriocin activity, alone or in combination. Only LBA1800 induced lactacin B production in broth cultures.

CONCLUSIONS

Lactacin B production is encoded within the 9.5-kb lab operon of 12 genes that are transcribed in a single transcript. LBA1800 is an inducing peptide of bacteriocin production.

SIGNIFICANCE AND IMPACT OF THE STUDY

A three-component regulatory system common to class II bacteriocins regulates the production of this bacteriocin by Lact. acidophilus.

Inhibition of bacteriophage replication in Streptococcus thermophilus by subunit poisoning of primase

Invariant and highly conserved amino acids within a primase consensus sequence were targeted by site-specific mutations within the putative primase of Streptococcus thermophilus phage kappa3. PCR products containing the desired mutation(s) within putative ATPase/helicase and/or oligomerization domains of the kappa3-encoded primase gene were cloned into a high-copy-number vector and expressed in S. thermophilus NCK1125. The majority of the plasmid constructs failed to alter phage sensitivity; however, four of the constructs conferred strong phage resistance upon the host. Expression of the K238(A/T) and RR340-341AA mutant proteins in trans suppressed the function of the native phage primase protein in a dominant negative fashion via a proposed subunit poisoning mechanism. These constructs completely inhibited phage genome synthesis and reduced the efficiencies of plaquing and centre of infection formation by more than 9 and 3.5 logs, respectively. Amber mutations introduced upstream of the transdominant RR340-341AA and K238(A/T) mutations restored phage genome replication and sensitivity of the host, indicating that translation was required to confer phage resistance. Introduction of an E437A mutation in a putative oligomerization domain located downstream of the transdominant K238T mutation also completely suppressed phage resistance. This study appears to represent the first use of transdominant proteins to inhibit phages that are disruptive to cultures used in industrial fermentations.

Efficient method for generation of bacteriophage insensitive mutants of Streptococcus thermophilus yoghurt and mozzarella strains

Bacteriophage infection of Streptococcus thermophilus is becoming increasingly problematic in many industry fermentations such as yoghurt and mozzarella manufacture. This study describes the development of an efficient and rapid 3-step approach for the generation of bacteriophage insensitive mutants (BIMs) of these starter strains. The method initially involves infection of a culture in solid media at a multiplicity of infection (M.O.I.) of 10 which is then incubated in milk overnight. BIMs are then isolated following successive rounds (20-25) of growth in 10% reconstituted skimmed milk (RSM) in the presence of high phage titres. The method selects for BIMs which can grow efficiently in milk. Using this approach BIMs of two industrial strains were generated, whose starter performance was comparable to the parent starters in terms of performance in milk. Genomic fingerprinting used to validate the identity of each BIM, revealed a number of restriction fragment length polymorphisms (RFLPs) in two of the resultant BIMs. This method provides a simple and reliable method for generation of BIMs of industrial starters which does not require any specialised equipment and should be widely applicable.

Evaluation of the metS and murB loci for antibiotic discovery using targeted antisense RNA expression analysis in Bacillus anthracis

The biowarfare-relevant bacterial pathogen Bacillus anthracis contains two paralogs each of the metS and murB genes, which encode the important antibiotic target functions methionyl-tRNA synthetase and UDP-N-acetylenolpyruvoylglucosamine reductase, respectively. Empirical screens were conducted to detect and characterize gene fragments of each of these four genes that could cause growth reduction of B. anthracis when inducibly expressed from a plasmid-borne promoter. Numerous such gene fragments that were overwhelmingly in the antisense orientation were identified for the metS1 and murB2 alleles, while no such orientation bias was seen for the metS2 and murB1 alleles. Gene replacement mutagenesis was used to confirm the essentiality of the metS1 and murB2 alleles, and the nonessentiality of the metS2 and murB1 alleles, for vegetative growth. Induced transcription of RNA from metS1 and murB2 antisense-oriented gene fragments resulted in specific reduction of mRNA of their cognate genes. Attenuation of MetS1 enzyme expression hypersensitized B. anthracis cells to a MetS-specific antimicrobial compound but not to other antibiotics that affect cell wall assembly, fatty acid biosynthesis, protein translation, or DNA replication. Antisense-dependent reduction of MurB2 enzyme expression caused hypersensitivity to beta-lactam antibiotics, a synergistic response that has also been noted for the MurA-specific antibiotic fosfomycin. These experiments form the basis of mode-of-action detection assays that can be used in the discovery of novel MetS- or MurB-specific antibiotic drugs that are effective against B. anthracis or other gram-positive bacterial pathogens.

Role of cystathionine beta-lyase in catabolism of amino acids to sulfur volatiles by genetic variants of Lactobacillus helveticus CNRZ 32

Catabolism of sulfur-containing amino acids plays an important role in the development of cheese flavor. During ripening, cystathionine beta-lyase (CBL) is believed to contribute to the formation of volatile sulfur compounds (VSCs) such as methanethiol and dimethyl disulfide. However, the role of CBL in the generation of VSCs from the catabolism of specific sulfur-containing amino acids is not well characterized. The objective of this study was to investigate the role of CBL in VSC formation by Lactobacillus helveticus CNRZ 32 using genetic variants of L. helveticus CNRZ 32 including the CBL-null mutant, complementation of the CBL-null mutant, and the CBL overexpression mutant. The formation of VSCs from methionine, cystathionine, and cysteine was determined in a model system using gas chromatography-mass spectrometry with solid-phase microextraction. With methionine as a substrate, CBL overexpression resulted in higher VSC production than that of wild-type L. helveticus CNRZ 32 or the CBL-null mutant. However, there were no differences in VSC production between the wild type and the CBL-null mutant. With cystathionine, methanethiol production was detected from the CBL overexpression variant and complementation of the CBL-null mutant, implying that CBL may be involved in the conversion of cystathionine to methanethiol. With cysteine, no differences in VSC formation were observed between the wild type and genetic variants, indicating that CBL does not contribute to the conversion of cysteine.

Transcriptional profiling of bacteriophage BFK20: Coexpression interrogated by “guilt-by-association” algorithm

Global gene expression profiling of bacteriophage BFK20 infecting the industrial L-lysine producer Brevibacterium flavum CCM 251 was performed using DNA microarray. The relative gene expressions were measured in fourteen time samples collected during phage development. Phage genes were classified as early, middle, late or unassigned based on complex expression patterns during infection. Temporal classification of BFK20 genes was in concordance with previous predictions. However, proposed late regulatory genes were reclassified and new functional assignments for ORF55 were strongly suggested. Furthermore, we consider possible functions of other genes and their products regarding coexpression pattern by using "guilt-by-association" algorithm. Microarray results were validated using real-time RT-PCR. The detailed description of phage BFK20 transcriptional profile can answer the basic questions of its life cycle and it also can help to prevent phage contamination during industrial fermentation. In addition, this work presents the first complete microarray time course study of gene expression utilizing loop design.

Abortive phage resistance mechanism AbiZ speeds the lysis clock to cause premature lysis of phage-infected Lactococcus lactis

The conjugative plasmid pTR2030 has been used extensively to confer phage resistance in commercial Lactococcus starter cultures. The plasmid harbors a 16-kb region, flanked by insertion sequence (IS) elements, that encodes the restriction/modification system LlaI and carries an abortive infection gene, abiA. The AbiA system inhibits both prolate and small isometric phages by interfering with the early stages of phage DNA replication. However, abiA alone does not account for the full abortive activity reported for pTR2030. In this study, a 7.5-kb region positioned within the IS elements and downstream of abiA was sequenced to reveal seven additional open reading frames (ORFs). A single ORF, designated abiZ, was found to be responsible for a significant reduction in plaque size and an efficiency of plaquing (EOP) of 10(-6), without affecting phage adsorption. AbiZ causes phage phi31-infected Lactococcus lactis NCK203 to lyse 15 min early, reducing the burst size of phi31 100-fold. Thirteen of 14 phages of the P335 group were sensitive to AbiZ, through reduction in either plaque size, EOP, or both. The predicted AbiZ protein contains two predicted transmembrane helices but shows no significant DNA homologies. When the phage phi31 lysin and holin genes were cloned into the nisin-inducible shuttle vector pMSP3545, nisin induction of holin and lysin caused partial lysis of NCK203. In the presence of AbiZ, lysis occurred 30 min earlier. In holin-induced cells, membrane permeability as measured using propidium iodide was greater in the presence of AbiZ. These results suggest that AbiZ may interact cooperatively with holin to cause premature lysis.

Engineered bacteriophage-defence systems in bioprocessing

Bacteriophages (phages) have the potential to interfere with any industry that produces bacteria as an end product or uses them as biocatalysts in the production of fermented products or bioactive molecules. Using microorganisms that drive food bioprocesses as an example, this review will describe a set of genetic tools that are useful in the engineering of customized phage-defence systems. Special focus will be given to the power of comparative genomics as a means of streamlining target selection, providing more widespread phage protection, and increasing the longevity of these industrially important bacteria in the bioprocessing environment.

Clustered regularly interspaced short palindrome repeats (CRISPRs) have spacers of extrachromosomal origin

Numerous prokaryote genomes contain structures known as clustered regularly interspaced short palindromic repeats (CRISPRs), composed of 25-50 bp repeats separated by unique sequence spacers of similar length. CRISPR structures are found in the vicinity of four genes named cas1 to cas4. In silico analysis revealed another cluster of three genes associated with CRISPR structures in many bacterial species, named here as cas1B, cas5 and cas6, and also revealed a certain number of spacers that have homology with extant genes, most frequently derived from phages, but also derived from other extrachromosomal elements. Sequence analysis of CRISPR structures from 24 strains of Streptococcus thermophilus and Streptococcus vestibularis confirmed the homology of spacers with extrachromosomal elements. Phage sensitivity of S. thermophilus strains appears to be correlated with the number of spacers in the CRISPR locus the strain carries. The authors suggest that the spacer elements are the traces of past invasions by extrachromosomal elements, and hypothesize that they provide the cell immunity against phage infection, and more generally foreign DNA expression, by coding an anti-sense RNA. The presence of gene fragments in CRISPR structures and the nuclease motifs in cas genes of both cluster types suggests that CRISPR formation involves a DNA degradation step.

Global gene expression analysis of two Streptococcus thermophilus bacteriophages using DNA microarray

A custom microarray was developed to study the temporal gene expression of the two groups of phages infecting the Gram-positive lactic acid bacterium Streptococcus thermophilus. The complete genomic sequence of the virulent cos-type phage DT1 (34,815 bp) and the pac-type phage 2972 (34,704 bp) were used for the construction of the microarray. Gene expression was measured at nine time intervals (0, 2, 7, 12, 17, 22, 27, 32 and 37 min) during phage infection and an expression curve was determined for each gene. Each phage gene was then classified into one of the three traditional transcription classes and these data were used to generate the complete transcriptional map of DT1 and 2972. Phage DT1 possesses 18 early genes, 12 middle genes and 12 late-expressed genes whereas 2972 has 16 early, 11 middle and 14 late genes. The trends of the phage gene expression profiles were also confirmed by slot blot hybridizations. Significant differences were observed when comparing the transcriptional maps of DT1 and 2972 with those already available for the S. thermophilus phages Sfi19 and Sfi21. To our knowledge, this report presents the first complete transcription analysis of bacteriophages infecting Gram-positive bacteria using the DNA microarray technology.

Genomic organization and molecular analysis of virulent bacteriophage 2972 infecting an exopolysaccharide-producing Streptococcus thermophilus strain

The Streptococcus thermophilus virulent pac-type phage 2972 was isolated from a yogurt made in France in 1999. It is a representative of several phages that have emerged with the industrial use of the exopolysaccharide-producing S. thermophilus strain RD534. The genome of phage 2972 has 34,704 bp with an overall G+C content of 40.15%, making it the shortest S. thermophilus phage genome analyzed so far. Forty-four open reading frames (ORFs) encoding putative proteins of 40 or more amino acids were identified, and bioinformatic analyses led to the assignment of putative functions to 23 ORFs. Comparative genomic analysis of phage 2972 with the six other sequenced S. thermophilus phage genomes confirmed that the replication module is conserved and that cos- and pac-type phages have distinct structural and packaging genes. Two group I introns were identified in the genome of 2972. They interrupted the genes coding for the putative endolysin and the terminase large subunit. Phage mRNA splicing was demonstrated for both introns, and the secondary structures were predicted. Eight structural proteins were also identified by N-terminal sequencing and/or matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Detailed analysis of the putative minor tail proteins ORF19 and ORF21 as well as the putative receptor-binding protein ORF20 showed the following interesting features: (i) ORF19 is a hybrid protein, because it displays significant identity with both pac- and cos-type phages; (ii) ORF20 is unique; and (iii) a protein similar to ORF21 of 2972 was also found in the structure of the cos-type phage DT1, indicating that this structural protein is present in both S. thermophilus phage groups. The implications of these findings for phage classification are discussed.

Characterization of the cro-ori region of the Streptococcus thermophilus virulent bacteriophage DT1

The virulent cos-type Streptococcus thermophilus phage DT1 was previously isolated from a mozzarella whey sample, and its complete genomic sequence is available. The putative ori of phage DT1 is characterized by three inverted and two direct repeats located in a noncoding region between orf36 and orf37. As the replication ability of the putative ori and flanking genes could not be established, its ability to confer phage resistance was tested. When ori is cloned on a high-copy-number plasmid, it provides protection to S. thermophilus strains against phage infection during milk fermentation. This protection is phage specific and strain dependent. Then, a detailed transcriptional map was established for the region located between the cro-like gene (orf29) and the ori. The results of the Northern blots indicated that the transcription of this region started 5 min after the onset of phage infection. Comparative analysis of the expression of the cro-ori region in the three S. thermophilus cos-type phages DT1, Sfi19 (virulent), and Sfi21 (temperate) reveals significant differences in the number and size of transcripts. The promoter upstream of orf29 was further investigated by primer extension analysis, and its activity was confirmed by a chloramphenicol acetyltransferase assay, which showed that the phage promoter is more efficient than the constitutive bacterial promoter of the S. thermophilus operon encoding the general proteins of the phosphoenolpyruvate:sugar phosphotransferase system. However, the phage promoter is less efficient than the pts promoter in Lactococcus lactis and in Escherichia coli.

The impact of bacteriophage genomics

The discovery of (bacterio)phages revolutionised microbiology and genetics, while phage research has been integral to answering some of the most fundamental biological questions of the twentieth century. The susceptibility of bacteria to bacteriophage attack can be undesirable in some cases, especially in the dairy industry, but can be desirable in others, for example, the use of bacteriophage therapy to eliminate pathogenic bacteria. The relative ease with which entire bacteriophage genome sequences can now be elucidated has had a profound impact on the study of these bacterial parasites.

Controlled expression of an rpoS antisense RNA can inhibit RpoS function in Escherichia coli

We show that an inducible rpoS antisense RNA complementary to the rpoS message can inhibit expression of RpoS in both exponential and stationary phases and can attenuate expression of the rpoS regulon in Escherichia coli. Plasmids containing rpoS antisense DNA expressed under the control of the T7lac promoter and T7 RNA polymerase were constructed, and expression of the rpoS antisense RNA was optimized in the pET expression system. rpoS antisense RNA levels could be manipulated to effectively control the expression of RpoS and RpoS-dependent genes. RpoS expression was inhibited by the expression of rpoS antisense RNA in both exponential and stationary phases in E. coli. RpoS-dependent catalase HPII was also downregulated, as determined by catalase activity assays and with native polyacrylamide gels stained for catalase. Induced RpoS antisense expression also reduced the level of RpoS-dependent glycogen synthesis. These results demonstrate that controlled expression of antisense RNA can be used to attenuate expression of a regulator required for the expression of host adaptation functions and may offer a basis for designing effective antimicrobial agents.

Antisense RNA targeting of primase interferes with bacteriophage replication in Streptococcus thermophilus

The putative primase gene and other genes associated with the Sfi21-prototype genome replication module are highly conserved in Streptococcus thermophilus bacteriophages. Expression of antisense RNAs complementary to the putative primase gene (pri3.1) from S. thermophilus phage kappa 3 provided significant protection from kappa 3 and two other Sfi21-type phages. Expression of pri3.10-AS, an antisense RNA that covered the entire primase gene, reduced the efficiency of plaquing (EOP) of kappa 3 to 3 x 10(-3) and reduced its burst size by 20%. Mutant phages capable of overcoming antisense inhibition were not recovered. Thirteen primase-specific antisense cassettes of different lengths (478 to 1,512 bp) were systematically designed to target various regions of the gene. Each cassette conferred some effect, reducing the EOP to between 0.8 and 3 x 10(-3). The largest antisense RNAs (1.5 kb) were generally found to confer the greatest reductions in EOP, but shorter (0.5 kb) antisense RNAs were also effective, especially when directed to the 5' region of the gene. The impacts of primase-targeted antisense RNAs on phage development were examined. The expression of pri3.10-AS resulted in reductions in target RNA abundance and the number of phage genomes synthesized. Targeting a key genome replication function with antisense RNA provided effective phage protection in S. thermophilus.

Bacteriophage-resistance systems in dairy starter strains: molecular analysis to application

Starter inhibition by bacteriophage infection in dairy fermentations can limit the usage of specific bacterial strains used in the manufacture of Cheddar, Mozzarella and other cheeses and can result in substantial economic losses. A variety of practical measures to alleviate the problem of phage infection have been adopted over the years but has invariably resulted in a very limited number of strains which can withstand intensive usage in industry. The application of genetic techniques to improve the phage-resistance of starter cultures for dairy fermentations has been intensively studied for the last 20 years to a point where this approach now has significant potential to alleviate the problem. This paper highlights the recent findings and developments that have been described in the literature that will have an impact on improvement of the phage-resistance of starter cultures.