Under Subbarao, Benjamin Duggar made his discovery of the world's first tetracycline antibiotic, chlortetracycline, in 1945. Duggar identified the antibiotic as the product of an actinomycete he cultured from a soil sample collected from Sanborn Field at the University of Missouri.[21]
Soil Microbiology Subba Rao Pdf 100
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Adsorption of bacteria in soils is often considered in terms of soil physical properties, and the degree of adsorption between microorganisms and soil particles is broadly related to the surface area and surface charge properties of the particles (5). The major soil components that affect bacterial adsorption are the organic matter and clay fractions. Both these components possess large surface areas and consist primarily of negatively charged particles.
Microorganisms are the major agents mineralizing organic pollutants in terrestrial and aquatic environments (1), and their adsorption on soil colloids is of great importance. Previous studies have shown biodegradation of pollutants in the presence of either humic acids (2) or clays (11). However, no data exist on the effect of humic acid-clay complexes on the microbial degradation of hydrophobic pollutants. Hence, a study was conducted to compare rates of biodegradation of phenanthrene, a polycyclic aromatic hydrocarbon widely present in contaminated soils, in the presence of soil colloids and to reveal possible mechanisms by which microorganisms mineralize polycyclic aromatic hydrocarbons in soils.
The phenanthrene-degrading bacterium was isolated from a Cambisol soil (pH 6.4; 15.5% organic matter and 13.4% clay) from Guadalajara (Spain). A few grams of soil were incubated in 100 ml of an inorganic salts solution (pH 5.6) (whose composition was described previously [14]) containing 0.1 g of phenanthrene per ml. The enrichment culture was incubated for 2 weeks at 31C on a rotary shaker operating at 120 rpm. After two serial transfers to fresh medium, the enrichment culture was streaked on plates containing 0.3% (wt/vol) Trypticase soy broth, 1.5% agar, and the inorganic salts solution. A pure culture was obtained from a single colony after incubation.
Humic fractions were isolated from soils. Humic acid was extracted from a Typic Xerochrept soil, and fulvic acid was extracted from a Typic Xerorthent soil. A detailed description of methods and chemical characteristics of the samples has been published elsewhere (17). Prior to incubation, 1-mg/ml stock solutions of humic acid were prepared in 0.1 M NaOH, and the pH was adjusted to 6 with HCl. The desired concentrations were obtained by diluting the stock solution with sterile salt medium (pH 6). P. fluorescens was unable to use the isolated humic fractions as the sole source of carbon and energy for growth.
Humic fractions (humic and fulvic acids) and clays often coexist in soil not as separate components but in a close relationship within the soil matrix. Therefore, the influence of the combination of different concentrations of humic fractions and clay on the biodegradation of phenanthrene was investigated. Stock solutions of fulvic acid were prepared by dissolving it with sterile mineral medium. Five-milliliter suspensions containing known amounts of humic fractions and montmorillonite were introduced into 10-ml screw-cap tubes and equilibrated overnight at 21C on a rotary shaker operating at 150 rpm. These suspensions were then used for mineralization in the same way as clay suspensions in the procedure described above.
Phenanthrene is a hydrophobic chemical, and this causes its strong tendency to sorb to soil surfaces when it is initially dissolved in the aqueous phase. A reduction in the water concentration due to sorption can modify the bioavailability of organic chemicals for microbial degradation (1). To assess whether sorption was occurring in our system, the aqueous concentration of phenanthrene in the aqueous phase at equilibrium was measured in humic acid solutions, clay suspensions, and controls identical to those used in mineralization experiments, but without bacteria. Aqueous phenanthrene concentration was measured in humic acid solutions by a reverse-phase separation technique (12). Six milliliters of the equilibrated solutions was passed through C18 Sep-Pak cartridges (Waters Associates), and the nonsorbed phenanthrene, which was retained in the cartridge, was subsequently eluted with dichloromethane and quantified by liquid scintillation counting. Aqueous concentrations in clay suspensions were measured by centrifugation and liquid scintillation counting of the supernatant. Equilibrated suspensions containing both humic acid and clay showed upon centrifugation a marked decrease in coloration of the supernatant, suggesting that most of the humic acid was sorbed onto clay. This fraction was quantified by the adsorbance at 285 nm of the supernatants after centrifugation. Controls without particles showed that humic acid remained in solution after centrifugation. This method was also used to measure the amount of humic acid adsorbed onto bacterial cells in suspensions with dissolved humic acid at 100 μg/ml. This fraction accounted for 20% of the humic acid initially in solution. The concentration of phenanthrene in the aqueous phase of suspensions of humic acid and clay was measured after centrifugation and passage of the supernatant through a Sep-Pak cartridge to discard the phenanthrene associated with humic acid in solution, i.e., not sorbed to the clay.
It has been reported that, in soils, primary particles combine into aggregates of varying size, and these aggregates are important factors in retarding soil organic matter decomposition. Also, soil humic fractions are mainly stabilized not as a result of their complex and recalcitrant structure but probably by association with metal ions and clays and aggregation (7). Evidence shown in this paper indicates that phenanthrene mineralization is enhanced in the presence of dissolved humic acids and humic acid-clay complex.
The mechanisms of the effects of these soil components on the transformation can be understood by postulating direct access by attachment of bacteria to the pool of sorbed compound. Soil microorganisms produce extracellular products which have been associated with the attachment of cells to surfaces (10). Therefore, it is suggested that direct bacterial contact with humic acid and montmorillonite particles facilitated the biodegradation of the sorbed compound. The stimulation observed may be the result of an increased concentration of substrate in the vicinity of the bacterial cells, caused by the direct contact with humic acid and humic acid-clay complexes. Humic acids, which harbor both hydrophilic and hydrophobic moieties, play a key role in facilitating better access of microbes to phenanthrene sorbed to humic acid-clay complexes. Alternately, humic acid could have induced the production of enzymes involved in phenanthrene mineralization, a possibility that was not excluded by our results.
Myxobacteria are micropredators in the soil ecosystem with the capacity to move and feed cooperatively. Some myxobacterial strains have been used to control soil-borne fungal phytopathogens. However, interactions among myxobacteria, plant pathogens, and the soil microbiome are largely unexplored. In this study, we aimed to investigate the behaviors of the myxobacterium Corallococcus sp. strain EGB in the soil and its effect on the soil microbiome after inoculation for controlling cucumber Fusarium wilt caused by Fusarium oxysporum f. sp. cucumerinum (FOC).
A greenhouse and a 2-year field experiment demonstrated that the solid-state fermented strain EGB significantly reduced the cucumber Fusarium wilt by 79.6% (greenhouse), 66.0% (2015, field), and 53.9% (2016, field). Strain EGB adapted to the soil environment well and decreased the abundance of soil-borne FOC efficiently. Spatiotemporal analysis of the soil microbial community showed that strain EGB migrated towards the roots and root exudates of the cucumber plants via chemotaxis. Cooccurrence network analysis of the soil microbiome indicated a decreased modularity and community number but an increased connection number per node after the application of strain EGB. Several predatory bacteria, such as Lysobacter, Microvirga, and Cupriavidus, appearing as hubs or indicators, showed intensive connections with other bacteria.
The predatory myxobacterium Corallococcus sp. strain EGB controlled cucumber Fusarium wilt by migrating to the plant root and regulating the soil microbial community. This strain has the potential to be developed as a novel biological control agent of soil-borne Fusarium wilt.
However, the use of registered BCAs is limited because of their poor efficacy and unstable performance [3]. Once applied to the soil, BCAs interact with the host plants, native soil microbes, target soil-borne pathogens, and the edaphic environment [5]. Understanding these ecological interactions is critical for the commercial development of BCAs [6]. In Trichoderma hamatum strain GD12, knockout of the N-acetyl-b-d-glucosaminidase gene reduced its competitive saprotrophic fitness and impaired its biocontrol ability [7]. Furthermore, plant microbiomes are crucial for plant health [8]. Biocontrol of plant soil-borne pathogens can also be achieved through the regulation of soil microbial communities [9, 10]. It has been reported that the reduction in soil microbial diversity was responsible for the burst of soil-borne plant diseases [11]. There has been an increased interest in the microbial communities of disease-suppressive soils, and these suppressive effects have been attributed to the enrichment of specific groups of soil microbes [12,13,14,15]. The fact that a community composed of non-antagonistic bacteria from multiple parallel mineralization systems could also suppress Fusarium wilt disease indicates the importance of the microbial community structure to the biocontrol capacity of BCAs [16]. 2ff7e9595c
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