The Experts below are selected from a list of 23715 Experts worldwide ranked by ideXlab platform
Chao Liang - One of the best experts on this subject based on the ideXlab platform.
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divergent accumulation of microbial necromass and plant lignin components in grassland soils
Nature Communications, 2018Co-Authors: Shanshan Zhu, Zhiheng Wang, Dima Chen, Guohua Dai, Bowei Feng, Wenxuan Han, Chao Liang, Yongfei Bai, Xiaojuan FengAbstract:The means through which microbes and plants contribute to soil organic carbon (SOC) accumulation remain elusive due to challenges in disentangling the complex components of SOC. Here we use Amino Sugars and lignin phenols as tracers for microbial necromass and plant lignin components, respectively, and investigate their distribution in the surface soils across Mongolian grasslands in comparison with published data for other grassland soils of the world. While lignin phenols decrease, Amino Sugars increase with SOC contents in all examined grassland soils, providing continental-scale evidence for the key role of microbial necromass in SOC accumulation. Moreover, in contrast to clay’s control on Amino Sugar accumulation in fine-textured soils, aridity plays a central role in Amino Sugar accrual and lignin decomposition in the coarse-textured Mongolian soils. Hence, aridity shifts may have differential impacts on microbial-mediated SOC accumulation in grassland soils of varied textures. It remains unclear how microbes and plants contribute to soil organic carbon (SOC) accrual. Here, using biomarkers, the authors show that microbial necromass and plant-derived lignin components have divergent accumulation mechanisms and that microbial necromass plays a key role in SOC accumulation.
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switchgrass rhizospheres stimulate microbial biomass but deplete microbial necromass in agricultural soils of the upper midwest usa
Soil Biology & Biochemistry, 2016Co-Authors: Chao Liang, David S Duncan, Randall D Jackson, Ederson Da Conceicao Jesus, John F Quensen, Teri C BalserAbstract:Abstract Rhizosphere microbial communities play an essential role in determining plant productivity, particularly in agriculturally marginal environments. Perennial plants like switchgrass (Panicum virgatum) are thought to particularly influence microbial community composition and function within their rhizosphere. We compared microbial communities in switchgrass rhizospheres and their associated bulk soils in two regions of the U.S. upper Midwest (Michigan and Wisconsin) with contrasting soil types, and at two site types with differing switchgrass establishment ages and management intensities. We characterized microbial communities with a range of culture-independent methods, including amplicon sequencing of 16S/18S rRNA and nifH genes, and membrane lipid profiling. In addition, we quantified abundances of soil Amino Sugars, a time-integrative indicator of microbial necromass. We found that Amino Sugar contents and microbial lipid profiles differed between rhizosphere and bulk soils, while DNA-based assays did not provide this discriminatory power. Differences between rhizosphere and bulk soils were not significantly affected by region or site type. Rhizosphere soils had higher microbial lipid abundances, particularly those associated with arbuscular mycorrhizal fungi and Gram-negative bacteria, while Amino Sugar abundances decreased in the rhizosphere. Our findings suggest switchgrass rhizospheres systematically stimulate microbial growth and microbial residue turnover.
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microbial lipid and Amino Sugar responses to long term simulated global environmental changes in a california annual grassland
Frontiers in Microbiology, 2015Co-Authors: Chao Liang, Jessica L M Gutknecht, Teri C BalserAbstract:Global environmental change is predicted to have major consequences for carbon cycling and the functioning of soil ecosystems. However, we have limited knowledge about its impacts on the microorganisms, which act as a “valve” between carbon sequestered in soils versus released into the atmosphere. In this study we examined microbial response to continuous 9-year manipulation of three global change factors (elevated CO2, warming, and nitrogen deposition), singly and in combination using two methods: lipid and Amino Sugar biomarkers at the Jasper Ridge Global Change Experiment (JRGCE). The two methods yielded important distinctions. There were limited microbial lipid differences, but many significant effects for microbial Amino Sugars. We found that CO2 was not a direct factor influencing soil carbon and major Amino Sugar pools, but had a positive impact on bacterial-derived muramic acid. Likewise, warming and nitrogen deposition appeared to enrich residues specific to bacteria despite an overall depletion in total Amino Sugars. The results indicate that elevated CO2, warming, and nitrogen deposition all appeared to increase bacterial-derived residues, but this accumulation effect was far offset by a corresponding decline in fungal residues. The sensitivity of microbial residue biomarker Amino Sugars to warming and nitrogen deposition may have implications for our predictions of global change impacts on soil stored carbon.
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soil microbial residue storage linked to soil legacy under biofuel cropping systems in southern wisconsin usa
Soil Biology & Biochemistry, 2013Co-Authors: Chao Liang, Teri C Balser, David S Duncan, James M Tiedje, Randall D JacksonAbstract:Abstract Microbial residues can be a significant component of soil organic matter, and their component Amino Sugars are integrative indicators of biologically relevant ecosystem properties. We evaluate the impact of soil attributes, microbial functional group biomass, and cropping system type on soil Amino Sugar profiles in three model biofuel cropping systems in southern Wisconsin, USA. Total soil carbon and clay content explained differences in soil Amino Sugar profiles, with glucosamine and galactosamine more strongly related to soil carbon and muramic acid associated with clay content. Amino Sugars were not correlated to cropping system or to microbial functional group lipid abundance, suggesting Amino Sugar differences among locations were due to differences in soil legacy properties such as total carbon, clay content and culture age rather than to current biotic drivers. The disconnect between the current biotic composition and legacies of past microbial activity suggests that microbial residues should be considered as trajectories over time rather than static system properties.
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net microbial Amino Sugar accumulation process in soil as influenced by different plant material inputs
Biology and Fertility of Soils, 2007Co-Authors: Xu Dong Zhang, Chao Liang, Teri C BalserAbstract:Identifying the impact of plant material inputs on soil Amino Sugar synthesis may advance our knowledge of microbial transformation processes in soils. In a 12-week laboratory microcosm incubation, 1, 2, 4, and 6% (w/w) soybean leaf or maize stalk were initially added to soil, respectively, whereas soil without plant addition was used as a control. The results showed that adding organic materials to the soil led to a net accumulation of Amino Sugars, because of greater microbial synthesis. The ratios of glucosamine to galactosamine and of glucosamine to muramic acid, two indicators differentiating the relative contribution to soil organic matter of fungi and bacteria, showed substantial variance across the gradient of substrate addition. Our results suggest that the amount of nutrients in a given substrate is the primary attribute determining microbial net accumulation of soil Amino Sugars, especially in the relatively short term, whereas the composition of nutrients might be more important in the relatively long term when nutrients are not sufficient. The use of the two ratios (glucosamine to galactosamine and glucosamine to muramic acid) reflects different dynamics of galactosamine and muramic acid during the decomposition of organic substrates in soils. Muramic acid, compared with galactosamine, is more likely to accumulate in the soil active organic fraction under abundant nutrient conditions, whereas it would be decomposed along with active organic matter when the nutrients are scarce and remain in minor quantities in the clay fraction without being attacked by microbes.
Xu Dong Zhang - One of the best experts on this subject based on the ideXlab platform.
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variations of soil viable and necromass carbon affected by biochar incorporation frequencies
Archives of Agronomy and Soil Science, 2021Co-Authors: Yulan Zhang, Hongtu Xie, Fuping Wang, Xu Dong ZhangAbstract:Little is known about how biochar incorporation frequency affect soil C sequestration and stabilization, particularly through living microbial carbon (MBC) and dead microbial carbon (Amino Sugar C,...
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effects of different fertilization managements on microbial necromass and plant lignin accumulation in a mollisol
Journal of Applied Ecology, 2020Co-Authors: Hainan Huo, Xiaochen Zhang, Ping Zhu, Lichun Wang, Yuanliang Shi, Xu Dong ZhangAbstract:Fertilization is an effective management to maintain and increase soil organic carbon (SOC) level in agroecosystems. Both microbial metabolism and plant component retention control SOC sequestration. Here, we used Amino Sugars and lignin as biomarkers to investigate the responses of distribution of microbial necromass and plant debris in a long-term cultivated soil (30 years) and SOC accumulation to different fertilization regime. The results showed that, compared with unfertilized treatment, inorganic fertilizer application (N fertilizer-only or the combination of organic or inorganic fertilizers) increased crop production and soil Amino Sugar accumulation, but did not affect the concentrations of lignin and SOC, indicating that inorganic fertilizer stimulated the assimilation of microbial substrate and accelerated the turnover of SOC and lignin in the plough layer. Compared with inorganic fertilizer treatment, long-term organic fertilizer application promoted SOC accumulation (38.3%), but did not affect Amino Sugar concentration in SOC, which indicated that soil could reach a 'saturation' state with respect to microbial residue accumulation. In contrast, the application of organic fertilizer increased the proportion of lignin in SOC,indicating that the contribution of plant residues to SOC persistence was enhanced. Compared with the manure-only treatment, organic-inorganic combined application mainly increased the contribution of Amino Sugar to SOC accumulation. Our findings indicated that long-term fertilization could affect SOC dynamics through modulating the accumulation processes of microbial necromass and plant debris.
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Physical, biochemical, and microbial controls on Amino Sugar accumulation in soils under long-term cover cropping and no-tillage farming
Soil Biology and Biochemistry, 2019Co-Authors: Candace B. Wilson, Xu Dong Zhang, Feng Zhou, Sean M. SchaefferAbstract:Abstract Understanding the processes controlling Amino Sugar accumulation in soil is essential for predicting the contribution of microbial residues to soil organic matter (SOM). The accumulation of Amino Sugars in soil is affected by multiple factors. Seldom are those factors examined together. We measured Amino Sugar concentration, extracellular enzyme activity, microbial respiration rate, and soil aggregate composition in an agricultural soil under 33-years of conservation management. The accumulation patterns of different Amino Sugars under the effects of no-tillage farming and cover cropping were compared and contrasted. The relative importance of physical, biochemical, and microbial controls of Amino Sugar accumulation was quantified using structural equation modelling. Our results show that although different types of Amino Sugars exhibited similar accumulation patterns in soil, their stabilization mechanisms might vary as demonstrated by structural equation models. The structural equation models indicate that macroaggregates had the largest total effect (0.59, P
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long term impacts of manure straw and fertilizer on Amino Sugars in a silty clay loam soil under temperate conditions
Biology and Fertility of Soils, 2013Co-Authors: Xueli Ding, Xiaozeng Han, Xu Dong ZhangAbstract:There is increasing evidence that microorganisms participate in soil C sequestration and stabilization in the form of resistant microbial residues. The type of fertilizers influences microbial activity and community composition; however, little is known about its effect on the microbial residues and their relative contribution to soil C storage. The aim of this study was to investigate the long-term impact (21 years) of different fertilizer treatments (chemical fertilizer, crop straw, and organic manure) on microbial residues in a silty clay loam soil (Udolls, USDA Soil Taxonomy). Amino Sugars were used to indicate the presence and origin of microbial residues. The five treatments were: CK, unfertilized control; NPK, chemical fertilizer NPK; NPKS1, NPK plus crop straw; NPKS2, NPK plus double amounts of straw; and NPKM, NPK plus pig manure. Long-term application of inorganic fertilizers and organic amendments increased the total Amino Sugar concentrations (4.4–8.4 %) as compared with the control; and this effect was more evident in the plots that continuously received pig manure (P < 0.05). The increase in total Amino Sugar stock was less pronounced in the straw-treated plots than the NPKM. These results indicate that the accumulation of soil Amino Sugars is largely influenced by the type of organic fertilizers entering the soil. Individual Amino Sugar enrichment in soil organic carbon was differentially influenced by the various fertilizer treatments, with a preferential accumulation of bacterial-derived Amino Sugars compared with fungal-derived glucosamine in manured soil.
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Kinetics of Amino Sugar formation from organic residues of different quality
Soil Biology and Biochemistry, 2013Co-Authors: Zhen Bai, Xu Dong Zhang, Samuel Bodé, Dries Huygens, Pascal BoeckxAbstract:Abstract Amino Sugars are key compounds of microbial cell walls, which have been widely used as biomarker of microbial residues to investigate soil microbial communities and organic residue cycling processes. However, the formation dynamics of Amino Sugar is not well understood. In this study, two agricultural Luvisols under distinct tillage managements were amended with uniformly 13 C-labeled wheat residues of different quality (grain, leaf and root). The isotopic composition of individual Amino Sugars and CO 2 emission were measured over a 21-day incubation period using liquid chromatography–isotope ratio mass spectrometry (LC–IRMS) and trace gas IRMS. Results showed that, the amount of residue derived Amino Sugars increased exponentially and reached a maximum within days after residue addition. Glucosamine and galactosamine followed different formation kinetics. The maxima of residue derived Amino Sugars formation ranged from 14 nmol g −1 dry soil for galactosamine (0.8% of the original concentration) to 319 nmol g −1 dry soil for glucosamine (11% of the original concentration). Mean production times of residue derived Amino Sugars ranged from 2.1 to 9.3 days for glucosamine and galactosamine, respectively. In general, larger amounts of Amino Sugars were formed at a higher rate with increasing plant residue quality. The microbial community of the no-till soil was better adapted to assimilate low quality plant residues (i.e. leaf and root). All together, the formation dynamics of microbial cell wall components was component-specific and determined by residue quality and soil microbial community.
Christian Bailly - One of the best experts on this subject based on the ideXlab platform.
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dna sequence recognition by the indolocarbazole antitumor antibiotic at2433 b1 and its diastereoisomer
Nucleic Acids Research, 2002Co-Authors: Carolina Carrasco, Michael Facompre, John D Chisholm, David L Van Vranken, David W Wilson, Christian BaillyAbstract:The antibiotic AT2433-B1 belongs to a therapeutically important class of antitumor agents. This natural product contains an indolocarbazole aglycone connected to a unique disaccharide consisting of a methoxyglucose and an Amino Sugar subunit, 2,4-dideoxy-4-methylAmino-l-xylose. The configuration of the Amino Sugar distinguishes AT2433-B1 from its diastereoisomer iso-AT2433-B1. Here we have investigated the interaction of these two disaccharide indolocarbazole derivatives with different DNA sequences by means of DNase I footprinting and surface plasmon resonance (SPR). Accurate binding measurements performed at 4 and 25°C using the BIAcore SPR method revealed that AT2433-B1 binds considerably more tightly to a hairpin oligomer containing a [CG]4 block than to an oligomer with a central [AT]4 tract. The kinetic analysis shows that the antibiotic dissociates much more slowly from the GC sequence compared to the AT one. Preferential binding of AT2433-B1 to GC-rich sequences in DNA was independently confirmed by DNase I footprinting experiments performed with a 117 bp DNA restriction fragment. The specific binding sequence 5′-AACGCCAG identified from the footprints was then converted into a biotin-labeled DNA hairpin duplex and compound interactions with this specific sequence were characterized by high resolution BIAcore SPR experiments. Such a combined approach provided a detailed understanding of the molecular basis of DNA recognition. The discovery that the glycosyl antibiotic AT2433-B1 preferentially recognizes defined sequences offers novel opportunities for the future design of sequence-specific DNA-reading small molecules.
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dna sequence recognition by the indolocarbazole antitumor antibiotic at2433 b1 and its diastereoisomer escholarship
2002Co-Authors: Carolina Carrasco, Michael Facompre, John D Chisholm, David L Van Vranken, David W Wilson, Christian BaillyAbstract:The antibiotic AT2433-B1 belongs to a therapeutically important class of antitumor agents. This natural product contains an indolocarbazole aglycone connected to a unique disaccharide consisting of a methoxyglucose and an Amino Sugar subunit, 2,4-dideoxy-4-methylAmino-l-xylose. The configuration of the Amino Sugar distinguishes AT2433-B1 from its diastereoisomer iso-AT2433-B1. Here we have investigated the interaction of these two disaccharide indolocarbazole derivatives with different DNA sequences by means of DNase I footprinting and surface plasmon resonance (SPR). Accurate binding measurements performed at 4 and 25°C using the BIAcore SPR method revealed that AT2433-B1 binds considerably more tightly to a hairpin oligomer containing a [CG]4 block than to an oligomer with a central [AT]4 tract. The kinetic analysis shows that the antibiotic dissociates much more slowly from the GC sequence compared to the AT one. Preferential binding of AT2433-B1 to GC-rich sequences in DNA was independently confirmed by DNase I footprinting experiments performed with a 117 bp DNA restriction fragment. The specific binding sequence 5′-AACGCCAG identified from the footprints was then converted into a biotin-labeled DNA hairpin duplex and compound interactions with this specific sequence were characterized by high resolution BIAcore SPR experiments. Such a combined approach provided a detailed understanding of the molecular basis of DNA recognition. The discovery that the glycosyl antibiotic AT2433-B1 preferentially recognizes defined sequences offers novel opportunities for the future design of sequence-specific DNA-reading small molecules.
Teri C Balser - One of the best experts on this subject based on the ideXlab platform.
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switchgrass rhizospheres stimulate microbial biomass but deplete microbial necromass in agricultural soils of the upper midwest usa
Soil Biology & Biochemistry, 2016Co-Authors: Chao Liang, David S Duncan, Randall D Jackson, Ederson Da Conceicao Jesus, John F Quensen, Teri C BalserAbstract:Abstract Rhizosphere microbial communities play an essential role in determining plant productivity, particularly in agriculturally marginal environments. Perennial plants like switchgrass (Panicum virgatum) are thought to particularly influence microbial community composition and function within their rhizosphere. We compared microbial communities in switchgrass rhizospheres and their associated bulk soils in two regions of the U.S. upper Midwest (Michigan and Wisconsin) with contrasting soil types, and at two site types with differing switchgrass establishment ages and management intensities. We characterized microbial communities with a range of culture-independent methods, including amplicon sequencing of 16S/18S rRNA and nifH genes, and membrane lipid profiling. In addition, we quantified abundances of soil Amino Sugars, a time-integrative indicator of microbial necromass. We found that Amino Sugar contents and microbial lipid profiles differed between rhizosphere and bulk soils, while DNA-based assays did not provide this discriminatory power. Differences between rhizosphere and bulk soils were not significantly affected by region or site type. Rhizosphere soils had higher microbial lipid abundances, particularly those associated with arbuscular mycorrhizal fungi and Gram-negative bacteria, while Amino Sugar abundances decreased in the rhizosphere. Our findings suggest switchgrass rhizospheres systematically stimulate microbial growth and microbial residue turnover.
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microbial lipid and Amino Sugar responses to long term simulated global environmental changes in a california annual grassland
Frontiers in Microbiology, 2015Co-Authors: Chao Liang, Jessica L M Gutknecht, Teri C BalserAbstract:Global environmental change is predicted to have major consequences for carbon cycling and the functioning of soil ecosystems. However, we have limited knowledge about its impacts on the microorganisms, which act as a “valve” between carbon sequestered in soils versus released into the atmosphere. In this study we examined microbial response to continuous 9-year manipulation of three global change factors (elevated CO2, warming, and nitrogen deposition), singly and in combination using two methods: lipid and Amino Sugar biomarkers at the Jasper Ridge Global Change Experiment (JRGCE). The two methods yielded important distinctions. There were limited microbial lipid differences, but many significant effects for microbial Amino Sugars. We found that CO2 was not a direct factor influencing soil carbon and major Amino Sugar pools, but had a positive impact on bacterial-derived muramic acid. Likewise, warming and nitrogen deposition appeared to enrich residues specific to bacteria despite an overall depletion in total Amino Sugars. The results indicate that elevated CO2, warming, and nitrogen deposition all appeared to increase bacterial-derived residues, but this accumulation effect was far offset by a corresponding decline in fungal residues. The sensitivity of microbial residue biomarker Amino Sugars to warming and nitrogen deposition may have implications for our predictions of global change impacts on soil stored carbon.
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soil microbial residue storage linked to soil legacy under biofuel cropping systems in southern wisconsin usa
Soil Biology & Biochemistry, 2013Co-Authors: Chao Liang, Teri C Balser, David S Duncan, James M Tiedje, Randall D JacksonAbstract:Abstract Microbial residues can be a significant component of soil organic matter, and their component Amino Sugars are integrative indicators of biologically relevant ecosystem properties. We evaluate the impact of soil attributes, microbial functional group biomass, and cropping system type on soil Amino Sugar profiles in three model biofuel cropping systems in southern Wisconsin, USA. Total soil carbon and clay content explained differences in soil Amino Sugar profiles, with glucosamine and galactosamine more strongly related to soil carbon and muramic acid associated with clay content. Amino Sugars were not correlated to cropping system or to microbial functional group lipid abundance, suggesting Amino Sugar differences among locations were due to differences in soil legacy properties such as total carbon, clay content and culture age rather than to current biotic drivers. The disconnect between the current biotic composition and legacies of past microbial activity suggests that microbial residues should be considered as trajectories over time rather than static system properties.
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net microbial Amino Sugar accumulation process in soil as influenced by different plant material inputs
Biology and Fertility of Soils, 2007Co-Authors: Xu Dong Zhang, Chao Liang, Teri C BalserAbstract:Identifying the impact of plant material inputs on soil Amino Sugar synthesis may advance our knowledge of microbial transformation processes in soils. In a 12-week laboratory microcosm incubation, 1, 2, 4, and 6% (w/w) soybean leaf or maize stalk were initially added to soil, respectively, whereas soil without plant addition was used as a control. The results showed that adding organic materials to the soil led to a net accumulation of Amino Sugars, because of greater microbial synthesis. The ratios of glucosamine to galactosamine and of glucosamine to muramic acid, two indicators differentiating the relative contribution to soil organic matter of fungi and bacteria, showed substantial variance across the gradient of substrate addition. Our results suggest that the amount of nutrients in a given substrate is the primary attribute determining microbial net accumulation of soil Amino Sugars, especially in the relatively short term, whereas the composition of nutrients might be more important in the relatively long term when nutrients are not sufficient. The use of the two ratios (glucosamine to galactosamine and glucosamine to muramic acid) reflects different dynamics of galactosamine and muramic acid during the decomposition of organic substrates in soils. Muramic acid, compared with galactosamine, is more likely to accumulate in the soil active organic fraction under abundant nutrient conditions, whereas it would be decomposed along with active organic matter when the nutrients are scarce and remain in minor quantities in the clay fraction without being attacked by microbes.
Ashu Sharma - One of the best experts on this subject based on the ideXlab platform.
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peptidoglycan synthesis in tannerella forsythia scavenging is the modus operandi
Molecular Oral Microbiology, 2018Co-Authors: Angela Ruscitto, Ashu SharmaAbstract:: Tannerella forsythia is a Gram-negative oral pathogen strongly associated with periodontitis. This bacterium has an absolute requirement for exogenous N-acetylmuramic acid (MurNAc), an Amino Sugar that forms the repeating disaccharide unit with Amino Sugar N-acetylglucosamine (GlcNAc) of the peptidoglycan backbone. In silico genome analysis indicates that T. forsythia lacks the key biosynthetic enzymes needed for the de novo synthesis of MurNAc, and so relies on alternative ways to meet its requirement for peptidoglycan biosynthesis. In the subgingival niche, the bacterium can acquire MurNAc and peptidoglycan fragments (muropeptides) released by the cohabiting bacteria during their cell wall breakdown associated with cell division. Tannerella forsythia is able to also use host sialic acid (Neu5Ac) in lieu of MurNAc or muropeptides for its survival during the biofilm growth. Evidence suggests that the bacterium might be able to shunt sialic acid into a metabolic pathway leading to peptidoglycan synthesis. In this review, we explore the mechanisms by which T. forsythia is able to scavenge MurNAc, muropeptide and sialic acid for its peptidoglycan synthesis, and the impact of these scavenging activities on pathogenesis.
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Peptidoglycan synthesis in Tannerella forsythia: Scavenging is the modus operandi.
Molecular oral microbiology, 2018Co-Authors: Angela Ruscitto, Ashu SharmaAbstract:Tannerella forsythia is a Gram-negative oral pathogen strongly associated with periodontitis. This bacterium has an absolute requirement for exogenous Nacetylmuramic acid (MurNAc), an Amino Sugar which forms the repeating disaccharide unit with Amino Sugar N-acetylglucosamine (GlcNAc) of the peptidoglycan backbone. In silico genome analysis indicates that T. forsythia lacks the key biosynthetic enzymes needed for the de novo synthesis of MurNAc, and thus relies on alternative ways to meet its requirement for peptidoglycan biosynthesis. In the subgingival niche, the bacterium can acquire MurNAc and peptidoglycan fragments (muropeptides) released by the cohabiting bacteria during their cell wall breakdown associated with cell division.T. forsythia is able to also utilize host sialic acid (Neu5Ac) in lieu of MurNAc or muropeptides for its survival during the biofilm growth. The evidence suggests that the bacterium might be able to shunt sialic acid into a metabolic pathway leading to peptidoglycan synthesis. In this review, we explore the mechanisms by which T. forsythia is able to scavenge MurNAc, muropeptide, and sialic acid for its peptidoglycan synthesis, and the impact of these scavenging activities on pathogenesis.