Abstract:
A drop-collapse method has been refined for use as both a qualitative assay to screen for surfactant-producing microbes, and as a quantitative assay to determine surfactant concentration. The assay is rapid, easy to perform, reproducible and requires little specialized equipment. The assay is performed in a 96-microwell plate, where each well is thinly coated with oil. A 5 μL sample droplet is added to the center of a well and observed after 1 min. The droplet will either bead up, spread out slightly or collapse, depending on the amount of surfactant in the sample. The basis for this method is the type of oil used to coat each well. In the qualitative method, each well is coated with 1.8 μL of Pennzoil® and either the drop collapses, indicating the presence of surfactant (a positive result), or the drop remains beaded, indicating the absence of surfactant (a negative response). In the quantitative method, each well is coated with 2 μL of mineral oil, and a dissecting microscope is used to measure the diameter of the droplet at 1 min. Results with both a test biosurfactant (rhamnolipid) and a test synthetic surfactant (sodium dodecyl sulfate) indicate a direct linear correlation between droplet diameter and surfactant concentration. The drop-collapse method has several advantages over commonly used methods that measure surface tension, such as the du Nouy ring method; a smaller volume is required (5 μL vs. 20 mL), the effective range of measurement is greater and it does not require specialized equipment. Copyright (C) 1998 Elsevier Science B.V.
PMID: 10919779;PMCID: PMC92143;Abstract:
Little is known about the interaction of biosurfactants with bacterial cells. Recent work in the area of biodegradation suggests that there are two mechanisms by which biosurfactants enhance the biodegradation of slightly soluble organic compounds. First, biosurfactants can solubilize hydrophobic compounds within micelle structures, effectively increasing the apparent aqueous solubility of the organic compound and its availability for uptake by a cell. Second, biosurfactants can cause the cell surface to become more hydrophobic, thereby increasing the association of the cell with the slightly soluble substrate. Since the second mechanism requires very low levels of added biosurfactant, it is the more intriguing of the two mechanisms from the perspective of enhancing the biodegradation process. This is because, in practical terms, addition of low levels of biosurfactants will be more cost-effective for bioremediation. To successfully optimize the use of biosurfacrants in the bioremediation process, their effect on cell surfaces must be understood. We report here that rhamnolipid biosurfactant causes the cell surface of Pseudomonas spp. to become hydrophobic through release of lipopolysaccharide (LPS). In this study, two Pseudomonas aeruginosa strains were grown on glucose and hexadecane to investigate the chemical and structural changes that occur in the presence of a rhamnolipid biosurfactant. Results showed that rhamnolipids caused an overall loss in cellular fatty acid content. Loss of fatty acids was due to release of LPS from the outer membrane, as demonstrated by 2-keto-3-deoxyoctonic acid and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and further confirmed by scanning electron microscopy. The amount of LPS loss was found to be dependent on rhamnolipid concentration, but significant loss occurred even at concentrations less than the critical micelle concentration. We conclude that rhamnolipid-induced LPS release is the probable mechanism of enhanced cell surface hydrophobicity.
PMID: 12826480;PMCID: PMC1241557;Abstract:
Forty percent of hazardous waste sites in the United States are co-contaminated with organic and metal pollutants. Data from both aerobic and anaerobic systems demonstrate that biodegradation of the organic component can be reduced by metal toxicity. Metal bioavailability, determined primarily by medium composition/soil type and pH, governs the extent to which metals affect biodegradation. Failure to consider bioavailability rather than total metal likely accounts for much of the enormous variability among reports of inhibitory concentrations of metals. Metals appear to affect organic biodegradation through impacting both the physiology and ecology of organic degrading microorganisms. Recent approaches to increasing organic biodegradation in the presence of metals involve reduction of metal bioavailability and include the use of metal-resistant bacteria, treatment additives, and clay minerals. The addition of divalent cations and adjustment of pH are additional strategies currently under investigation.
Abstract:
The operation and exploitation of the Pilares breccia-pipe copper ore deposit and its subsequent cessation of activities in 1945 generated approximately 20 million tons in mine tailings and waste, currently covering a 19 hectares surface located within the urbanized area of Nacozari de Garcia, Sonora. The studied mine tailings are unconfined, with unstable slopes of 50°, lack of vegetation and seasonal efflorescent salts haved formed onthe surface. Such characteristics generate high eolian and water erosion and the possiblecollapse of material. In this work, calculated erosion is classified as severe to very severe. Granulometric characteristics of the tailings mark significant differences in theintensity of water erosion. The physical characterization of the tailings shows two areas of different granulometry that influence in the oxidation of sulfides and metal release: an area of large particle size, and another of fine particle size produced by the flotation process. Climate indexes were estimated for a time period from 1960 to 2011. The aridity index of Martonne shows a rank from 47 to 66, varying from humid in thé60s and́70s to perhumid in thé80s, and humid at present. The Lang index shows values from 15.2 to 22.8, which classifies as a steppe. Copper contents in residential soils and airborne dust collected at two meters height exceed the geochemical background.
PMID: 18409640;Abstract:
Eolian dispersion of mine tailings in arid and semiarid environments is an emerging global issue for which economical remediation alternatives are needed. Phytostabilization, the revegetation of these sites with native plants, is one such alternative. Revegetation often requires the addition of bulky amendments such as compost which greatly increases cost. We report the use of plant growth-promoting bacteria (PGPB) to enhance the revegetation of mine tailings and minimize the need for compost amendment. Twenty promising PGPB isolates were used as seed inoculants in a series of greenhouse studies to examine revegetation of an extremely acidic, high metal content tailings sample previously shown to require 15% compost amendment for normal plant growth. Several isolates significantly enhanced growth of two native species, quailbush and buffalo grass, in tailings. In this study, PGPB/compost outcomes were plant specific; for quailbush, PGPB were most effective in combination with 10% compost addition while for buffalo grass, PGPB enhanced growth in the complete absence of compost. Results indicate that selected PGPB can improve plant establishment and reduce the need for compost amendment. Further, PGPB activities necessary for aiding plant growth in mine tailings likely include tolerance to acidic pH and metals. © 2008 American Chemical Society.
