Structure and Chemical Analysis of Major Specialized Metabolites Produced by the Lichen Evernia prunastri

We performed comparative profiling of four specialized metabolites in the lichen Evernia prunastri, collected at three different geographic locations, California and Maine, USA, and Yoshkar Ola, Mari El, Russia. Among the compounds produced at high concentrations that were identified in all three specimens, evernic acid, usnic acid, lecanoric acid and chloroatranorin, evernic acid was the most abundant. Two depsidones, salazinic acid and physodic acid, were detected in the Yoshkar-Ola collection only. The crystalline structure of evernic acid (2-hydroxy-4-[(2-hydroxy-4-methoxy-6-methylbenzoyl)oxy]-6-methylbenzoate) (hmb) revealed two crystallographically and conformationally distinct hmb anions, along with two monovalent sodium atoms. One hmb moiety contained an exotetradentate binding mode to sodium, whereas the other exhibited an exohexadentate binding mode to sodium. Embedded edge-sharing {Na₂ O₈ }_n sodium-oxygen chains connected the hmb anions into the full three-dimensional crystal structure of the title compound. The crystal used for single-crystal X-ray diffraction exhibited non-merohedral twinning. The data suggest the importance of the acetyl-polymalonyl pathway products to processes of maintaining integrity of the lichen holobiont community.

The Fungal bZIP Transcription Factor AtfB Controls Virulence-Associated Processes in Aspergillus parasiticus

Fungal basic leucine zipper (bZIP) transcription factors mediate responses to oxidative stress. The ability to regulate stress response pathways in Aspergillus spp. was postulated to be an important virulence-associated cellular process, because it helps establish infection in humans, plants, and animals. Previous studies have demonstrated that the fungal transcription factor AtfB encodes a protein that is associated with resistance to oxidative stress in asexual conidiospores, and AtfB binds to the promoters of several stress response genes. Here, we conducted a gene silencing of AtfB in Aspergillus parasiticus, a well-characterized fungal pathogen of plants, animals, and humans that produces the secondary metabolite and carcinogen aflatoxin, in order to determine the mechanisms by which AtfB contributes to virulence. We show that AtfB silencing results in a decrease in aflatoxin enzyme levels, the down-regulation of aflatoxin accumulation, and impaired conidiospore development in AtfB-silenced strains. This observation is supported by a decrease of AtfB protein levels, and the down-regulation of many genes in the aflatoxin cluster, as well as genes involved in secondary metabolism and conidiospore development. Global expression analysis (RNA Seq) demonstrated that AtfB functionally links oxidative stress response pathways to a broader and novel subset of target genes involved in cellular defense, as well as in actin and cytoskeleton arrangement/transport. Thus, AtfB regulates the genes involved in development, stress response, and secondary metabolism in A. parasiticus. We propose that the bZIP regulatory circuit controlled by AtfB provides a large number of excellent cellular targets to reduce fungal virulence. More importantly, understanding key players that are crucial to initiate the cellular response to oxidative stress will enable better control over its detrimental impacts on humans.

Aflatoxin levels in sunflower seeds and cakes collected from micro- and small-scale sunflower oil processors in Tanzania

Aflatoxin, a mycotoxin found commonly in maize and peanuts worldwide, is associated with liver cancer, acute toxicosis, and growth impairment in humans and animals. In Tanzania, sunflower seeds are a source of snacks, cooking oil, and animal feed. These seeds are a potential source of aflatoxin contamination. However, reports on aflatoxin contamination in sunflower seeds and cakes are scarce. The objective of the current study was to determine total aflatoxin concentrations in sunflower seeds and cakes from small-scale oil processors across Tanzania. Samples of sunflower seeds (n = 90) and cakes (n = 92) were collected across two years, and analyzed for total aflatoxin concentrations using a direct competitive enzyme-linked immunosorbent assay (ELISA). For seed samples collected June-August 2014, the highest aflatoxin concentrations were from Dodoma (1.7-280.6 ng/g), Singida (1.4-261.8 ng/g), and Babati-Manyara (1.8-162.0 ng/g). The highest concentrations for cakes were from Mbeya (2.8-97.7 ng/g), Dodoma (1.9-88.2 ng/g), and Singida (2.0-34.3 ng/g). For seed samples collected August-October 2015, the highest concentrations were from Morogoro (2.8-662.7 ng/g), Singida (1.6-217.6 ng/g) and Mbeya (1.4-174.2 ng/g). The highest concentrations for cakes were from Morogoro (2.7-536.0 ng/g), Dodoma (1.4-598.4 ng/g) and Singida (3.2-52.8 ng/g). In summary, humans and animals are potentially at high risk of exposure to aflatoxins through sunflower seeds and cakes from micro-scale millers in Tanzania; and location influences risk.

Effects of Zinc Chelators on Aflatoxin Production in Aspergillus parasiticus

Zinc concentrations strongly influence aflatoxin accumulation in laboratory media and in food and feed crops. The presence of zinc stimulates aflatoxin production, and the absence of zinc impedes toxin production. Initial studies that suggested a link between zinc and aflatoxin biosynthesis were presented in the 1970s. In the present study, we utilized two zinc chelators, N,N,N′,N′-tetrakis (2-pyridylmethyl) ethane-1,2-diamine (TPEN) and 2,3-dimercapto-1-propanesulfonic acid (DMPS) to explore the effect of zinc limitation on aflatoxin synthesis in Aspergillus parasiticus. TPEN but not DMPS decreased aflatoxin biosynthesis up to six-fold depending on whether A. parasiticus was grown on rich or minimal medium. Although we observed significant inhibition of aflatoxin production by TPEN, no detectable changes were observed in expression levels of the aflatoxin pathway gene ver-1 and the zinc binuclear cluster transcription factor, AflR. Treatment of growing A. parasiticus solid culture with a fluorescent zinc probe demonstrated an increase in intracellular zinc levels assessed by increases in fluorescent intensity of cultures treated with TPEN compared to controls. These data suggest that TPEN binds to cytoplasmic zinc therefore limiting fungal access to zinc. To investigate the efficacy of TPEN on food and feed crops, we found that TPEN effectively decreases aflatoxin accumulation on peanut medium but not in a sunflower seeds-derived medium. From an application perspective, these data provide the basis for biological differences that exist in the efficacy of different zinc chelators in various food and feed crops frequently contaminated by aflatoxin.

VeA is associated with the response to oxidative stress in the aflatoxin producer Aspergillus flavus

Survival of fungal species depends on the ability of these organisms to respond to environmental stresses. Osmotic stress or high levels of reactive oxygen species (ROS) can cause stress in fungi resulting in growth inhibition. Both eukaryotic and prokaryotic cells have developed numerous mechanisms to counteract and survive the stress in the presence of ROS. In many fungi, the HOG signaling pathway is crucial for the oxidative stress response as well as for osmotic stress response. This study revealed that while the osmotic stress response is only slightly affected by the master regulator veA, this gene, also known to control morphological development and secondary metabolism in numerous fungal species, has a profound effect on the oxidative stress response in the aflatoxin-producing fungus Aspergillus flavus. We found that the expression of A. flavus homolog genes involved in the HOG signaling pathway is regulated by veA. Deletion of veA resulted in a reduction in transcription levels of oxidative stress response genes after exposure to hydrogen peroxide. Furthermore, analyses of the effect of VeA on the promoters of cat1 and trxB indicate that the presence of VeA alters DNA-protein complex formation. This is particularly notable in the cat1 promoter, where the absence of VeA results in abnormally stronger complex formation with reduced cat1 expression and more sensitivity to ROS in a veA deletion mutant, suggesting that VeA might prevent binding of negative transcription regulators to the cat1 promoter. Our study also revealed that veA positively influences the expression of the transcription factor gene atfB and that normal formation of DNA-protein complexes in the cat1 promoter is dependent on AtfB.

Association with AflR in endosomes reveals new functions for AflJ in aflatoxin biosynthesis

Aflatoxins are the most potent naturally occurring carcinogens of fungal origin. Biosynthesis of aflatoxin involves the coordinated expression of more than 25 genes. The function of one gene in the aflatoxin gene cluster, aflJ, is not entirely understood but, because previous studies demonstrated a physical interaction between the Zn₂Cys₆ transcription factor AflR and AflJ, AflJ was proposed to act as a transcriptional co-activator. Image analysis revealed that, in the absence of aflJ in A. parasiticus, endosomes cluster within cells and near septa. AflJ fused to yellow fluorescent protein complemented the mutation in A. parasiticus ΔaflJ and localized mainly in endosomes. We found that AflJ co-localizes with AflR both in endosomes and in nuclei. Chromatin immunoprecipitation did not detect AflJ binding at known AflR DNA recognition sites suggesting that AflJ either does not bind to these sites or binds to them transiently. Based on these data, we hypothesize that AflJ assists in AflR transport to or from the nucleus, thus controlling the availability of AflR for transcriptional activation of aflatoxin biosynthesis cluster genes. AflJ may also assist in directing endosomes to the cytoplasmic membrane for aflatoxin export.

Oxidative stress-related transcription factors in the regulation of secondary metabolism

There is extensive and unequivocal evidence that secondary metabolism in filamentous fungi and plants is associated with oxidative stress. In support of this idea, transcription factors related to oxidative stress response in yeast, plants, and fungi have been shown to participate in controlling secondary metabolism. Aflatoxin biosynthesis, one model of secondary metabolism, has been demonstrated to be triggered and intensified by reactive oxygen species buildup. An oxidative stress-related bZIP transcription factor AtfB is a key player in coordinate expression of antioxidant genes and genes involved in aflatoxin biosynthesis. Recent findings from our laboratory provide strong support for a regulatory network comprised of at least four transcription factors that bind in a highly coordinated and timely manner to promoters of the target genes and regulate their expression. In this review, we will focus on transcription factors involved in co-regulation of aflatoxin biosynthesis with oxidative stress response in aspergilli, and we will discuss the relationship of known oxidative stress-associated transcription factors and secondary metabolism in other organisms. We will also talk about transcription factors that are involved in oxidative stress response, but have not yet been demonstrated to be affiliated with secondary metabolism. The data support the notion that secondary metabolism provides a secondary line of defense in cellular response to oxidative stress.

Evidence that a transcription factor regulatory network coordinates oxidative stress response and secondary metabolism in aspergilli

The mycotoxin aflatoxin is a secondary metabolite and potent human carcinogen. We investigated one mechanism that links stress response with coordinate activation of genes involved in aflatoxin biosynthesis in Aspergillus parasiticus. Electrophoretic mobility shift assays demonstrated that AtfB, a basic leucine zipper (bZIP) transcription factor, is a master co-regulator that binds promoters of early (fas-1), middle (ver-1), and late (omtA) aflatoxin biosynthetic genes as well as stress-response genes (mycelia-specific cat1 and mitochondria-specific Mn sod) at cAMP response element motifs. A novel conserved motif 5′-T/GNT/CAAG CCNNG/AA/GC/ANT/C-3′ was identified in promoters of the aflatoxin biosynthetic and stress-response genes. A search for transcription factors identified SrrA as a transcription factor that could bind to the motif. Moreover, we also identified a STRE motif (5′-CCCCT-3′) in promoters of aflatoxin biosynthetic and stress-response genes, and competition EMSA suggested that MsnA binds to this motif. Our study for the first time provides strong evidence to suggest that at least four transcription factors (AtfB, SrrA, AP-1, and MsnA) participate in a regulatory network that induces aflatoxin biosynthesis as part of the cellular response to oxidative stress in A. parasiticus.

Passage of Campylobacter jejuni through the chicken reservoir or mice promotes phase variation in contingency genes Cj0045 and Cj0170 that strongly associates with colonization and disease in a mouse model

Human illness due to Camplyobacter jejuni infection is closely associated with consumption of poultry products. We previously demonstrated a 50 % shift in allele frequency (phase variation) in contingency gene Cj1139 (wlaN) during passage of C. jejuni NCTC11168 populations through Ross 308 broiler chickens. We hypothesized that phase variation in contingency genes during chicken passage could promote subsequent colonization and disease in humans. To test this hypothesis, we passaged C. jejuni strains NCTC11168, 33292, 81-176, KanR4 and CamR2 through broiler chickens and analysed the ability of passaged and non-passaged populations to colonize C57BL6 IL-10-deficient mice, our model for human colonization and disease. We utilized fragment analysis and nucleotide sequence analysis to measure phase variation in contingency genes. Passage through the chicken reservoir promoted phase variation in five specific contingency genes, and these 'successful' populations colonized mice. When phase variation did not occur in these same five contingency genes during chicken passage, these 'unsuccessful' populations failed to colonize mice. Phase variation during chicken passage generated small insertions or deletions (indels) in the homopolymeric tract (HT) in contingency genes. Single-colony isolates of C. jejuni strain KanR4 carrying an allele of contingency gene Cj0170 with a10G HT colonized mice at high frequency and caused disease symptoms, whereas single-colony isolates carrying the 9G allele failed to colonize mice. Supporting results were observed for the successful 9G allele of Cj0045 in strain 33292. These data suggest that phase variation in Cj0170 and Cj0045 is strongly associated with mouse colonization and disease, and that the chicken reservoir can play an active role in natural selection, phase variation and disease.

Analysis of volatile compounds emitted by filamentous fungi using solid-phase microextraction-gas chromatography/mass spectrometry

Here, we describe a solid-phase microextraction-gas chromatography/mass spectrometry (SPME-GC/MS) analytical approach that identifies and analyzes volatile compounds in the headspace above a live fungal culture. This approach is a sensitive, solvent-free, robust technique; most importantly from a practical standpoint, this approach is noninvasive and requires minimal sample handling. Aliquots of liquid fungal cultures are placed into vials equipped with inert septa and equilibrated at a constant temperature, and headspace gases are sampled using an SPME fiber inserted through the septum into the headspace above the fungal culture for a standardized period of time. The outer polymer coating of a fused silica fiber absorbs volatiles from the headspace; the volatiles are then desorbed in the hot GC inlet and chromatographed in the usual manner. The separated compounds are subsequently identified by mass spectrometry. All steps in volatile profiling of a single sample from volatile sorption on a fiber to obtaining a list of volatiles can take as little as 15 min or can be extended to several hours if longer sorption is required for compounds present at very low levels and/or have low rates of diffusion.

Proteomic and biochemical evidence support a role for transport vesicles and endosomes in stress response and secondary metabolism in Aspergillus parasiticus

Aflatoxin is among the most potent naturally occurring carcinogens known. Previous studies demonstrated that endosomes in the filamentous fungus Aspergillus parasiticus carry enzymes that catalyze the final two steps in aflatoxin synthesis, and these structures also play a role in aflatoxin storage and export. We hypothesized that endosomes house a complete and functional aflatoxin biosynthetic pathway. To address this hypothesis, we purified a cellular fraction containing endosomes, transport vesicles, and vacuoles (V fraction) from A. parasiticus grown under aflatoxin inducing and noninducing conditions. We also added (fed) aflatoxin pathway intermediates to V fraction to test the functional status of aflatoxin pathway enzymes. High throughput LC-MS/MS analysis of proteins in V fraction detected 8 aflatoxin enzymes with high reliability and 8 additional enzymes at lower reliability, suggesting that most aflatoxin pathway enzymes are present. Purified V fraction synthesized aflatoxin and addition of the pathway intermediate versicolorin A increased aflatoxin synthesis, confirming that middle and late aflatoxin enzymes in V fraction are functional. Of particular significance, proteomic and biochemical analysis strongly suggested that additional secondary metabolic pathways as well as proteins involved in response to heat, osmotic, and oxidative stress are housed in V fraction.

Stress-related transcription factor AtfB integrates secondary metabolism with oxidative stress response in aspergilli

In filamentous fungi, several lines of experimental evidence indicate that secondary metabolism is triggered by oxidative stress; however, the functional and molecular mechanisms that mediate this association are unclear. The basic leucine zipper (bZIP) transcription factor AtfB, a member of the bZIP/CREB family, helps regulate conidial tolerance to oxidative stress. In this work, we investigated the role of AtfB in the connection between oxidative stress response and secondary metabolism in the filamentous fungus Aspergillus parasiticus. This well characterized model organism synthesizes the secondary metabolite and carcinogen aflatoxin. Chromatin immunoprecipitation with specific anti-AtfB demonstrated AtfB binding at promoters of seven genes in the aflatoxin gene cluster that carry CREs. Promoters lacking CREs did not show AtfB binding. The binding of AtfB to the promoters occurred under aflatoxin-inducing but not under aflatoxin-noninducing conditions and correlated with activation of transcription of the aflatoxin genes. Deletion of veA, a global regulator of secondary metabolism and development, nearly eliminated this binding. Electrophoretic mobility shift analysis demonstrated that AtfB binds to the nor-1 (an early aflatoxin gene) promoter at a composite regulatory element that consists of highly similar, adjacent CRE1 and AP-1-like binding sites. The five nucleotides immediately upstream from CRE1, AGCC(G/C), are highly conserved in five aflatoxin promoters that demonstrate AtfB binding. We propose that AtfB is a key player in the regulatory circuit that integrates secondary metabolism and cellular response to oxidative stress.

Willow volatiles influence growth, development, and secondary metabolism in Aspergillus parasiticus

Aflatoxin is a mycotoxin and the most potent naturally occurring carcinogen in many animals. Aflatoxin contamination of food and feed crops causes a significant global burden on human and animal health. However, available methods to eliminate aflatoxin from food and feed are not fully effective. Our goal is to discover novel, efficient, and practical methods to control aflatoxin contamination in crops during storage. In the present study, we tested the effect of volatiles produced by willow (Salix acutifolia and Salix babylonica) and maple (Acer saccharinum) bark on fungal growth, development, and aflatoxin production by the fungus Aspergillus parasiticus, one economically important aflatoxin producer. S. acutifolia bark volatiles nearly eliminated aflatoxin accumulation (>90% reduction) by A. parasiticus grown on a minimal agar medium. The decrease in aflatoxin accumulation correlated with a twofold reduction in ver-1 (encodes a middle aflatoxin pathway enzyme) transcript level. Expression data also indicate that one histone H4 acetyltransferase, MYST3, may play a role in epigenetic control of aflatoxin gene transcription in response to volatile exposure. Volatiles derived from wood bark samples also increased fungal growth up to 20% and/or enhanced conidiospore development. Solid-phase microextraction-gas chromatographic-mass spectrometric analysis of bark samples identified sets of shared and unique volatile compounds that may mediate the observed regulatory effects on growth, development, and aflatoxin synthesis. This work provides an experimental basis for the use of willow industry by-products to control aflatoxin contamination in food and feed crops.

Volatile profiling reveals intracellular metabolic changes in Aspergillus parasiticus: veA regulates branched chain amino acid and ethanol metabolism

Background

Filamentous fungi in the genus Aspergillus produce a variety of natural products, including aflatoxin, the most potent naturally occurring carcinogen known. Aflatoxin biosynthesis, one of the most highly characterized secondary metabolic pathways, offers a model system to study secondary metabolism in eukaryotes. To control or customize biosynthesis of natural products we must understand how secondary metabolism integrates into the overall cellular metabolic network. By applying a metabolomics approach we analyzed volatile compounds synthesized by Aspergillus parasiticus in an attempt to define the association of secondary metabolism with other metabolic and cellular processes.

Results

Volatile compounds were examined using solid phase microextraction - gas chromatography/mass spectrometry. In the wild type strain Aspergillus parasiticus SU-1, the largest group of volatiles included compounds derived from catabolism of branched chain amino acids (leucine, isoleucine, and valine); we also identified alcohols, esters, aldehydes, and lipid-derived volatiles. The number and quantity of the volatiles produced depended on media composition, time of incubation, and light-dark status. A block in aflatoxin biosynthesis or disruption of the global regulator veA affected the volatile profile. In addition to its multiple functions in secondary metabolism and development, VeA negatively regulated catabolism of branched chain amino acids and synthesis of ethanol at the transcriptional level thus playing a role in controlling carbon flow within the cell. Finally, we demonstrated that volatiles generated by a veA disruption mutant are part of the complex regulatory machinery that mediates the effects of VeA on asexual conidiation and sclerotia formation.

Conclusions

1) Volatile profiling provides a rapid, effective, and powerful approach to identify changes in intracellular metabolic networks in filamentous fungi. 2) VeA coordinates the biosynthesis of secondary metabolites with catabolism of branched chain amino acids, alcohol biosynthesis, and β-oxidation of fatty acids. 3) Intracellular chemical development in A. parasiticus is linked to morphological development. 4) Understanding carbon flow through secondary metabolic pathways and catabolism of branched chain amino acids is essential for controlling and customizing production of natural products.

Compartmentalization and molecular traffic in secondary metabolism: a new understanding of established cellular processes

Great progress has been made in understanding the regulation of expression of genes involved in secondary metabolism. Less is known about the mechanisms that govern the spatial distribution of the enzymes, cofactors, and substrates that mediate catalysis of secondary metabolites within the cell. Filamentous fungi in the genus Aspergillus synthesize an array of secondary metabolites and provide useful systems to analyze the mechanisms that mediate the temporal and spatial regulation of secondary metabolism in eukaryotes. For example, aflatoxin biosynthesis in Aspergillus parasiticus has been studied intensively because this mycotoxin is highly toxic, mutagenic, and carcinogenic in humans and animals. Using aflatoxin synthesis to illustrate key concepts, this review focuses on the mechanisms by which sub-cellular compartmentalization and intra-cellular molecular traffic contribute to the initiation and completion of secondary metabolism within the cell. We discuss the recent discovery of aflatoxisomes, specialized trafficking vesicles that participate in the compartmentalization of aflatoxin synthesis and export of the toxin to the cell exterior; this work provides a new and clearer understanding of how cells integrate secondary metabolism into basic cellular metabolism via the intra-cellular trafficking machinery.

Genetic diversity in Campylobacter jejuni is associated with differential colonization of broiler chickens and C57BL/6J IL10-deficient mice

Previous studies have demonstrated that Campylobacter jejuni, the leading causative agent of bacterial food-borne disease in the USA, exhibits high-frequency genetic variation that is associated with changes in cell-surface antigens and ability to colonize chickens. To expand our understanding of the role of genetic diversity in the disease process, we analysed the ability of three C. jejuni human disease isolates (strains 11168, 33292 and 81-176) and genetically marked derivatives to colonize Ross 308 broilers and C57BL/6J IL10-deficient mice. C. jejuni colonized broilers at much higher efficiency (all three strains, 23 of 24 broilers) than mice (11168 only, 8 of 24 mice). C. jejuni 11168 genetically marked strains colonized mice at very low efficiency (2 of 42 mice); however, C. jejuni reisolated from mice colonized both mice and broilers at high efficiency, suggesting that this pathogen can adapt genetically in the mouse. We compared the genome composition in the three wild-type C. jejuni strains and derivatives by microarray DNA/DNA hybridization analysis; the data demonstrated a high degree of genetic diversity in three gene clusters associated with synthesis and modification of the cell-surface structures capsule, flagella and lipo-oligosaccharide. Finally, we analysed the frequency of mutation in homopolymeric tracts associated with the contingency genes wlaN (GC tract) and flgR (AT tracts) in culture and after passage through broilers and mice. C. jejuni adapted genetically in culture at high frequency and the degree of genetic diversity was increased by passage through broilers but was nearly eliminated in the gastrointestinal tract of mice. The data suggest that the broiler gastrointestinal tract provides an environment which promotes outgrowth and genetic variation in C. jejuni; the enhancement of genetic diversity at this location may contribute to its importance as a human disease reservoir.

Prevalence and patterns of antimicrobial resistance in Campylobacter spp isolated from pigs reared under antimicrobial-free and conventional production methods in eight states in the Midwestern United States

OBJECTIVE

To compare apparent prevalence and patterns of antimicrobial resistance in Campylobacter spp in feces collected from pigs reared with antimicrobial-free versus conventional production methods in 8 states in the Midwestern United States.

DESIGN

Cross-sectional study.

SAMPLE POPULATION

95 swine farms that used antimicrobial-free (n = 35) or conventional (60) production methods.

PROCEDURES

Fecal samples from 15 pigs/farm were collected. Biochemical and multiplex-PCR analyses were used to identify Campylobacter spp. The minimal inhibitory concentrations of erythromycin, azithromycin, ciprofloxacin, nalidixic acid, gentamicin, and tetracycline for these organisms were determined by use of a commercially available antimicrobial gradient strip. The data were analyzed by use of population-averaged statistical models.

RESULTS

Campylobacter spp were isolated from 512 of 1,422 pigs. A subset (n = 464) of the 512 isolates was available for antimicrobial susceptibility testing. The apparent prevalence of Campylobacter spp isolates from pigs on conventional farms (35.8%) and antimicrobial-free farms (36.4%) did not differ significantly. Resistances to azithromycin, erythromycin, and tetracycline were significantly higher on conventional farms (70.0%, 68.3%, and 74.5%, respectively) than antimicrobial-free farms (20.1%, 21.3%, and 48.8%, respectively). Resistances to azithromycin, erythromycin, and tetracycline declined as the number of years that a farm was antimicrobial-free increased.

CONCLUSIONS AND CLINICAL RELEVANCE

Production method did not affect the apparent prevalence of Campylobacter spp on swine farms. However, antimicrobial-free farms had a significantly lower prevalence of antimicrobial resistance. Although cessation of antimicrobial drug use will lower resistance over time, investigation of other interventions designed to reduce resistance levels is warranted.

Functional expression and sub-cellular localization of the early aflatoxin pathway enzyme Nor-1 in Aspergillus parasiticus

Aflatoxin biosynthesis in Aspergillus parasiticus requires at least 17 enzyme activities (from acetate). Although the activities of most aflatoxin biosynthetic enzymes have been established, the mechanisms that govern transport and sub-cellular localization of these enzymes are not clear. We developed plasmid constructs that express Nor-1 fused to a green fluorescent protein reporter (EGFP) to monitor transport and localization of this early pathway enzyme in real time in Aspergillus parasiticus. Plasmids expressing EGFP fused to Nor-1 were introduced into A. parasiticus B62 (carries non-functional Nor-1). Transformants were screened for increased aflatoxin accumulation (restored Nor-1 activity) on coconut agar medium and for EGFP expression using fluorescence microscopy. Increased aflatoxin accumulation was confirmed by TLC and ELISA. Nor-1 fused to EGFP at either the N- or C- terminus functionally complemented non-functional Nor-1 in B62 and increased aflatoxin synthesis to wild-type (N-terminus) or lower levels (C-terminus). We detected full-length Nor-1 fusion proteins in transformants with increased aflatoxin accumulation (Western blot) and determined that the expression plasmid integrated at the nor-1 locus in these cells (Southern blot). Confocal laser scanning microscopy (CLSM) demonstrated that Nor-1 fusion proteins localized in the cytoplasm and vacuoles of fungal hyphae grown on aflatoxin-inducing solid media for 48h; control EGFP (no Nor-1) did not localize to vacuoles until 72h. The highest rate of aflatoxin synthesis coincided with the highest rate of transport of Nor-1 fusion proteins to the vacuole strongly suggesting that Nor-1 is synthesized in the cytoplasm and transported to the vacuole to carry out an early step in aflatoxin synthesis.

Purification of a vesicle-vacuole fraction functionally linked to aflatoxin synthesis in Aspergillus parasiticus

Current studies in our laboratory demonstrate a functional link between vesicles, vacuoles and aflatoxin biosynthesis in the filamentous fungus, Aspergillus parasiticus. Under aflatoxin inducing conditions in liquid yeast-extract sucrose medium, A. parasiticus undergoes a shift from vacuole biogenesis to accumulation of an enhanced number of vesicles which exhibit significant heterogeneity in size and density. As a first step in conducting a detailed analysis of the role of these organelles in aflatoxin synthesis, we developed a novel method to purify the vesicle and vacuole fraction using protoplasts prepared from cells harvested during aflatoxin synthesis. The method includes the following steps: 1] preparation of protoplasts from mycelia grown for 36 h under aflatoxin inducing conditions; 2] release of vesicles and vacuoles from purified protoplasts in the presence of Triton X-100; and 3] fractionation of the vesicles and vacuoles using a "one-step high density cushion". The vesicle-vacuole fraction showed a 35 fold enrichment in alpha-mannosidase activity (vacuole marker) and non-detectable succinate dehydrogenase and lactate dehydrogenase activities (mitochondrial and cytoplasmic markers, respectively). Confocal laser scanning microscopy with the vacuole dyes MDY-64 and CMAC demonstrated that the fraction contained pure vesicles and vacuoles and was devoid of membranous debris. Transmission electron microscopy (TEM) confirmed that no mitochondria or unbroken protoplasts contaminated the purified fraction. The purified organelles exhibited significant size heterogeneity with a range of sizes similar to that observed in whole cells and protoplasts.

Evidence that a wortmannin-sensitive signal transduction pathway regulates aflatoxin biosynthesis

A signal transduction pathway involving cAMP and protein kinase A (PKA) regulates aflatoxin accumulation and nor-1 and ver-1 (aflatoxin structural genes) promoter function in Aspergillus parasiticus by modulating expression of a key transcriptional activator, AflR. To understand the function of this pathway in greater detail we treated A. parasiticus in culture with wortmannin, a frequently used probe of phosphatidyl inositol (PI)-3 kinase activity. A. parasiticus D8D3 (nor-1::GUS reporter) and I4 (ver-1::GUS reporter) were grown on a defined solid growth medium (GMS agar) under aflatoxin-inducing conditions. GMS containing wortmannin (1 microM) reduced aflatoxin B1 accumulation up to 15-fold accompanied by a similarly large decrease in ver-1 and nor-1 promoter activity. Wortmannin inhibited growth (colony diameter) and asexual sporulation but to a much smaller extent. Wortmannin treatment increased intracellular cAMP levels up to 25-fold; total PKA activity also increased within 10 min of wortmannin exposure. These data support a regulatory model in which PI-3 kinase activity modulates intracellular cAMP accumulation and PKA activity. This in turn regulates AflR expression and activity, aflatoxin gene expression and aflatoxin accumulation.

Altered hydrophobicity and membrane composition in stress-adapted Listeria innocua

Exposure of Listeria innocua to acid and starvation stress decreases sensitivity to the quaternary ammonium compound cetrimide, whereas exposure to cold and heat stress increases sensitivity to this compound. Changes in membrane lipids occur in response to certain types of stress, and these changes likely impact cell sensitivity to chemical sanitizers. The present study included an assessment of the effects of acid, starvation, cold, and heat stress on net cell hydrophobicity and fatty acid composition in L. innocua. Net cell hydrophobicity was determined by measuring absorbance of stress-adapted cell suspensions after partitioning with the nonpolar solvent n-hexadecane. Free fatty acids extracted from stress-adapted suspensions were analyzed by gas chromatography. Adaptation to acid and starvation increased net cell hydrophobicity and decreased membrane fluidity, which was correlated with reductions in anteiso fatty acids and in ratios of anteiso to iso fatty acids. Conversely, cold-stressed populations exhibited decreased net cell hydrophobicity and increased membrane fluidity with a corresponding increase in C15:C17 and anteiso:iso ratios and in C18 unsaturated fatty acids. No significant changes in net cell hydrophobicity or membrane fluidity were observed in heat-stressed cells, which exhibited increased sensitivity to cetrimide, suggesting another mechanism for altered cell sensitivity. These findings indicate that the efficacy of cetrimide against Listeria is partially dependent on the physiological state of the organism following exposure to various environmental stresses.

The initiation and pattern of spread of histone H4 acetylation parallel the order of transcriptional activation of genes in the aflatoxin cluster

The 27 genes involved in aflatoxin biosynthesis are clustered within a 70 kb region in the Aspergillus parasiticus genome. Using chromatin immunoprecipitation, we demonstrated a positive correlation between the initiation and spread of histone H4 acetylation in aflatoxin promoters and the onset of accumulation of aflatoxin proteins and aflatoxin. Histone H4 acetylation in the pksA (encodes an 'early' biosynthetic pathway enzyme) promoter peaked at 30 h, prior to the increased acetylation in the omtA and ordA (encode 'late' enzymes) promoters detected at 40 h. The specific order in which pksA, ver-1 (encodes a 'middle' enzyme) and omtA transcripts accumulated in cells paralleled the pattern of spread of histone H4 acetylation. Binding of AflR, a positive regulator of aflatoxin biosynthesis, to the ordA promoter showed a positive correlation with the spread of histone H4 acetylation. The data suggest that the order of genes within the aflatoxin cluster determines the timing and order of transcriptional activation, and that the site of initiation and spread of histone H4 acetylation mediate this process. Our data indicate that the aflatoxin and adjacent sugar utilization clusters are part of a larger 'regulatory unit'.

Aspergillus volatiles regulate aflatoxin synthesis and asexual sporulation in Aspergillus parasiticus

Aspergillus parasiticus is one primary source of aflatoxin contamination in economically important crops. To prevent the potential health and economic impacts of aflatoxin contamination, our goal is to develop practical strategies to reduce aflatoxin synthesis on susceptible crops. One focus is to identify biological and environmental factors that regulate aflatoxin synthesis and to manipulate these factors to control aflatoxin biosynthesis in the field or during crop storage. In the current study, we analyzed the effects of aspergillus volatiles on growth, development, aflatoxin biosynthesis, and promoter activity in the filamentous fungus A. parasiticus. When colonies of Aspergillus nidulans and A. parasiticus were incubated in the same growth chamber, we observed a significant reduction in aflatoxin synthesis and asexual sporulation by A. parasiticus. Analysis of the headspace gases demonstrated that A. nidulans produced much larger quantities of 2-buten-1-ol (CA) and 2-ethyl-1-hexanol (EH) than A. parasiticus. In its pure form, EH inhibited growth and increased aflatoxin accumulation in A. parasiticus at all doses tested; EH also stimulated aflatoxin transcript accumulation. In contrast, CA exerted dose-dependent up-regulatory or down-regulatory effects on aflatoxin accumulation, conidiation, and aflatoxin transcript accumulation. Experiments with reporter strains carrying nor-1 promoter deletions and mutations suggested that the differential effects of CA were mediated through separate regulatory regions in the nor-1 promoter. The potential efficacy of CA as a tool for analysis of transcriptional regulation of aflatoxin biosynthesis is discussed. We also identify a novel, rapid, and reliable method to assess norsolorinic acid accumulation in solid culture using a Chroma Meter CR-300 apparatus.

C57BL/6 and congenic interleukin-10-deficient mice can serve as models of Campylobacter jejuni colonization and enteritis

Campylobacter jejuni is a globally distributed cause of human food-borne enteritis and has been linked to chronic joint and neurological diseases. We hypothesized that C. jejuni 11168 colonizes the gastrointestinal tract of both C57BL/6 mice and congenic C57BL/6 interleukin-10-deficient (IL-10(-/-)) mice and that C57BL/6 IL-10(-/-) mice experience C. jejuni 11168-mediated clinical signs and pathology. Individually housed mice were challenged orally with C. jejuni 11168, and the course of infection was monitored by clinical examination, bacterial culture, C. jejuni-specific PCR, gross pathology, histopathology, immunohistochemistry, and anti-C. jejuni-specific serology. Ceca of C. jejuni 11168-infected mice were colonized at high rates: ceca of 50/50 wild-type mice and 168/170 IL-10(-/-) mice were colonized. In a range from 2 to 35 days after infection with C. jejuni 11168, C57BL/6 IL-10(-/-) mice developed severe typhlocolitis best evaluated at the ileocecocolic junction. Rates of colonization and enteritis did not differ between male and female mice. A dose-response experiment showed that as little as 10(6) CFU produced significant disease and pathological lesions similar to responses seen in humans. Immunohistochemical staining demonstrated C. jejuni antigens within gastrointestinal tissues of infected mice. Significant anti-C. jejuni plasma immunoglobulin levels developed by day 28 after infection in both wild-type and IL-10-deficient animals; antibodies were predominantly T-helper-cell 1 (Th1)-associated subtypes. These results indicate that the colonization of the mouse gastrointestinal tract by C. jejuni 11168 is necessary but not sufficient for the development of enteritis and that C57BL/6 IL-10(-/-) mice can serve as models for the study of C. jejuni enteritis in humans.

Detection of Escherichia coli in packaged alfalfa sprouts with an electronic nose and an artificial neural network

A rapid method for the detection of Escherichia coli (ATCC 25922) in packaged alfalfa sprouts was developed. Volatile compounds from the headspace of packaged alfalfa sprouts, inoculated with E. coli and incubated at 10 degrees C for 1, 2, and 3 days, were collected and analyzed. Uninoculated sprouts were used as control samples. An electronic nose with 12 metal oxide electronic sensors was used to monitor changes in the composition of the gas phase of the package headspace with respect to volatile metabolites produced by E. coli. The electronic nose was able to differentiate between samples with and without E. coli. To predict the number of E. coli in packaged alfalfa sprouts, an artificial neural network was used, which included an input layer, a hidden layer, and an output layer, with a hyperbolic tangent sigmoidal transfer function in the hidden layer and a linear transfer function in the output layer. The network was shown to be capable of correlating voltametric responses with the number of E. coli. A good prediction was possible, as measured by a regression coefficient (R2 = 0.903) between the actual and predicted data. In conjunction with the artificial neural network, the electronic nose proved to have the ability to detect E. coli in packaged alfalfa sprouts.

Hexanoate synthase, a specialized type I fatty acid synthase in aflatoxin B1 biosynthesis

In fungi, fatty acids are biosynthesized by large multifunctional enzyme complexes, the fatty acid synthases (FASs), which catalyze chain assembly in an iterative manner. Many fungal secondary metabolites contain fatty acid moieties, and it is often unclear whether they are recruited from primary metabolism or are biosynthesized de novo by secondary metabolic FASs. The most convincing evidence of such a dedicated FAS comes from the biosyntheses of aflatoxin (AF) and sterigmatocystin (ST) in certain species of the filamentous fungus Aspergillus. Incorporation studies in AF and genetic analyses of ST and AF biosynthesis strongly suggest that their biosyntheses begin with the production of a C6 fatty acid by a specialized FAS. The genes encoding the alpha (hexA) and beta (hexB) subunits of this hexanoate synthase (HexS) from the AF pathway in Aspergillus parsiticus SU-1 were cloned and both their gDNAs and cDNAs were sequenced and their transcriptional ends analyzed. Translated amino acid sequences are predicted to result in proteins of 181.3 and 210.5 kDa, for HexA and HexB, respectively. Comparison of the HexA and HexB sequences with those of the ST FAS subunits and primary metabolic FASs indicated that the secondary metabolic enzymes are members of a well-defined subclass of the FAS family. Phylogenetic predictions and an analysis of GC-bias in AF and ST pathway genes compared with primary metabolic Aspergillus genes were used as a basis to propose a route for the evolution of the AF and ST clusters.

Role of cis-acting sites NorL, a TATA box, and AflR1 in nor-1 transcriptional activation in Aspergillus parasiticus

The transcription factor AflR is required for up-regulation of specific pathway genes involved in aflatoxin biosynthesis in the filamentous fungus Aspergillus. nor-1 encodes an early aflatoxin pathway enzyme; its promoter contains a consensus AflR binding site (AflR1). Proteins in Aspergillus parasiticus cell extracts and AflR expressed in Escherichia coli do not bind to A. parasiticus AflR1 in vitro, so it was not clear if this site was required for nor-1 expression or if other transcription factors contributed to gene regulation. In this study we defined the role of AflR1 in nor-1 expression in A. parasiticus and identified additional cis-acting sites required for maximum nor-1 transcriptional activation. Deletion and substitution of AflR1 in the nor-1 promoter in A. parasiticus nor-1::GUS reporter strains showed that this site is required for nor-1 transcriptional activation in vivo. Substitution of a putative TATA box in the nor-1 promoter resulted in nondetectable beta-glucuronidase (GUS) activity, demonstrating that this TATA box is functional in vivo. We also identified a novel cis-acting site, designated NorL, between residues -210 and -238 that was required for maximum nor-1 transcriptional activation in A. parasiticus grown in liquid medium and on solid medium. Using an electrophoretic mobility shift assay, we identified a specific NorL-dependent DNA-protein complex that relies on a functional AflR, either directly or indirectly, for maximum binding capacity. Because the NorL site appears only once in the aflatoxin gene cluster, its association with the nor-1 promoter may have important implications for the overall regulatory scheme for the aflatoxin pathway.

Regulation of aflatoxin synthesis by FadA/cAMP/protein kinase A signaling in Aspergillus parasiticus

Analysis of fadA and pkaA mutants in the filamentous fungus Aspergillus nidulans demonstrated that FadA (Galpha) stimulates cyclic AMP (cAMP)-dependent protein kinase A (PKA) activity resulting, at least in part, in inhibition of conidiation and sterigmatocystin (ST) biosynthesis. In contrast, cAMP added to the growth medium stimulates aflatoxin (AF) synthesis in Aspergillus parasiticus. Our goal was to explain these conflicting reports and to provide mechanistic detail on the role of FadA, cAMP, and PKA in regulation of AF synthesis and conidiation in A. parasiticus. cAMP or dibutyryl-cAMP (DcAMP) were added to a solid growth medium and intracellular cyclic nucleotide levels, PKA activity, and nor-1 promoter activity were measured in A. parasiticus D8D3 (nor1::GUS reporter) and TJYP1-22 (fadAGA2R, activated allele). Similar to Tice and Buchanan [34], cAMP or DcAMP stimulated AF synthesis (and conidiation) associated with an AflR-dependent increase in nor-1 promoter activity. However, treatment resulted in a 100-fold increase in intracellular cAMP/DcAMP accompanied by a 40 to 80 fold decrease in total PKA activity. ThefadAG42R allele in TJYP1-22 decreased AF synthesis and conidiation, increased basal PKA activity 10 fold, and decreased total PKA activity 2 fold. In TJYP1-22, intracellular cAMP increased 2 fold without cAMP or DcAMP treatment; treatment did not stimulate conidiation or AF synthesis. Based on these data, we conclude that: (1) FadA/PKA regulate toxin synthesis and conidiation via similar mechanisms in Aspergillus spp.; and (2) intracellular cAMP levels, at least in part, mediate a PKA-dependent regulatory influence on conidiation and AF synthesis.

Identification of two alphaRYR alleles and characterization of alphaRYR transcript variants in turkey skeletal muscle

We hypothesized that a mutation in the turkey skeletal muscle ryanodine receptor alpha isoform (alphaRYR) underlies turkey meat quality problems which are strikingly similar to pale, soft, exudative (PSE) pork. RT-PCR analysis of turkey alphaRYR mRNA covering amino acids 376 to 615 (numbered according to the human sequence) revealed at least three transcript variants. One transcript was homologous to the mammalian skeletal muscle RYR1 sequence in this region. The second transcript variant (AS-81) was characterized by the absence of 81 bases located at the beginning of exon 13, while the third transcript variant (AS-193) carried a deletion of 193 bases, corresponding to the entire exon 13. Two alphaRYR genomic DNA alleles (alphaRYR-I and alphaRYR-II) carrying the region of deletions in the turkey cDNA sequences were identified. Nucleotide sequence analysis demonstrated that the two alleles are identical in exon sequences but different in intron sequences. Comparison of genomic and cDNA sequences indicated that both AS-81 and AS-193 transcript variants probably arose via alternative splicing. Consistent with this mechanism, the last eight nucleotides of the 81 bases form a consensus sequence for a splice acceptor site. Both alleles could give rise to the AS-81 and AS-193 transcript variants via alternative splicing. Birds homozygous for alphaRYR-II tended to have superior meat quality indicators including significantly higher muscle pH at 15-min post mortem and lower muscle exudate at 24-h post mortem, compared to birds homozygous for alphaRYR-I.

Rapid method for prediction of Escherichia coli numbers using an electronic sensor array and an artificial neural network

An electronic sensor array with 12 nonspecific metal oxide sensors was evaluated for its ability to monitor volatile compounds in super broth alone and in super broth inoculated with Escherichia coli (ATCC 25922) at 37 degrees C for 2 to 12 h. Using discriminant function analysis, it was possible to differentiate super broth alone from that containing E. coli when cell numbers were 10(5) CFU or more. There was a good agreement between the volatile profiles from the electronic sensor array and a gas chromatography-mass spectrometer method. The potential to predict the number of E. coli and the concentration of specific metabolic compounds was investigated using an artificial neural network (ANN). The artificial neural network was composed of an input layer, one hidden layer, and an output layer, with a hyperbolic tangent sigmoidal transfer function in the hidden layer and a linear transfer function in the output layer. Good prediction was found as measured by a regression coefficient (R2 = 0.999) between actual and predicted data.

Solid-phase microextraction, gas chromatography, and mass spectrometry coupled with discriminant factor analysis and multilayer perceptron neural network for detection of Escherichia coli

This study was performed to investigate the ability of using discriminant factor analysis (DFA) and an artificial neural network (ANN) to identify and quantify the number of Escherichia coli (ATCC 25922) in nutrient media from data generated by analysis of E. coli volatile metabolic compounds using solid-phase microextraction (SPME) coupled with gas chromatography (GC) and mass spectrometry (MS). E. coli was grown in super broth and incubated at 37 degrees C for 2 to 12 h. Numbers of E. coli were followed using a colony counting method. An SPME device was used to collect the volatiles from the headspace above the samples, and the volatiles were identified using GC-MS. DFA was used to classify the samples from different incubation times. From DFA, it was possible to differentiate super broth from media containing E. coli when cell numbers were 10(5) CFU or more. The potential to predict the number of E. coli from the SPME-GC-MS data was investigated using a multilayer perceptron (MLP) neural network with back propagation training. The MLP comprised an input layer, one hidden layer, and an output layer, with a hyperbolic tangent sigmoidal transfer function in the hidden layer and a linear transfer function in the output layer. Good prediction was found as measured by a regression coefficient (R2 = 0.996) between actual and predicted data.

Distribution and sub-cellular localization of the aflatoxin enzyme versicolorin B synthase in time-fractionated colonies of Aspergillus parasiticus

Aflatoxins are highly toxic and carcinogenic fungal secondary metabolites. At least 18 enzyme activities are required for aflatoxin biosynthesis in the filamentous fungus Aspergillus parasiticus. One of these enzymes, versicolorin B synthase (VBS), catalyzes bisfuran ring closure in versiconal hemiacetal (a reaction near the middle of the pathway) to form versicolorin B. This reaction is required for the subsequent activation to aflatoxin B1-8,9 epoxide, a highly reactive and toxic aflatoxin metabolite, and is important for aflatoxin toxicity. We analyzed the localization of VBS in the aflatoxin-producing strain A. parasiticus SU-1 grown on solid media using a colony fractionation technique developed previously. A highly specific polyclonal antibody, raised against a maltose-binding protein-VBS fusion protein synthesized in Escherichia coli, was used to detect VBS in SU-1 grown on a rich solid medium via immunofluorescence confocal laser scanning microscopy (CLSM) and immunogold transmission electron microscopy (TEM). VBS was detected in both vegetative hyphae and in asexual developmental structures, called conidiophores. Western blot and CLSM analyses demonstrated the highest abundance of VBS in colony fraction S2 consisting of cells that had grown for 24-48 h; this fraction also contained the highest levels of newly developed conidiophores and the highest abundance of aflatoxin B1, consistent with VBS abundance. At the subcellular level, CLSM and TEM detected VBS distributed throughout the cytoplasm and concentrated in ring-like structures surrounding nuclei. It is uncertain whether enzymatically active VBS is present in either or both locations.

A novel cAMP-response element, CRE1, modulates expression of nor-1 in Aspergillus parasiticus

The level of aflatoxin accumulation in the filamentous fungus Aspergillus parasiticus is modulated by a variety of environmental cues. The presence of glucose (a preferred carbon source) in liquid and solid glucose minimal salts (GMS) growth media strongly stimulated aflatoxin accumulation. Peptone (a non-preferred carbon source) in peptone minimal salts (PMS) media stimulated only low levels of aflatoxin accumulation. Glucose stimulated transcription of the aflatoxin structural genes ver-1 and nor-1 to similar intermediate levels in liquid GMS, while on solid media, ver-1 transcription was stimulated to 20-fold higher levels than nor-1. PMS liquid and solid media stimulated very low or non-detectable levels of transcription of both genes. Electrophoretic mobility shift analysis using a nor-1 promoter fragment (norR) and A. parasiticus cell protein extracts revealed specific DNA-protein complexes of different mobility on GMS and PMS solid and liquid media. An imperfect cAMP-response element, CRE1, was identified in norR that mediated formation of the specific DNA-protein complexes. Mutation in CRE1 or AflR1 (AflR cis-acting site) caused up to a 3-fold decrease in cAMP-mediated stimulation of nor-1 promoter activity on GMS agar. South-Western blot analysis identified a 32-kDa protein that specifically bound to norR. p32 could be co-immunoprecipitated by anti-AflR antibody and co-purified with an AflR-maltose-binding protein fusion demonstrating a physical interaction between AflR and p32 in vitro. We hypothesize that p32 assists AflR in binding to the nor-1 promoter, thereby modulating nor-1 gene expression in response to environmental cues.

Subcellular localization of aflatoxin biosynthetic enzymes Nor-1, Ver-1, and OmtA in time-dependent fractionated colonies of Aspergillus parasiticus

The biosynthesis of aflatoxin in Aspergillus parasiticus is a complex process that involves the activities of at least 18 pathway enzymes. The distribution of these enzymes within fungal colonies and fungal cells is not clearly understood. The objective of this study was to investigate the distribution and subcellular location of Nor-1, Ver-1, and OmtA, which represent early, middle, and late enzymatic activities, respectively, in the aflatoxin biosynthetic pathway. The distribution of these three enzymes within A. parasiticus SU-1 was analyzed in time-fractionated, 72-h fungal colonies (fraction 1, 48-72 h; fraction 2, 24-48 h; fraction 3, 0-24 h). Western blot analysis and immunofluorescence microscopy demonstrated the highest abundance of Nor-1, Ver-1, and OmtA in colony fraction 2. Fungal tissues in this fraction were analyzed by immunoelectron microscopy. Nor-1 and Ver-1 were primarily localized to the cytoplasm, suggesting that they are cytosolic enzymes. OmtA was also detected in the cytoplasm. However, in cells located near the basal (substrate) surface of the colony, OmtA was predominantly detected in organelles tentatively identified as vacuoles. The role of this organelle in toxin biosynthesis is unclear. The relative distribution of OmtA to the cytoplasm or to vacuole-like organelles may depend on the age and/or physiological condition of the fungal cells.

Ethylene modulates development and toxin biosynthesis in aspergillus possibly via an ethylene sensor-mediated signaling pathway

Ethylene, a biologically active natural compound, inhibited aflatoxin accumulation by Aspergillus parasiticus on a solid growth medium in a dose-dependent manner at concentrations of 0.1 to 150 ppm. The activity of the nor-1 promoter (an early aflatoxin gene) was reduced to nondetectable levels by similar quantities of ethylene, suggesting that the inhibitory effect on toxin synthesis occurred, at least in part, at the level of transcription. The inhibitory effect of ethylene on aflatoxin accumulation was also observed when A. parasiticus was grown on raw peanuts. Under similar growth conditions and doses, ethylene strongly inhibited development of asci and ascospores in Aspergillus nidulans, with no detectable effect on Hülle cell formation, conidiation, or sterigmatocystin accumulation. During early growth, A. parasiticus and A. nidulans produced ethylene with approximately twofold higher quantities measured in continuous light than in the dark. 1-Methylcyclopropene (an inhibitor of ethylene receptors in plants), light, CO2, temperature, and growth medium composition altered the effect of ethylene on A. nidulans and A. parasiticus. These observations are consistent with the existence of an ethylene sensor molecule that mediates the function of an ethylene-responsive signaling pathway(s) in Aspergillus.

Variation of the natural transformation frequency of Campylobacter jejuni in liquid shake culture

Natural transformation, a mechanism that generates genetic diversity in Campylobacter jejuni, was studied in a novel liquid shake culturing system that allowed an approximately 10 000-fold increase in cell density. C. jejuni transformation frequency was analysed in this system under 10 %, 5·0 % and 0·7 % CO2 atmospheres. At 5·0 % and 10 % CO2 concentrations, when purified isogenic chromosomal DNA was used to assess competence, transformation frequency ranged from 10−3 to 10−4 at low cell concentrations and declined as cell density increased. Transformation frequency under a 0·7 % CO2 atmosphere was more stable, maintaining 10−3 levels at high cell densities, and was 10- to 100-fold higher than that under a 10 % CO2 atmosphere. Three of four C. jejuni strains tested under a 5·0 % CO2 atmosphere were naturally competent for isogenic DNA; competent strains demonstrated a lack of barriers to intraspecies genetic exchange by taking up and incorporating chromosomal DNA from multiple C. jejuni donors. C. jejuni showed a preference for its own DNA at the species level, and co-cultivation demonstrated that DNA transfer via natural transformation occurred between isogenic populations during short periods of exposure in liquid medium when cell density and presumably DNA concentrations were low. Transformation frequency during co-cultivation of isogenic populations was also influenced by CO2 concentration. Under a 0·7 % CO2 atmosphere, co-cultivation transformation frequency increased approximately 500-fold in a linear fashion with regard to cell density, and was 1000- to 10 000-fold higher during late-exponential-phase growth when compared to cultures grown under a 10 % CO2 atmosphere.

Determination of ciprofloxacin and nalidixic acid resistance in Campylobacter jejuni with a fluorogenic polymerase chain reaction assay

A fluorogenic polymerase chain reaction assay for the gyrA gene was used to determine the frequency of a Thr-86 mutation in Campylobacter jejuni isolates from food animals and humans in northern Thailand and to investigate the correlation between this mutation and bacterial resistance to fluoroquinolones. Eighty-four isolates of C. jejuni were used: 65 from healthy chickens on farms, 16 from chickens at the slaughterhouse, 1 from chicken meat at the market, and 1 from a healthy farm worker. The microbroth dilution technique was used for in vitro susceptibility testing. MIC breakpoints established by the National Antimicrobial Resistance Monitoring System were used to categorize the resistance of C. jejuni to ciprofloxacin and nalidixic acid. Sixty of the 84 C. jejuni isolates tested carried the Thr-86 mutation in the gyrA gene. All isolates with ciprofloxacin MICs of > or = 2 mg/liter carried the mutation, and no isolates with nalidixic acid MICs of < or = 16 mg/liter carried the Thr-86-to-Ile mutation. There was a very strong association between ciprofloxacin resistance and the presence of the mutation (kappa = 0.971, P < 0.01). The association between the presence of the Thr-86-to-Ile mutation and nalidixic acid resistance was weaker (kappa 0.859: P < or = 0.01).

Function of native OmtA in vivo and expression and distribution of this protein in colonies of Aspergillus parasiticus

The activities of two enzymes, a 168-kDa protein and a 40-kDa protein, OmtA, purified from the filamentous fungus Aspergillus parasiticus were reported to convert the aflatoxin pathway intermediate sterigmatocystin to O-methylsterigmatocystin in vitro. Our initial goal was to determine if OmtA is necessary and sufficient to catalyze this reaction in vivo and if this reaction is necessary for aflatoxin synthesis. We generated A. parasiticus omtA-null mutant LW1432 and a maltose binding protein-OmtA fusion protein expressed in Escherichia coli. Enzyme activity analysis of OmtA fusion protein in vitro confirmed the reported catalytic function of OmtA. Feeding studies conducted with LW1432 demonstrated a critical role for OmtA, and the reaction catalyzed by this enzyme in aflatoxin synthesis in vivo. Because of a close regulatory link between aflatoxin synthesis and asexual sporulation (conidiation), we hypothesized a spatial and temporal association between OmtA expression and conidiospore development. We developed a novel time-dependent colony fractionation protocol to analyze the accumulation and distribution of OmtA in fungal colonies grown on a solid medium that supports both toxin synthesis and conidiation. OmtA-specific polyclonal antibodies were purified by affinity chromatography using an LW1432 protein extract. OmtA was not detected in 24-h-old colonies but was detected in 48-h-old colonies using Western blot analysis; the protein accumulated in all fractions of a 72-h-old colony, including cells (0 to 24 h) in which little conidiophore development was observed. OmtA in older fractions of the colony (24 to 72 h) was partly degraded. Fluorescence-based immunohistochemical analysis conducted on thin sections of paraffin-embedded fungal cells from time-fractionated fungal colonies demonstrated that OmtA is evenly distributed among different cell types and is not concentrated in conidiophores. These data suggest that OmtA is present in newly formed fungal tissue and then is proteolytically cleaved as cells in that section of the colony age.

Identification of Campylobacter jejuni isolates from cloacal and carcass swabs of chickens in Thailand by a 5' nuclease fluorogenic polymerase chain reaction assay

A rapid 5' nuclease fluorogenic polymerase chain reaction (PCR) assay for identifying Campylobacter jejuni was applied to Campylobacter isolates from chicken cloacal and carcass swabs collected from three chicken farms and a slaughterhouse in Thailand. The primers and the probe were based on the sequence of the gyrA gene in C jejuni. C. jejuni isolates were identified by fluorogenic PCR assay of bacterial cells directly from Campylobacter-selective agar medium. This assay allowed the identification of C. jejuni within 1 day after colonies appeared on selective media. The fluorogenic PCR assay yielded results comparable to those of the conventional test kit (kappa = 0.76) but required less time. When the two methods disagreed with regard to species identification, results were confirmed by PCR restriction fragment length polymorphism of 23S rRNA genes. In these instances, the fluorogenic PCR assay correctly identified more isolates of C. jejuni than did the conventional test kit (six of seven isolates were unidentifiable by the conventional test kit). The fluorogenic PCR assay is a rapid and specific method that outperforms the conventional test kit in the identification of C. jejuni from environmental samples.

Molecular and functional characterization of a second copy of the aflatoxin regulatory gene, aflR-2, from Aspergillus parasiticus

The genes required for the synthesis of aflatoxin (AF) in Aspergillus flavus and Aspergillus parasiticus have been shown to be clustered on a chromosome in these fungi. Transcription of most of these genes is dependent upon the activity of the aflR gene, also present on the gene cluster, which encodes a zinc binuclear cluster DNA-binding protein. While many strains of A. parasiticus have only one copy of aflR (aflR-1), many others contain a second copy of this gene (aflR-2) which resides on a duplicated region of the aflatoxin gene cluster. Targeted disruption of aflR-1 generated a number of non-aflatoxin producing transformants of A. parasiticus SU-1 which still harbored a wild-type aflR-2 gene. Southern and Northern hybridization analyses and ELISA assays demonstrated that aflR-1 had been successfully inactivated in strain AFS10. DNA sequence analysis showed that aflR-2 was capable of encoding a deduced 47 kDa protein. Northern and RT-PCR analysis of RNA from a toxin producing strain indicated that aflR-2 was transcribed at extremely low levels compared to aflR-1. RT-PCR analysis of RNA from AFS10 demonstrated that mRNAs of aflatoxin pathway genes were not processed to their mature forms. Functional analysis of aflr-2 protein in a yeast system showed that it was not activating transcription.

Chromosomal location plays a role in regulation of aflatoxin gene expression in Aspergillus parasiticus

The nor-1 gene in the filamentous fungus Aspergillus parasiticus encodes a ketoreductase involved in aflatoxin biosynthesis. To study environmental influences on nor-1 expression, we generated plasmid pAPGUSNNB containing a nor-1 promoter-beta-glucuronidase (GUS) (encoded by uidA) reporter fusion with niaD (encodes nitrate reductase) as a selectable marker. niaD transformants of A. parasiticus strain NR-1 (niaD) carried pAPGUSNNB integrated predominantly at the nor-1 or niaD locus. Expression of the native nor-1 and nor-1::GUS reporter was compared in transformants grown under aflatoxin-inducing conditions by Northern and Western analyses and by qualitative and quantitative GUS activity assays. The timing and level of nor-1 promoter function with pAPGUSNNB integrated at nor-1 was similar to that observed for the native nor-1 gene. In contrast, nor-1 promoter activity in pAPGUSNNB and a second nor-1::GUS reporter construct, pBNG3.0, was not detectable when integration occurred at niaD. Because niaD-dependent regulation could account for the absence of expression at niaD, a third chromosomal location was analyzed using pAPGUSNP, which contained nor-1::GUS plus pyrG (encodes OMP decarboxylase) as a selectable marker. GUS expression was detectable only when pAPGUSNP integrated at nor-1 and was not detectable at pyrG, even under growth conditions that required pyrG expression. nor-1::GUS is regulated similarly to the native nor-1 gene when it is integrated at its homologous site within the aflatoxin gene cluster but is not expressed at native nor-1 levels at two locations outside of the aflatoxin gene cluster. We conclude that the GUS reporter system can be used effectively to measure nor-1 promoter activity and that nor-1 is subject to position-dependent regulation in the A. parasiticus chromosome.

Isolation and analysis of fluP, a gene associated with hyphal growth and sporulation in Aspergillus parasiticus

Aflatoxins (AF) are polyketide-derived mycotoxins that frequently contaminate food and feed crops, causing health risks to animals and humans. The fluP gene was cloned by screening an Aspergillus parasiticus genomic DNA library with a cDNA probe encoding part of a polyketide synthase (PKS), the 6-methylsalicylic acid synthase (MSAS) from Penicillium patulum. FluP was hypothesized to function as a PKS in AF biosynthesis. The predicted amino acid sequence of FluP demonstrated a high degree of identity to MSAS (55%), moderate identity to another fungal PKS protein encoded by wA from A. nidulans (22%) and low identity (<5%) to fungal fatty acid synthase (FAS) proteins. Disruption of fluP in A. parasiticus resulted in the loss of fluP transcript, a 3- to 4-fold reduction in hyphal growth rate, the appearance of a fluffy, cotton-like hyphal morphology, reduction or elimination of asexual spores and spore-bearing structures, and a twofold reduction in aflatoxin accumulation. Removal of selective pressure on fluP knockout transformants resulted in frequent reversion (10%) to the wild-type genotype and phenotype, establishing a direct link between gene disruption and the associated phenotype. The data suggest that fluP encodes a novel PKS associated with hyphal growth and cell development (sporulation), whose activity indirectly influences aflatoxin accumulation in A. parasiticus.

Identification of ciprofloxacin-resistant Campylobacter jejuni by use of a fluorogenic PCR assay

Fluoroquinolones are one class of antimicrobial agents commonly used to treat severe Campylobacter jejuni infection. C. jejuni strains resistant to high levels of the fluoroquinolone ciprofloxacin (MIC >/=16 microg/ml) have been predominantly characterized with a C-->T transition in codon 86 of gyrA. The gyrA gene encodes one subunit of DNA gyrase, which is a primary target for fluoroquinolone antibiotics. This study establishes a rapid PCR-based TaqMan method for identifying ciprofloxacin-resistant C. jejuni strains that carry the C-->T transition in codon 86 of gyrA. The assay uses real-time detection, eliminating the need for gel electrophoresis. Optimization of the assay parameters using purified Campylobacter DNA resulted in the ability to detect femtogram levels of DNA. The method should be useful for monitoring the development of ciprofloxacin resistance in C. jejuni. Compiled nucleotide sequence data on the quinolone resistance-determining region of gyrA in Campylobacter indicate that sequence comparison of this region is a useful method for tentative identification of Campylobacter isolates at the species level.

Binding of aflatoxin B1 to bifidobacteria in vitro

Aflatoxins are mycotoxins that cause health and economic problems when they contaminate food and feed. One potential method for reducing human health effects due to aflatoxin ingestion is to block uptake via binding by bacteria that either make up the normal gut flora or are present in fermented foods in our diet. These bacteria would bind aflatoxin and make it unavailable for absorption in the intestinal tract. Bifidobacteria comprise a large fraction of the normal gut flora, are thought to provide many probiotic effects and are increasingly used in fermented dairy products. These qualities targeted bifidobacteria for studies to determine if various strains of heat-killed bifidobacteria can bind aflatoxin B1 (AFB1) in vitro. The AFB1 binding affinities of various strains of bifidobacteria, Staphylococcus aureus, and Escherichia coli were quantitated utilizing enzyme-linked immunosorbent and [3H]AFB1 binding assays. The bacteria analyzed were found to bind significant quantities of AFB1 ranging from 25% to nearly 60% of the added toxin. The data also suggest that there are reproducible strain differences in AFB1 binding capacity.