cremoris strain AM2 by renaturing polyacrylamide gel electrophoresis: identification of a prophage-encoded enzyme

Cell wall modifications that alter the exolytic activity of lactococcal phage endolysins have little impact on phage growth

Bacteriophages are a nuisance in the production of fermented dairy products driven by starter bacteria and strategies to reduce the risk of phage infection are permanently sought. Bearing in mind that the bacterial cell wall plays a pivotal role in host recognition and lysis, our goal was to elucidate to which extent modifications in the cell wall may alter endolysin activity and influence the outcome of phage infection in Lactococcus. Three lactococcal endolysins with distinct catalytic domains (CHAP, amidase and lysozyme) from phages 1,358, p2 and c2 respectively, were purified and their exolytic activity was tested against lactococcal mutants either overexpressing or lacking genes involved in the cell envelope stress (CES) response or in modifying peptidoglycan (PG) composition. After recombinant production in E. coli, Lys1358 (CHAP) and LysC2 (muramidase) were able to lyse lactococcal cells in turbidity reduction assays, but no activity of LysP2 was detected. The degree of PG acetylation, namely C₆-O-acetylation and de-N-acetylation influenced the exolytic activity, being LysC2 more active against cells depleted of the PG deacetylase PgdA and the O-acetyl transferase OatA. On the contrary, both endolysins showed reduced activity on cells with an induced CES response. By measuring several growth parameters of phage c2 on these lactococcal mutants (lytic score, efficiency of plaquing, plaque size and one-step curves), a direct link between the exolytic activity of its endolysin and phage performance could not be stablished.

The bacteriophage decides own tracks: When they are with or against the bacteria

Bacteriophages, bacteria-infecting viruses, are considered by many researchers a promising solution for antimicrobial resistance. On the other hand, some phages have shown contribution to bacterial resistance phenomenon by transducing antimicrobial resistance genes to their bacterial hosts. Contradictory consequences of infections are correlated to different phage lifecycles. Out of four known lifecycles, lysogenic and lytic pathways have been riddles since the uncontrolled conversion between them could negatively affect the intended use of phages. However, phages still can be engineered for applications against bacterial and viral infections to ensure high efficiency. This review highlights two main aspects: (1) the different lifecycles as well as the different factors that affect lytic-lysogenic switch are discussed, including the intracellular and molecular factors control this decision. In addition, different models which describe the effect of phages on the ecosystem are compared, besides the approaches to study the switch. (2) An overview on the contribution of the phage in the evolution of the bacteria, instead of eating them, as a consequence of different mode of actions. As well, how phage display has helped in restricting phage cheating and how it could open new gates for immunization and pandemics control will be tacked.

The resident TP712 prophage of Lactococcus lactis MG1363 provides extra holin functions to the new P335 phage CAP for effective host lysis

Prophages are widely present in Lactococcus lactis, a lactic acid bacterium (LAB) that plays a key role in dairy fermentations. L. lactis MG1363 is a laboratory strain used worldwide as a model LAB. Initially regarded as plasmid- and prophage-free, MG1363 carries two complete prophages TP712 and MG-3. Only TP712 seems to be inducible but unable to lyse the host. Several so-called TP712 lysogens able to lyse upon prophage induction were reported in the past, but the reason for their lytic phenotype remained unknown. In this work, we describe CAP, a new P335 prophage detected in the "lytic TP712 lysogens", which had remained unnoticed. CAP is able to excise after mitomycin C treatment, along with TP712, and able to infect L. lactis MG1363-like strains but not the lytic TP712 lysogens. Both phages cooperate for efficient host lysis. While the expression in trans of the CAP lytic genes was sufficient to trigger cell lysis, this process was boosted when the resident TP712 prophage was concomitantly induced. Introduction of mutations into the TP712 lytic genes revealed that its holin but not its endolysin plays a major role. Accordingly, it is shown that the lytic activity of the recombinant CAP endolysin relies on membrane depolarization. Revisiting the seminal work to generate the extensively used L. lactis MG1363 strain led us to conclude that the CAP phage was originally present in its ancestor L. lactis NCDO712 and our results solved long-standing mysteries around the MG1363 resident prophage TP712 reported in the "pre-sequencing" era. Importance Prophages are bacterial viruses that integrate in the chromosome of bacteria until an environmental trigger induces their lytic cycle ending with lysis of the host. Prophages present in dairy starters can compromise milk fermentation and represent a serious threat in dairy plants. In this work, we have discovered that two temperate phages TP712 and CAP infecting the laboratory strain Lactococcus lactis MG1363 join forces to lyse the host. Based on the in vitro lytic activity of the LysCAP endolysin, in combination with mutated versions of TP712 lacking either its holin or endolysin, we conclude that this cooperation relies on the combined activity of the holins of both phages that boost the activity of LysCAP. The presence of an additional prophage explains the lytic phenotype of the formerly thought to be single TP712 lysogens that had remained a mystery for many years.

Autolysis and Cell Death Is Affected by pH in L. reuteri DSM 20016 Cells

A key obstacle to the successful delivery of a probiotic to the consumer is maintaining viability of the live cells during storage, a challenge for the beneficial Lactibacillus reuteri. Three processes play a role in the reduction of viability: autolysis, cell death, and cell weakening. Using a phosphate induction model of autolysis, the initial aim of this project was to discover novel molecular determinants of autolysis in L. reuteri, with the long -term goal of elucidating new strategies for increasing viability. We employed a 2D Native/SDS-Page method to monitor changes in protein expression over time; however, the result was that excess phosphate did not induce noticeable changes in expression patterns. On the other hand, we found that pH affects both the rate of total viability and autolysis, as seen with other species of LAB. In addition, we found that the phosphate model of autolysis may not be sufficient to explain how autolysis is triggered in L. reuteri. Two parameters appear to modulate the pH in media containing L. reuteri cells: overall buffering capacity and the presence of a carbon source. Ultimately, phosphate sources appear to facilitate autolysis by maintaining pH in the media via a higher buffering capacity. In addition, the alkaline sugar free almond drink appears to be a promising possible preservative for L. reuteri.

Myxococcus xanthus predation of Gram-positive or Gram-negative bacteria is mediated by different bacteriolytic mechanisms

Myxococcus xanthus kills other species to use their biomass as energy source. Its predation mechanisms allow feeding on a broad spectrum of bacteria, but the identity of predation effectors and their mode of action remains largely unknown. We initially focused on the role of hydrolytic enzymes for prey killing and compared the activity of secreted M. xanthus proteins against four prey strains. 72 secreted proteins were identified by mass spectrometry, and among them a family 19 glycoside hydrolase that displayed bacteriolytic activity in vivo and in vitro This enzyme, which we name LlpM (lectin/lysozyme-like protein of M. xanthus), was not essential for predation, indicating that additional secreted components are required to disintegrate prey. Furthermore, secreted proteins lysed only Gram-positive, but not Gram-negative species. We thus compared the killing of different preys by cell-associated mechanisms: Individual M. xanthus cells killed all four test strains in a cell-contact dependent manner, but were only able to disintegrate Gram-negative, not Gram-positive cell envelopes. Thus, our data indicate that M. xanthus uses different, multifactorial mechanisms for killing and degrading different preys. Besides secreted enzymes, cell-associated mechanisms that have not been characterized so far, appear to play a major role for prey killing.IMPORTANCEPredation is an important survival strategy of the widespread myxobacteria, but it remains poorly understood on the mechanistic level. Without a basic understanding of how prey cell killing and consumption is achieved, it also remains difficult to investigate the role of predation for the complex myxobacterial lifestyle, reciprocal predator-prey relationships or the impact of predation on complex bacterial soil communities.We study predation in the established model organism Myxococcus xanthus, aiming to dissect the molecular mechanisms of prey cell lysis. In this study, we addressed the role of secreted bacteriolytic proteins, as well as potential mechanistic differences in the predation of Gram-positive and Gram-negative bacteria. Our observation shows that secreted enzymes are sufficient for killing and degrading Gram-positive species, but that cell-associated mechanisms may play a major role for killing Gram-negative and Gram-positive prey on fast timescales.

Living the Sweet Life: How Liquorilactobacillus hordei TMW 1.1822 Changes Its Behavior in the Presence of Sucrose in Comparison to Glucose

Liquorilactobacillus (L.) hordei (formerly Lactobacillus hordei) is one of the dominating lactic acid bacteria within the water kefir consortium, being highly adapted to survive in this environment, while producing high molecular weight dextrans from sucrose. In this work, we extensively studied the physiological response of L. hordei TMW 1.1822 to sucrose compared to glucose, applying label-free, quantitative proteomics of cell lysates and exoproteomes. This revealed the differential expression of 53 proteins within cellular proteomes, mostly associated with carbohydrate uptake and metabolism. Supported by growth experiments, this suggests that L. hordei TMW 1.1822 favors fructose over other sugars. The dextransucrase was expressed irrespectively of the present carbon source, while it was significantly more released in the presence of sucrose (log₂FC = 3.09), being among the most abundant proteins within exoproteomes of sucrose-treated cells. Still, L. hordei TMW 1.1822 expressed other sucrose active enzymes, predictively competing with the dextransucrase reaction. While osmolysis appeared to be unlikely, sucrose led to increased release of a multitude of cytoplasmic proteins, suggesting that biofilm formation in L. hordei is not only composed of a polysaccharide matrix but is also of proteinaceous nature. Therefore, our study highlights the intrinsic adaptation of water kefir-borne L. hordei to sucrose-rich habitats and provides fundamental knowledge for its use as a starter culture in plant-based food fermentations with in situ dextran formation.

The effect of sucrose-induced osmotic stress on the sensitivity ofEscherichia colito bacteriocins

Bacteriocins are antimicrobial peptides, produced by Gram-positive bacteria such as lactococci and staphylococci, that have limited bactericidal action against Gram-negative bacteria. The aim of this paper was to study the sensitivity of three strains of Escherichia coli to bacteriocins: nisin (as Nisaplin®) and two staphylococcal peptides (warnerin and hominin) during sucrose-induced osmotic stress. We found that all peptides in a 0.3 g·mL−1sucrose solution significantly reduced the number of viable E. coli. The most pronounced antibacterial effect was achieved by nisin against E. coli K-12 (3 log reduction). Slightly less bactericidal effects were observed with warnerin (1 mg·mL−1) and hominin (1 mg·mL−1) in sucrose solution. The lytic activity of staphylococcal peptides was detected by decreased optical density and viable cell counts. Moreover, it was confirmed by the increased amount of DNA and protein in the medium and the morphological changes detected by atomic force microscopy after 20 h of treatment. Zymographic analysis revealed the release of lytic enzymes from E. coli cells after treatment with staphylococcal peptides and sucrose. These results indicated that the antimicrobial action of peptides can be extended to Gram-negative bacteria via combination with high concentrations of sucrose.

Insight into the Lytic Functions of the Lactococcal Prophage TP712

The lytic cassette of Lactococcus lactis prophage TP712 contains a putative membrane protein of unknown function (Orf54), a holin (Orf55), and a modular endolysin with a N-terminal glycoside hydrolase (GH_25) catalytic domain and two C-terminal LysM domains (Orf56, LysTP712). In this work, we aimed to study the mode of action of the endolysin LysTP712. Inducible expression of the holin-endolysin genes seriously impaired growth. The growth of lactococcal cells overproducing the endolysin LysTP712 alone was only inhibited upon the dissipation of the proton motive force by the pore-forming bacteriocin nisin. Processing of a 26-residues signal peptide is required for LysTP712 activation, since a truncated version without the signal peptide did not impair growth after membrane depolarization. Moreover, only the mature enzyme displayed lytic activity in zymograms, while no lytic bands were observed after treatment with the Sec inhibitor sodium azide. LysTP712 might belong to the growing family of multimeric endolysins. A C-terminal fragment was detected during the purification of LysTP712. It is likely to be synthesized from an alternative internal translational start site located upstream of the cell wall binding domain in the lysin gene. Fractions containing this fragment exhibited enhanced activity against lactococcal cells. However, under our experimental conditions, improved in vitro inhibitory activity of the enzyme was not observed upon the supplementation of additional cell wall binding domains in. Finally, our data pointed out that changes in the lactococcal cell wall, such as the degree of peptidoglycan O-acetylation, might hinder the activity of LysTP712. LysTP712 is the first secretory endolysin from a lactococcal phage described so far. The results also revealed how the activity of LysTP712 might be counteracted by modifications of the bacterial peptidoglycan, providing guidelines to exploit the biotechnological potential of phage endolysins within industrially relevant lactococci and, by extension, other bacteria.

Expanding the Diversity of Myoviridae Phages Infecting Lactobacillus plantarum-A Novel Lineage of Lactobacillus Phages Comprising Five New Members

Lactobacillus plantarum is a bacterium with probiotic properties and promising applications in the food industry and agriculture. So far, bacteriophages of this bacterium have been moderately addressed. We examined the diversity of five new L. plantarum phages via whole genome shotgun sequencing and in silico protein predictions. Moreover, we looked into their phylogeny and their potential genomic similarities to other complete phage genome records through extensive nucleotide and protein comparisons. These analyses revealed a high degree of similarity among the five phages, which extended to the vast majority of predicted virion-associated proteins. Based on these, we selected one of the phages as a representative and performed transmission electron microscopy and structural protein sequencing tests. Overall, the results suggested that the five phages belong to the family Myoviridae, they have a long genome of 137,973-141,344 bp, a G/C content of 36.3-36.6% that is quite distinct from their host's, and surprisingly, 7 to 15 tRNAs. Only an average 41/174 of their predicted genes were assigned a function. The comparative analyses unraveled considerable genetic diversity for the five L. plantarum phages in this study. Hence, the new genus "Semelevirus" was proposed, comprising exclusively of the five phages. This novel lineage of Lactobacillus phages provides further insight into the genetic heterogeneity of phages infecting Lactobacillus sp. The five new Lactobacillus phages have potential value for the development of more robust starters through, for example, the selection of mutants insensitive to phage infections. The five phages could also form part of phage cocktails, which producers would apply in different stages of L. plantarum fermentations in order to create a range of organoleptic outputs.

Genomic and functional characterisation of two Enterococcus strains isolated from Cotija cheese and their potential role in ripening

Enterococcus spp. are present in the native microbiota of many traditional fermented foods. Their ability to produce antibacterial compounds, mainly against Listeria monocytogenes, has raised interest recently. However, there is scarce information about their proteolytic and lipolytic potential, and their biotechnological application is currently limited because enterococcal strains have been related to nosocomial infections. In this work, next-generation sequencing and optimised bioinformatic pipelines were used to annotate the genomes of two Enterococcus strains-one E. faecium and one E. faecalis-isolated from the Mexican artisanal ripened Cotija cheese. A battery of genes involved in their proteolytic system was annotated. Genes coding for lipases, esterases and other enzymes whose final products contribute to cheese aroma and flavour were identified as well. As for the production of antibacterial compounds, several peptidoglycan hydrolase- and bacteriocin-coding genes were identified in both genomes experimentally and by bioinformatic analyses. E. faecalis showed resistance to aminoglycosides and E. faecium to aminoglycosides and macrolides, as predicted by the genome functional annotation. No pathogenicity islands were found in any of the strains, although traits such as the ability of biofilm formation and cell aggregation were observed. Finally, a comparative genomic analysis was able to discriminate between the food strains isolated and nosocomial strains. In summary, pathogenic strains are resistant to a wide range of antibiotics and contain virulence factors that cause host damage; in contrast, food strains display less antibiotic resistance, include genes that encode class II bacteriocins and express virulence factors associated with host colonisation rather than invasion.

Reduced Binding of the Endolysin LysTP712 to Lactococcus lactis ΔftsH Contributes to Phage Resistance

Absence of the membrane protease FtsH in Lactococcus lactis hinders release of the bacteriophage TP712. In this work we have analyzed the mechanism responsible for the non-lytic phenotype of L. lactis ΔftsH after phage infection. The lytic cassette of TP712 contains a putative antiholin–pinholin system and a modular endolysin (LysTP712). Inducible expression of the holin gene demonstrated the presence of a dual start motif which is functional in both wildtype and L. lactis ΔftsH cells. Moreover, simulating holin activity with ionophores accelerated lysis of wildtype cells but not L. lactis ΔftsH cells, suggesting inhibition of the endolysin rather than a role of FtsH in holin activation. However, zymograms revealed the synthesis of an active endolysin in both wildtype and L. lactis ΔftsH TP712 lysogens. A reporter protein was generated by fusing the cell wall binding domain of LysTP712 to the fluorescent mCherry protein. Binding of this reporter protein took place at the septa of both wildtype and L. lactis ΔftsH cells as shown by fluorescence microscopy. Nonetheless, fluorescence spectroscopy demonstrated that mutant cells bound 40% less protein. In conclusion, the non-lytic phenotype of L. lactis ΔftsH is not due to direct action of the FtsH protease on the phage lytic proteins but rather to a putative function of FtsH in modulating the architecture of the L. lactis cell envelope that results in a lower affinity of the phage endolysin to its substrate.

Phenotypic and genotypic characterization of peptidoglycan hydrolases of Lactobacillus sakei

Lactobacillus sakei, a lactic acid bacterium naturally found in fresh meat and sea products, is considered to be one of the most important bacterial species involved in meat fermentation and bio-preservation. Several enzymes of Lb. sakei species contributing to microbial safeguarding and organoleptic properties of fermented-meat were studied. However, the specific autolytic mechanisms and associated enzymes involved in Lb. sakei are not well understood. The autolytic phenotype of 22 Lb. sakei strains isolated from Tunisian meat and seafood products was evaluated under starvation conditions, at pH 6.5 and 8.5, and in the presence of different carbon sources. A higher autolytic rate was observed when cells were grown in the presence of glucose and incubated at pH 6.5. Almost all strains showed high resistance to mutanolysin, indicating a minor role of muramidases in Lb. sakei cell lysis. Using Micrococcus lysodeikticus cells as a substrate in activity gels zymogram, peptidoglycan hydrolase (PGH) patterns for all strains was characterized by two lytic bands of ∼80 (B1) and ∼70 kDa (B2), except for strain BMG.167 which harbored two activity signals at a lower MW. Lytic activity was retained in high salt and in acid/basic conditions and was active toward cells of Lb. sakei, Listeria monocytogenes, Listeria ivanovii and Listeria innocua. Analysis of five putative PGH genes found in the Lb. sakei 23 K model strain genome, indicated that one gene, lsa1437, could encode a PGH (N-acetylmuramoyl-L-alanine amidase) containing B1 and B2 as isoforms. According to this hypothesis, strain BMG.167 showed an allelic version of lsa1437 gene deleted of one of the five LysM domains, leading to a reduction in the MW of lytic bands and the high autolytic rate of this strain. Characterization of autolytic phenotype of Lb. sakei should expand the knowledge of their role in fermentation processes where they represent the dominant species.

Bacteriophage-derived CHAP domain protein, P128, kills Staphylococcus cells by cleaving interpeptide cross-bridge of peptidoglycan

P128 is an anti-staphylococcal protein consisting of the Staphylococcus aureus phage-K-derived tail-associated muralytic enzyme (TAME) catalytic domain (Lys16) fused with the cell-wall-binding SH3b domain of lysostaphin. In order to understand the mechanism of action and emergence of resistance to P128, we isolated mutants of Staphylococcus spp., including meticillin-resistant Staphylococcus aureus (MRSA), resistant to P128. In addition to P128, the mutants also showed resistance to Lys16, the catalytic domain of P128. The mutants showed loss of fitness as shown by reduced rate of growth in vitro. One of the mutants tested was found to show reduced virulence in animal models of S. aureus septicaemia suggesting loss of fitness in vivo as well. Analysis of the antibiotic sensitivity pattern showed that the mutants derived from MRSA strains had become sensitive to meticillin and other β-lactams. Interestingly, the mutant cells were resistant to the lytic action of phage K, although the phage was able to adsorb to these cells. Sequencing of the femA gene of three P128-resistant mutants showed either a truncation or deletion in femA, suggesting that improper cross-bridge formation in S. aureus could be causing resistance to P128. Using glutathione S-transferase (GST) fusion peptides as substrates it was found that both P128 and Lys16 were capable of cleaving a pentaglycine sequence, suggesting that P128 might be killing S. aureus by cleaving the pentaglycine cross-bridge of peptidoglycan. Moreover, peptides corresponding to the reported cross-bridge of Staphylococcus haemolyticus (GGSGG, AGSGG), which were not cleaved by lysostaphin, were cleaved efficiently by P128. This was also reflected in high sensitivity of S. haemolyticus to P128. This showed that in spite of sharing a common mechanism of action with lysostaphin, P128 has unique properties, which allow it to act on certain lysostaphin-resistant Staphylococcus strains.

Biochemical characterization of the major N-acetylmuramidase from Lactobacillus buchneri

Bacterial cell wall hydrolases are essential for peptidoglycan remodelling in regard to bacterial cell growth and division. In this study, peptidoglycan hydrolases (PGHs) of different Lactobacillus buchneri strains were investigated. First, the genome sequence of L. buchneri CD034 and L. buchneri NRRL B-30929 was analysed in silico for the presence of PGHs. Of 23 putative PGHs with different predicted hydrolytic specificities, the glycosyl hydrolase family 25 domain-containing homologues LbGH25B and LbGH25N from L. buchneri CD034 and NRRL B-30929, respectively, were selected and characterized in detail. Zymogram analysis confirmed hydrolysing activity on bacterial cell walls for both enzymes. Subsequent reversed-phase HPLC and MALDI-TOF MS analysis of the peptidoglycan breakdown products from L. buchneri strains CD034 and NRRL B-30929, and from Lactobacillus rhamnosus GG, which served as a reference, revealed that LbGH25B and LbGH25N have N-acetylmuramidase activity. Both enzymes were identified as cell wall-associated proteins by means of immunofluorescence microscopy and cellular fractionation, as well as by the ability of purified recombinant LbGH25B and LbGH25N to bind to L. buchneri cell walls in vitro. Moreover, similar secondary structures mainly composed of β-sheets and nearly identical thermal stabilities with T m values around 49 °C were found for the two N-acetylmuramidases by far-UV circular dichroism spectroscopy. The functional and structural data obtained are discussed and compared to related PGHs. In this study, a major N-acetylmuramidase from L. buchneri was characterized in detail for the first time.

Properties and mutation studies of a bacteriophage-derived chimeric recombinant staphylolytic protein P128: Comparison to recombinant lysostaphin

P128 is a chimeric anti-staphylococcal protein having a catalytic domain from a Staphylococcus bacteriophage K tail associated structural protein and a cell wall targeting domain from the Staphylococcus bacteriocin-lysostaphin. In this study, we disclose additional properties of P128 and compared the same with lysostaphin. While lysostaphin was found to get inactivated by heat and was inactive on its parent strain S. simulans biovar staphylolyticus, P128 was thermostable and was lytic towards S. simulans biovar staphylolyticus demonstrating a difference in their mechanism of action. Selected mutation studies of the catalytic domain of P128 showed that arginine and cysteine, at 40th and 76th positions respectively, are critical for the staphylolytic activity of P128, although these amino acids are not conserved residues. In comparison to native P128, only the R40S mutant (P301) was catalytically active on zymogram gel and had a similar secondary structure, as assessed by circular dichroism analysis and in silico modeling with similar cell binding properties. Mutation of the arginine residue at 40th position of the P128 molecule caused dramatic reduction in the V_max (∆OD₆₀₀ [mg/min]) value (nearly 270 fold) and the recombinant lysostaphin also showed lesser V_max value (nearly 1.5 fold) in comparison to the unmodified P128 protein. The kinetic parameters such as apparent K_m (K_m^APP) and apparent K_cat (K_cat^APP) of the native P128 protein also showed significant differences in comparison to the values observed for P301 and lysostaphin.

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.

Biochemical characterization of an anti-Candida factor produced by Enterococcus faecalis

Background

Because Candida albicans is resistant to several antifungal antibiotics, there is a need to identify other less toxic natural products, particularly antimicrobial proteins, peptides or bacteriocin like inhibitory substances. An attempt has been made to purify and characterise an anti-Candida compound produced by Enterococcus faecalis.

Results

An anti-Candida protein (ACP) produced by E. faecalis active against 8 C. albicans strains was characterised and partially purified. The ACP showed a broad-spectrum activity against multidrug resistant C. albicans MTCC 183, MTCC 7315, MTCC 3958, NCIM 3557, NCIM 3471 and DI. It was completely inactivated by treatment with proteinase K and partially by pronase E.

The ACP retained biological stability after heat-treatment at 90°C for 20 min, maintained activity over a pH range 6–10, and remained active after treatment with α-amylase, lipase, organic solvents, and detergents. The antimicrobial activity of the E. faecalis strain was found exclusively in the extracellular filtrate produced in the late logarithmic growth phase. The highest activity (1600 AU mL^-1) against C. albicans MTCC 183 was recorded at 48 h of incubation, and activity decreased thereafter. The peptide showed very low haemagglutination and haemolytic activities against human red blood cells. The antimicrobial substance was purified by salt-fractionation and chromatography.

Partially purified ACP had a molecular weight of approximately 43 KDa in Tricine-PAGE analysis. The 12 amino acid N terminal sequence was obtained by Edman degradation. The peptide was de novo sequenced by ESI-MS, and the deduced combined sequence when compared to other bacteriocins and antimicrobial peptide had no significant sequence similarity.

Conclusions

The inhibitory activity of the test strain is due to the synthesis of an antimicrobial protein. To our knowledge, this is the first report on the isolation of a promising non-haemolytic anti-Candida protein from E. faecalis that might be used to treat candidiasis especially in immunocompromised patients.

Genetic and biochemical characterization of the cell wall hydrolase activity of the major secreted protein of Lactobacillus rhamnosus GG

Lactobacillus rhamnosus GG (LGG) produces two major secreted proteins, designated here Msp1 (LGG_00324 or p75) and Msp2 (LGG_00031 or p40), which have been reported to promote the survival and growth of intestinal epithelial cells. Intriguingly, although each of these proteins shares homology with cell wall hydrolases, a physiological function that correlates with such an enzymatic activity remained to be substantiated in LGG. To investigate the bacterial function, we constructed knock-out mutants in the corresponding genes aiming to establish a genotype to phenotype relation. Microscopic examination of the msp1 mutant showed the presence of rather long and overly extended cell chains, which suggests that normal daughter cell separation is hampered. Subsequent observation of the LGG wild-type cells by immunofluorescence microscopy revealed that the Msp1 protein accumulates at the septum of exponential-phase cells. The cell wall hydrolyzing activity of the Msp1 protein was confirmed by zymogram analysis. Subsequent analysis by RP-HPLC and mass spectrometry of the digestion products of LGG peptidoglycan (PG) by Msp1 indicated that the Msp1 protein has D-glutamyl-L-lysyl endopeptidase activity. Immunofluorescence microscopy and the failure to construct a knock-out mutant suggest an indispensable role for Msp2 in priming septum formation in LGG.

A novel bacteriophage Tail-Associated Muralytic Enzyme (TAME) from Phage K and its development into a potent antistaphylococcal protein

BACKGROUND

Staphylococcus aureus is a major cause of nosocomial and community-acquired infections. However, the rapid emergence of antibiotic resistance limits the choice of therapeutic options for treating infections caused by this organism. Muralytic enzymes from bacteriophages have recently gained attention for their potential as antibacterial agents against antibiotic-resistant gram-positive organisms. Phage K is a polyvalent virulent phage of the Myoviridae family that is active against many Staphylococcus species.

RESULTS

We identified a phage K gene, designated orf56, as encoding the phage tail-associated muralytic enzyme (TAME). The gene product (ORF56) contains a C-terminal domain corresponding to cysteine, histidine-dependent amidohydrolase/peptidase (CHAP), which demonstrated muralytic activity on a staphylococcal cell wall substrate and was lethal to S. aureus cells. We constructed N-terminal truncated forms of ORF56 and arrived at a 16-kDa protein (Lys16) that retained antistaphylococcal activity. We then generated a chimeric gene construct encoding Lys16 and a staphylococcal cell wall-binding SH3b domain. This chimeric protein (P128) showed potent antistaphylococcal activity on global clinical isolates of S. aureus including methicillin-resistant strains. In addition, P128 was effective in decolonizing rat nares of S. aureus USA300 in an experimental model.

CONCLUSIONS

We identified a phage K gene that encodes a protein associated with the phage tail structure. The muralytic activity of the phage K TAME was localized to the C-terminal CHAP domain. This potent antistaphylococcal TAME was combined with an efficient Staphylococcus-specific cell-wall targeting domain SH3b, resulting in the chimeric protein P128. This protein shows bactericidal activity against globally prevalent antibiotic resistant clinical isolates of S. aureus and against the genus Staphylococcus in general. In vivo, P128 was efficacious against methicillin-resistant S. aureus in a rat nasal colonization model.

Bacteriophages of lactic acid bacteria and their impact on milk fermentations

Every biotechnology process that relies on the use of bacteria to make a product or to overproduce a molecule may, at some time, struggle with the presence of virulent phages. For example, phages are the primary cause of fermentation failure in the milk transformation industry. This review focuses on the recent scientific advances in the field of lactic acid bacteria phage research. Three specific topics, namely, the sources of contamination, the detection methods and the control procedures will be discussed.

The nine peptidoglycan hydrolases genes in Lactobacillus helveticus are ubiquitous and early transcribed

Peptidoglycan hydrolases (PGHs) are bacterial enzymes that can hydrolyze the peptidoglycan in bacterial cell wall leading to autolysis. By releasing intracellular enzymes, autolysis of Lactobacillus helveticus has important applications in cheese ripening as its extent varied from strain to strain. Nine genes coding PGHs were previously annotated in the genome of the high autolytic strain L. helveticus DPC 4571. This study was conducted to evaluate the clone diversity of the nine PGHs genes within a collection of 24 L. helveticus strains, highly diverse in terms of origin, biotope and autolytic activity. Pulsed field gel electrophoresis was applied to assess the genomic diversity of the 24 strains. The presence or absence of nine PGHs genes was verified for all L. helveticus strains. Nucleotide and deduced amino acid sequence were compared for six relevant strains. Finally, gene expression was monitored by reverse transcription during growth and by zymogram for 12 strains. The nine PGHs genes are ubiquitous and transcripted early during growth. Zymograms were similar in terms of molecular size of the bands, but exhibited strain to strain variations in the number of bands revealing from 2 to 5 lytic bands per strain.

Absence of pneumococcal PcsB is associated with overexpression of LysM domain-containing proteins

The streptococcal protein required for cell separation B (PcsB) is predicted to play an important role in peptidoglycan metabolism, based on sequence motifs and altered phenotypes of gene deletion mutant cells exhibiting defects in cell separation. However, no enzymic activity has been demonstrated for PcsB so far. By generating gene deletion mutant strains in four different genetic backgrounds we could demonstrate that pcsB is not essential for cell survival in Streptococcus pneumoniae, but is essential for proper cell division. Deletion mutant cells displayed cluster formation due to aberrant cell division, reduced growth and antibiotic sensitivity that were fully reverted by transformation with a plasmid carrying pcsB. Immunofluorescence staining revealed that PcsB was localized to the cell poles, similarly to PBP3 and LytB, enzymes with demonstrated peptidoglycan-degrading activity required for daughter cell separation. Similarly to other studies with PcsB homologues, we could not detect peptidoglycan-lytic activity with recombinant or native pneumococcal PcsB in vitro. In addition to defects in septum placement and separation, the absence of PcsB induced an increased release of several proteins, such as enolase, MalX and the SP0107 LysM domain protein. Interestingly, genes encoding both LysM domain-containing proteins that are present in the pneumococcal genome (SP0107 and SP2063) and predicted to be involved in cell wall metabolism were found to be highly overexpressed (14-33-fold increase) in ΔpcsB cells in two different genetic backgrounds. Otherwise, we detected very few changes in the global gene expression profile of cells lacking PcsB. Thus our data suggest that LysM domain proteins partially compensate for the lack of PcsB function and allow the survival and slow growth of the pneumococcus.

The genomes and comparative genomics of Lactobacillus delbrueckii phages

Lactobacillus delbrueckii phages are a great source of genetic diversity. Here, the genome sequences of Lb. delbrueckii phages LL-Ku, c5 and JCL1032 were analyzed in detail, and the genetic diversity of Lb. delbrueckii phages belonging to different taxonomic groups was explored. The lytic isometric group b phages LL-Ku (31,080 bp) and c5 (31,841 bp) showed a minimum nucleotide sequence identity of 90% over about three-fourths of their genomes. The genomic locations of their lysis modules were unique, and the genomes featured several putative overlapping transcription units of genes. LL-Ku and c5 virions displayed peptidoglycan hydrolytic activity associated with a ~36-kDa protein similar in size to the endolysin. Unexpectedly, the 49,433-bp genome of the prolate phage JCL1032 (temperate, group c) revealed a conserved gene order within its structural genes. Lb. delbrueckii phages representing groups a (a phage LL-H), b and c possessed only limited protein sequence homology. Genomic comparison of LL-Ku and c5 suggested that diversification of Lb. delbrueckii phages is mainly due to insertions, deletions and recombination. For the first time, the complete genome sequences of group b and c Lb. delbrueckii phages are reported.

Comparative transcriptome analysis of Lactococcus lactis subsp. cremoris strains under conditions simulating Cheddar cheese manufacture

Gene expression in response to technological variations can influence fermentation and flavor generation in Cheddar cheese, and can vary from one lactococcal strain to another, perceived as differences in starter performance. The aim of this study was to determine the influence of cheese cooking temperature at 38 °C and salting on the transcriptional profiles of four closely related strains of L. lactis subsp. cremoris under simulated conditions of Cheddar cheese manufacture. Two responses could be distinguished, a core gene expression, corresponding to the common response of all strains and strain-specific response during the Cheddar simulating process. For the core gene expression after heating of inoculated milk at 38 °C, two groups of differentially expressed genes were identified: i) stress response and ii) carbohydrate and amino acid metabolism. The response to combined stresses of heat, acid and salt resulted in: i) general decrease of functions linked to cell division and metabolism, ii) specific responses related to stress such as the induction of genes coding for chaperones and proteases and iii) expression of prophage lytic systems for certain strains. Strain-specific responses were mainly observed in three of the four tested strains. These responses were the induction of genes related to osmotic stress or the release of CodY repression leading to the activation of oligopeptide transporters as well as the bcaT gene, related to amino acid degradation for the production of flavor. Comparing transcriptomes provides a core expression profile that contributes to understanding gene expression responses to environmental variations. The strain-specific responses identify predictive markers for the transcriptional state of starter strains before they enter the cheese ripening phase.

Streptococcus pyogenes Ser/Thr kinase-regulated cell wall hydrolase is a cell division plane-recognizing and chain-forming virulence factor

Cell division and cell wall synthesis are closely linked complex phenomena and play a crucial role in the maintenance and regulation of bacterial virulence. Eukaryotic-type Ser/Thr kinases reported in prokaryotes, including that in group A Streptococcus (GAS) (Streptococcus pyogenes Ser/Thr kinase (SP-STK)), regulate cell division, growth, and virulence. The mechanism of this regulation is, however, unknown. In this study, we demonstrated that SP-STK-controlled cell division is mediated under the positive regulation of secretory protein that possesses a cysteine and histidine-dependent aminohydrolases/peptidases (CHAP) domain with functionally active cell wall hydrolase activity (henceforth named as CdhA (CHAP-domain-containing and chain-forming cell wall hydrolase). Deletion of the CdhA-encoding gene resulted in severe cell division and growth defects in GAS mutants. The mutant expressing the truncated CdhA (devoid of the CHAP domain), although displayed no such defects, it became attenuated for virulence in mice and highly susceptible to cell wall-acting antibiotics, as observed for the mutant lacking CdhA. When CdhA was overexpressed in the wild-type GAS as well as in heterologous strains, Escherichia coli and Staphylococcus aureus, we observed a distinct increase in bacterial chain length. Our data reveal that CdhA is a multifunctional protein with a major function of the N-terminal region as a cell division plane-recognizing domain and that of the C-terminal CHAP domain as a virulence-regulating domain. CdhA is thus an important therapeutic target.

Functional genomic analysis of two Staphylococcus aureus phages isolated from the dairy environment

The genomes of the two lytic mutant Staphylococcus aureus bacteriophages, vB_SauS-phiIPLA35 (phiIPLA35) and vB_SauS-phiIPLA88 (phiIPLA88), isolated from milk have been analyzed. Their genomes are 45,344 bp and 42,526 bp long, respectively, and contain 62 and 61 open reading frames (ORFS). Enzymatic analyses and sequencing revealed that the phiIPLA35 DNA molecule has 3'-protruding cohesive ends (cos) 10 bp long, whereas phiIPLA88 DNA is 4.5% terminally redundant and most likely is packaged by a headful mechanism. N-terminal amino acid sequencing, mass spectrometry, bioinformatic analyses, and functional analyses enabled the assignment of putative functions to 58 gene products, including DNA packaging proteins, morphogenetic proteins, lysis components, and proteins necessary for DNA recombination, modification, and replication. Point mutations in their lysogeny control-associated genes explain their strictly lytic behavior. Muralytic activity associated with other structural components has been detected in virions of both phages. Comparative analysis of phiIPLA35 and phiIPLA88 genome structures shows that they resemble those of phi12 and phi11, respectively, both representatives of large genomic groupings within the S. aureus-infecting phages.

Antimicrobial activity of a chimeric enzybiotic towards Staphylococcus aureus

Phage lytic enzymes (enzybiotics) have gained attention as prospective tools to eradicate Gram-positive pathogens resistant to antibiotics. Attempts to purify the P16 endolysin of Staphylococcus aureus phage P68 were unsuccessful owing to the poor solubility of the protein. To overcome this limitation, we constructed a chimeric endolysin (P16-17) comprised of the inferred N-terminal d-alanyl-glycyl endopeptidase domain and the C-terminal cell wall targeting domain of the S. aureus phage P16 endolysin and the P17 minor coat protein, respectively. The domain swapping approach and the applied purification procedure resulted in soluble P16-17 protein, which exhibited antimicrobial activity towards S. aureus. In addition, P16-17 augmented the antimicrobial efficacy of the antibiotic gentamicin. This synergistic effect could be useful to reduce the effective dose of aminoglycoside antibiotics.

Surface physicochemical analysis of natural Lactococcus lactis strains reveals the existence of hydrophobic and low charged strains with altered adhesive properties

The cell surface physicochemical properties of 50 Lactococcus lactis strains of different subspecies and isolated from different origins (dairy, vegetal and animal) were examined. Cell surface hydrophobicity and Lewis acid-base properties were evaluated by affinity measurements to solvents in a partitioning test, while the global electrical charge of the cells was assessed by micro-electrophoresis using a laser zeta-meter. A global multivariate analysis of the results revealed a high natural diversity of L. lactis cell surface properties. While 52% of the strains present a hydrophilic and electronegative cell wall surface, a group of strikingly hydrophobic strains (12% of the strains) and a group of strains with unusual low charged surface (18%) were identified. Adhesion on polystyrene microtitre plates was evaluated for twelve strains selected from the multivariate analysis as representatives of the various observed cell wall surface physicochemical patterns. A significant correlation between adhesion, hydrophobicity and low electronegativity was observed when adhesion was performed in a low ionic strength suspending medium. The most adhesive strains were hydrophobic or low charged. The presence of repulsive electrostatic interactions led to a decrease in adhesion of the most negatively charged hydrophilic strains. The present study highlights the diversity of L. lactis cell surface physicochemical properties, diversity that could not be connected to the origin or to the subspecies of the strains.

Phenotypic, genotypic and technological characterization of predominant lactic acid bacteria in Pecorino cheese from central Italy

AIMS

Identification and biotyping of lactic acid bacteria (LAB) isolated from raw-milk Pecorino cheese manufactured in the Marche region (central Italy) for selection of suitable starter cultures or adjuncts.

METHODS AND RESULTS

Preliminary characterization with morphological and biochemical assays were undertaken for 112 Gram-positive and catalase-negative isolates. Unequivocal identification of the isolates was obtained through restriction analysis of the amplified 16S rRNA gene and sequencing of 360-380 bp amplicons. Fifty-nine isolates belonging to LAB species generally recognized as safe and potentially utilized as starters or flavour-producing adjuncts were preselected and tested for their acidifying, proteolitic and autolytic activities. Fifty-five of these isolates were also subject to RAPD (randomly amplified polymorphic DNA) fingerprinting and unweighted pair-group method with arithmetic averages (UPGMA) cluster analysis for the estimation of genotypic intra-species variation. As a result, in Pecorino cheese, a heterogeneous lactic acid bacteria population, which includes strains with metabolic characteristics of technological interest, was characterized.

CONCLUSIONS

The polyphasic approach proposed allows the bacterial ecology of Pecorino cheese to be investigated and allows to assess the potential role of autochthonous LAB strains for the dairy industry.

SIGNIFICANCE AND IMPACT OF THE STUDY

The great economic importance of Pecorino cheese encouraged a deeper knowledge of its microbiota, which is known to influence the peculiar sensory properties of this cheese, also in view of its exploitation.

Bacteriophages in dairy products: pros and cons

Since the time bacteriophages were first identified as a major cause of fermentation failure in the dairy industry, researchers have been struggling to develop strategies to exclude them from the dairy environment. Over 70 years of research has led to huge improvements in the consistency and quality of fermented dairy products, while also facilitating an appreciation of the beneficial properties of bacteriophages with respect to dairy product development. With specific reference to Lactococcus lactis and cheese production, this review outlines some recently reported novel methods aimed at limiting the bacteriophage infection as well as highlighting some beneficial aspects of bacteriophage activity.

Lactococcus lactis gene yjgB encodes a gamma-D-glutaminyl-L-lysyl-endopeptidase which hydrolyzes peptidoglycan

YjgB is one of five peptidoglycan hydrolases previously identified in Lactococcus lactis. Analysis of its amino acid sequence revealed that YjgB contains an NlpC/P60 domain, whereas no specific cell wall binding domain or motif could be identified. The NlpC/P60 family is characterized by three conserved residues, a cysteine, a histidine, and a polar residue. In agreement with the presence of a Cys residue in the catalytic site of YjgB, its enzymatic activity was enhanced in the presence of dithiothreitol. Peptidoglycan-hydrolyzing activity of YjgB was detected in growing cells of an L. lactis strain overexpressing YjgB, as revealed by the presence of disaccharide (DS)-dipeptide in the muropeptide composition of the overexpressing strain. YjgB hydrolyzes the peptide chains of L. lactis muropeptides between gamma-D-Gln and L-Lys residues. Its hydrolytic activity was detected on DSs with tetra- and pentapeptide chains, whereas hydrolytic activity was very low on DS-tripeptides. Thus, we demonstrated that YjgB is an endopeptidase which cleaves gamma-D-Gln-L-Lys bonds in peptide chains of L. lactis peptidoglycan.

SpxB Regulates O-Acetylation-dependent Resistance of Lactococcus lactis Peptidoglycan to Hydrolysis

Endogenous peptidoglycan (PG)-hydrolyzing enzymes, the autolysins, are needed to relax the rigid PG sacculus to allow bacterial cell growth and separation. PGs of pathogens and commensal bacteria may also be degraded by hydrolases of animal origin (lysozymes), which act as antimicrobials. The genetic mechanisms regulating PG resistance to hydrolytic degradation were dissected in the Gram-positive bacterium Lactococcus lactis. We found that the ability of L. lactis to counteract PG hydrolysis depends on the degree of acetylation. Overexpression of PG O-acetylase (encoded by oatA) led to bacterial growth arrest, indicating the potential lethality of oatA and a need for its tight regulation. A novel regulatory factor, SpxB (previously denoted as YneH), exerted a positive effect on oatA expression. Our results indicate that SpxB binding to RNA polymerase constitutes a previously missing link in the multistep response to cell envelope stress, provoked by PG hydrolysis with lysozyme. We suggest that the two-component system CesSR responds to this stress by inducing SpxB, thus favoring its interactions with RNA polymerase. Induction of PG O-acetylation by this cascade renders it resistant to hydrolysis.

Two 2[5H]-furanones as possible signaling molecules in Lactobacillus helveticus

Two 2[5H]-furanones, in association with medium-chain fatty acids, were released in whey by Lactobacillus helveticus exposed to oxidative and heat stresses. This species plays an important role in cheese technology, particularly for Swiss-type cheeses and Grana cheese. Moreover, it significantly contributes to cheese ripening by means of an early autolysis and the release of enzymes during processing. Experimental evidence of the involvement of the two 2[5H]-furanones, detected by a gas chromatography-mass spectrometry/solid-phase microextraction technique, in the autolysis phenomenon has been obtained. Zymograms performed by using renaturing sodium dodecyl sulfate-polyacrylamide gels were used to detect the bioactivity of the supernatants containing the two furanones on fresh cells of the same strain. In addition to bands corresponding to known autolysins, new autolysins were detected concomitant with the exposure of Lactobacillus helveticus to the supernatants, which can be regarded as conditioned media (CM), and to a commercial furanone, 5-ethyl-3-hydroxy-4-methyl-2[5H]-furanone (HEMFi), having spectral data similar to those of the newly described 2[5H]-furanones. Morphological changes were observed when fresh cells were exposed to CM containing the two 2[5H]-furanones and HEMFi. The two furanones produced by Lactobacillus helveticus, which met a number of criteria to be included in cell-cell signaling molecules, have a presumptive molecular mass lower than those of already known 3[2H]-furanones having an autolytic activity and being produced by gram-negative bacteria. Moreover, they present a different chemical structure with respect to the furanones already identified as products of Lactococcus lactis subsp. cremoris or to those identified in some cheeses with Lactobacillus helveticus as a starter culture.

Functional analysis of the lysis genes of Staphylococcus aureus phage P68 in Escherichia coli

Double-stranded DNA phages of both Gram-positive and Gram-negative bacteria typically use a holin-endolysin system to achieve lysis of their host. In this study, the lysis genes of Staphylococcus aureus phage P68 were characterized. P68 gene lys16 was shown to encode a cell-wall-degrading enzyme, which causes cell lysis when externally added to clinical isolates of S. aureus. Another gene, hol15, was identified embedded in the -1 reading frame at the 3' end of lys16. The deduced Hol15 protein has three putative transmembrane domains, and thus resembles class I holins. An additional candidate holin gene, hol12, was found downstream of the endolysin gene lys16 based on two predicted transmembrane domains of the encoded protein, which is a typical trait of class II holins. The synthesis of either Hol12 or Hol15 resulted in growth retardation of Escherichia coli, and both hol15 and hol12 were able to complement a phage lambda Sam mutation. The hol15 gene has a dual start motif beginning with the codons Met1-Lys2-Met3.... Evidence is presented that the hol15 gene encodes a lysis inhibitor (anti-holin) and a lysis effector (actual holin). As depolarization of the membrane converted the anti-holin to a functional holin, these studies suggested that hol15 functions as a typical dual start motif class I holin. The unusual arrangement of the P68 lysis genes is discussed.

Phage P68 virion-associated protein 17 displays activity against clinical isolates of Staphylococcus aureus

Phage-encoded murein hydrolases are either part of the lysis cassette or found as structural components of the phage virion. Here, we show that Staphylococcus aureus bacteriophage P68 contains a virion-associated muralytic enzyme. Protein 17 has a composite structure. The N-terminal part comprises the muralytic activity, whereas the C-terminal part is required for binding to the cell surface. A high multiplicity of infection with phage P68 caused rapid lysis, and purified protein 17 triggered premature lysis when added to S. aureus cells prior to infection with P68, suggesting that it functions to weaken the murein at the site of phage DNA entry. Protein 17 displayed activity against clinical S. aureus isolates, which are resistant to infection by phage P68, demonstrating that the protein targets surface structures distinct from the phage receptor. This broad activity spectrum of protein 17 could qualify virion-associated muralytic enzymes as attractive antimicrobials.

Analysis of the peptidoglycan hydrolase complement of Lactococcus lactis: identification of a third N-acetylglucosaminidase, AcmC

The peptidoglycan hydrolase (PGH) complement of Lactococcus lactis was identified by amino acid sequence similarity searching of the L. lactis IL-1403 complete genome sequence. Five PGHs that are not encoded by prophages were detected, including the previously characterized AcmA and AcmB proteins. Four of these PGHs, AcmA to AcmD, contain a catalytic domain homologous to that of enterococcal muramidase, but they have different domain structures. The fifth one (YjgB) has sequence similarity with the active-site domain of peptidoglycan-specific endopeptidases. The three new PGH-encoding genes identified in this study are all actively transcribed in L. lactis subsp. cremoris MG1363. The relative abundance of their transcripts varied during growth and was maximal during the early exponential growth phase. The three encoded proteins have peptidoglycan-hydrolyzing activities which are detected only at acidic pHs by zymography. Like AcmA and AcmB, AcmC has N-acetylglucosaminidase activity rather than the N-acetylmuramidase activity predicted by sequence similarity.

Characterisation of autolytic enzymes in Lactobacillus pentosus

AIMS

To characterize autolysis and autolytic system of the lactic acid bacterium Lactobacillus pentosus.

METHODS AND RESULTS

Autolysis of nine Lact. pentosus strains was evaluated in buffer solution. Their peptidoglycan hydrolase profiles were examined by renaturing SDS-PAGE and revealed two major activity bands at 58 and 112 kDa. Specificity analysis indicated the presence of at least two different types of peptidoglycan hydrolase activities in Lact. pentosus 1091.

CONCLUSIONS

Autolysis of Lact. pentosus was shown to be strain dependent and involvement of at least two different autolysins was evidenced.

SIGNIFICANCE AND IMPACT OF THE STUDY

The autolytic system of Lact. pentosus was characterized for the first time and the data obtained could be used in the selection of strains of technological interest.

Bacteriophage Tuc2009 encodes a tail-associated cell wall-degrading activity

Tuc2009 is a P335-type member of the tailed-phage supergroup Siphoviridae and was originally identified as a resident prophage of the gram-positive bacterium Lactococcus lactis UC509. A Tuc2009 gene designated tal2009 which is located within the morphogenic module was shown to specify a lytic activity within the 3' portion of its coding region. Comparative sequence analysis indicated that the cell wall-degrading part of Tal2009 is a member of the M37 protein family and that Tal2009 lacks a cell-binding domain, a finding supported by binding studies. Tal2009 appears to undergo self-mediated posttranslational processing in both L. lactis and Escherichia coli. Antibodies directed against a purified C-terminal portion of Tal2009 were used for immunoelectron microscopy, which showed that Tal2009 is located at the tail tip of Tuc2009. Antibody neutralization studies demonstrated that Tal2009-directed antibodies inhibited the ability of phage to mediate host lysis by more than 100-fold. These data indicate that tal2009 encodes a tail-associated lysin involved in localized cell wall degradation, thus allowing the Tuc2009 DNA injection machinery access to the membrane of its bacterial host.

Complete genome sequence of the Lactococcus lactis temperate phage φLC3: comparative analysis of φLC3 and its relatives in lactococci and streptococci

Complete genome sequencing of the P335 temperate Lactococcus lactis bacteriophage phiLC3 (32, 172 bp) revealed fifty-one open reading frames (ORFs). Four ORFs did not show any homology to other proteins in the database and twenty-one ORFs were assigned a putative biological function. phiLC3 contained a unique replication module and orf201 was identified as the putative replication initiator protein-encoding gene. phiLC3 was closely related to the L. lactis r1t phage (73% DNA identity). Similarity was also shared with other lactococcal P335 phages and the Streptococcus pyogenes prophages 370.3, 8232.4 and 315.5 over the non-structural genes and the genes involved in DNA packaging/phage morphogenesis, respectively. phiLC3 contained small homologous regions distributed among lactococcal phages suggesting that these regions might be involved in mediating genetic exchange. Two regions of 30 and 32 bp were conserved among the streptococcal and lactococcal r1t-like phages. These two regions, as well as other homologous regions, were located at mosaic borders and close to putative transcriptional terminators indicating that such regions together might attract recombination. The conserved regions found among lactococcal and streptococcal phages might be used for identification of phages/prophages/prophage remnants in their hosts.

Characterization of AcmB, an N-acetylglucosaminidase autolysin from Lactococcus lactis

A gene encoding a putative peptidoglycan hydrolase, named acmB, which is a paralogue of the major autolysin acmA gene, was identified in the Lactococcus lactis genome sequence. The acmB gene is transcribed in L. lactis MG1363 and its expression is modulated during cellular growth. The encoded AcmB protein has a modular structure with three domains: an N-terminal domain, especially rich in Ser, Thr, Pro and Asn residues, resembling a cell-wall-associated domain; a central domain homologous to the Enterococcus hirae muramidase catalytic domain; and a C-terminal domain of unknown function. A recombinant AcmB derivative, devoid of its N-terminal domain, was expressed in Escherichia coli. It exhibited hydrolysing activity on the peptidoglycan of several Gram-positive bacteria, including L. lactis. Though showing sequence similarity with enterococcal muramidase, AcmB has N-acetylglucosaminidase specificity. The acmB gene was inactivated in order to evaluate the role of the enzyme. AcmB does not appear to be involved in cell separation but contributes to cellular autolysis.

Plant lectin-like bacteriocin from a rhizosphere-colonizing Pseudomonas isolate

Rhizosphere isolate Pseudomonas sp. strain BW11M1, which belongs to the Pseudomonas putida cluster, secretes a heat- and protease-sensitive bacteriocin which kills P. putida GR12-2R3. The production of this bacteriocin is enhanced by DNA-damaging treatment of producer cells. We isolated a TnMod mutant of strain BW11M1 that had lost the capacity to inhibit the growth of strain GR12-2R3. A wild-type genomic fragment encompassing the transposon insertion site was shown to confer the bacteriocin phenotype when it was introduced into Escherichia coli cells. The bacteriocin structural gene was identified by defining the minimal region required for expression in E. coli. This gene was designated llpA (lectin-like putidacin) on the basis of significant homology of its 276-amino-acid product with mannose-binding lectins from monocotyledonous plants. LlpA is composed of two monocot mannose-binding lectin (MMBL) domains. Several uncharacterized bacterial genes encoding diverse proteins containing one or two MMBL domains were identified. A phylogenetic analysis of the MMBL domains present in eukaryotic and prokaryotic proteins assigned the putidacin domains to a new bacterial clade within the MMBL-containing protein family. Heterologous expression of the llpA gene also conveyed bacteriocin production to several Pseudomonas fluorescens strains. In addition, we demonstrated that strain BW11M1 and heterologous hosts secrete LlpA into the growth medium without requiring a cleavable signal sequence. Most likely, the mode of action of this lectin-like bacteriocin is different from the modes of action of previously described Pseudomonas bacteriocins.

Use of real-time quantitative PCR for the analysis of phiLC3 prophage stability in lactococci

Bacteriophages are a common and constant threat to proper milk fermentation. It has become evident that lysogeny is widespread in lactic acid bacteria, and in this work the temperate lactococcal bacteriophage phi LC3 was used as a model to study prophage stability in lactococci. The stability was analyzed in six phi LC3 lysogenic Lactococcus lactis subsp. cremoris host strains when they were growing at 15 and 30 degrees C. In order to perform these analyses, a real-time PCR assay was developed. The stability of the phi LC3 prophage was found to vary with the growth phase of its host L. lactis IMN-C1814, in which the induction rate increased during the exponential growth phase and reached a maximum level when the strain was entering the stationary phase. The maximum spontaneous induction frequency of the phi LC3 prophage varied between 0.32 and 9.1% (28-fold) in the six lysogenic strains. No correlation was observed between growth rates of the host cells and the spontaneous prophage induction frequencies. Furthermore, the level of extrachromosomal phage DNA after induction of the prophage varied between the strains (1.9 to 390%), and the estimated burst sizes varied up to eightfold. These results show that the host cells have a significant impact on the lytic and lysogenic life styles of temperate bacteriophages. The present study shows the power of the real-time PCR technique in the analysis of temperate phage biology and will be useful in work to reveal the impact of temperate phages and lysogenic bacteria in various ecological fields.