Cereulide production capacities and genetic properties of 31 emetic Bacillus cereus group strains.

The highly potent toxin cereulide is a frequent cause of foodborne intoxications. This extremely resistant toxin is produced by Bacillus cereus group strains carrying the plasmid encoded cesHPTABCD gene cluster. It is known that the capacities to produce cereulide vary greatly between different strains but the genetic background of these variations is not clear. In this study, cereulide production capacities were associated with genetic characteristics. For this, cereulide levels in cultures of 31 strains were determined after incubation in tryptic soy broth for 24 h at 24 °C, 30 °C and 37 °C. Whole genome sequencing based data were used for an in-depth characterization of gene sequences related to cereulide production. The taxonomy, population structure and phylogenetic relationships of the strains were evaluated based on average nucleotide identity, multi-locus sequence typing (MLST), core genome MLST and single nucleotide polymorphism analyses. Despite a limited strain number, the approach of a genome wide association study (GWAS) was tested to link genetic variation with cereulide quantities. Our study confirms strain-dependent differences in cereulide production. For most strains, these differences were not explainable by sequence variations in the cesHPTABCD gene cluster or the regulatory genes abrB, spo0A, codY and pagRBc. Likewise, the population structure and phylogeny of the tested strains did not comprehensively reflect the cereulide production capacities. GWAS yielded first hints for associated proteins, while their possible effect on cereulide synthesis remains to be further investigated.

PsrA is a novel regulator contributes to antibiotic synthesis, bacterial virulence, cell motility and extracellular polysaccharides production in Serratia marcescens.

Serratia marcescens is a Gram-negative bacterium of the Enterobacteriaceae family that can produce numbers of biologically active secondary metabolites. However, our understanding of the regulatory mechanisms behind secondary metabolites biosynthesis in S. marcescens remains limited. In this study, we identified an uncharacterized LysR family transcriptional regulator, encoding gene BVG90_12635, here we named psrA, that positively controlled prodigiosin synthesis in S. marcescens. This phenotype corresponded to PsrA positive control of transcriptional of the prodigiosin-associated pig operon by directly binding to a regulatory binding site (RBS) and an activating binding site (ABS) in the promoter region of the pig operon. We demonstrated that L-proline is an effector for the PsrA, which enhances the binding affinity of PsrA to its target promoters. Using transcriptomics and further experiments, we show that PsrA indirectly regulates pleiotropic phenotypes, including serrawettin W1 biosynthesis, extracellular polysaccharide production, biofilm formation, swarming motility and T6SS-mediated antibacterial activity in S. marcescens. Collectively, this study proposes that PsrA is a novel regulator that contributes to antibiotic synthesis, bacterial virulence, cell motility and extracellular polysaccharides production in S. marcescens and provides important clues for future studies exploring the function of the PsrA and PsrA-like proteins which are widely present in many other bacteria.

Ammoides pusilla Essential Oil: A Potent Inhibitor of the Growth of Fusarium avenaceum and Its Enniatin Production.

Enniatins are mycotoxins produced by Fusarium species contaminating cereals and various agricultural commodities. The co-occurrence of these mycotoxins in large quantities with other mycotoxins such as trichothecenes and the possible synergies in toxicity could lead to serious food safety problems. Using the agar dilution method, Ammoides pusilla was selected among eight Tunisian plants for the antifungal potential of its essential oil (EO) on Fusarium avenaceum mycelial growth and its production of enniatins. Two EO batches were produced and analyzed by GC/MS-MS. Their activities were measured using both contact assays and fumigant tests (estimated IC50 were 0.1 µL·mL-1 and 7.6 µL·L-1, respectively). The A. pusilla EOs and their volatiles inhibited the germination of spores and the mycelial growth, showing a fungistatic but not fungicidal activity. The accumulation of enniatins was also significantly reduced (estimated IC50 were 0.05 µL·mL-1 for the contact assays and 4.2 µL·L-1 for the fumigation assays). The most active batch of EO was richer in thymol, the main volatile compound found. Thymol used as fumigant showed a potent fungistatic activity but not a significant antimycotoxigenic activity. Overall, our data demonstrated the bioactivity of A. pusilla EO and its high potential to control F. avenaceum and its enniatins production in agricultural commodities.

The regulation of BbLaeA on the production of beauvericin and bassiatin in Beauveria bassiana.

Beauvericin and bassiatin are two valuable compounds with various bioactivities biosynthesized by the supposedly same nonribosomal peptide synthetase BbBEAS in entomopathogenic fungus Beauveria bassiana. To evaluate the regulatory effect of global regulator LaeA on their production, we constructed BbLaeA gene deletion and overexpression mutants, respectively. Deletion of BbLaeA resulted in a decrease of the beauvericin titer, while overexpression of BbLaeA increased its production by 1-2.26 times. No bassiatin could be detected in ΔBbLaeA and wild type strain of B. bassiana, but 4.26-5.10 µg/mL bassiatin was produced in OE::BbLaeA. Furthermore, additional metabolites with increased production in OE::BbLaeA were isolated and identified as primary metabolites. Among them, 4-hydroxyphenylacetic acid showed antibacterial bioactivity against Ralstonia solanacearum. These results indicated that BbLaeA positively regulates the production of beauvericin, bassiatin and various bioactive primary metabolites.

Stereodivergent Nitrocyclopropane Formation during Biosynthesis of Belactosins and Hormaomycins.

Belactosins and hormaomycins are peptide natural products containing 3-(2-aminocyclopropyl)alanine and 3-(2-nitrocyclopropyl)alanine residues, respectively, with opposite stereoconfigurations of the cyclopropane ring. Herein we demonstrate that the heme oxygenase-like enzymes BelK and HrmI catalyze the N-oxygenation of l-lysine to generate 6-nitronorleucine. The nonheme iron enzymes BelL and HrmJ then cyclize the nitroalkane moiety to the nitrocyclopropane ring with the desired stereochemistry found in the corresponding natural products. We also show that both cyclopropanases remove the 4-proS-H of 6-nitronorleucine during the cyclization, establishing the inversion and retention of the configuration at C4 during the BelL and HrmJ reactions, respectively. This study reveals the unique strategy for stereocontrolled cyclopropane synthesis in nature.

Engineered Biosynthesis of Pharmaceutically Important Compounds.

Natural products are an important source of medicinal seeds. The discovery of novel biosynthetic enzymes from nature is important for their use as biocatalysts for the enzymatic synthesis of useful natural products. In addition, genetics and structural biology developments have enabled the engineering of enzymes for the production of unnatural analogs of bioactive natural products. In this review, I describe the recent research on these two topics, the exploitation of a novel secondary metabolite enzyme involved in the biosynthesis of the sulfonamide natural product antibiotic SB-203208, and the production of unnatural bioactive depsipeptides by reconstruction of the modular enzyme assembly lines in the microbial host.

Unprecedented Noncanonical Features of the Nonlinear Nonribosomal Peptide Synthetase Assembly Line for WS9326A Biosynthesis.

Systematic inactivation of nonribosomal peptide synthetase (NRPS) domains and translocation of the thioesterase (TE) domain revealed several unprecedented nonlinear NRPS assembly processes during the biosynthesis of the cyclodepsipeptide WS9326A in Streptomyces sp. SNM55. First, two sets of type ΙΙ TE (TEΙΙ)-like enzymes mediate the shuttling of activated amino acids between two sets of stand-alone adenylation (A)-thiolation (T) didomain modules and an "A-less" condensation (C)-T module with distinctive specificities and flexibilities. This was confirmed by the elucidation of the affinities of the A-T didomains for the TEΙΙs and its structure. Second, the C-T didomain module operates iteratively and independently from other modules in the same protein to catalyze two chain elongation cycles. Third, this biosynthetic pathway includes the first example of module skipping, where the interpolated C and T domains are required for chain transfer.

Structural Revision of Natural Cyclic Depsipeptide MA026 Established by Total Synthesis and Biosynthetic Gene Cluster Analysis.

A revised structure of natural 14-mer cyclic depsipeptide MA026, isolated from Pseudomonas sp. RtlB026 in 2002 was established by physicochemical analysis with HPLC, MS/MS, and NMR and confirmed by total solid-phase synthesis. The revised structure differs from that previously reported in that two amino acid residues, assigned in error, have been replaced. Synthesized MA026 with the revised structure showed a tight junction (TJ) opening activity like that of the natural one in a cell-based TJ opening assay. Bioinformatic analysis of the putative MA026 biosynthetic gene cluster (BGC) of RtIB026 demonstrated that the stereochemistry of each amino acid residue in the revised structure can be reasonably explained. Phylogenetic analysis with xantholysin BGC indicates an exceptionally high homology (ca. 90 %) between xantholysin and MA026. The TJ opening activity of MA026 when binding to claudin-1 is a key to new avenues for transdermal administration of large hydrophilic biologics.

Bacillus weihenstephanensis can readily evolve for increased endospore heat resistance without compromising its thermotype.

Proper elimination of bacterial endospores in foods and food processing environment is challenging because of their extreme resistance to various stresses. Often, sporicidal treatments prove insufficient to eradicate the contaminating endospore population as a whole, and might therefore serve as a selection pressure for enhanced endospore resistance. In the sporeforming Bacillus cereus group, Bacillus weihenstephanensis is an important food spoilage organism and potential cereulide producing pathogen, due to its psychrotolerant growth ability at 7 °C. Although the endospores of B. weihenstephanensis are generally less heat resistant compared to their mesophilic or thermotolerant relatives, our data now show that non-emetic B. weihenstephanensis strain LMG 18989T can readily and reproducibly evolve to acquire much enhanced endospore heat resistance. In fact, one of the B. weihenstephanensis mutants from directed evolution by wet heat in this study yielded endospores displaying a > 4-fold increase in D-value at 91 °C compared to the parental strain. Moreover, these mutant endospores retained their superior heat resistance even when sporulation was performed at 10 °C. Interestingly, increased endospore heat resistance did not negatively affect the vegetative growth capacities of the evolved mutants at lower (7 °C) and upper (37 °C) growth temperature boundaries, indicating that the correlation between cardinal growth temperatures and endospore heat resistance which is observed among bacterial sporeformers is not necessarily causal.

Thioesterase-mediated side chain transesterification generates potent Gq signaling inhibitor FR900359.

The potent and selective Gq protein inhibitor depsipeptide FR900359 (FR), originally discovered as the product of an uncultivable plant endosymbiont, is synthesized by a complex biosynthetic system comprising two nonribosomal peptide synthetase (NRPS) assembly lines. Here we characterize a cultivable bacterial FR producer, enabling detailed investigations into biosynthesis and attachment of the functionally important FR side chain. We reconstitute side chain assembly by the monomodular NRPS FrsA and the non-heme monooxygenase FrsH, and characterize intermolecular side chain transesterification to the final macrocyclic intermediate FR-Core, mediated by the FrsA thioesterase domain. We harness FrsA substrate promiscuity to generate FR analogs with altered side chains and demonstrate indispensability of the FR side chain for efficient Gq inhibition by comparative bioactivity, toxicity and docking studies. Finally, evolution of FR and side chain biosynthesis is discussed based on bioinformatics analyses. Side chain transesterification boosts potency and target affinity of selective Gq inhibitor natural products.

Synthesis and Biological Activities of Cyclodepsipeptides of Aurilide Family from Marine Origin.

Aurilides are a class of depsipeptides occurring mainly in marine cyanobacteria. Members of the aurilide family have shown to exhibit strong cytotoxicity against various cancer cell lines. These compounds bear a pentapeptide, a polyketide, and an α-hydroxy ester subunit in their structure. A large number of remarkable studies on aurilides have emerged since 1996. This comprehensive account summarizes the biological activities and total syntheses of natural compounds of the aurilide family as well as their synthetic analogues.

Structures and Biosynthetic Pathway of Coprisamides C and D, 2-Alkenylcinnamic Acid-Containing Peptides from the Gut Bacterium of the Carrion Beetle Silpha perforata.

Coprisamides C and D (1 and 2) were isolated from a gut bacterium, Micromonospora sp. UTJ3, of the carrion beetle Silpha perforata. Based on the combined analysis of UV, MS, and NMR spectral data, the planar structures of 1 and 2 were elucidated to be unreported derivatives of coprisamides A and B, cyclic depsipeptides bearing a 2-alkenylcinnamic acid unit and the unusual amino acids ß-methylaspartic acid and 2,3-diaminopropanoic acid. The absolute configuration of 1 was determined using the advanced Marfey's method, phenylglycine methyl ester derivatization, and J-based configuration analysis. The biosynthetic gene clusters for the coprisamides were investigated based on genomic data from coprisamide-producing strains Micromonospora sp. UTJ3 and Streptomyces sp. SNU533. Coprisamide C (1) was active against the Mycobacterium tuberculosis mc2 6230 strain.

Fusarium Cyclodepsipeptide Mycotoxins: Chemistry, Biosynthesis, and Occurrence.

Most of the fungi from the Fusarium genus are pathogenic to cereals, vegetables, and fruits and the products of their secondary metabolism mycotoxins may accumulate in foods and feeds. Non-ribosomal cyclodepsipeptides are one of the main mycotoxin groups and include beauvericins (BEAs), enniatins (ENNs), and beauvenniatins (BEAEs). When ingested, even small amounts of these metabolites significantly affect human and animal health. On the other hand, in view of their antimicrobial activities and cytotoxicity, they may be used as components in drug discovery and processing and are considered as suitable candidates for anti-cancer drugs. Therefore, it is crucial to expand the existing knowledge about cyclodepsipeptides and to search for new analogues of these compounds. The present manuscript aimed to highlight the extensive variability of cyclodepsipeptides by describing chemistry, biosynthesis, and occurrence of BEAs, ENNs, and BEAEs in foods and feeds. Moreover, the co-occurrence of Fusarium species was compared to the amounts of toxins in crops, vegetables, and fruits from different regions of the world.

Investigating the Efficiency of Hydroxycinnamic Acids to Inhibit the Production of Enniatins by Fusarium avenaceum and Modulate the Expression of Enniatins Biosynthetic Genes.

Enniatins (ENNs) that belong to the group of emerging mycotoxins are widespread contaminants of agricultural commodities. There is currently insufficient evidence to rule out health concerns associated with long-term exposure to ENNs and efforts must be strengthened to define a control strategy. While the potential of plant compounds to counteract the contamination with legislated mycotoxins has been reported, little remains known regarding ENNs. The present study evidenced for the first time the efficiency of hydroxycinnamic acids to inhibit the fungal growth and ENNs yield by Fusarium avenaceum. Notably, 0.5 mM of exogenous ferulic, caffeic, and p-coumaric acids led to a drastic reduction of ENNs synthesis in pH4 broths, with ferulic acid being the most potent. The ENNs production inhibitory activity of ferulic acid was shown to be associated with a significant down-regulation of the expression of ENNs biosynthetic genes. To further investigate the bioactivity of ferulic acid, its metabolic fate was characterized in fungal broths and the capacity of F. avenaceum to metabolize it through a C2-cleavage type degradation was demonstrated. Overall, our data support the promising use of ferulic acid in ENNs control strategies, either as part of an environmentally friendly plant-care product or as a biomarker of plant resistance.

Biosynthesis of depsipeptides, or Depsi: The peptides with varied generations.

Depsipeptides are compounds that contain both ester bonds and amide bonds. Important natural product depsipeptides include the piscicide antimycin, the K+ ionophores cereulide and valinomycin, the anticancer agent cryptophycin, and the antimicrobial kutzneride. Furthermore, database searches return hundreds of uncharacterized systems likely to produce novel depsipeptides. These compounds are made by specialized nonribosomal peptide synthetases (NRPSs). NRPSs are biosynthetic megaenzymes that use a module architecture and multi-step catalytic cycle to assemble monomer substrates into peptides, or in the case of specialized depsipeptide synthetases, depsipeptides. Two NRPS domains, the condensation domain and the thioesterase domain, catalyze ester bond formation, and ester bonds are introduced into depsipeptides in several different ways. The two most common occur during cyclization, in a reaction between a hydroxy-containing side chain and the C-terminal amino acid residue in a peptide intermediate, and during incorporation into the growing peptide chain of an α-hydroxy acyl moiety, recruited either by direct selection of an α-hydroxy acid substrate or by selection of an α-keto acid substrate that is reduced in situ. In this article, we discuss how and when these esters are introduced during depsipeptide synthesis, survey notable depsipeptide synthetases, and review insight into bacterial depsipeptide synthetases recently gained from structural studies.

Cereulide Synthetase Acquisition and Loss Events within the Evolutionary History of Group III Bacillus cereus Sensu Lato Facilitate the Transition between Emetic and Diarrheal Foodborne Pathogens.

Cereulide-producing members of Bacillus cereussensu lato group III (also known as emetic B. cereus) possess cereulide synthetase, a plasmid-encoded, nonribosomal peptide synthetase encoded by the ces gene cluster. Despite the documented risks that cereulide-producing strains pose to public health, the level of genomic diversity encompassed by emetic B. cereus has never been evaluated at a whole-genome scale. Here, we employ a phylogenomic approach to characterize group III B. cereussensu lato genomes which possess ces (ces positive) alongside their closely related, ces-negative counterparts (i) to assess the genomic diversity encompassed by emetic B. cereus and (ii) to identify potential ces loss and/or gain events within the evolutionary history of the high-risk and medically relevant sequence type (ST) 26 lineage often associated with emetic foodborne illness. Using all publicly available ces-positive group III B. cereussensu lato genomes and the ces-negative genomes interspersed among them (n = 159), we show that emetic B. cereus is not clonal; rather, multiple lineages within group III harbor cereulide-producing strains, all of which share an ancestor incapable of producing cereulide (posterior probability = 0.86 to 0.89). Members of ST 26 share an ancestor that existed circa 1748 (95% highest posterior density [HPD] interval = 1246.89 to 1915.64) and first acquired the ability to produce cereulide before 1876 (95% HPD = 1641.43 to 1946.70). Within ST 26 alone, two subsequent ces gain events were observed, as well as three ces loss events, including among isolates responsible for B. cereussensu lato toxicoinfection (i.e., "diarrheal" illness).IMPORTANCEB. cereus is responsible for thousands of cases of foodborne disease each year worldwide, causing two distinct forms of illness: (i) intoxication via cereulide (i.e., emetic syndrome) or (ii) toxicoinfection via multiple enterotoxins (i.e., diarrheal syndrome). Here, we show that emetic B. cereus is not a clonal, homogenous unit that resulted from a single cereulide synthetase gain event followed by subsequent proliferation; rather, cereulide synthetase acquisition and loss is a dynamic, ongoing process that occurs across lineages, allowing some group III B. cereussensu lato populations to oscillate between diarrheal and emetic foodborne pathogens over the course of their evolutionary histories. We also highlight the care that must be taken when selecting a reference genome for whole-genome sequencing-based investigation of emetic B. cereussensu lato outbreaks, since some reference genome selections can lead to a confounding loss of resolution and potentially hinder epidemiological investigations.

Does the Future of Antibiotics Lie in Secondary Metabolites Produced by Xenorhabdus spp.? A Review.

The over-prescription of antibiotics for treatment of infections is primarily to blame for the increase in bacterial resistance. Added to the problem is the slow rate at which novel antibiotics are discovered and the many processes that need to be followed to classify antimicrobials safe for medical use. Xenorhabdus spp. of the family Enterobacteriaceae, mutualistically associated with entomopathogenic nematodes of the genus Steinernema, produce a variety of antibacterial peptides, including bacteriocins, depsipeptides, xenocoumacins and PAX (peptide antimicrobial-Xenorhabdus) peptides, plus additional secondary metabolites with antibacterial and antifungal activity. The secondary metabolites of some strains are active against protozoa and a few have anti-carcinogenic properties. It is thus not surprising that nematodes invaded by a single strain of a Xenorhabdus species are not infected by other microorganisms. In this review, the antimicrobial compounds produced by Xenorhabdus spp. are listed and the gene clusters involved in synthesis of these secondary metabolites are discussed. We also review growth conditions required for increased production of antimicrobial compounds.

Discovery and Biosynthesis of Bolagladins: Unusual Lipodepsipeptides from Burkholderia gladioli Clinical Isolates*.

Two Burkholderia gladioli strains isolated from the lungs of cystic fibrosis patients were found to produce unusual lipodepsipeptides containing a unique citrate-derived fatty acid and a rare dehydro-ß-alanine residue. The gene cluster responsible for their biosynthesis was identified by bioinformatics and insertional mutagenesis. In-frame deletions and enzyme activity assays were used to investigate the functions of several proteins encoded by the biosynthetic gene cluster, which was found in the genomes of about 45 % of B. gladioli isolates, suggesting that its metabolic products play an important role in the growth and/or survival of the species. The Chrome Azurol S assay indicated that these metabolites bind ferric iron, which suppresses their production when added to the growth medium. Moreover, a gene encoding a TonB-dependent ferric-siderophore receptor is adjacent to the biosynthetic genes, suggesting that these metabolites may function as siderophores in B. gladioli.

Thioesterase from Cereulide Biosynthesis Is Responsible for Oligomerization and Macrocyclization of a Linear Tetradepsipeptide.

Cereulide is a toxic cyclic depsidodecapeptide produced in Bacillus cereus by two nonribosomal peptide synthetases, CesA and CesB. While highly similar in structure to valinomycin and with a homologous biosynthetic gene cluster, recent work suggests that cereulide is produced via a different mechanism that relies on a noncanonical coupling of two didepsipeptide-peptidyl carrier protein (PCP) bound intermediates. Ultimately this alternative mechanism generates a tetradepsipeptide-PCP bound intermediate that differs from the tetradepsipeptide-PCP intermediate predicted from canonical activity of CesA and CesB. To differentiate between the mechanisms, both tetradepsipeptides were prepared as N-acetyl cysteamine thioesters (SNAC), and the ability of the purified recombinant terminal CesB thioesterase (CesB TE) to oligomerize and macrocyclize each substrate was probed. Only the canonical substrate is converted to cereulide, ruling out the alternative mechanism. It was demonstrated that CesB TE can use related tetradepsipeptide substrates, such as the valinomycin tetradespipetide and a hybrid cereulide-valinomycin tetradepsipetide in conjunction with its native substrate to generate chimeric natural products. This work clarifies the biosynthetic origins of cereulide and provides a powerful biocatalyst to access analogues of these ionophoric natural products.

Biosynthesis and Chemical Diversification of Verucopeptin Leads to Structural and Functional Versatility.

A synthesis program for structurally complex macrocycles is very challenging. Herein, we propose a biosynthesis pathway of the pyranylated cyclodepsipeptide verucopeptin to make enough supply and to diversify verucopeptin by genetic manipulation and one-step semisynthesis. The synthesis relies on the intrinsic reactivity of the interchangeable hemiketal pyrane and opened keto along with adjacent alkene. Biological evaluation of verucopeptin-oriented analogs delivers a potent AMP-activated protein kinase (AMPK) agonist, antibacterial agent, and selective NFκB modulator.