Siderophores from the Entomopathogenic Fungus Beauveria bassiana

We report NMR- and MS-based structural characterizations of siderophores and related compounds from Beauveria bassiana (Balsamo-Crivelli) Vuillemin, including ten new chemical entities (2-4, 6-9, 11-12, and 15) and five known compounds, (1, 5, 10, 13, and 14). The siderophore mixture from ARSEF strain #2680 included two compounds in which N⁵-mevalonyl-N⁵-hydroxyornithine replaces both (2) or one (3) of the N⁵-anhydromevalonyl-N⁵-hydroxyornithine units of dimerumic acid (1). Mevalonolactone (14) was present as a degradation product of 2 and 3. ARSEF #2860 also produced compounds that have mannopyranose (5, 6) or 4-O-methyl-mannopyranose units (4, 7), two compounds (8, 9) that can be rationalized as 4-O-methyl-mannopyranosyl analogues of the esterifying acid moieties of metachelins A and B, respectively, and two probable decomposition products of 1, a nitro compound (11) and a formate (12). Beauverichelin A (15), a coprogen-type siderophore that represents the di-4-O-methyl-mannopyranosyl analogue of metachelin A, was detected in crude extracts of ARSEF #2860, but only in trace amounts. ARSEF strains #252 and #1955 yielded beauverichelin A in quantities that were sufficient for NMR analysis. Only the di- (1-7) and trihydroxamate (15) siderophores showed iron-binding activity in the CAS assay and, when ferrated, showed strong ESIMS signals consistent with 1:1 ligand/iron complexes.

Starvation and Imidacloprid Exposure Influence Immune Response by Anoplophora glabripennis (Coleoptera: Cerambycidae) to a Fungal Pathogen

In several insect systems, fungal entomopathogens synergize with neonicotinoid insecticides which results in accelerated host death. Using the Asian longhorned beetle, Anoplophora glabripennis (Motschulsky), an invasive woodborer inadvertently introduced into North America and Europe, we investigated potential mechanisms in the synergy between the entomopathogenic fungus Metarhizium brunneum Petch and the insecticide imidacloprid. A potential mechanism underlying this synergy could be imidacloprid's ability to prevent feeding shortly after administration. We investigated whether starvation would have an impact similar to imidacloprid exposure on the mortality of fungal-inoculated beetles. Using real-time PCR to quantify fungal load in inoculated beetles, we determined how starvation and pesticide exposure impacted beetles' ability to tolerate or resist a fungal infection. The effect of starvation and pesticide exposure on the encapsulation and melanization immune responses of the beetles was also quantified. Starvation had a similar impact on the survival of M. brunneum-inoculated beetles compared to imidacloprid exposure. The synergy, however, was not completely due to starvation, as imidacloprid reduced the beetles' melanotic encapsulation response and capsule area, while starvation did not significantly reduce these immune responses. Our results suggest that there are multiple interacting mechanisms involved in the synergy between M. brunneum and imidacloprid.

Insights into Adaptations to a Near-Obligate Nematode Endoparasitic Lifestyle from the Finished Genome of Drechmeria coniospora

Nematophagous fungi employ three distinct predatory strategies: nematode trapping, parasitism of females and eggs, and endoparasitism. While endoparasites play key roles in controlling nematode populations in nature, their application for integrated pest management is hindered by the limited understanding of their biology. We present a comparative analysis of a high quality finished genome assembly of Drechmeria coniospora, a model endoparasitic nematophagous fungus, integrated with a transcriptomic study. Adaptation of D. coniospora to its almost completely obligate endoparasitic lifestyle led to the simplification of many orthologous gene families involved in the saprophytic trophic mode, while maintaining orthologs of most known fungal pathogen-host interaction proteins, stress response circuits and putative effectors of the small secreted protein type. The need to adhere to and penetrate the host cuticle led to a selective radiation of surface proteins and hydrolytic enzymes. Although the endoparasite has a simplified secondary metabolome, it produces a novel peptaibiotic family that shows antibacterial, antifungal and nematicidal activities. Our analyses emphasize the basic malleability of the D. coniospora genome: loss of genes advantageous for the saprophytic lifestyle; modulation of elements that its cohort species utilize for entomopathogenesis; and expansion of protein families necessary for the nematode endoparasitic lifestyle.

Metachelins, mannosylated and N-oxidized coprogen-type siderophores from Metarhizium robertsii

Under iron-depleted culture conditions, the entomopathogenic fungus Metarhizium robertsii (Bischoff, Humber, and Rehner) (= M. anisopliae) produces a complex of extracellular siderophores including novel O-glycosylated and N-oxidized coprogen-type compounds as well as the known fungal siderophores N(α)-dimethylcoprogen (NADC) and dimerumic acid (DA). Metachelin A (1), the most abundant component in the M. robertsii siderophore mixture, was characterized as a 1094 Da analogue of NADC that is O-glycosylated by β-mannose at both terminal hydroxyl groups and N-oxidized at the dimethylated α-nitrogen. The mixture also contained a 1078 Da analogue, metachelin B (2), which lacks the N-oxide modification. Also characterized were the aglycone of 1, i.e., the N-oxide of NADC (3), and the monomannoside of DA (6). N-Oxide and O-glycosyl substituents are unprecedented among microbial siderophores. At high ESIMS source energy and at room temperature in DMSO, 1 underwent Cope elimination, resulting in loss of the N(α)-dimethyl group and dehydration of the α-β bond. High-resolution ESIMS data confirmed that all tri- and dihydroxamate siderophores (1-6) complex with trivalent Fe, Al, and Ga. In a chrome azurol S assay, all of the M. robertsii siderophores showed iron-binding activity roughly equivalent to that of desferrioxamine B.

Discovering the secondary metabolite potential encoded within entomopathogenic fungi

This highlight discusses the secondary metabolite potential of the insect pathogensMetarhiziumandBeauveria, including a bioinformatics analysis of secondary metabolite genes for which no products are yet identified. (Top picture is a mole cricket infected withBeauveria bassianaand the bottom picture is a wasp infected withBeauveria bassiana.)

Resistant ticks inhibit Metarhizium infection prior to haemocoel invasion by reducing fungal viability on the cuticle surface

We studied disease progression of, and host responses to, four species in the Metarhizium anisopliae complex expressing green fluorescent protein (GFP). We compared development and determined their relative levels of virulence against two susceptible arthropods, the cattle tick Rhipicephalus annulatus and the lepidopteran Galleria mellonella, and two resistant ticks, Hyalomma excavatum and Rhipicephalus sanguineus. Metarhizium brunneum Ma7 caused the greatest mortality of R. annulatus, Metarhizium robertsii ARSEF 2575 and Metarhizium pingshaense PPRC51 exhibited intermediate levels of virulence, and Metarhizium majus PPRC27 caused low mortality of cattle ticks. Conidia of all four species germinated on all hosts examined, but on resistant hosts, sustained hyphal growth was inhibited and GFP emission steadily and significantly decreased over time, suggesting a loss of fungal viability. Cuticle penetration was observed only for the three most virulent species infecting susceptible hosts. Cuticles of resistant and susceptible engorged female ticks showed significant increases in red autofluorescence at sites immediately under fungal hyphae. This is the first report (i) of tick mortality occurring after cuticle penetration but prior to haemocoel colonization and (ii) that resistant ticks do not support development of Metarhizium germlings on the outer surface of the cuticle. Whether reduced Metarhizium viability on resistant tick cuticles is due to antibiosis or limited nutrient availability is unknown.

Hydrophobin genes of the entomopathogenic fungus, Metarhizium brunneum, are differentially expressed and corresponding mutants are decreased in virulence

Hydrophobins are small, cysteine-rich, secreted proteins, ubiquitously produced by filamentous fungi that are speculated to function in fungal growth, cell surface properties, and development, although this has been rigorously tested for only a few species. Herein, we report identification of three hydrophobin genes from the entomopathogenic fungus, Metarhizium brunneum, and functional characterization of strains lacking these genes. One gene (HYD1/ssgA) encodes a class I hydrophobin identified previously. Two new genes, HYD3 and HYD2, encode a class I and class II hydrophobin, respectively. To examine function, we deleted all three separately, from the M. brunneum strain KTU-60 genome, using Agrobacterium tumefaciens-mediated transformation. Deletion strains were screened for alterations in developmental phenotypes including growth, sporulation, pigmentation, colony surface properties, and virulence to insects. All deletion strains were reduced in their ability to sporulate and showed alterations in wild-type pigmentation, but all retained wild-type hydrophobicity, except for one individual hyd3 mutant. Complementation with the wild-type HYD3 gene restored hydrophobicity. Each gene, present as a single copy in the genome, showed differential expression patterns dependent on the developmental stage of the fungus. When Spodoptera exigua (beet armyworm) larvae were treated with either conidia or blastospores of each hyd mutant, reductions in virulence and delayed mortality were observed as compared to WT. Together, these results suggest that hydrophobins are differentially expressed and may have distinct, but compensating roles, in conidiation, pigmentation, hydrophobicity, and virulence.

Metacridamides A and B, macrocycles from conidia of the entomopathogenic fungus Metarhizium acridum

Metarhizium acridum, an entomopathogenic fungus, has been commercialized and used successfully for biocontrol of grasshopper pests in Africa and Australia. Its conidia produce two novel 17-membered macrocycles, metacridamides A and B, which consist of a Phe unit condensed with a nonaketide. Planar structures were elucidated by a combination of mass spectrometric and NMR techniques. Following hydrolysis of 1, chiral amino acid analysis assigned the L-configuration to the Phe unit. A crystal structure established the absolute configuration of the eight remaining stereogenic centers in 1. Metacridamide A showed cytotoxicity to three cancer lines with IC₅₀'s of 6.2, 11.0, and 10.8 μM against Caco-2 (epithelial colorectal adenocarcinoma), MCF-7 (breast cancer), and HepG2/C3A (hepatoma) cell lines, respectively. In addition, metacridamide B had an IC₅₀ of 18.2 μM against HepG2/C3A, although it was inactive at 100 μM against Caco-2 and MCF-7. Neither analogue showed antimicrobial, phytotoxic, or insecticidal activity.

Serinocyclins A and B, cyclic heptapeptides from Metarhizium anisopliae

Two new cyclic heptapeptides, serinocyclins A (1) and B (2), were isolated from conidia of the entomopathogenic fungus Metarhizium anisopliae. Structures were elucidated by a combination of mass spectrometric, NMR, and X-ray diffraction techniques. Serinocyclin A (1) contains three serine units, a hydroxyproline (Hyp), a beta-alanine (beta-Ala), and two uncommon nonproteinogenic amino acids, 1-aminocyclopropane-1-carboxylic acid (Acc) and gamma-hydroxylysine (HyLys). The peptide sequence established for 1 by NMR is cyclo-(Acc-Hyp-Ser1-HyLys-beta-Ala-Ser2-Ser3). Serinocyclin B (2) has Lys in place of the HyLys unit found in 1. Chiral amino acid analysis indicated the presence in both compounds of one (2 S,4 R)-Hyp, two L-Ser, and one D-Ser residue. A Lys found in the hydrolyzate of 2 was established as D-configured. A crystal structure of 1 established the position of the D-Ser (Ser2) and the absolute configuration of the HyLys unit (2 R,4 S). The absence of methyl groups is unusual among fungal peptides and, along with the charged lysyl side chain and multiple hydroxyl groups, contributes to the polar nature of the compounds. Serinocyclin A produced a sublethal locomotory defect in mosquito larvae at an EC 50 of 59 ppm.

Production of mutagenic metabolites by Metarhizium anisopliae

NG-391 (1) and NG-393 (2), previously reported from undescribed Fusarium species as nerve-cell growth stimulants, were identified from fermentation extracts of the entomopathogenic fungus Metarhizium anisopliae. These compounds are 7-desmethyl analogues of fusarin C and (8Z)-fusarin C, mutagenic toxins from Fusarium species that contaminate corn. A mutant strain of M. anisopliae (KOB1-3) overproduces 1 and 2 by ca. 10-fold relative to the wild-type strain, ARSEF 2575, from which it was derived. Overproduction of these compounds in KOB1-3 imparts a yellow pigmentation to the culture medium of the fungus. These compounds were inactive at 100 mug/disk in antimicrobial disk diffusion assays. Compound 1 was inactive at 100 ppm in a mosquitocidal assay. However, like their fusarin analogues, 1 and 2 exhibited potent S9-dependent mutagenic activity in the Salmonella mutagenicity test. Discovery of these highly mutagenic mycotoxins in M. anisopliae suggests that screening for production of NG-391 and NG-393 in strains that are used as biocontrol agents would be a prudent course of action. The impact of these findings on the use of M. anisopliae as a biocontrol agent is currently unknown and requires further investigation.

Enhanced enzymatic hydrolysis of langostino shell chitin with mixtures of enzymes from bacterial and fungal sources

A combination of enzyme preparations from Trichoderma atroviride and Serratia marcescens was able to completely degrade high concentrations (100 g/L) of chitin from langostino crab shells to N-acetylglucosamine (78%), glucosamine (2%), and chitobiose (10%). The result was achieved at 32 degrees C in 12 days with no pre-treatment (size reduction or swelling) of the substrate and without removal of the inhibitory end-products from the mixture. Enzymatic degradation of three forms of chitin by Serratia/Trichoderma and Streptomyces/Trichoderma blends was carried out according to a simplex-lattice mixture design. Fitted polynomial models indicated that there was synergy between prokaryotic and fungal enzymes for both hydrolysis of crab chitin and reduction of turbidity of colloidal chitin (primarily endo-type activity). Prokaryotic/fungal enzymes were not synergistic in degrading chitosan. Enzymes from prokaryotic sources had much lower activity against chitosan than enzymes from T. atroviride.