Molecular analysis of BcrR, a membrane-bound bacitracin sensor and DNA-binding protein from Enterococcus faecalis

Microcosm experiments revealed resistome coalescence of sewage treatment plant effluents in river environment

Sewage treatment plant (STP) effluents are important contributors of antibiotic resistance (AR) pollution in rivers. Effluent discharging into rivers causes resistome coalescence. However, their mechanisms and dynamic processes are poorly understood, especially for the effects of dilution, diffusion, and sunlight-induced attenuation on coalescence. In this study, we have constructed microcosmic experiments based on in-situ investigation to explore these issues. The first batch experiment revealed the effects of dilution and diffusion. The coverage of water coalesced resistomes ranged 66.26∼152.18 × /Gb and was positively correlated with effluent volume (Mann-Kendall test, p < 0.01). Principal coordinate analysis (PCoA) and source tracking analysis demonstrated that dilution and diffusion stepwise reduced AR pollution. The second batch experiment explored the temporal dynamics and sunlight attenuation on coalesced resistomes. Under natural light, the coverage and diversity of water resistomes posed decreasing trends, primarily attributed to drastic erasure of effluent traces. The proportion of effluent-specific ARGs in coalesced resistomes significantly declined over time (Spearman's r = -0.83 and -0.94 in coverage and richness). While under dark condition, the coverage and diversity increased. Sunlight radiation intensified the interactions between water and sediment resistomes, as evidenced by more shared ARGs and less dissimilarities across niches. Network analysis, metagenome-assembled genome (MAG) analysis and variation partitioning analysis (VPA) showed that microbiome controlled resistome coalescence, explaining 56.5% and 58.4% of resistomes in water and sediment, respectively. Biotic and abiotic factors synergistically explained 40% of water resistomes. This study offers a comprehensive understanding of AR transmission and provides theoretical bases for grasping AR pollution and developing effective suppression strategies.

Transmembrane Transcription Regulators Are Widespread in Bacteria and Archaea

To adapt and proliferate, bacteria must sense and respond to the ever-changing environment. Transmembrane transcription regulators (TTRs) are a family of one-component transcription regulators that respond to extracellular information and influence gene expression from the cytoplasmic membrane. How TTRs function to modulate expression of their target genes while localized to the cytoplasmic membrane remains poorly understood. In part, this is due to a lack of knowledge regarding the prevalence of TTRs among prokaryotes. Here, we show that TTRs are highly diverse and prevalent throughout bacteria and archaea. Our work demonstrates that TTRs are more common than previously appreciated and are enriched within specific bacterial and archaeal phyla and that many TTRs have unique transmembrane region properties that can facilitate association with detergent-resistant membranes. IMPORTANCE One-component signal transduction systems are the major class of signal transduction systems among bacteria and are commonly cytoplasmic. TTRs are a group of unique one-component signal transduction systems that influence transcription from the cytoplasmic membrane. TTRs have been implicated in a wide array of biological pathways critical for both pathogens and human commensal organisms but were considered to be rare. Here, we demonstrate that TTRs are in fact highly diverse and broadly distributed in bacteria and archaea. Our findings suggest that transcription factors can access the chromosome and influence transcription from the membrane in both archaea and bacteria. This study challenges thus the commonly held notion that signal transduction systems require a cytoplasmic transcription factor and highlights the importance of the cytoplasmic membrane in directly influencing signal transduction.

Two-component systems regulate ABC transporters in antimicrobial peptide production, immunity and resistance

Bacteria offer resistance to a broad range of antibiotics by activating their export channels of ATP-binding cassette transporters. These transporters perform a central role in vital processes of self-immunity, antibiotic transport and resistance. The majority of ATP-binding cassette transporters are capable of detecting the presence of antibiotics in an external vicinity and are tightly regulated by two-component systems. The presence of an extracellular loop and an adjacent location of both the transporter and two-component system offers serious assistance to induce a quick and specific response against antibiotics. Both systems have demonstrated their ability of sensing such agents, however, the exact mechanism is not yet fully established. This review highlighted the three key functions of antibiotic resistance, transport and self-immunity of ATP-binding cassette transporters and an adjacent two-component regulatory system.

Agricultural Origins of a Highly Persistent Lineage of Vancomycin-Resistant Enterococcus faecalis in New Zealand

Historical antimicrobial use in NZ agriculture has driven the evolution of ST108, a VRE lineage carrying a range of clinically relevant antimicrobial resistances. The persistence of this lineage in NZ for over a decade indicates that coselection may be an important stabilizing mechanism for its persistence.

Enterococcus faecalis and Enterococcus faecium are human and animal gut commensals. Vancomycin-resistant enterococci (VRE) are important opportunistic pathogens with limited treatment options. Historically, the glycopeptide antibiotics vancomycin and avoparcin selected for the emergence of vancomycin resistance in human and animal isolates, respectively, resulting in global cessation of avoparcin use between 1997 and 2000. To better understand human- and animal-associated VRE strains in the postavoparcin era, we sequenced the genomes of 231 VRE isolates from New Zealand (NZ; 75 human clinical, 156 poultry) cultured between 1998 and 2009. E. faecium lineages and their antibiotic resistance carriage patterns strictly delineated between agricultural and human reservoirs, with bacitracin resistance ubiquitous in poultry but absent in clinical E. faecium strains. In contrast, one E. faecalis lineage (ST108) predominated in both poultry and human isolates in the 3 years following avoparcin discontinuation. Both phylogenetic and antimicrobial susceptibility (i.e., ubiquitous bacitracin resistance in both poultry and clinical ST108 isolates) analyses suggest an agricultural origin for the ST108 lineage. VRE isolate resistomes were carried on multiple, heterogeneous plasmids. In some isolate genomes, bacitracin, erythromycin, and vancomycin resistance elements were colocalized, indicating multiple potentially linked selection mechanisms.

IMPORTANCE Historical antimicrobial use in NZ agriculture has driven the evolution of ST108, a VRE lineage carrying a range of clinically relevant antimicrobial resistances. The persistence of this lineage in NZ for over a decade indicates that coselection may be an important stabilizing mechanism for its persistence.

Functional characterization of BcrR: a one-component transmembrane signal transduction system for bacitracin resistance

Bacitracin is a cell wall targeting antimicrobial with clinical and agricultural applications. With the growing mismatch between antimicrobial resistance and development, it is essential we understand the molecular mechanisms of resistance in order to prioritize and generate new effective antimicrobials. BcrR is a unique membrane-bound one-component system that regulates high-level bacitracin resistance in Enterococcus faecalis. In the presence of bacitracin, BcrR activates transcription of the bcrABD operon conferring resistance through a putative ATP-binding cassette (ABC) transporter (BcrAB). BcrR has three putative functional domains, an N-terminal helix-turn-helix DNA-binding domain, an intermediate oligomerization domain and a C-terminal transmembrane domain. However, the molecular mechanisms of signal transduction remain unknown. Random mutagenesis of bcrR was performed to generate loss- and gain-of-function mutants using transcriptional reporters fused to the target promoter PbcrA. Fifteen unique mutants were isolated across all three proposed functional domains, comprising 14 loss-of-function and one gain-of-function mutant. The gain-of-function variant (G64D) mapped to the putative dimerization domain of BcrR, and functional analyses indicated that the G64D mutant constitutively expresses the PbcrA-luxABCDE reporter. DNA-binding and membrane insertion were not affected in the five mutants chosen for further characterization. Homology modelling revealed putative roles for two key residues (R11 and S33) in BcrR activation. Here we present a new model of BcrR activation and signal transduction, providing valuable insight into the functional characterization of membrane-bound one-component systems and how they can coordinate critical bacterial responses, such as antimicrobial resistance.

Expression of Genes Involved in Bacteriocin Production and Self-Resistance in Lactobacillus brevis 174A Is Mediated by Two Regulatory Proteins

We have previously shown that the lactic acid bacterium Lactobacillus brevis 174A, isolated from Citrus iyo fruit, produces a bacteriocin designated brevicin 174A, which is comprised of two antibacterial polypeptides (designated brevicins 174A-β and 174A-γ). We have also found a gene cluster, composed of eight open reading frames (ORFs), that contains genes for the biosynthesis of brevicin 174A, self-resistance to its own bacteriocin, and two transcriptional regulatory proteins. Some lactic acid bacterial strains have a system to start the production of bacteriocin at an adequate stage of growth. Generally, the system consists of a membrane-bound histidine protein kinase (HPK) that senses a specific environmental stimulus and a corresponding response regulator (RR) that mediates the cellular response. We have previously shown that although the HPK- and RR-encoding genes are not found on the brevicin 174A biosynthetic gene cluster in the 174A strain, two putative regulatory genes, designated breD and breG, are in the gene cluster. In the present study, we demonstrate that the expression of brevicin 174A production and self-resistance is positively controlled by two transcriptional regulatory proteins, designated BreD and BreG. BreD is expressed together with BreE as the self-resistance determinant of L. brevis 174A. DNase I footprinting analysis and a promoter assay demonstrated that BreD binds to the breED promoter as a positive autoregulator. The present study also demonstrates that BreG, carrying a transmembrane domain, binds to the common promoter of breB and breC, encoding brevicins 174A-β and 174A-γ, respectively, for positive regulation.IMPORTANCE The problem of the appearance of bacteria that are resistant to practical antibiotics and the increasing demand for safe foods have increased interest in replacing conventional antibiotics with bacteriocin produced by the lactic acid bacteria. This antibacterial substance can inhibit the growth of pathogenic bacteria without side effects on the human body. The bacteriocin that is produced by a Citrus iyo-derived Lactobacillus brevis strain inhibits the growth of pathogenic bacteria such as Listeria monocytogenes, Staphylococcus aureus, and Streptococcus mutans In general, lactic acid bacterial strains have a system to start the production of bacteriocin at an adequate stage of growth, which is called a quorum-sensing system. The system consists of a membrane-bound histidine protein kinase that senses a specific environmental stimulus and a corresponding response regulator that mediates the cellular response. The present study demonstrates that the expression of the genes encoding bacteriocin biosynthesis and the self-resistance determinant is positively controlled by two transcriptional regulatory proteins.

Vibrio parahaemolyticus VtrA is a membrane-bound regulator and is activated via oligomerization

Vibrio parahaemolyticus is a Gram-negative pathogen that causes food-borne gastroenteritis. A major virulence determinant of the organism is a type III secretion system (T3SS2) encoded on a pathogenicity island, Vp-PAI. Vp-PAI gene expression is regulated by two transcriptional regulators, VtrA and VtrB, whose N-terminal regions share homology with an OmpR-family DNA-binding domain. VtrA activates the gene expression of VtrB, which in turn activates Vp-PAI gene expression; however, the mechanism of this transcriptional activation by VtrA is not well understood. In this study, we determined that VtrA is a membrane protein with a transmembrane (TM) domain, which was required for its transcriptional regulatory activity. Although the N-terminal region of VtrA alone is insufficient for its transcriptional regulatory activity, forced oligomerization using the leucine-zipper dimerization domain of yeast GCN4 conferred transcriptional regulatory activity and a greater affinity for the promoter region of vtrB. A ToxR-based assay demonstrated that VtrA oligomerizes in vivo. We also showed that bile, a host-derived activator of VtrA, induces the oligomerization of VtrA, which requires the C-terminal domain. The promoter region of vtrB contained repetitive T-rich DNA elements, which are important for vtrB transcriptional activation and are conserved among T3SS2-possessing Vibrio species. These findings propose that VtrA is active as oligomers, which may facilitate its N-terminus binding the target DNA, thus enhancing its transcriptional regulatory activity.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

The cell envelope stress response of Bacillus subtilis: from static signaling devices to dynamic regulatory network

The cell envelope stress response (CESR) encompasses all regulatory events that enable a cell to protect the integrity of its envelope, an essential structure of any bacterial cell. The underlying signaling network is particularly well understood in the Gram-positive model organism Bacillus subtilis. It consists of a number of two-component systems (2CS) and extracytoplasmic function σ factors that together regulate the production of both specific resistance determinants and general mechanisms to protect the envelope against antimicrobial peptides targeting the biogenesis of the cell wall. Here, we summarize the current picture of the B. subtilis CESR network, from the initial identification of the corresponding signaling devices to unraveling their interdependence and the underlying regulatory hierarchy within the network. In the course of detailed mechanistic studies, a number of novel signaling features could be described for the 2CSs involved in mediating CESR. This includes a novel class of so-called intramembrane-sensing histidine kinases (IM-HKs), which-instead of acting as stress sensors themselves-are activated via interprotein signal transfer. Some of these IM-HKs are involved in sensing the flux of antibiotic resistance transporters, a unique mechanism of responding to extracellular antibiotic challenge.

Role of the Transporter-Like Sensor Kinase CbrA in Histidine Uptake and Signal Transduction

CbrA is an atypical sensor kinase found in Pseudomonas. The autokinase domain is connected to a putative transporter of the sodium/solute symporter family (SSSF). CbrA functions together with its cognate response regulator, CbrB, and plays an important role in nutrient acquisition, including regulation of hut genes for the utilization of histidine and its derivative, urocanate. Here we report on the findings of a genetic and biochemical analysis of CbrA with a focus on the function of the putative transporter domain. The work was initiated with mutagenesis of histidine uptake-proficient strains to identify histidine-specific transport genes located outside the hut operon. Genes encoding transporters were not identified, but mutations were repeatedly found in cbrA. This, coupled with the findings of [(3)H]histidine transport assays and further mutagenesis, implicated CbrA in histidine uptake. In addition, mutations in different regions of the SSSF domain abolished signal transduction. Site-specific mutations were made at four conserved residues: W55 and G172 (SSSF domain), H766 (H box), and N876 (N box). The mutations W55G, G172H, and N876G compromised histidine transport but had minimal effects on signal transduction. The H766G mutation abolished both transport and signal transduction, but the capacity to transport histidine was restored upon complementation with a transport-defective allele of CbrA, most likely due to interdomain interactions. Our combined data implicate the SSSF domain of CbrA in histidine transport and suggest that transport is coupled to signal transduction.

IMPORTANCE

Nutrient acquisition in bacteria typically involves membrane-bound sensors that, via cognate response regulators, determine the activity of specific transporters. However, nutrient perception and uptake are often coupled processes. Thus, from a physiological perspective, it would make sense for systems that couple the process of signaling and transport within a single protein and where transport is itself the stimulus that precipitates signal transduction to have evolved. The CbrA regulator in Pseudomonas represents a unique type of sensor kinase whose autokinase domain is connected to a transporter domain. We present genetic and biochemical evidence that suggests that CbrA plays a dual role in histidine uptake and sensing and that transport is dependent on signal transduction.

Accumulation of heptaprenyl diphosphate sensitizes Bacillus subtilis to bacitracin: implications for the mechanism of resistance mediated by the BceAB transporter

Heptaprenyl diphosphate (C35 -PP) is an isoprenoid intermediate in the synthesis of both menaquinone and the sesquarterpenoids. We demonstrate that inactivation of ytpB, encoding a C35 -PP utilizing enzyme required for sesquarterpenoid synthesis, leads to an increased sensitivity to bacitracin, an antibiotic that binds undecaprenyl pyrophosphate (C55 -PP), a key intermediate in cell wall synthesis. Genetic studies indicate that bacitracin sensitivity is due to accumulation of C35 -PP, rather than the absence of sesquarterpenoids. Sensitivity is accentuated in a ytpB menA double mutant, lacking both known C35 -PP consuming enzymes, and in a ytpB strain overexpressing the HepST enzyme that synthesizes C35 -PP. Conversely, sensitivity in the ytpB background is suppressed by mutation of hepT or by supplementation with 1,4-dihydroxy-2-naphthoate, a co-substrate with C35 -PP for MenA. Bacitracin sensitivity results from impairment of the BceAB and BcrC resistance mechanisms by C35 -PP: in a bceAB bcrC double mutant disruption of ytpB no longer increases bacitracin sensitivity. These results suggest that C35 -PP inhibits both BcrC (a C55 -PP phosphatase) and BceAB (an ABC transporter that confers bacitracin resistance). These findings lead to a model in which BceAB protects against bacitracin by transfer of the target, C55 -PP, rather than the antibiotic across the membrane.

Bacillus subtilis as a platform for molecular characterisation of regulatory mechanisms of Enterococcus faecalis resistance against cell wall antibiotics

To combat antibiotic resistance of Enterococcus faecalis, a better understanding of the molecular mechanisms, particularly of antibiotic detection, signal transduction and gene regulation is needed. Because molecular studies in this bacterium can be challenging, we aimed at exploiting the genetically highly tractable Gram-positive model organism Bacillus subtilis as a heterologous host. Two fundamentally different regulators of E. faecalis resistance against cell wall antibiotics, the bacitracin sensor BcrR and the vancomycin-sensing two-component system VanSB-VanRB, were produced in B. subtilis and their functions were monitored using target promoters fused to reporter genes (lacZ and luxABCDE). The bacitracin resistance system BcrR-BcrAB of E. faecalis was fully functional in B. subtilis, both regarding regulation of bcrAB expression and resistance mediated by the transporter BcrAB. Removal of intrinsic bacitracin resistance of B. subtilis increased the sensitivity of the system. The lacZ and luxABCDE reporters were found to both offer sensitive detection of promoter induction on solid media, which is useful for screening of large mutant libraries. The VanSB-VanRB system displayed a gradual dose-response behaviour to vancomycin, but only when produced at low levels in the cell. Taken together, our data show that B. subtilis is a well-suited host for the molecular characterization of regulatory systems controlling resistance against cell wall active compounds in E. faecalis. Importantly, B. subtilis facilitates the careful adjustment of expression levels and genetic background required for full functionality of the introduced regulators.

Identification and characterization of a bacitracin resistance network in Enterococcus faecalis

Resistance of Enterococcus faecalis against antimicrobial peptides, both of host origin and produced by other bacteria of the gut microflora, is likely to be an important factor in the bacterium's success as an intestinal commensal. The aim of this study was to identify proteins with a role in resistance against the model antimicrobial peptide bacitracin. Proteome analysis of bacitracin-treated and untreated cells showed that bacitracin stress induced the expression of cell wall-biosynthetic proteins and caused metabolic rearrangements. Among the proteins with increased production, an ATP-binding cassette (ABC) transporter with similarity to known peptide antibiotic resistance systems was identified and shown to mediate resistance against bacitracin. Expression of the transporter was dependent on a two-component regulatory system and a second ABC transporter, which were identified by genome analysis. Both resistance and the regulatory pathway could be functionally transferred to Bacillus subtilis, proving the function and sufficiency of these components for bacitracin resistance. Our data therefore show that the two ABC transporters and the two-component system form a resistance network against antimicrobial peptides in E. faecalis, where one transporter acts as the sensor that activates the TCS to induce production of the second transporter, which mediates the actual resistance.

Undecaprenyl pyrophosphate phosphatase confers low-level resistance to bacitracin in Enterococcus faecalis

OBJECTIVES

Undecaprenyl pyrophosphate phosphatases (UppPs) have been implicated in bacitracin resistance in some bacterial genera and the aim of this study was to determine the role of UppPs in mediating low-level bacitracin resistance in Enterococcus faecalis.

METHODS

The uppP gene was identified in the genomes of laboratory (JH2-2) and clinical (V583) strains of E. faecalis. Gene fusions (uppP-lacZ) and 5'-RACE were used to study uppP expression. The uppP gene in both strains was inactivated and mutants were studied for antimicrobial susceptibility and their susceptibilities to various stress agents.

RESULTS

The UppP protein from E. faecalis showed high sequence identity to the Escherichia coli BacA-type UppP and was predicted to be a hydrophobic protein with eight transmembrane helices. The expression of uppP-lacZ was constitutive and not affected by bacitracin or cell wall-active antimicrobials. E. faecalis uppP mutants showed no significant changes in growth rate, colony morphology and biofilm formation. The uppP mutants exhibited increased susceptibility to bacitracin (MICs=3-6 mg/L) relative to the isogenic wild-type (MICs=32-48 mg/L). When uppP was expressed in a wild-type background, the MIC of bacitracin increased to 128-≥256 mg/L. The MICs of cefoxitin, teicoplanin, vancomycin, gentamicin, enrofloxacin and d-cycloserine were unaltered in the uppP mutant relative to the wild-type, as were susceptibilities to other stress agents (glycine, lysozyme, NaCl, SDS, low and high pH, oxidative stress and ethanol).

CONCLUSIONS

The results demonstrate that low-level bacitracin resistance in E. faecalis is mediated by a BacA-type UppP.

ABC transporters of antimicrobial peptides in Firmicutes bacteria - phylogeny, function and regulation

Antimicrobial peptides (AMPs) are a group of antibiotics that mainly target the cell wall of Gram-positive bacteria. Resistance is achieved by a variety of mechanisms including target alterations, changes in the cell's surface charge, expression of immunity peptides or by dedicated ABC transporters. The latter often provide the greatest level of protection. Apart from resistance, ABC transporters are also required for the export of peptides during biosynthesis. In this review the different AMP transporters identified to date in Firmicutes bacteria were classified into five distinct groups based on their domain architecture, two groups with a role in biosynthesis, and three involved in resistance. Comparison of the available information for each group regarding function, transport mechanism and gene regulation revealed distinguishing characteristics as well as common traits. For example, a strong correlation between transporter group and mode of gene regulation was observed, with three different types of two-component systems as well as XRE family transcriptional regulators commonly associated with individual transporter groups. Furthermore, the presented summary of the state-of-the-art on AMP transport in Firmicutes bacteria, discussed in the context of transporter phylogeny, provides insights into the mechanisms of substrate translocation and how this may result in resistance against compounds that bind extracellular targets.

Regulation of proline metabolism in mycobacteria and its role in carbon metabolism under hypoxia

Genes with a role in proline metabolism are strongly expressed when mycobacterial cells are exposed to nutrient starvation and hypoxia. Here we show that proline metabolism in mycobacteria is mediated by the monofunctional enzymes Δ(1) -pyrroline-5-carboxylate dehydrogenase (PruA) and proline dehydrogenase (PruB). Proline metabolism was controlled by a unique membrane-associated DNA-binding protein PruC. Under hypoxia, addition of proline led to higher biomass production than in the absence of proline despite excess carbon and nitrogen. To identify the mechanism responsible for this enhanced growth, microarray analysis of wild-type Mycobacterium smegmatis versus pruC mutant was performed. Expression of the DNA repair machinery and glyoxalases was increased in the pruC mutant. Glyoxalases are proposed to degrade methylglyoxal, a toxic metabolite produced by various bacteria due to an imbalance in intermediary metabolism, suggesting the pruC mutant was under methylglyoxal stress. Consistent with this notion, pruB and pruC mutants were hypersensitive to methylglyoxal. Δ(1) -pyrroline-5-carboxylate is reported to react with methylglyoxal to form non-toxic 2-acetyl-1-pyrroline, thus providing a link between proline metabolism and methylglyoxal detoxification. In support of this mechanism, we show that proline metabolism protects mycobacterial cells from methylglyoxal toxicity and that functional proline dehydrogenase, but not Δ(1) -pyrroline-5-carboxylate dehydrogenase, is essential for this protective effect.

Combined effect of synthetic enterocin CRL35 with cell wall, membrane-acting antibiotics and muranolytic enzymes against Listeria cells

AIMS

To evaluate the inhibition effectiveness of enterocin CRL35 in combination with cell wall, membrane-acting antibiotics and muranolytic enzymes against the foodborne pathogen Listeria.

METHODS AND RESULTS

Synthetic enterocin CRL35 alone and in combination with monensin, bacitracin, gramicidin, mutanolysin and lysozyme were used in this study. Minimal inhibitory concentration (MIC) and fractional inhibitory concentration (FIC) index assays were performed using Listeria innocua 7 and Listeria monocytogenes FBUNT as sensitive strains. Antibiotics showed positive interactions with the bacteriocin in both strains tested. On the other hand, when mutanolysin and enterocin CRL35 were added to resting cells in a buffer system, the lytic effect of mutanolysin was enhanced. However, the addition of mutanolysin showed no effect on the growth of L. innocua 7 cells in a culture medium. Moreover, mutanolysin allowed the overgrowth of L. innocua 7 cells to an OD similar to control cells in the presence of inhibitory concentration of enterocin CRL35. In contrast, the combination of lysozyme and enterocin CRL35 resulted in a 50% inhibition of the L. innocua 7 growth.

CONCLUSIONS

Based on our results, we conclude that the combination of synthetic enterocin CRL35 with some antibiotics is effective against L. innocua 7 and L. monocytogenes FBUNT cells, and more importantly the amount of these agents to be used was considerably reduced. The effectiveness of the combination of synthetic enterocin CRL35 with muramidases seems to depend on complex environments, and more detailed studies need to be performed to elucidate this issue.

SIGNIFICANCE AND IMPACT OF THE STUDY

Enterocin CRL35 represents a promising agent that not only can ensure the quality and safety of food but it can also be combined with several antimicrobial agents important in the medical field.

Identification of DysI, the immunity factor of the streptococcal bacteriocin dysgalacticin

DysI is identified as the protein that confers specific immunity to dysgalacticin, a plasmid-encoded streptococcal bacteriocin. dysI is transcribed as part of the copG-repB-dysI replication-associated operon. DysI appears to function at the membrane level to prevent the inhibitory effects of dysgalacticin on glucose transport, membrane integrity, and intracellular ATP content.

The Lon protease from the haloalkaliphilic archaeon Natrialba magadii is transcriptionally linked to a cluster of putative membrane proteases and displays DNA-binding activity

The ATP-dependent Lon protease is universally distributed in bacteria, eukaryotic organelles and archaea. In comparison with bacterial and eukaryal Lon proteases, the biology of the archaeal Lon has been studied to a limited extent. In this study, the gene encoding the Lon protease of the alkaliphilic haloarchaeon Natrialba magadii (Nmlon) was cloned and sequenced, and the genetic organization of Nmlon was examined at the transcriptional level. Nmlon encodes a 84 kDa polypeptide with a pI of 4.42 which contains the ATPase, protease and membrane targeting domains of the archaeal-type LonB proteases. Nmlon is part of an operon that encodes membrane proteases and it is transcribed as a polycistronic mRNA in N. magadii cells at different growth stages. Accordingly, NmLon was detected in cell membranes of N. magadii throughout growth by Western blot analysis using specific anti-NmLon antibodies. Interestingly, in electrophoretic mobility shift assays, purified NmLon bound double stranded as well as single stranded DNA in the presence of elevated salt concentrations. This finding shows that DNA-binding is conserved in the LonA and LonB subfamilies and suggests that Lon-DNA interaction may be relevant for its function in haloarchaea.

Direct stimulus perception and transcription activation by a membrane-bound DNA binding protein

Few membrane proteins with a role in transcriptional regulation have been studied, and none are able to perceive their respective stimuli and activate transcription of their regulons without the aid of auxiliary proteins. The bacitracin resistance regulator, BcrR, of Enterococcus faecalis is a membrane-bound DNA binding protein and is required for bacitracin-dependent expression of the bacitracin resistance genes, bcrABD. Here, we show that BcrR interacts directly with Zn2+ bacitracin (Kd = 2-5 micropM), but not metal-free bacitracin. A solution-based DNA binding assay demonstrated that the affinity of BcrR for its target DNA is much higher (Kd = 40 nM) than previously found for transmembrane regulators and is comparable to that of soluble DNA binding proteins. A construct of BcrR that lacked the transmembrane domain was unable to bind to DNA, indicating that membrane localization was important for DNA binding. Bacitracin did not cause a change in the DNaseI footprint of BcrR on the bcrA promoter, but in vitro transcription assays with BcrR proteoliposomes showed bacitracin-dependent activation of transcription. These findings demonstrate that BcrR is a bona fide one-component transmembrane signal transduction system, which perceives an extracellular stimulus (presence of bacitracin) and relays it to an intracellular transcriptional response independent of any auxiliary proteins.