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Abstract
Hemolytic assay, bacterial adhesion to hydrocarbons (BATH) assay, drop collapse assay, oil spreading assay, emulsification assay and surface tension measurement were compared for their reliability and ease of use to screen biosurfactant producing bacteria. 105 bacterial strains were screened for their biosurfactant production. Results from screening methods revealed that hemolytic and BATH assays are not reliable methods to check biosurfactant production because hemolytic assay relies on lysis of blood cells, which can be caused by compounds produced by microbes other than the biosurfactants. In that case hemolytic assay may include strains that do not produce biosurfactant. BATH assay relies on hydrophobicity or adherence of cells with hydrophobic compounds. Results obtained in BATH assay for three bacterial isolates, Branhamella catarrhalis, Citrobacter intermedius and Klebsiella ozaenae were positive, but when the same isolates screened for drop collapse, oil spreading, and surface tension measurement the results were negative. This indicates that these bacterial cells are either acting as biosurfactant themselves (cell surface) or they don’t have the ability to produce biosurfactants. In conclusion, drop collapse and oil spreading assays are reliable methods to screen large number of samples. However, for confirmation surface tension measurement can be used at the end. Another interesting finding in this study was the use of crude oil as hydrophobic substrate for the isolation of biosurfactant producing bacterial strains; this approach may be useful during the initial isolation of biosurfactant producing bacteria to reduce the number of strains to be screened.
Highlights
Bacteria produce a wide range of extracellular products with many properties and applications
In this study six different methods were used to screen bacterial isolates for biosurfactant production and it was found that drop collapse and oil spreading methods are reliable methods to screen large number of samples for biosurfactant production because these methods require only 5-10 μl of biosurfactant solution and have direct correlation with each other as reposted in this study and in other earlier studies
After screening the strains for biosurfactant production using oil spreading and drop collapse assay, for confirmation surface tension method can be included in the screening criteria
Summary
Bacteria produce a wide range of extracellular products with many properties and applications. Biosurfactants are one such extracellular amphiphilic compounds produced by bacteria, yeast and fungi, especially when they are grown on hydrophobic substrates. Biosurfactants have received considerable attention in the field of environmental remediation processes such as biodegradation, soil washing and soil flushing. Biosurfactants influence these processes because of their efficacy as dispersion and remediation agents and their environmentally friendly characteristics such as low toxicity and high biodegradability [5,6,7,8,9]. Biosurfactants are potential antimicrobial agents to specific microbes and can be exploited for its antimicrobial applications against plant, animal and human microbial pathogens [1]
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