Comparative in vitro activities of AC98-6446, a novel semisynthetic glycopeptide derivative of the natural product mannopeptimycin alpha, and other antimicrobial agents against gram-positive clinical isolates

Structural and Biochemical Characterization of MppQ, an L-Enduracididine Biosynthetic Enzyme from Streptomyces hygroscopicus

MppQ is an enzyme of unknown function from Streptomyces hygroscopicus (ShMppQ) that operates in the biosynthesis of the nonproteinogenic amino acid L-enduracididine (L-End). Since L-End is a component of several peptides showing activity against antibiotic-resistant pathogens, understanding its biosynthetic pathway could facilitate the development of chemoenzymatic routes to novel antibiotics. Herein, we report on the crystal structures of ShMppQ complexed with pyridoxal-5'-phosphate (PLP) and pyridoxamine-5'-phosphate (PMP). ShMppQ is similar to fold-type I PLP-dependent aminotransferases like aspartate aminotransferase. The tertiary structure of ShMppQ is composed of an N-terminal extension, a large domain, and a small domain. The active site is placed at the junction of the large and small domains and includes residues from both protomers of the homodimer. We also report the first functional characterization of MppQ, which we incubated with the enzymatically produced 2-ketoenduracidine and observed the conversion to L-End, establishing ShMppQ as the final enzyme in L-End biosynthesis. Additionally, we have observed that MppQ has a relatively high affinity for 2-keto-5-guanidinovaleric acid (i.e., 2-ketoarginine), a shunt product of MppP, indicating the potential role of MppQ in increasing the efficiency of L-End biosynthesis by converting 2-ketoarginine back to the starting material, l-arginine. A panel of potential amino-donor substrates was tested for the transamination activity against a saturating concentration of 2-ketoarginine in end-point assays. Most l-Arg was produced with l-ornithine as the donor substrate. Steady-state kinetic analysis of the transamination reaction with l-Orn and 2-ketoarginine shows that the kinetic constants are in line with those for the amino donor substrate of other fold-type I aminotransferases.

Synthesis, Biocidal and Antibiofilm Activities of New Isatin-Quinoline Conjugates against Multidrug-Resistant Bacterial Pathogens along with Their In Silico Screening

Isatin-quinoline conjugates 10a-f and 11a-f were assembled by the reaction of N-(bromobutyl) isatin derivatives 3a, b with aminoquinolines 6a-c and their corresponding hydrazinyl 9a-c in good yields. The structures of the resulting conjugates were established by spectroscopic tools and showed data consistent with the proposed structures. In vitro antibacterial activity against different bacterial strains was evaluated. All tested conjugates showed significant biocidal activity with lower MIC than the first line drugs chloramphenicol and ampicillin. Conjugates 10a, 10b and 10f displayed the most potent activity against all clinical isolates. The antibiofilm activity for all tested conjugates was screened against the reference drug vancomycin using the MRSA strain. The results revealed that all conjugates had an inhibitory activity against biofilm formation and conjugate. Conjugate 11a showed 83.60% inhibition at 10 mg/mL. In addition, TEM studies were used to prove the mechanism of antibacterial action of conjugates 10a and 11a against (MRSA). Modeling procedures were performed on 10a-f and 11a-f and interestingly the results were nearly consistent with the biological activities. In addition, in silico pharmacokinetic evaluation was performed and revealed that the synthesized compounds 10a-f and 11a-f were considered drug-like molecules with promising bioavailability and high GI absorption. The results confirmed that the title compounds caused the disruption of bacterial cell membranes and could be used as potential leads for the further development and optimization of antibacterial agents.

Multi-omic characterisation of Streptomyces hygroscopicus NRRL 30439: detailed assessment of its secondary metabolic potential

The emergence of multidrug-resistant pathogenic bacteria creates a demand for novel antibiotics with distinct mechanisms of action. Advances in next-generation genome sequencing promised a paradigm shift in the quest to find new bioactive secondary metabolites. Genome mining has proven successful for predicting putative biosynthetic elements in secondary metabolite superproducers such as Streptomycetes. However, genome mining approaches do not inform whether biosynthetic gene clusters are dormant or active under given culture conditions. Here we show that using a multi-omics approach in combination with antiSMASH, it is possible to assess the secondary metabolic potential of a Streptomyces strain capable of producing mannopeptimycin, an important cyclic peptide effective against Gram-positive infections. The genome of Streptomyces hygroscopicus NRRL 30439 was first sequenced using PacBio RSII to obtain a closed genome. A chemically defined medium was then used to elicit a nutrient stress response in S. hygroscopicus NRRL 30439. Detailed extracellular metabolomics and intracellular proteomics were used to profile and segregate primary and secondary metabolism. Our results demonstrate that the combination of genomics, proteomics and metabolomics enables rapid evaluation of a strain's performance in bioreactors for industrial production of secondary metabolites.

Total Synthesis of Mannopeptimycin β via β-Hydroxyenduracididine Ligation

Nonribosomal peptide synthesis in bacteria has endowed cyclic peptides with fascinating structural complexity via incorporating nonproteinogenic amino acids. These bioactive cyclic peptides provide interesting structural motifs for exploring total synthesis and medicinal chemistry studies. Cyclic glycopeptide mannopeptimycins exhibit antibacterial activity against antibiotic-resistant Gram-positive pathogens and act as the lipid II binder to stop bacterial cell wall biosynthesis. Here, we report a strategy streamlining solution phase-solid phase synthesis and chemical ligation-mediated peptide cyclization for the total synthesis of mannopeptimycin β.

Cyclofaulknamycin with the Rare Amino Acid D-capreomycidine Isolated from a Well-Characterized Streptomyces albus Strain

Targeted genome mining is an efficient method of biosynthetic gene cluster prioritization within constantly growing genome databases. Using two capreomycidine biosynthesis genes, alpha-ketoglutarate-dependent arginine beta-hydroxylase and pyridoxal-phosphate-dependent aminotransferase, we identified two types of clusters: one type containing both genes involved in the biosynthesis of the abovementioned moiety, and other clusters including only arginine hydroxylase. Detailed analysis of one of the clusters, the flk cluster from Streptomyces albus, led to the identification of a cyclic peptide that contains a rare D-capreomycidine moiety for the first time. The absence of the pyridoxal-phosphate-dependent aminotransferase gene in the flk cluster is compensated by the XNR_1347 gene in the S. albus genome, whose product is responsible for biosynthesis of the abovementioned nonproteinogenic amino acid. Herein, we report the structure of cyclofaulknamycin and the characteristics of its biosynthetic gene cluster, biosynthesis and bioactivity profile.

Antibacterial activities of the extracts, fractions and isolated compounds from Canarium patentinervium Miq. against bacterial clinical isolates

Background

Canarium patentinervium leaves are used by the local indigenous people of Malaysia for wound healing. The current study is undertaken to screen the comprehensive antibacterial activity of the leaves and barks extracts, fractions and isolated compounds from this plant. Bioassay guided fractionation was also undertaken to deeply evaluate the antibacterial activity of the water fraction of the leaves extract. This is to provide preliminary scientific evidence to the ethnopharmacology usage of this plant by investigating antibacterial properties of the plant and its isolated constituents.

Methods

Bio-assay guided fractionation and subsequent isolation of compounds using open column chromatography. The antibacterial activity against gram positive and gram negative ATCC strain and resistant clinical strains were evaluated using microtiter broth dilution method to determine minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and time-kill assay. The chemical structure of the isolated compounds from the water fraction of the ethanol extract of leaves was elucidated using Nuclear Magnetic Resonance (NMR).

Results

The ethanol extract of the leaves and barks showed antimicrobial activity against all four ATCC and eight clinical isolates. The ethanol extract of the leaves and the corresponding water fraction had good activity against MRSA S. aureus. (MIC: 250 μg/ml) and had bactericidal effect on eight of the clinical strains (MSSA,MRSA, oxacillin-resistant CONS, oxacillin-sensitive CONS, Enterococcus faecalis, Klebsiela species, Kleb pneumoniae ESBL and Candida parapsilosis). Further phytochemical investigation of the water fraction of the crude ethanol extract of leaves afforded compound 7 (hyperin) and compound 8 (cynaroside) that had bactericidal activity against tested bacterial species (MIC 50 μg/ml and 100 μg/ml). The two compounds were isolated from this genus for the first time.

Conclusions

These results may provide a rational support for the traditional use of Canarium patentinervium Miq. in infections and wound healing, since the antimicrobial compounds isolated were also present in the leaves extract.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Molecular identification and antibacterial properties of an ericoid associated mycorrhizal fungus

Background

The quest for novel sources of antibacterial compounds have necessitated the inclusion of ericoid mycorrhizal fungi (ERM) commonly found within the root of ericaceous plants. Agar-well diffusion method was used to detect antibacterial activity and was followed by the microbroth diffusion method [minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC)].

Results

The results of the phytochemical screening indicated that only alkaloids, flavonoids, phenols, saponins, cardiac glycosides and terpenoids were present, while steroids and tannins were absent. The MIC of the extracts ranged between 2 and 16 mg/mL, and the lowest MIC was obtained with Staphylococcus aureus. Also, the result of the MBC study indicated that the fungal extract was most active at concentrations of 2 and 4 mg/mL against Bacillus subtilis and S. aureus, respectively.

Conclusions

This bioassay showed, for the first time, antibacterial activity of L. incrustata against some bacterial species. Subsequently, ERM fungi should be given attention when searching for antimicrobial agents because they could provide a solution to solve problems associated with conventional disease treatments (i.e. pathogenic microorganisms resistance).

Novel vancomycin-peptide conjugate as potent antibacterial agent against vancomycin-resistant Staphylococcus aureus

Background

Increase in vancomycin (Van)-resistant bacterial strains including vancomycin-resistant Staphylococcus aureus (VRSA) and lack of new effective antibiotics have become a formidable health problem.

Materials and methods

We designed a new conjugate composed of Van and a peptide Hecate (Hec; Van/Hec), and its potential antimicrobial activity was evaluated.

Results

Results from disk diffusion test, time-kill assay, determination of minimum inhibitory concentration (MIC), microscopy, and comet assay showed strong antimicrobial effects of Van/Hec against wild-type, methicillin-resistant Staphylococcus aureus (MRSA) and VRSA. Microscopy revealed that the exposure to Van/Hec results in disruption of bacterial cell integrity in all tested strains, which was not observed in case of Van or Hec alone.

Conclusion

Overall, we showed that the preparation of conjugates from antibiotics and biologically active peptides could help us to overcome the limitation of the use of antibiotic in the treatment of infections caused by multidrug-resistant bacteria.

Antibacterial Effect of Copaifera duckei Dwyer Oleoresin and Its Main Diterpenes against Oral Pathogens and Their Cytotoxic Effect

This study evaluates the antibacterial activity of the Copaifera duckei Dwyer oleoresin and two isolated compounds [eperu-8(20)-15,18-dioic acid and polyalthic acid] against bacteria involved in primary endodontic infections and dental caries and assesses the cytotoxic effect of these substances against a normal cell line. MIC and MBC assays pointed out the most promising metabolites for further studies on bactericidal kinetics, antibiofilm activity, and synergistic antibacterial action. The oleoresin and polyalthic acid but not eperu-8(20)-15,18-dioic provided encouraging MIC and MBC results at concentrations lower than 100 μg mL⁻¹. The oleoresin and polyalthic acid activities depended on the evaluated strain. A bactericidal effect on Lactobacillus casei (ATCC 11578 and clinical isolate) emerged before 8 h of incubation. For all the tested bacteria, the oleoresin and polyalthic acid inhibited biofilm formation by at least 50%. The oleoresin and polyalthic acid gave the best activity against Actinomyces naeslundii (ATCC 19039) and L. casei (ATCC 11578), respectively. The synergistic assays combining the oleoresin or polyalthic acid with chlorhexidine did not afford interesting results. We examined the cytotoxicity of C. duckei oleoresin, eperu-8(20)-15,18-dioic acid, and polyalthic acid against Chinese hamster lung fibroblasts. The oleoresin and polyalthic acid were cytotoxic at concentrations above 78.1 μg mL⁻¹, whereas eperu-8(20)-15,18-dioic displayed cytotoxicity at concentrations above 312.5 μg mL⁻¹. In conclusion, the oleoresin and polyalthic acid are potential sources of antibacterial agents against bacteria involved in primary endodontic infections and dental caries in both the sessile and the planktonic modes at concentrations that do not cause cytotoxicity.

Diversity, Phylogeny, anticancer and antimicrobial potential of fungal endophytes associated with Monarda citriodora L

BACKGROUND

Present study focuses on diversity and distribution analysis of endophytic fungi associated with different tissues of the Monarda citriodora Cerv. ex Lag. (Lamiaceae/Labiatae). Anticancer and antimicrobial potential of isolated endophytes have also been investigated.

RESULTS

A total of twenty eight fungal endophytes belonging to 11 different genera were isolated from this plant. All the endophytic fungi belonged to the Ascomycota phylum. The leaves were immensely rich in fungal species, while roots showed the highest tissue specific fungal dominance. Out of 28 fungal species, 72% endophytic extracts were found cytotoxic against one or more human cancer cell lines. The most prominent anticancer activity (IC₅₀ value <10 μg/mL) was shown by MC-14 L (Fusarium oxysporum), MC-14 F (F. oxysporum), MC-18 L (Aspergillus fumigatus), MC-24 L (Cladosporium tenuissimum), MC-25 L (Fusarium sp.), MC-26 F (F. oxysporum) extracts. 75% of the extracts showed antimicrobial activities in agar disc-diffusion assay and 27% in the tube dilution method (MIC <100 μg/mL) respectively against the tested pathogens. Extracts of MC-14 L (F. oxysporum) and MC-18 L (A. fumigatus) displayed broad spectrum antimicrobial activity.

CONCLUSIONS

These results indicated that M. citriodora harbors a rich fungal endophytic community with anticancer and antimicrobial activities. The isolated endophyte MC-24 L (C. tenuissimum) has the potential to be a source of novel cytotoxic/antimicrobial compounds. This is the first report of diversity of fungal endophytes isolated from M. citriodora.

Evaluation of bioactive secondary metabolites from endophytic fungus Pestalotiopsis neglecta BAB-5510 isolated from leaves of Cupressus torulosa D.Don

Six endophytic fungi were isolated from Cupressus torulosa D.Don and identified phenotypically and genotypically. The fungal cultures were further grown and the culture was extracted by two organic solvents methanol and ethyl acetate. The screening was carried out using the agar well diffusion method against human pathogen such as Escherichia coli, Salmonella typhimurium, Bacillus subtilis and Staphylococcus aureus. Isolated strain of Pestalotiopsis sp. was showing prominent antibacterial activity. The crude methanol and ethyl acetate extract of Pestalotiopsis sp. showed MIC of 6.25 mg/mL for S. typhimurium and S. aureus which showed its efficacy as a potent antimicrobial. The phytochemical screening revealed the existence of a diverse group of secondary metabolites in the crude extracts of the endophytic fungi that resembled those in the host plant extracts. On the basis of phenotypic characteristics and rDNA sequencing of the ITS region of the endophyte was identified as P. neglecta which turned out to be a promising source of bioactive compounds. There is little known about endophytes from C. torulosa D.Don. In this paper we studied in detail the identification of isolated endophytic fungi P. neglecta from C. torulosa D.Don and characterization of its active metabolite compounds. The partially purified second fraction (PPF) extracted from the fungal culture supernatant was subjected to gas chromatography followed by mass spectrometry which revealed the presence of many phytochemicals. These results indicate that endophytic fungi P. neglecta isolated from medicinal plants could be a potential source for bioactive compounds and may find potential use in pharmaceutical industry.

Enduracididine, a rare amino acid component of peptide antibiotics: Natural products and synthesis

Rising resistance to current clinical antibacterial agents is an imminent threat to global public health and highlights the demand for new lead compounds for drug discovery. One such potential lead compound, the peptide antibiotic teixobactin, was recently isolated from an uncultured bacterial source, and demonstrates remarkably high potency against a wide range of resistant pathogens without apparent development of resistance. A rare amino acid residue component of teixobactin, enduracididine, is only known to occur in a small number of natural products that also possess promising antibiotic activity. This review highlights the presence of enduracididine in natural products, its biosynthesis together with a review of analogues of enduracididine. Reported synthetic approaches to the cyclic guanidine structure of enduracididine are discussed, illustrating the challenges encountered to date in the development of efficient synthetic routes to facilitate drug discovery efforts inspired by the discovery of teixobactin.

Assessment of the antibacterial, cytotoxic and mutagenic potential of the phenolic-rich hydroalcoholic extract from Copaifera trapezifolia Hayne leaves

Copaifera trapezifolia Hayne occurs in the Atlantic Rainforest, which is considered one of the most important and endangered tropical forests on the planet. Although literature works have described many Copaifera spp., their biological activities remain little known. In the present study, we aimed to evaluate (1) the potential of the hydroalcoholic extract from C. trapezifolia leaves (CTE) to act against the causative agents of tooth decay and apical periodontitis and (2) the cytotoxicity and mutagenicity of CTE to ensure that it is safe for subsequent application. Concerning the tested bacteria, the MIC and the minimum bactericidal concentration of CTE varied between 100 and 400 µg ml-1. The time-kill assay conducted at a CTE concentration of 100 µg ml-1 evidenced bactericidal activity against Porphyromonas gingivalis (ATCC 33277) and Peptostreptococcus micros (clinical isolate) within 72 h. CTE at 200 µg ml-1 inhibited Porphyromonas gingivalis and Peptostreptococcus micros biofilm formation by at least 50 %. A combination of CTE with chlorhexidine dichlorohydrate did not prompt any synergistic effects. The colony-forming assay conducted on V79 cells showed that CTE was cytotoxic at concentrations above 156 µg ml-1. CTE exerted mutagenic effect on V79 cells, but the micronucleus test conducted on Swiss mice and the Ames test did not reveal any mutagenicity. Therefore, the use of standardized and safe extracts could be an important strategy to develop novel oral care products with antibacterial action. These extracts could also serve as a source of compounds for the discovery of new promising biomolecules.

New Found Hope for Antibiotic Discovery: Lipid II Inhibitors

Research into antibacterial agents has recently gathered pace in light of the disturbing crisis of antimicrobial resistance. The development of modern tools offers the opportunity of reviving the fallen era of antibacterial discovery through uncovering novel lead compounds that target vital bacterial cell components, such as lipid II. This paper provides a summary of the role of lipid II as well as an overview and insight into the structural features of macrocyclic peptides that inhibit this bacterial cell wall component. The recent discovery of teixobactin, a new class of lipid II inhibitor has generated substantial research interests. As such, the significant progress that has been achieved towards its development as a promising antibacterial agent is discussed.

Total Synthesis of Mannopeptimycins α and β

The mannopeptimycins are a class of glycopeptide natural products with unusual structures and potent antibiotic activity against a range of Gram-positive multidrug-resistant bacteria. Their cyclic hexapeptide core features a pair of unprecedented β-hydroxyenduracididines (L- and D-βhEnd), an O-glycosylated D-Tyr carrying an α-linked dimannose, and a β-methylated Phe residue. The D-βhEnd unit also carries an α-linked mannopyranose at the most hindered N of its cyclic guanidine ring. Herein, we report the first total synthesis of mannopeptimycin α and β with fully elaborated N- and O-linked sugars. Critically, a gold-catalyzed N-glycosylation of a D-βhEnd substrate with a mannosyl ortho-alkynylbenzoate donor enabled the synthesis of the most challenging N-Man-D-βhEnd unit with excellent efficiency and stereoselectivity. The L-βMePhe unit was prepared using a Pd-catalyzed C-H arylation method. The L-βhEnd, D-Tyr(di-Man), and L-βMePhe units were prepared in gram quantities. A convergent assembly of the cyclic peptide scaffold and a single global hydrogenolysis deprotection operation provided mannopeptimycin α and β.

The evolving role of chemical synthesis in antibacterial drug discovery

The discovery and implementation of antibiotics in the early twentieth century transformed human health and wellbeing. Chemical synthesis enabled the development of the first antibacterial substances, organoarsenicals and sulfa drugs, but these were soon outshone by a host of more powerful and vastly more complex antibiotics from nature: penicillin, streptomycin, tetracycline, and erythromycin, among others. These primary defences are now significantly less effective as an unavoidable consequence of rapid evolution of resistance within pathogenic bacteria, made worse by widespread misuse of antibiotics. For decades medicinal chemists replenished the arsenal of antibiotics by semisynthetic and to a lesser degree fully synthetic routes, but economic factors have led to a subsidence of this effort, which places society on the precipice of a disaster. We believe that the strategic application of modern chemical synthesis to antibacterial drug discovery must play a critical role if a crisis of global proportions is to be averted.

Alternative strategies for proof-of-principle studies of antibacterial agents

The proof that a new antibacterial agent is not only active in vitro but also effective in vivo under clinically relevant conditions is currently provided (i) by using appropriate nonclinical models of infection and pharmacokinetic-pharmacodynamic (PK-PD) analysis providing evidence of the likelihood of clinical efficacy and (ii) by examining the study drug in exploratory clinical trials, as well as dose and schedule finding during phase II of clinical development. This approach is both time-consuming and costly. Furthermore, PK-PD targets for any novel antibacterial agent cannot be derived from studies with experimental animals. Therefore, alternative strategies have to be identified to prove the principle that a novel antibacterial agent is active under clinically relevant conditions. This review summarizes evidence that the quantitative analysis of shifts in the viable counts of pathogens in infected patients or the evaluation of the PD effect of an investigational agent on indicator organisms of the human resident microflora or colonizers of healthy volunteers, if paralleled with PK monitoring of serum and the target site, provides an alternative to a classical proof-of-principle study in the course of a phase II study program.

Cytotoxic and antimicrobial activities of endophytic fungi isolated from Bacopa monnieri (L.) Pennell (Scrophulariaceae)

Background

Endophytes, which reside in plant tissues, have the potential to produce novel metabolites with immense benefits for health industry. Cytotoxic and antimicrobial activities of endophytic fungi isolated from Bacopa monnieri (L.) Pennell were investigated.

Methods

Endophytic fungi were isolated from the Bacopa monnieri. Extracts from liquid cultures were tested for cytotoxicity against a number of cancer cell lines using the MTT assay. Antimicrobial activity was determined using the micro dilution method.

Results

22% of the examined extracts showed potent (IC₅₀ of <20 μg/ml) cytotoxic activity against HCT-116 cell line. 5.5%, 11%, 11% of the extracts were found to be cytotoxic for MCF-7, PC-3, and A-549 cell lines respectively. 33% extracts displayed antimicrobial activity against at least one test organism with MIC value 10–100 μg/ml. The isolate B9_Pink showed the most potent cytotoxic activity for all the cell lines examined and maximum antimicrobial activity against the four pathogens examined which was followed by B19.

Conclusions

Results indicated the potential for production of bioactive agents from endophytes of Bacopa monnieri.

Structural and functional characterization of MppR, an enduracididine biosynthetic enzyme from streptomyces hygroscopicus: functional diversity in the acetoacetate decarboxylase-like superfamily

The nonproteinogenic amino acid enduracididine is a critical component of the mannopeptimycins, cyclic glycopeptide antibiotics with activity against drug-resistant pathogens, including methicillin-resistant Staphylococcus aureus. Enduracididine is produced in Streptomyces hygroscopicus by three enzymes, MppP, MppQ, and MppR. On the basis of primary sequence analysis, MppP and MppQ are pyridoxal 5'-phosphate-dependent aminotransferases; MppR shares a low, but significant, level of sequence identity with acetoacetate decarboxylase. The exact reactions catalyzed by each enzyme and the intermediates involved in the route to enduracididine are currently unknown. Herein, we present biochemical and structural characterization of MppR that demonstrates a catalytic activity for this enzyme and provides clues about its role in enduracididine biosynthesis. Bioinformatic analysis shows that MppR belongs to a previously uncharacterized family within the acetoacetate decarboxylase-like superfamily (ADCSF) and suggests that MppR-like enzymes may catalyze reactions diverging from the well-characterized, prototypical ADCSF decarboxylase activity. MppR shares a high degree of structural similarity with acetoacetate decarboxylase, though the respective quaternary structures differ markedly and structural differences in the active site explain the observed loss of decarboxylase activity. The crystal structure of MppR in the presence of a mixture of pyruvate and 4-imidazolecarboxaldehyde shows that MppR catalyzes the aldol condensation of these compounds and subsequent dehydration. Surprisingly, the structure of MppR in the presence of "4-hydroxy-2-ketoarginine" shows the correct 4R enantiomer of "2-ketoenduracididine" bound to the enzyme. These data, together with bioinformatic analysis of MppR homologues, identify a novel family within the acetoacetate decarboxylase-like superfamily with divergent active site structure and, consequently, biochemical function.

Antistaphylococcal activity of bacteriophage derived chimeric protein P128

Background

Bacterial drug resistance is one of the most significant challenges to human health today. In particular, effective antibacterial agents against methicillin-resistant Staphylococcus aureus (MRSA) are urgently needed. A causal relationship between nasal commensal S. aureus and infection has been reported. Accordingly, elimination of nasal S. aureus reduces the risk of infection. Enzymes that degrade bacterial cell walls show promise as antibacterial agents. Bacteriophage-encoded bacterial cell wall-degrading enzymes exhibit intrinsic bactericidal activity. P128 is a chimeric protein that combines the lethal activity of the phage tail-associated muralytic enzyme of Phage K and the staphylococcal cell wall targeting-domain (SH3b) of lysostaphin.

Here we report results of in vitro studies evaluating the susceptibility of staphylococcal strains to this novel protein.

Results

Using the broth microdilution method adapted for lysostaphin, we found that P128 is effective against S. aureus clinical strains including MRSA, methicillin-sensitive S. aureus (MSSA), and a mupirocin-resistant S. aureus. Minimum bactericidal concentrations and minimum inhibitory concentrations of P128 (1-64 μg/mL) were similar across the 32 S. aureus strains tested, demonstrating its bactericidal nature.

In time-kill assays, P128 reduced colony-forming units by 99.99% within 1 h and inhibited growth up to 24 h.

In an assay simulating topical application of P128 to skin or other biological surfaces, P128 hydrogel was efficacious when layered on cells seeded on solid media. P128 hydrogel was lethal to Staphylococci recovered from nares of healthy people and treated without any processing or culturing steps, indicating its in situ efficacy. This methodology used for in vitro assessment of P128 as an agent for eradicating nasal carriage is unique.

Conclusions

The novel chimeric protein P128 is a staphylococcal cell wall-degrading enzyme under development for clearance of S. aureus nasal colonization and MRSA infection. The protein is active against globally prevalent antibiotic-resistant clinical isolates and other clinically significant staphylococcal species including S. epidermidis. The P128 hydrogel formulation was bactericidal against Staphylococci including S. aureus recovered from the nares of 31 healthy people, demonstrating its in situ efficacy.

Synthetic studies toward the mannopeptimycins: synthesis of orthogonally protected beta-hydroxyenduracididines

The asymmetric synthesis of the nonproteinogenic amino acids (2S,3S,4'S)-beta-hydroxyenduracididine 3 and (2R,3S,4'S)-beta-hydroxyenduracididine 4 in orthogonally protected form in 15 total steps from Garner's aldehyde is reported. The former and N-glycosylated form of the latter are found in the glycopeptide antibiotic mannopeptimycin.

Biological activities of the fermentation extract of the endophytic fungus Alternaria alternata isolated from Coffea arabica L

A total of 22 endophytic fungi isolated from coffee (Coffea arabica L.) were cultivated in vitro and their crude extracts tested. The screening was carried out using the agar diffusion method against Staphylococcus aureus, Escherichia coli and Candida albicans. The most effective isolate was Alternaria alternata, and subsequently, its extract was assayed. The total phenolic content was 3.44 μg GAE/mg of the crude extract. For the antibacterial and antifungal activity assays, minimum inhibitory concentrations (MIC) and minimum bactericidal and fungicidal concentrations (MBC and MFC) were determined. The ranges of MIC values were 50-100 μg/mL for S. aureus and 400-800 μg/mL for E. coli. The extract did not show activity in the tested concentrations for C. albicans. The fungal crude extract was assayed for antioxidant activities. Its ability to scavenge DPPH radicals and antioxidant activity by β-carotene/linoleic acid system oxidation was not significant. In addition, antitumor activity was studied using the MTT assay. At a dilution of 400 μg/mL, the extract displayed a cytotoxic activity of approximately 50% towards HeLa cells in vitro. The results indicate that endophytic fungi could be a promising source of bioactive compounds and warrant further study.

Antibacterial cyclic D,L-alpha-glycopeptides

We report the design, synthesis, membrane activity, biophysical characterization, and in vitro antibacterial activities of cationic cyclic D,L-alpha-glycopeptides bearing d-glucosamine (GlcNH(2)), D-galactose (Gal), or D-mannose (Man) glycosyl side chains.

In vitro antibacterial activities of tigecycline and comparative agents by time-kill kinetic studies in fresh Mueller-Hinton broth

Time-kill kinetics performed with tigecycline, in fresh MHB, demonstrated a consistent 1 to 2 log(10) CFU/ml reduction in bacterial counts against the majority of clinically relevant pathogens tested. Although classified as a bacteriostatic agent, tigecycline shows bactericidal activity against select isolates associated with serious infection. In general, vancomycin and imipenem demonstrated bactericidal activity.

Synthesis of aza-analogues of the glycosylated tyrosine portion of mannopeptimycin-E

Two C4' amido disaccharide analogues of mannopeptimycin-E were synthesized via an iterative palladium glycosylation sequence. The stereoselective synthesis of the C4' acylated 1,4-alpha,alpha-manno,manno-amido disaccharide has been achieved in ten steps from a protected d-tyrosine. The path relies upon a regio- and diastereoselective palladium-catalyzed azide substitution reaction. The competence of the synthesis is demonstrated by the good overall yield (21%) from protected tyrosine.

Formation and properties of in vitro biofilms of ica-negative Staphylococcus epidermidis clinical isolates

Coagulase-negative Staphylococcus epidermidis has become the leading cause of foreign-body infections due to its biofilm formation on all kinds of medical-device surfaces. The biofilm development of S. epidermidis includes two steps: the initial attachment phase and the accumulative phase. In the accumulative phase, the polysaccharide intercellular adhesin (PIA), encoded by the icaADBC locus, is the major component mediating intercellular adhesion. However, recent studies have revealed the emergence of biofilm-positive/ica-negative staphylococcal clinical isolates. In this report, two ica-negative S. epidermidis clinical strains, SE1 and SE4, exhibited their heterogeneity in biofilm architecture under static and flow conditions, compared with the biofilm-positive/ica-positive RP62A strain. Strains with this type of absence of PIA from biofilms also displayed intermediate resistance to vancomycin. More importantly, the cells of both SE1 and SE4 strains were more tolerant than those of RP62A to exposure to lysostaphin and vancomycin. Based on the results, it is suggested that the biofilm-positive/ica-negative strain represents a newly emergent subpopulation of S. epidermidis clinical strains, arising from selection by antibiotics in the nosocomial milieu, which displays a survival advantage in its host environment. Recent epidemiological data support this suggestion, by showing a tendency towards an increasing proportion of this subpopulation in staphylococci-associated infections.

Synthetic studies toward mannopeptimycin-E: synthesis of the O-linked tyrosine 1,4-alpha,alpha-manno,manno-pyranosyl pyranoside

[structure: see text] The enantioselective synthesis of the C-4' acylated 1,4-alpha,alpha-manno,manno-disaccharide fragment of mannopeptimycin-E has been achieved in seven steps from d-tyrosine. The route relies upon diastereoselective palladium-catalyzed glycosylation, diastereoselective reduction, and diastereoselective bis-dihydroxylation. The efficiency of the synthesis is demonstrated by the high overall yield (37%) and the preparation of various analogues.

Biosynthetic pathway for mannopeptimycins, lipoglycopeptide antibiotics active against drug-resistant gram-positive pathogens

The mannopeptimycins are a novel class of lipoglycopeptide antibiotics active against multidrug-resistant pathogens with potential as clinically useful antibacterials. This report is the first to describe the biosynthesis of this novel class of mannosylated lipoglycopeptides. Included here are the cloning, sequencing, annotation, and manipulation of the mannopeptimycin biosynthetic gene cluster from Streptomyces hygroscopicus NRRL 30439. Encoded by genes within the mannopeptimycin biosynthetic gene cluster are enzymes responsible for the generation of the hexapeptide core (nonribosomal peptide synthetases [NRPS]) and tailoring reactions (mannosylation, isovalerylation, hydroxylation, and methylation). The NRPS system is noncanonical in that it has six modules utilizing only five amino acid-specific adenylation domains and it lacks a prototypical NRPS macrocyclizing thioesterase domain. Analysis of the mannopeptimycin gene cluster and its engineering has elucidated the mannopeptimycin biosynthetic pathway and provides the framework to make new and improved mannopeptimycins biosynthetically.

Lipid II as a target for antibiotics

Lipid II is a membrane-anchored cell-wall precursor that is essential for bacterial cell-wall biosynthesis. The effectiveness of targeting Lipid II as an antibacterial strategy is highlighted by the fact that it is the target for at least four different classes of antibiotic, including the clinically important glycopeptide antibiotic vancomycin. However, the growing problem of bacterial resistance to many current drugs, including vancomycin, has led to increasing interest in the therapeutic potential of other classes of compound that target Lipid II. Here, we review progress in understanding of the antibacterial activities of these compounds, which include lantibiotics, mannopeptimycins and ramoplanin, and consider factors that will be important in exploiting their potential as new treatments for bacterial infections.

Targeting the forgotten transglycosylases

Forty years ago, moenomycin was reported as a representative of a novel natural product class with strong antibacterial activity against Gram-positive organisms. Moenomycin was developed as an antimicrobial growth promoter in animal feeds. Mechanistically, moenomycin acts via inhibition of the transglycosylation process at the final stage of the peptidoglycan biosynthesis, in particular through binding directly to the transglycosylase enzymes, thereby preventing polymerisation of lipid II into linear peptidoglycan. Despite moenomycin's success, no developments of direct transglycosylase enzyme inhibitors were reported for over 30 years, probably due to the complexities and uncertainties surrounding the transglycosylation process, in particular the number of enzymes involved in the process and their specific roles. The development of better research tools and an improved understanding of the transglycosylation process, together with the increasing threat presented by multidrug-resistant bacteria, have led to a resurfacing of interest in targeting the forgotten transglycosylases. In addition, several new generation glycopeptides in clinical development inhibit the transglycosylation process, adding further value to the approach. In this paper, we summarise some of the developments in the area of transglycosylase inhibitors over the last 10 years.

Natural products--the future scaffolds for novel antibiotics?

Natural products have played a pivotal role in antibiotic drug discovery with most antibacterial drugs being derived from a natural product or natural product lead. However, the rapid onset of resistance to most antibacterial drugs diminishes their effectiveness considerably and necessitates a constant supply of new antibiotics for effective treatment of infections. The natural product templates of actinonin, pleuromutilin, ramoplanin and tiacumicin B, which are compounds undergoing clinical evaluation, represent templates not found in currently marketed antibacterial drugs. In addition, the new templates present in the recently discovered lead antibacterials arylomycin, GE23077, mannopeptimycin, muraymycin/caprazamycin, nocathiacin and ECO-0501, are discussed. Despite extensive efforts to identify antibiotic leads from molecular targets, only the peptide deformylase inhibitor LBM-415 is currently in clinical trials. It is proposed that new antibacterial assays which combine cell-based screening with molecular targets could offer better prospects for lead discovery.

Mannopeptimycins, a novel class of glycopeptide antibiotics active against gram-positive bacteria

Mannopeptimycins alpha-epsilon, novel glycopeptides with activity against methicillin-resistant staphylococci and vancomycin-resistant enterococci, are purified from the fermentation broth of a strain of Streptomyces hygroscopicus, LL-AC98, and their structures characterized using spectroscopic analyses and chemical methods. The SAR data of the natural and synthetic esters demonstrate that the presence of hydrophobic groups near the terminal mannosyl moiety is critical for antibacterial potency. Scalable syntheses of 4,6-cyclic acetals and ketals on this moiety are used to produce significant quantities of the respective mannopeptimycin derivatives. These acetal and ketal derivatives exhibit potent activities against susceptible and resistant Gram-positive bacteria in both in vitro and in vivo experiments, comparable with or exceeding the activity of vancomycin. Studies on the mechanism of action suggest that the mannopeptimycins interfere with the late stages of bacterial cell wall biosynthesis. It is believed that these antibiotics inhibit the transglycosylation by binding to the transglycosylase substrate, lipid II.

Hydrophobic Acetal and Ketal Derivatives of Mannopeptimycin-α and Desmethylhexahydromannopeptimycin-α: Semisynthetic Glycopeptides with Potent Activity Against Gram-Positive Bacteria

The effect of introducing hydrophobic groups onto the disaccharide portion of the mannopeptimycins has been examined. Under acid-catalyzed conditions dimethyl acetals and ketals react on the terminal mannose of the disaccharide moiety of mannopeptimycin-alpha and the cyclohexylalanyl analogue 2. The preferentially formed monofunctionalized 4,6-acetals and -ketals display potent antibacterial activities against Gram-positive microorganisms, including MRSA, PRSP, and VRE pathogens.