High efficiency introduction of plasmid DNA into glycine treated Enterococcus faecalis by electroporation

Metformin reduced the alkaline resistance of Enterococcus faecalis against calcium hydroxide via Man-PTS EII: in vitro and in vivo studies

OBJECTIVES

The mannose phosphotransferase system (Man-PTS) plays crucial roles in the adaptive metabolic activity of Enterococcus faecalis (E. faecalis) in adverse environments. The aim of this study was to evaluate the role of Man-PTS in the alkaline resistance of E. faecalis against calcium hydroxide (CH) and the effect of metformin (Met) on the alkaline resistance of E. faecalis to CH.

MATERIALS AND METHODS

The regulatory role of Man-PTS EII in the alkaline resistance of E. faecalis was firstly investigated using a wild-type highly alkaline-resistant E. faecalis XS 003, standard ATCC 29212 and Man-PTS EIID gene deficient (△mptD) and overexpressing (+mptD)  strains of E. faecalis. RNA sequencing of Met-treated E. faecalis was performed to further validate the effect of Met on Man-PTS. The effect of Met on CH resistance of E. faecalis was verified by evaluating the survival, membrane potential and permeability, intracellular pH and ATP, and the expression of Man-PTS EII and membrane transporter-related genes of E. faecalis. The effect of Met on the ability of CH to remove E. faecalis biofilm on the dentin surface was also tested. The in vivo therapeutic effect of Met plus CH (CHM) was further investigated in a rat apical periodontitis model induced by E. faecalis XS 003.

RESULTS

Man-PTS EII significantly promoted the survival ability of E. faecalis in CH and enhanced its resistance to CH. The inhibition of Man-PTS EII by Met resulted in reduced alkaline resistance of E. faecalis in the presence of CH, while also enhancing the antimicrobial properties of CH against E. faecalis biofilm on dentin. Additionally, Met plus CH showed the synergistically promoted intra-canal E. faecalis infection control and healing of periapical lesion in rats.

CONCLUSIONS

Met could significantly reduce the alkaline resistance of E. faecalis against CH through the modulation of Man-PTS EII, and improved the antibacterial effect of CH against E. faecalis infection both in vitro and in vivo.

CLINICAL RELEVANCE

Met could significantly enhance the ability of CH to control E. faecalis infection through reducing the alkaline resistance of E. faecalis.

Regulatory interactions between daptomycin- and bacitracin-responsive pathways coordinate the cell envelope antibiotic resistance response of Enterococcus faecalis

Enterococcal infections frequently show high levels of antibiotic resistance, including to cell envelope-acting antibiotics like daptomycin (DAP). While we have a good understanding of the resistance mechanisms, less is known about the control of such resistance genes in enterococci. Previous work unveiled a bacitracin resistance network, comprised of the sensory ABC transporter SapAB, the two-component system (TCS) SapRS and the resistance ABC transporter RapAB. Interestingly, components of this system have recently been implicated in DAP resistance, a role usually regulated by the TCS LiaFSR. To better understand the regulation of DAP resistance and how this relates to mutations observed in DAP-resistant clinical isolates of enterococci, we here explored the interplay between these two regulatory pathways. Our results show that SapR regulates an additional resistance operon, dltXABCD, a known DAP resistance determinant, and show that LiaFSR regulates the expression of sapRS. This regulatory structure places SapRS-target genes under dual control, where expression is directly controlled by SapRS, which itself is up-regulated through LiaFSR. The network structure described here shows how Enterococcus faecalis coordinates its response to cell envelope attack and can explain why clinical DAP resistance often emerges via mutations in regulatory components.

Identification of Multiple Iron Uptake Mechanisms in Enterococcus faecalis and Their Relationship to Virulence

Among the unfavorable conditions bacteria encounter within the host is restricted access to essential trace metals such as iron. To overcome iron deficiency, bacteria deploy multiple strategies to scavenge iron from host tissues, with abundant examples of iron acquisition systems being implicated in bacterial pathogenesis. Yet the mechanisms utilized by the major nosocomial pathogen Enterococcus faecalis to maintain intracellular iron balance are poorly understood. In this study, we conducted a systematic investigation to identify and characterize the iron acquisition mechanisms of E. faecalis and to determine their contribution to virulence. Bioinformatic analysis and literature surveys revealed that E. faecalis possesses three conserved iron uptake systems. Through transcriptomics, we discovered two novel ABC-type transporters that mediate iron uptake. While inactivation of a single transporter had minimal impact on the ability of E. faecalis to maintain iron homeostasis, inactivation of all five systems (Δ5Fe strain) disrupted intracellular iron homeostasis and considerably impaired cell growth under iron deficiency. Virulence of the Δ5Fe strain was generally impaired in different animal models but showed niche-specific variations in mouse models, leading us to suspect that heme can serve as an iron source to E. faecalis during mammalian infections. Indeed, heme supplementation restored growth of Δ5Fe under iron depletion and virulence in an invertebrate infection model. This study revealed that the collective contribution of five iron transporters promotes E. faecalis virulence and that the ability to acquire and utilize heme as an iron source is critical to the systemic dissemination of E. faecalis.

Barriers to genetic manipulation of Enterococci: Current Approaches and Future Directions

Enterococcus faecalis and Enterococcus faecium are Gram-positive commensal gut bacteria that can also cause fatal infections. To study clinically relevant multi-drug resistant E. faecalis and E. faecium strains, methods are needed to overcome physical (thick cell wall) and enzymatic barriers that limit the transfer of foreign DNA and thus prevent facile genetic manipulation. Enzymatic barriers to DNA uptake identified in E. faecalis and E. faecium include type I, II and IV restriction modification systems and CRISPR-Cas. This review examines E. faecalis and E. faecium DNA defence systems and the methods with potential to overcome these barriers. DNA defence system bypass will allow the application of innovative genetic techniques to expedite molecular-level understanding of these important, but somewhat neglected, pathogens.

c-di-AMP Is Essential for the Virulence of Enterococcus faecalis

Second messenger nucleotides are produced by bacteria in response to environmental stimuli and play a major role in the regulation of processes associated with bacterial fitness, including but not limited to osmoregulation, envelope homeostasis, central metabolism, and biofilm formation. In this study, we uncovered the biological significance of c-di-AMP in the opportunistic pathogen Enterococcus faecalis by isolating and characterizing strains lacking genes responsible for c-di-AMP synthesis (cdaA) and degradation (dhhP and gdpP). Using complementary approaches, we demonstrated that either complete loss of c-di-AMP (ΔcdaA strain) or c-di-AMP accumulation (ΔdhhP, ΔgdpP, and ΔdhhP ΔgdpP strains) drastically impaired general cell fitness and virulence of E. faecalis. In particular, the ΔcdaA strain was highly sensitive to envelope-targeting antibiotics, was unable to multiply and quickly lost viability in human serum or urine ex vivo, and was virtually avirulent in an invertebrate (Galleria mellonella) and in two catheter-associated mouse infection models that recapitulate key aspects of enterococcal infections in humans. In addition to evidence linking these phenotypes to altered activity of metabolite and peptide transporters and inability to maintain osmobalance, we found that the attenuated virulence of the ΔcdaA strain also could be attributed to a defect in Ebp pilus production and activity that severely impaired biofilm formation under both in vitro and in vivo conditions. Collectively, these results demonstrate that c-di-AMP signaling is essential for E. faecalis pathogenesis and a desirable target for drug development.

Type II fatty acid synthesis pathway and cyclopropane ring formation are dispensable during Enterococcus faecalis systemic infection

Enterococcus faecalis, a multi-antibiotic-resistant Gram-positive bacterium, has emerged as a serious nosocomial pathogen. Here, we used a genetic approach to characterize the strategies used by E. faecalis to fulfill its requirements for endogenous fatty acid (FA) synthesis in vitro and in vivo. The FA synthesis (FASII) pathway is encoded by two operons and two monocistronic genes. Expression of all these genes is repressed by exogenous FAs, which are incorporated in the E. faecalis membrane and modify its composition. Deletion of nine genes of the 12-gene operon abolished growth in a FA-free medium. Addition of serum, which is lipid-rich, restored growth. Interestingly, the E. faecalis membrane contains cyclic fatty acids that modify membrane properties, but are unavailable in host serum. The cfa gene that encodes the cyclopropanation process, is located in a locus independent of the FASII genes. Its deletion did not alter growth under the conditions tested, but yielded bacteria devoid of cyclic FAs. No differences were observed between mice infected with wild-type, or FASII or cyclopropanation mutant strains, in terms of bacterial loads in blood, liver, spleen or kidneys. We conclude that in E. faecalis, neither FASII nor cyclopropanation enzymes are suitable antibiotic targets. Importance Membrane lipid homeostasis is crucial for bacterial physiology, adaptation, and virulence. Fatty acids are constituents of the phospholipids that are essential membrane components. Most bacteria incorporate exogenous fatty acids into their membranes. Enterococcus faecalis has emerged as a serious nosocomial pathogen, which is responsible for urinary tract infections, bacteremia and endocarditis, and is intrinsically resistant to numerous antibiotics. E. faecalis synthesizes saturated and unsaturated fatty acids, but also cyclic fatty acids that are not found in the human host. We characterized mutant strains deficient in fatty acid synthesis and modification using genetic, biochemical, and in vivo approaches. We conclude that neither the fatty acid synthesis pathway nor the cyclopropanation enzyme are suitable targets for E. faecalis antibiotic development.

Charged Residues Flanking the Transmembrane Domain of Two Related Toxin-Antitoxin System Toxins Affect Host Response

A majority of toxins produced by type I toxin–antitoxin (TA-1) systems are small membrane-localized proteins that were initially proposed to kill cells by forming non-specific pores in the cytoplasmic membrane. The examination of the effects of numerous TA-1 systems indicates that this is not the mechanism of action of many of these proteins. Enterococcus faecalis produces two toxins of the Fst/Ldr family, one encoded on pheromone-responsive conjugative plasmids (Fst_pAD1) and the other on the chromosome, Fst_EF0409. Previous results demonstrated that overexpression of the toxins produced a differential transcriptomic response in E. faecalis cells. In this report, we identify the specific amino acid differences between the two toxins responsible for the differential response of a gene highly induced by Fst_pAD1 but not Fst_EF0409. In addition, we demonstrate that a transporter protein that is genetically linked to the chromosomal version of the TA-1 system functions to limit the toxicity of the protein.

The Large pBS32/pLS32 Plasmid of Ancestral Bacillus subtilis

The ancestral strain of Bacillus subtilis NCIB3610 (3610) bears a large, low-copy-number plasmid, called pBS32, that was lost during the domestication of laboratory strain derivatives. Selection against pBS32 may have been because it encodes a potent inhibitor of natural genetic competence (ComI), as laboratory strains were selected for high-frequency transformation. Previous studies have shown that pBS32 and its sibling, pLS32 in Bacillus subtilis subsp. natto, encode a replication initiation protein (RepN), a plasmid partitioning system (AlfAB), a biofilm inhibitor (RapP), and an alternative sigma factor (SigN) that can induce plasmid-mediated cell death in response to DNA damage. Here, we review the literature on pBS32/pLS32, the genes found on it, and their associated phenotypes.

Involvement of Chromosomally Encoded Homologs of the RRNPP Protein Family in Enterococcus faecalis Biofilm Formation and Urinary Tract Infection Pathogenesis

Enterococcus faecalis is an opportunistic pathogen capable of causing infections, including endocarditis and urinary tract infections (UTI). One of the well-characterized quorum-sensing pathways in E. faecalis involves coordination of the conjugal transfer of pheromone-responsive plasmids by PrgX, a member of the RRNPP protein family. Members of this protein family in various Firmicutes have also been shown to contribute to numerous cellular processes, including sporulation, competence, conjugation, nutrient sensing, biofilm formation, and virulence. As PrgX is a plasmid-encoded RRNPP family member, we surveyed the genome of the multidrug-resistant strain V583 for additional RRNPP homologs using computational searches and refined those identified hits for predicted structural similarities to known RRNPP family members. This led us to investigate the contribution of the chromosomally encoded RRNPP homologs to biofilm processes and pathogenesis in a catheter-associated urinary tract infection (CAUTI) model. In this study, we identified five such homologs and report that 3 of the 5 homologs, EF0073, EF1599, and EF1316, affect biofilm formation as well as outcomes in the CAUTI model.IMPORTANCE Enterococcus faecalis causes health care-associated infections and displays resistance to a variety of broad-spectrum antibiotics by acquisition of resistance traits as well as the ability to form biofilms. Even though a growing number of factors related to biofilm formation have been identified, mechanisms that contribute to biofilm formation are still largely unknown. Members of the RRNPP protein family regulate a diverse set of biological reactions in low-G+C Gram-positive bacteria (Firmicutes). Here, we identify three predicted structural homologs of the RRNPP family, EF0073, EF1599, and EF1316, which affect biofilm formation and CAUTI pathogenesis.

Antisense peptide nucleic acids againstftsZ andefaA genes inhibit growth and biofilm formation of Enterococcusfaecalis

Enterococcus faecalis is one of the important causes of nosocomial infections. Nowadays, increasing prevalence of antibiotic-resistant bacteria and slow progress in recognizing new antimicrobial agents has limited the efficiency of conventional antibiotics, which cause to find novel strategies to overcome bacteria. Therefore, in this study, we aimed to assess the role of efaA gene in the biofilm formation and the role of ftsZ gene in the controlling of bacterial growth by the anti-sense PNAs(Peptide Nucleic Acid).E. faecalis ATCC® 29212™was used for the study of PNAs designed to targeting the start codon section of the ftsZ andefaA genes. PNA attachment to RNA was confirmed by blotting. Electroporation technique was used for the intracellular transfer of anti-ftsZ PNAs. The spot-plating method was used to the assessment of alteration in bacterial growth. Biofilm formation assay and real-time PCR were used for detection of biofilm inhibitory effect of cell penetrating peptide (CPP) conjugated to anti-efaA PNAs.ByftsZ PNAs treatment, no growth was seen from the strain in agar by a spot plating method and the inhibition zone of anti-ftsZ PNAs was not seen. PNAs against the efaA gene decreased by 95% the expression of the efaA gene and biofilm formation. In addition, the(3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) MTT assay showed no toxicity on MCF7 cells for both of anti-ftsZand anti-efaA PNAs.This study used new genetic and molecular tools to inhibit pathogenicity and infection by E. faecalis. In this study, we suggested that efaA gene plays a critical role in the biofilm formation and anti-efaA PNAs could decrease the formation of biofilm, as well as, anti-ftsZ PNAs could eliminate bacterial growth.

Genomewide Profiling of the Enterococcus faecalis Transcriptional Response to Teixobactin Reveals CroRS as an Essential Regulator of Antimicrobial Tolerance

Teixobactin is a new antimicrobial with no known mechanisms of resistance. Understanding how resistance could develop will be crucial to the success and longevity of teixobactin as a new potent antimicrobial. Antimicrobial tolerance has been shown to facilitate the development of resistance, and we show that E. faecalis is intrinsically tolerant to teixobactin at high concentrations. We subsequently chose E. faecalis as a model to elucidate the molecular mechanism underpinning teixobactin tolerance and how this may contribute to the development of teixobactin resistance.

Teixobactin is a new antimicrobial of significant interest. It is active against a number of multidrug-resistant pathogens, including Staphylococcus aureus and Enterococcus faecalis, with no reported mechanisms of teixobactin resistance. However, historically, mechanisms of resistance always exist and arise upon introduction of a new antimicrobial into a clinical setting. Therefore, for teixobactin to remain effective long term, we need to understand how mechanisms of resistance could develop. Here we demonstrate that E. faecalis shows a remarkable intrinsic tolerance to high concentrations of teixobactin. This is of critical importance, as antimicrobial tolerance has been shown to precede the development of antimicrobial resistance. To identify potential pathways responsible for this tolerance, we determined the genomewide expression profile of E. faecalis strain JH2-2 in response to teixobactin using RNA sequencing. A total of 573 genes were differentially expressed (2.0-fold log₂ change in expression) in response to teixobactin, with genes involved in cell wall biogenesis and division and transport/binding being among those that were the most upregulated. Comparative analyses of E. faecalis cell wall-targeting antimicrobial transcriptomes identified CroRS, LiaRS, and YclRK to be important two-component regulators of antimicrobial-mediated stress. Further investigation of CroRS demonstrated that deletion of croRS abolished tolerance to teixobactin and to other cell wall-targeting antimicrobials. This highlights the crucial role of CroRS in controlling the molecular response to teixobactin.

IMPORTANCE Teixobactin is a new antimicrobial with no known mechanisms of resistance. Understanding how resistance could develop will be crucial to the success and longevity of teixobactin as a new potent antimicrobial. Antimicrobial tolerance has been shown to facilitate the development of resistance, and we show that E. faecalis is intrinsically tolerant to teixobactin at high concentrations. We subsequently chose E. faecalis as a model to elucidate the molecular mechanism underpinning teixobactin tolerance and how this may contribute to the development of teixobactin resistance.

PolyGlcNAc-containing exopolymers enable surface penetration by non-motile Enterococcus faecalis

Bacterial pathogens have evolved strategies that enable them to invade tissues and spread within the host. Enterococcus faecalis is a leading cause of local and disseminated multidrug-resistant hospital infections, but the molecular mechanisms used by this non-motile bacterium to penetrate surfaces and translocate through tissues remain largely unexplored. Here we present experimental evidence indicating that E. faecalis generates exopolysaccharides containing β-1,6-linked poly-N-acetylglucosamine (polyGlcNAc) as a mechanism to successfully penetrate semisolid surfaces and translocate through human epithelial cell monolayers. Genetic screening and molecular analyses of mutant strains identified glnA, rpiA and epaX as genes critically required for optimal E. faecalis penetration and translocation. Mechanistically, GlnA and RpiA cooperated to generate uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) that was utilized by EpaX to synthesize polyGlcNAc-containing polymers. Notably, exogenous supplementation with polymeric N-acetylglucosamine (PNAG) restored surface penetration by E. faecalis mutants devoid of EpaX. Our study uncovers an unexpected mechanism whereby the RpiA-GlnA-EpaX metabolic axis enables production of polyGlcNAc-containing polysaccharides that endow E. faecalis with the ability to penetrate surfaces. Hence, targeting carbohydrate metabolism or inhibiting biosynthesis of polyGlcNAc-containing exopolymers may represent a new strategy to more effectively confront enterococcal infections in the clinic.

Enterococcus faecalis is a microbial inhabitant of the human gastrointestinal tract that can cause lethal infections. Typically classified as a non-motile bacterium, E. faecalis can readily migrate and translocate across epithelial barriers to invade distant organs. Nevertheless, the molecular pathways driving enterococcal invasive attributes remain poorly understood. In this study, we uncover that E. faecalis produces a polyGlcNAc-containing extracellular glycopolymer to efficiently migrate into semisolid surfaces and translocate through human epithelial cell monolayers. Our work provides evidence that non-motile bacterial pathogens can exploit endogenous carbohydrate metabolic pathways to penetrate surfaces. Thus, targeting glycopolymer biosynthetic programs might be useful to control infections by Gram-positive cocci in the clinic.

Loss-of-Function Mutations in epaR Confer Resistance to ϕNPV1 Infection in Enterococcus faecalis OG1RF

Enterococcus faecalis is a Gram-positive opportunistic pathogen that inhabits the human gastrointestinal tract. Because of the high frequency of antibiotic resistance among Enterococcus clinical isolates, interest in using phage to treat enterococcal infections and to decolonize high-risk patients for antibiotic-resistant Enterococcus is rising. Bacteria can evolve phage resistance, but there is little published information on these mechanisms in E. faecalis In this report, we identified genetic determinants of E. faecalis resistance to phage NPV1 (ϕNPV1). We found that loss-of-function mutations in epaR confer ϕNPV1 resistance by blocking phage adsorption. We attribute the inability of the phage to adsorb to the modification or loss of an extracellular polymer in strains with inactivated epaR Phage-resistant epaR mutants exhibited increased daptomycin and osmotic stress susceptibilities. Our results demonstrate that in vitro spontaneous resistance to ϕNPV1 comes at a cost in E. faecalis OG1RF.

Manganese acquisition is essential for virulence of Enterococcus faecalis

Manganese (Mn) is an essential micronutrient that is not readily available to pathogens during infection due to an active host defense mechanism known as nutritional immunity. To overcome this nutrient restriction, bacteria utilize high-affinity transporters that allow them to compete with host metal-binding proteins. Despite the established role of Mn in bacterial pathogenesis, little is known about the relevance of Mn in the pathophysiology of E. faecalis. Here, we identified and characterized the major Mn acquisition systems of E. faecalis. We discovered that the ABC-type permease EfaCBA and two Nramp-type transporters, named MntH1 and MntH2, work collectively to promote cell growth under Mn-restricted conditions. The simultaneous inactivation of EfaCBA, MntH1 and MntH2 (ΔefaΔmntH1ΔmntH2 strain) led to drastic reductions (>95%) in cellular Mn content, severe growth defects in body fluids (serum and urine) ex vivo, significant loss of virulence in Galleria mellonella, and virtually complete loss of virulence in rabbit endocarditis and murine catheter-associated urinary tract infection (CAUTI) models. Despite the functional redundancy of EfaCBA, MntH1 and MntH2 under in vitro or ex vivo conditions and in the invertebrate model, dual inactivation of efaCBA and mntH2 (ΔefaΔmntH2 strain) was sufficient to prompt maximal sensitivity to calprotectin, a Mn- and Zn-chelating host antimicrobial protein, and for the loss of virulence in mammalian models. Interestingly, EfaCBA appears to play a prominent role during systemic infection, whereas MntH2 was more important during CAUTI. The different roles of EfaCBA and MntH2 in these sites could be attributed, at least in part, to the differential expression of efaA and mntH2 in cells isolated from hearts or from bladders. Collectively, this study demonstrates that Mn acquisition is essential for the pathogenesis of E. faecalis and validates Mn uptake systems as promising targets for the development of new antimicrobials.

Examination of Enterococcus faecalis Toxin-Antitoxin System Toxin Fst Function Utilizing a Pheromone-Inducible Expression Vector with Tight Repression and Broad Dynamic Range

Tools for regulated gene expression in Enterococcus faecalis are extremely limited. In this report, we describe the construction of an expression vector for E. faecalis, designated pCIE, utilizing the P_Q pheromone-responsive promoter of plasmid pCF10. We demonstrate that this promoter is tightly repressed, responds to nanogram quantities of the peptide pheromone, and has a large dynamic range. To demonstrate its utility, the promoter was used to control expression of the toxic peptides of two par family toxin-antitoxin (TA) loci present in E. faecalis, par_pAD1 of the pAD1 plasmid and par_EF0409 located on the E. faecalis chromosome. The results demonstrated differences in the modes of regulation of toxin expression and in the effects of toxins of these two related systems. We anticipate that this vector will be useful for further investigation of par TA system function as well as the regulated expression of other genes in E. faecalisIMPORTANCEE. faecalis is an important nosocomial pathogen and a model organism for examination of the genetics and physiology of Gram-positive cocci. While numerous genetic tools have been generated for the manipulation of this organism, vectors for the regulated expression of cloned genes remain limited by high background expression and the use of inducers with undesirable effects on the cell. Here we demonstrate that the P_Q pheromone-responsive promoter is repressed tightly enough to allow cloning of TA system toxins and evaluate their effects at very low induction levels. This tool will allow us to more fully examine TA system function in E. faecalis and to further elucidate its potential roles in cell physiology.

Regulation of Cell Wall Plasticity by Nucleotide Metabolism in Lactococcus lactis

To ensure optimal cell growth and separation and to adapt to environmental parameters, bacteria have to maintain a balance between cell wall (CW) rigidity and flexibility. This can be achieved by a concerted action of peptidoglycan (PG) hydrolases and PG-synthesizing/modifying enzymes. In a search for new regulatory mechanisms responsible for the maintenance of this equilibrium in Lactococcus lactis, we isolated mutants that are resistant to the PG hydrolase lysozyme. We found that 14% of the causative mutations were mapped in the guaA gene, the product of which is involved in purine metabolism. Genetic and transcriptional analyses combined with PG structure determination of the guaA mutant enabled us to reveal the pivotal role of the pyrB gene in the regulation of CW rigidity. Our results indicate that conversion of l-aspartate (l-Asp) to N-carbamoyl-l-aspartate by PyrB may reduce the amount of l-Asp available for PG synthesis and thus cause the appearance of Asp/Asn-less stem peptides in PG. Such stem peptides do not form PG cross-bridges, resulting in a decrease in PG cross-linking and, consequently, reduced PG thickness and rigidity. We hypothesize that the concurrent utilization of l-Asp for pyrimidine and PG synthesis may be part of the regulatory scheme, ensuring CW flexibility during exponential growth and rigidity in stationary phase. The fact that l-Asp availability is dependent on nucleotide metabolism, which is tightly regulated in accordance with the growth rate, provides L. lactis cells the means to ensure optimal CW plasticity without the need to control the expression of PG synthesis genes.

Deletion of fabN in Enterococcus faecalis results in unsaturated fatty acid auxotrophy and decreased release of inflammatory cytokines

The Gram-positive bacterium Enterococcus faecalis can cause life-threatening infections and is resistant to several commonly used antibiotics. The type II fatty acid pathway in bacteria is discussed as a potential target for antimicrobial therapy. However, it was shown that inhibition or deletion of its enzymes can be rescued in Gram-positive bacteria by supplementation with fatty acids. Here we show that by deletion of the fabN gene, which is essential for unsaturated fatty acid (UFA) synthesis in E. faecalis, growth is impaired but can be rescued by supplementation with oleic acid or human serum. Nonetheless, we demonstrate alterations of the UFA profile after supplementation with oleic acid in the ΔfabN mutant using a specific glycolipid. In addition, we demonstrate that cytokine release in vitro is almost abolished after stimulation of mouse macrophages by the mutant in comparison to the wild type. The results indicate that fabN is not a suitable target for antimicrobials as UFA auxotrophy can be overcome. However, deletion of fabN resulted in a decreased inflammatory response indicating that fabN and resulting UFA synthesis are relevant for virulence.

Identification and functional characterization of the putative polysaccharide biosynthesis protein (CapD) of Enterococcus faecium U0317

Most bacterial species produce capsular polysaccharides that contribute to disease pathogenesis through evasion of the host innate immune system and are also involved in inhibiting leukocyte killing. In the present study, we identified a gene in Enterococcus faecium U0317 with homologies to the polysaccharide biosynthesis protein CapD that is made up of 336 amino acids and putatively catalyzes N-linked glycosylation. A capD deletion mutant was constructed and complemented by homologous recombination that was confirmed by PCR and sequencing. The mutant revealed different growth behavior and morphological changes compared to wild-type by scanning electron microscopy, also the capD mutant showed a strong hydrophobicity and that was reversed in the reconstituted mutant. For further characterization and functional analyses, in-vitro cell culture and in-vivo a mouse infection models were used. Antibodies directed against alpha lipotechoic acid (αLTA) and the peptidyl-prolyl cis-trans isomerase (αPpiC), effectively mediated the opsonophagocytic killing in the capD knock-out mutant, while this activity was not observed in the wild-type and reconstituted mutant. By comparison more than 2-fold decrease was seen in mutant colonization and adherence to both T24 and Caco2 cells. However, a significant higher bacterial colonization was observed in capD mutant during bacteremia in the animal model, while virulence in a mouse UTI (urinary tract infection) model, there were no obvious differences. Further studies are needed to elucidate the function of capsular polysaccharide synthesis gene clusters and its involvement in the disease pathogenesis with the aim to develop targeted therapies to treat multidrug-resistant E. faecium infections.

The Native Plasmid pML21 Plays a Role in Stress Tolerance in Enterococcus faecalis ML21, as Analyzed by Plasmid Curing Using Plasmid Incompatibility

To investigate the role of the native plasmid pML21 in Enterococcus faecalis ML21's response to abiotic stresses, the plasmid pML21 was cured based on the principle of plasmid incompatibility and segregational instability, generating E. faecalis mutant strain ML0. The mutant and the wild strains were exposed to abiotic stresses: bile salts, low pH, H2O2, ethanol, heat, and NaCl, and their survival rate was measured. We found that curing of pML21 lead to reduced tolerance to stress in E. faecalis ML0, especially oxidative and osmotic stress. Complementation analysis suggested that the genes from pML21 played different role in stress tolerance. The result indicated that pML21 plays a role in E. faecalis ML21's response to abiotic stresses.

RovS and its associated signaling peptide form a cell-to-cell communication system required for Streptococcus agalactiae pathogenesis

Bacteria can communicate with each other to coordinate their biological functions at the population level. In a previous study, we described a cell-to-cell communication system in streptococci that involves a transcriptional regulator belonging to the Rgg family and short hydrophobic peptides (SHPs) that act as signaling molecules. Streptococcus agalactiae, an opportunistic pathogenic bacterium responsible for fatal infections in neonates and immunocompromised adults, has one copy of the shp/rgg locus. The SHP-associated Rgg is called RovS in S. agalactiae. In this study, we found that the SHP/RovS cell-to-cell communication system is active in the strain NEM316 of S. agalactiae, and we identified different partners that are involved in this system, such as the Eep peptidase, the PptAB, and the OppA1-F oligopeptide transporters. We also identified a new target gene controlled by this system and reexamined the regulation of a previously proposed target gene, fbsA, in the context of the SHP-associated RovS system. Furthermore, our results are the first to indicate the SHP/RovS system specificity to host liver and spleen using a murine model, which demonstrates its implication in streptococci virulence. Finally, we observed that SHP/RovS regulation influences S. agalactiae's ability to adhere to and invade HepG2 hepatic cells. Hence, the SHP/RovS cell-to-cell communication system appears to be an essential mechanism that regulates pathogenicity in S. agalactiae and represents an attractive target for the development of new therapeutic strategies.

IMPORTANCE

Rgg regulators and their cognate pheromones, called small hydrophobic peptides (SHPs), are present in nearly all streptococcal species. The general pathways of the cell-to-cell communication system in which Rgg and SHP take part are well understood. However, many other players remain unidentified, and the direct targets of the system, as well as its link to virulence, remain unclear. Here, we identified the different players involved in the SHP/Rgg system in S. agalactiae, which is the leading agent of severe infections in human newborns. We have identified a direct target of the Rgg regulator in S. agalactiae (called RovS) and examined a previously proposed target, all in the context of associated SHP. For the first time, we have also demonstrated the implication of the SHP/RovS mechanism in virulence, as well as its host organ specificity. Thus, this cell-to-cell communication system may represent a future target for S. agalactiae disease treatment.

An ABC transporter is required for secretion of peptide sex pheromones in Enterococcus faecalis

Enterococci are leading causes of hospital-acquired infection in the United States and continue to develop resistances to new antibiotics. Many Enterococcus faecalis isolates harbor pheromone-responsive plasmids that mediate horizontal transfer of even large blocks of chromosomal genes, resulting in hospital-adapted strains over a quarter of whose genomes consist of mobile elements. Pheromones to which the donor cells respond derive from lipoprotein signal peptides. Using a novel bacterial killing assay dependent on the presence of sex pheromones, we screened a transposon mutant library for functions that relate to the production and/or activity of the effector pheromone. Here we describe a previously uncharacterized, but well-conserved, ABC transporter that contributes to pheromone production. Using three distinct pheromone-dependent mating systems, we show that mutants defective in expressing this transporter display a 5- to 6-order-of-magnitude reduction in conjugation efficiency. In addition, we demonstrate that the ABC transporter mutant displays an altered biofilm architecture, with a significant reduction in biofilm biomass compared to that of its isogenic parent, suggesting that pheromone activity also influences biofilm development. The conservation of this peptide transporter across the Firmicutes suggests that it may also play an important role in cell-cell communication in other species within this important phylum.

Enterococcus faecalis ranks as one of the leading causes of hospital-associated infections. Strains possessing resistance to multiple antibiotics are becoming all too common in clinical settings. Pheromone-responsive plasmids play an important role in harboring and disseminating these antibiotic resistance genes. Here we have identified a novel ABC transporter that is responsible for the secretion of peptide pheromones, which enables communication between cells to mediate plasmid transfer. We have also shown that this transporter is important for biofilm formation, providing a strong rationale for its use as a viable therapeutic target which could be targeted to curb infection, as well as the spread of existing drug resistance.

Pyruvate formate-lyase is essential for fumarate-independent anaerobic glycerol utilization in the Enterococcus faecalis strain W11

Although anaerobic glycerol metabolism in Enterococcus faecalis requires exogenous fumarate for NADH oxidation, E. faecalis strain W11 can metabolize glycerol in the absence of oxygen without exogenous fumarate. In this study, metabolic end product analyses and reporter assays probing the expression of enzymes involved in pyruvate metabolism were performed to investigate this fumarate-independent anaerobic metabolism of glycerol in W11. Under aerobic conditions, the metabolic end products of W11 cultured with glycerol were similar to those of W11 cultured with glucose. However, when W11 was cultured anaerobically, most of the glucose was converted to l-lactate, but glycerol was converted to ethanol and formate. During anaerobic culture with glycerol, the expression of the l-lactate dehydrogenase and pyruvate dehydrogenase E1αβ genes in W11 was downregulated, whereas the expression of the pyruvate formate-lyase (Pfl) and aldehyde/alcohol dehydrogenase genes was upregulated. These changes in the expression levels caused the change in the composition of end products. A pflB gene disruptant (Δpfl mutant) of W11 could barely utilize glycerol under anaerobic conditions, but the growth of the Δpfl mutant cultured with either glucose or dihydroxyacetone (DHA) under anaerobic conditions was the same as that of W11. Glucose metabolism and DHA generates one NADH molecule per pyruvate molecule, whereas glycerol metabolism in the dehydrogenation pathway generates two NADH molecules per pyruvate molecule. These findings demonstrate that NADH generated from anaerobic glycerol metabolism in the absence of fumarate is oxidized through the Pfl-ethanol fermentation pathway. Thus, Pfl is essential to avoid the accumulation of excess NADH during fumarate-independent anaerobic glycerol metabolism.

Analysis of the tolerance of pathogenic enterococci and Staphylococcus aureus to cell wall active antibiotics

OBJECTIVES

Tolerance refers to the phenomenon that bacteria do not significantly die when exposed to bactericidal antibiotics. Enterococci are known for their high tolerance to these drugs, but the molecular reasons why they resist killing are not understood. In a previous study we showed that the superoxide dismutase (SOD) is implicated in this tolerance. This conclusion was based on the results obtained with one particular strain of Enterococcus faecalis and therefore the objective of the present communication was to analyse whether dependence of tolerance on active SOD is a general phenomenon for enterococci and another Gram-positive pathogen, Staphylococcus aureus.

METHODS

Mutants deficient in SOD activity were constructed in pathogenic enterococci. The wild-type sodA gene was cloned into an expression vector and transformed into SOD-deficient strains for complementation with varying levels of SOD activity. Previously constructed SOD-deficient strains of S. aureus were also included in this study. Tolerance to vancomycin and penicillin was then tested.

RESULTS

We demonstrated that the dependence on SOD of tolerance to vancomycin and penicillin is a common trait of antibiotic-susceptible pathogenic enterococci. By varying the levels of expression we could also show that tolerance to vancomycin is directly correlated to SOD activity. Interestingly, deletion of the sodA gene in a non-tolerant Enterococcus faecium strain did not further sensitize the mutant to bactericidal antibiotics. Finally, we showed that the SOD enzymes of S. aureus are also implicated in tolerance to vancomycin.

CONCLUSION

High tolerance of enterococci to cell wall active antibiotics can be reversed by SOD deficiency.

Eep confers lysozyme resistance to enterococcus faecalis via the activation of the extracytoplasmic function sigma factor SigV

Enterococcus faecalis is a commensal bacterium found in the gastrointestinal tract of most mammals, including humans, and is one of the leading causes of nosocomial infections. One of the hallmarks of E. faecalis pathogenesis is its unusual ability to tolerate high concentrations of lysozyme, which is an important innate immune component of the host. Previous studies have shown that the presence of lysozyme leads to the activation of SigV, an extracytoplasmic function (ECF) sigma factor in E. faecalis, and that the deletion of sigV increases the susceptibility of the bacterium toward lysozyme. Here, we describe the contribution of Eep, a membrane-bound zinc metalloprotease, to the activation of SigV under lysozyme stress by its effects on the stability of the anti-sigma factor RsiV. We demonstrate that the Δeep mutant phenocopies the ΔsigV mutant in lysozyme, heat, ethanol, and acid stress susceptibility. We also show, using an immunoblot analysis, that in an eep deletion mutant, the anti-sigma factor RsiV is only partially degraded after lysozyme exposure, suggesting that RsiV is processed by unknown protease(s) prior to the action of Eep. An additional observation is that the deletion of rsiV, which results in constitutive SigV expression, leads to chaining of cells, suggesting that SigV might be involved in regulating cell wall-modifying enzymes important in cell wall turnover. We also demonstrate that, in the absence of eep or sigV, enterococci bind significantly more lysozyme, providing a plausible explanation for the increased sensitivity of these mutants toward lysozyme.

Modular evolution of TnGBSs, a new family of integrative and conjugative elements associating insertion sequence transposition, plasmid replication, and conjugation for their spreading

Integrative and conjugative elements (ICEs) have a major impact on gene flow and genome dynamics in bacteria. The ICEs TnGBS1 and TnGBS2, first identified in Streptococcus agalactiae, use a DDE transposase, unlike most characterized ICEs, which depend on a phage-like integrase for their mobility. Here we identified 56 additional TnGBS-related ICEs by systematic genome analysis. Interestingly, all except one are inserted in streptococcal genomes. Sequence comparison of the proteins conserved among these ICEs defined two subtypes related to TnGBS1 or TnGBS2. We showed that both types encode different conjugation modules: a type IV secretion system, a VirD4 coupling protein, and a relaxase and its cognate oriT site, shared with distinct lineages of conjugative elements of Firmicutes. Phylogenetic analysis suggested that TnGBSs evolved from two conjugative elements of different origins by the successive recruitment of a transposition module derived from insertion sequences (ISs). Furthermore, TnGBSs share replication modules with different plasmids. Mutational analyses and conjugation experiments showed that TnGBS1 and TnGBS2 combine replication and transposition upstream promoters for their transfer and stabilization. Despite an evolutionarily successful horizontal dissemination within the genus Streptococcus, these ICEs have a restricted host range. However, we reveal that for TnGBS1 and TnGBS2, this host restriction is not due to a transfer incompatibility linked to the conjugation machineries but most likely to their ability for transient maintenance through replication after their transfer.

Characterization of the effects of an rpoC mutation that confers resistance to the Fst peptide toxin-antitoxin system toxin

Overexpression of the Fst toxin in Enterococcus faecalis strain OG1X leads to defects in chromosome segregation, cell division and, eventually, membrane integrity. The M7 mutant derivative of OG1X is resistant to most of these effects but shows a slight growth defect in the absence of Fst. Full-genome sequencing revealed two differences between M7 and its OG1X parent. First, OG1X contains a frameshift mutation that inactivates the etaR response regulator gene, while M7 is a wild-type revertant for etaR. Second, the M7 mutant contains a missense mutation in the rpoC gene, which encodes the β' subunit of RNA polymerase. Mutagenesis experiments revealed that the rpoC mutation was primarily responsible for the resistance phenotype. Microarray analysis revealed that a number of transporters were induced in OG1X when Fst was overexpressed. These transporters were not induced in M7 in response to Fst, and further experiments indicated that this had a direct protective effect on the mutant cells. Therefore, exposure of cells to Fst appears to have a cascading effect, first causing membrane stress and then potentiation of these effects by overexpression of certain transporters.

The lysozyme-induced peptidoglycan N-acetylglucosamine deacetylase PgdA (EF1843) is required for Enterococcus faecalis virulence

Lysozyme is a key component of the innate immune response in humans that provides a first line of defense against microbes. The bactericidal effect of lysozyme relies both on the cell wall lytic activity of this enzyme and on a cationic antimicrobial peptide activity that leads to membrane permeabilization. Among Gram-positive bacteria, the opportunistic pathogen Enterococcus faecalis has been shown to be extremely resistant to lysozyme. This unusual resistance is explained partly by peptidoglycan O-acetylation, which inhibits the enzymatic activity of lysozyme, and partly by d-alanylation of teichoic acids, which is likely to inhibit binding of lysozyme to the bacterial cell wall. Surprisingly, combined mutations abolishing both peptidoglycan O-acetylation and teichoic acid alanylation are not sufficient to confer lysozyme susceptibility. In this work, we identify another mechanism involved in E. faecalis lysozyme resistance. We show that exposure to lysozyme triggers the expression of EF1843, a protein that is not detected under normal growth conditions. Analysis of peptidoglycan structure from strains with EF1843 loss- and gain-of-function mutations, together with in vitro assays using recombinant protein, showed that EF1843 is a peptidoglycan N-acetylglucosamine deacetylase. EF1843-mediated peptidoglycan deacetylation was shown to contribute to lysozyme resistance by inhibiting both lysozyme enzymatic activity and, to a lesser extent, lysozyme cationic antimicrobial activity. Finally, EF1843 mutation was shown to reduce the ability of E. faecalis to cause lethality in the Galleria mellonella infection model. Taken together, our results reveal that peptidoglycan deacetylation is a component of the arsenal that enables E. faecalis to thrive inside mammalian hosts, as both a commensal and a pathogen.

The highly dynamic CRISPR1 system of Streptococcus agalactiae controls the diversity of its mobilome

Clustered regularly interspaced short palindromic repeats (CRISPR) confer immunity against mobile genetic elements (MGEs) in prokaryotes. Streptococcus agalactiae, a leading cause of neonatal infections contains in its genome two CRISPR/Cas systems. We show that type 1-C CRISPR2 is present in few strains but type 2-A CRISPR1 is ubiquitous. Comparative sequence analysis of the CRISPR1 spacer content of 351 S. agalactiae strains revealed that it is extremely diverse due to the acquisition of new spacers, spacer duplications and spacer deletions that witness the dynamics of this system. The spacer content profile mirrors the S. agalactiae population structure. Transfer of a conjugative transposon targeted by CRISPR1 selected for spacer rearrangements, suggesting that deletions and duplications pre-exist in the population. The comparison of protospacers located within MGE or the core genome and protospacer-associated motif-shuffling demonstrated that the GG motif is sufficient to discriminate self and non-self and for spacer selection and integration. Strikingly more than 40% of the 949 different CRISPR1 spacers identified target MGEs found in S. agalactiae genomes. We thus propose that the S. agalactiae type II-A CRISPR1/Cas system modulates the cohabitation of the species with its mobilome, as such contributing to the diversity of MGEs in the population.

First report of the multidrug resistance gene cfr in Enterococcus faecalis of animal origin

The multiresistance gene cfr was identified for the first time in an Enterococcus faecalis isolate of animal origin. The 32,388-bp plasmid pEF-01, which carried the cfr gene, was sequenced completely. Three copies of the insertion sequence IS1216 were identified in pEF-01, and the detection of a cfr- and IS1216-containing amplicon by inverse PCR suggests that IS1216 may play a role in the dissemination of cfr by a recombination process.

Deletion of σ(54) (rpoN) alters the rate of autolysis and biofilm formation in Enterococcus faecalis

Transcription initiation is a critical step in bacterial gene regulation and is often controlled by transcription regulators. The alternate sigma factor (σ(54)) is one such regulator that facilitates activator-dependent transcription initiation and thus modulates the expression of a variety of genes involved in metabolism and pathogenesis in bacteria. This study describes the role of σ(54) in the nosocomial pathogen Enterococcus faecalis. Biofilm formation is one of the important pathogenic mechanisms of E. faecalis, as it elevates the organism's potential to cause surgical site and urinary tract infections. Lysis of bacterial cells within the population contributes to biofilm formation by providing extracellular DNA (eDNA) as a key component of the biofilm matrix. Deletion of rpoN rendered E. faecalis resistant to autolysis, which in turn impaired eDNA release. Despite the significant reduction in eDNA levels compared to the parental strain, the rpoN mutant formed more robust biofilms as observed using laser scanning confocal microscopy and Comstat analysis, indicating and emphasizing the presence of other matrix components. Initial adherence to a polystyrene surface was also enhanced in the mutant. Proteinase K treatment at early stages of biofilm development significantly reduced the accumulation of biofilm by the rpoN mutant. In conclusion, our data indicate that other factors in addition to eDNA might contribute to the overall composition of the enterococcal biofilm and that the regulatory role of σ(54) governs the nature and composition of the biofilm matrix.

Persistent, toxin-antitoxin system-independent, tetracycline resistance-encoding plasmid from a dairy Enterococcus faecium isolate

A tetracycline-resistant (Tet(r)) dairy Enterococcus faecium isolate designated M7M2 was found to carry both tet(M) and tet(L) genes on a 19.6-kb plasmid. After consecutive transfer in the absence of tetracycline, the resistance-encoding plasmid persisted in 99% of the progenies. DNA sequence analysis revealed that the 19.6-kb plasmid contained 28 open reading frames (ORFs), including a tet(M)-tet(L)-mob gene cluster, as well as a 10.6-kb backbone highly homologous (99.9%) to the reported plasmid pRE25, but without an identified toxin-antitoxin (TA) plasmid stabilization system. The derived backbone plasmid without the Tet(r) determinants exhibited a 100% retention rate in the presence of acridine orange, suggesting the presence of a TA-independent plasmid stabilization mechanism, with its impact on the persistence of a broad spectrum of resistance-encoding traits still to be elucidated. The tet(M)-tet(L) gene cluster from M7M2 was functional and transmissible and led to acquired resistance in Enterococcus faecalis OG1RF by electroporation and in Streptococcus mutans UA159 by natural transformation. Southern hybridization showed that both the tet(M) and tet(L) genes were integrated into the chromosome of S. mutans UA159, while the whole plasmid was transferred to and retained in E. faecalis OG1RF. Quantitative real-time reverse transcription-PCR (RT-PCR) indicated tetracycline-induced transcription of both the tet(M) and tet(L) genes of pM7M2. The results indicated that multiple mechanisms might have contributed to the persistence of antibiotic resistance-encoding genes and that the plasmids pM7M2, pIP816, and pRE25 are likely correlated evolutionarily.

Transcription antitermination by a phosphorylated response regulator and cobalamin-dependent termination at a B₁₂ riboswitch contribute to ethanolamine utilization in Enterococcus faecalis

The ability of bacteria to utilize ethanolamine (EA) as a carbon and nitrogen source may confer an advantage for survival, colonization, and pathogenicity in the human intestinal tract. Enterococcus faecalis, a Gram-positive human commensal organism, depends on a two-component signaling system (TCS-17) for sensing EA and regulating the expression of the ethanolamine utilization genes. Multiple promoters participate in eut gene expression in the presence of EA as the sole carbon source and cobalamin (CoB12), an essential cofactor in the enzymatic degradation process. By means of in vivo and in vitro approaches, this study characterized the transcriptional activity identified in the eutT-eutG intergenic region of the E. faecalis eut cluster. Two novel promoters in this region were shown to be active in vivo. The distal P2-1 promoter was associated with a B12 riboswitch that terminated transcription in the presence of CoB12. Transcription elongation from the proximal P2-2 promoter was regulated by antitermination mediated by the phosphorylated form of the response regulator of TCS-17 (RR17). 3'-Rapid amplification of cDNA ends (RACE) analyses of the terminated RNA products allowed precise identification of the hairpin loop structures involved in termination/antitermination. The results uncovered the role of the B12 riboswitch and RR17 in eut gene expression, adding to the complexity of this regulatory pathway and extending the knowledge of possible means of transcription regulation in Gram-positive organisms.

Enterococcus faecalis virulence regulator FsrA binding to target promoters

The FsrABDC signal transduction system is a major virulence regulator in Enterococcus faecalis. The FsrC sensor histidine kinase, upon activation by the gelatinase biosynthesis-activating pheromone (GBAP) peptide encoded by the fsrBD genes, phosphorylates the FsrA response regulator required for the transcription of the fsrBDC and the gelE-sprE genes from the fsrB promoter and the gelE promoter, respectively. FsrA belongs to the LytTR family of proteins, which includes other virulence regulators, such as AgrA of Staphylococcus aureus, AlgR of Pseudomonas aeruginosa, and VirR of Clostridium perfringens. The LytTR DNA-binding domain that characterizes these proteins generally binds to two imperfect direct repeats separated by a number of bases that place the repeats on the same face of the DNA helix. In this study, we demonstrated that FsrA also binds to two imperfect direct repeats separated by 13 bp, based on the consensus sequence of FsrA, T/AT/CAA/GGGAA/G, which is consistent with the binding characteristics of LytTR domains.

Plasmid Transfer into the Homoacetogen Acetobacterium woodii by Electroporation and Conjugation

Shuttle vectors (pMS3 and pMS4) which replicated in Escherichia coli and in gram-positive Acetobacterium woodii were constructed by ligating the replication origin of plasmid pAMbeta1 with the E. coli cloning vector pUC19 and the tetM gene of streptococcal transposon Tn916. Electrotransformation of A. woodii was achieved at frequencies of 4.5 x 10 transformants per mug of plasmid DNA. For conjugal plasmid transfer, the mobilizable shuttle vector pKV12 was constructed by cloning the tetM gene into pAT187. Mating of E. coli containing pKV12 with A. woodii resulted in transfer frequencies of 3 x 10 to 7 x 10 per donor or recipient.

Transformation of Kluyveromyces lactis by Electroporation

The physical and biological parameters involved in efficient transformation of Kluyveromyces lactis by electroporation have been analyzed. By using an optimum voltage and a constant volume of cell suspension in a cuvette, the efficiency of transformation increased with increases in cell numbers and plasmid concentration. However, the most important parameter was the time of the pulse. Changes of 1 ms decreased the efficiency of transformation more than 70 to 80%. Under our best conditions, between 10 and 10 transformants per mug of plasmid DNA could be obtained. Under certain conditions, the size of the plasmid also affected electroporation efficiency. In any case, we did not obtain integrative transformation with an autonomously replicating plasmid.

Capsular polysaccharide production in Enterococcus faecalis and contribution of CpsF to capsule serospecificity

Many bacterial species produce capsular polysaccharides that contribute to pathogenesis through evasion of the host innate immune system. The gram-positive pathogen Enterococcus faecalis was previously reported to produce one of four capsule serotypes (A, B, C, or D). Previous studies describing the four capsule serotypes of E. faecalis were based on immunodetection methods; however, the underlying genetics of capsule production did not fully support these findings. Previously, it was shown that capsule production for serotype C (Maekawa type 2) was dependent on the presence of nine open reading frames (cpsC to cpsK). Using a novel genetic system, we demonstrated that seven of the nine genes in the cps operon are essential for capsule production, indicating that serotypes A and B do not make a capsular polysaccharide. In support of this observation, we showed that serotype C and D capsule polysaccharides mask lipoteichoic acid from detection by agglutinating antibodies. Furthermore, we determined that the genetic basis for the difference in antigenicity between serotypes C and D is the presence of cpsF in serotype C strains. High-pH anion-exchange chromatography with pulsed amperometric detection analysis of serotype C and D capsules indicated that cpsF is responsible for glucosylation of serotype C capsular polysaccharide in E. faecalis.

A fratricidal mechanism is responsible for eDNA release and contributes to biofilm development of Enterococcus faecalis

Extracellular DNA (eDNA), a by-product of cell lysis, was recently established as a critical structural component of the Enterococcus faecalis biofilm matrix. Here, we describe fratricide as the governing principle behind gelatinase (GelE)-mediated cell death and eDNA release. GFP reporter assays confirmed that GBAP (gelatinase biosynthesis-activating pheromone) quorum non-responders (GelE-SprE-) were a minority subpopulation of prey cells susceptible to the targeted fratricidal action of the quorum responsive predatorial majority (GelE+SprE+). The killing action is dependent on GelE, and the GelE producer population is protected from self-destruction by the co-production of SprE as an immunity protein. Targeted gene inactivation and protein interaction studies demonstrate that extracellular proteases execute their characteristic effects following downstream interactions with the primary autolysin, AtlA. Finally, we address a mechanism by which GelE and SprE may modify the cell wall affinity of proteolytically processed AtlA resulting in either a pro- or anti-lytic outcome.

Tetracycline resistome of the organic pig gut

The occurrence of genes conferring resistance to tetracyclines in the organic pig gut was assessed through the metagenomic approach. Of 9,000 bacterial artificial chromosome clones analyzed, 10 were identified as carrying the known tet(C), tet(W), and tet(40) genes, as well as novel genes encoding resistance to the tetracyclines minocycline and doxycycline. The latter are different from the known tet genes and are homologous to genes encoding UDP-glucose 4-epimerases, with the domain structure characteristic for these enzymes. The majority of the resistance genes were associated with putative mobile genetic elements. The sequence of a novel 9.7-kb plasmid carrying tet(W) and tet(40) was also identified. Conserved flanking regions identified around the tet(W) and tet(40) genes in our metagenomic library may play a role in genetic exchange of these genes. This is the first report describing the occurrence of tet(40) outside the human intestine. The maintenance of antibiotic resistance genes in apparently antibiotic-free animals is probably due to their presence on mobile genetic elements, the fitness cost of which for the cell is ameliorated during the previous antibiotic selection.

Can pharmacokinetic-pharmacodynamic parameters provide dosing regimens that are less vulnerable to resistance?

Dissemination of antibiotic resistance in bacteria is associated with prescription of the corresponding drugs. Various pharmacokinetic-pharmacodynamic parameters have been developed with the intention of reducing the spread of resistance. In this review, it is considered whether dosing regimens based on these parameters can delay this spread. The evolution of bacterial resistance to antibiotics involves two successive but distinct and independent mechanisms. The first occurs by mutation in the genome, including the host chromosome and mobile accessory genetic elements such as plasmids or transposons, or, following acquisition of a resistance determinant from another bacterium, by horizontal gene transfer. These two genetic events happen by chance, which means that they do not rely on the presence of an antibiotic in the environment; that is, they are not induced, but simply revealed and propagated by the drugs. The second step is dissemination of resistance which can be due to the spread of bacteria (clonal epidemics), of replicons (plasmid epidemics) or of resistance determinants (gene epidemics). Resistance dissemination by each one of these three levels which superimpose in nature, is not only infectious but also exponential, since all three are associated with DNA replication (duplication) of the host chromosome, of a plasmid, or of a transposon. As opposed to emergence, dissemination is clearly associated with the selective pressure exerted by antibiotic prescription [1,2]. The consequence of this dual evolutionary pathway is that proper use of antibiotics will, at best, delay the spread of resistance. In this review, the pharmacokinetic-pharmacodynamic (PK-PD) parameters that are intended to lower resistance dissemination are considered exclusively.

Phage production and maintenance of stocks, including expected stock lifetimes

In microbiology, preservation of an archival stock or a "master stock" of a given microorganism is essential for many reasons including scientific research, conservation of the genetic resources and providing the foundation for several biotechnological processes. The objective is to preserve the initial characteristics of the microorganism and to avoid the genetic drift that occurs when the organism is maintained indefinitely in an actively growing state. The same holds true in phage biology and it is of particular interest when a collection of phages is to be maintained. The aim of this chapter is to provide phage biologists with general procedures to prepare and maintain bacteriophage stocks on a long-term basis. The protocols described below should be considered as general guidelines because although many phages and bacterial strains can be propagated and stored in these conditions, specific media and/or growth and storage conditions must be evaluated for each phage and bacterium. Since it was not the scope of this chapter to provide an exhaustive list of these particular conditions, we instead highlighted the main factors affecting phage amplification and storage. We hope this will help phage biologists to develop their own strategies for their preferred phages.

Ethanolamine activates a sensor histidine kinase regulating its utilization in Enterococcus faecalis

Enterococcus faecalis is a gram-positive commensal bacterium of the human intestinal tract. Its opportunistic pathogenicity has been enhanced by the acquisition of multiple antibiotic resistances, making the treatment of enterococcal infections an increasingly difficult problem. The extraordinary capacity of this organism to colonize and survive in a wide variety of ecological niches is attributable, at least in part, to signal transduction pathways mediated by two-component systems (TCS). Here, the ability of E. faecalis to utilize ethanolamine as the sole carbon source is shown to be dependent upon the RR-HK17 (EF1633-EF1632) TCS. Ethanolamine is an abundant compound in the human intestine, and thus, the ability of bacteria to utilize it as a source of carbon and nitrogen may provide an advantage for survival and colonization. Growth of E. faecalis in a synthetic medium with ethanolamine was abolished in the response regulator RR17 mutant strain. Transcription of the response regulator gene was induced by the presence of ethanolamine. Ethanolamine induced a 15-fold increase in the rate of autophosphorylation in vitro of the HK17 sensor histidine kinase, indicating that this is the ligand recognized by the sensor domain of the kinase. These results assign a role to the RR-HK17 TCS as coordinator of the enterococcal response to specific nutritional conditions existing at the site of bacterial invasion, the intestinal tract of an animal host.

Regulation of autolysis-dependent extracellular DNA release by Enterococcus faecalis extracellular proteases influences biofilm development

Enterococci are major contributors of hospital-acquired infections and have emerged as important reservoirs for the dissemination of antibiotic resistance traits. The ability to form biofilms on medical devices is an important aspect of pathogenesis in the hospital environment. The Enterococcus faecalis Fsr quorum system has been shown to regulate biofilm formation through the production of gelatinase, but the mechanism has been hitherto unknown. Here we show that both gelatinase (GelE) and serine protease (SprE) contribute to biofilm formation by E. faecalis and provide clues to how the activity of these proteases governs this developmental process. Confocal imaging of biofilms suggested that GelE(-) mutants were significantly reduced in biofilm biomass compared to the parental strain, whereas the absence of SprE appeared to accelerate the progression of biofilm development. The phenotype observed in a SprE(-) mutant was linked to an observed increase in autolytic rate compared to the parental strain. Culture supernatant analysis and confocal microscopy confirmed the inability of mutants deficient in GelE to release extracellular DNA (eDNA) in planktonic and biofilm cultures, whereas cells deficient in SprE produced significantly more eDNA as a component of the biofilm matrix. DNase I treatment of E. faecalis biofilms reduced the accumulation of biofilm, implying a critical role for eDNA in biofilm development. In conclusion, our data suggest that the interplay of two secreted and coregulated proteases--GelE and SprE--is responsible for regulating autolysis and the release of high-molecular-weight eDNA, a critical component for the development of E. faecalis biofilms.

Full activation of Enterococcus faecalis gelatinase by a C-terminal proteolytic cleavage

Enterococci account for nearly 10% of all nosocomial infections and constitute a significant treatment challenge due to their multidrug resistance properties. One of the well-studied virulence factors of Enterococcus faecalis is a secreted bacterial protease, termed gelatinase, which has been shown to contribute to the process of biofilm formation. Gelatinase belongs to the M4 family of bacterial zinc metalloendopeptidases, typified by thermolysin. Gelatinase is synthesized as a preproenzyme consisting of a signal sequence, a putative propeptide, and then the mature enzyme. We determined that the molecular mass of the mature protein isolated from culture supernatant was 33,030 Da, which differed from the predicted molecular mass, 34,570 Da, by over 1,500 Da. Using N-terminal sequencing, we confirmed that the mature protein begins at the previously identified sequence VGSEV, thus suggesting that the 1,500-Da molecular mass difference resulted from a C-terminal processing event. By using mutants with site-directed mutations within a predicted C-terminal processing site and mutants with C-terminal deletions fused to a hexahistidine tag, we determined that the processing site is likely to be between residues D304 and I305 and that it requires the Q306 residue. The results suggest that the E. faecalis gelatinase requires C-terminal processing for full activation of protease activity, making it a unique enzyme among the members of the M4 family of proteases of gram-positive bacteria.

The Group B Streptococcus NADH oxidase Nox-2 is involved in fatty acid biosynthesis during aerobic growth and contributes to virulence

Numerous Streptococcaceae produce an H2O-forming NADH oxidase, Nox-2, which has been generally implicated in aerobic survival. We examined the roles of Nox-2 in Group B Streptococcus (GBS), a leading agent of neonatal infections. While nox2 inactivation caused an aerobic growth arrest, no improvement was seen by addition of antioxidants to cultures, suggesting that this defect was not due to accumulation of toxic oxygen species. Using several approaches, we show that the observed inability of the nox2 mutant to grow aerobically is mainly due to an underlying defect in fatty acid (FA) biosynthesis: (i) the nox2 aerobic growth defect is fully and rapidly complemented by adding oleic acid to culture medium, and (ii) direct assimilation of this unsaturated FA in both wild type (WT) and nox2 GBS membranes is demonstrated and correlated with mutant growth rescue. We propose that NAD+ depletion in the nox2 mutant results in reduced acetyl-CoA production, which perturbs FA biosynthesis and hence blocks growth in aerobiosis. The nox2 aerobic growth defect was also complemented when GBS respiration metabolism was activated by exogenous haem and menaquinone. The membrane NADH oxidase activity generated by the functional respiratory chain thus compensates the cytoplasmic NADH oxidase deficiency. The nox2 mutant was attenuated for virulence, as assessed in lung, intraperitoneal and intravenous murine infection models. As the nox2 defect seems only to affect aerobic growth of GBS, its reduced virulence supports the suggestion that aerobic conditions and NADH oxidase activities are relevant to the GBS infection process.

Revised model for Enterococcus faecalis fsr quorum-sensing system: the small open reading frame fsrD encodes the gelatinase biosynthesis-activating pheromone propeptide corresponding to staphylococcal agrd

Gelatinase biosynthesis-activating pheromone (GBAP) is an autoinducing peptide involved in Enterococcus faecalis fsr quorum sensing, and its 11-amino-acid sequence has been identified in the C-terminal region of the 242-residue deduced fsrB product (J. Nakayama et al., Mol. Microbiol. 41:145-154, 2001). In this study, however, we demonstrated the existence of fsrD, encoding the GBAP propeptide, which is in frame with fsrB but is translated independently of fsrB. It was also demonstrated that FsrB', an FsrD segment-truncated FsrB, functions as a cysteine protease-like processing enzyme to generate GBAP from FsrD. This revised model is consistent with the staphylococcal agr system.

The recombination deficient Enterococcus faecalis UV202 strain is a recA mutant

The recA gene of the recombination deficient Enterococcus faecalis strain UV202 was sequenced and found to encode a glycine to aspartic acid mutation at amino acid 265. Both the UV sensitive and recombination deficient phenotypes of the UV202 strain were complemented by expression of the wild-type recA gene cloned under the control of the nisin-inducible promoter of an expression vector.

Respiration metabolism of Group B Streptococcus is activated by environmental haem and quinone and contributes to virulence

Group B Streptococcus (GBS) is a common constituent of the vaginal microflora, but its transmission to newborns can cause life-threatening sepsis, pneumonia and meningitis. Energy metabolism of this opportunist pathogen has been deduced to be strictly fermentative. We discovered that GBS undergoes respiration metabolism if its environment supplies two essential respiratory components: quinone and haem. Respiration metabolism led to significant changes in growth characteristics, including a doubling of biomass and an altered metabolite profile under the tested conditions. The GBS respiratory chain is inactivated by: (i) withdrawing haem and/or quinone, (ii) treating cultures with a respiration inhibitor or (iii) inactivating the cydA gene product, a subunit of cytochrome bd quinol oxidase, in all cases resulting in exclusively fermentative growth. cydA inactivation reduced GBS growth in human blood and strongly attenuated virulence in a neonatal rat sepsis model, suggesting that the animal host may supply the components that activate GBS respiration. These results suggest a role of respiration metabolism in GBS dissemination. Our findings show that environmental factors can increase the flexibility of GBS metabolism by activating a newly identified respiration chain. The need for two environmental factors may explain why GBS respiration metabolism was not found in previous studies.