AbstractBiosurfactants are surface active chemicals expressed by a range of organisms that reduce liquid (aqueous) surface tensions () of aqueous and aqueous-hydrocarbon (oil) mixtures. They are widely used in biotechnology, including agriculture, cosmetics, food, pharmacology and bioremediation, and many new biosurfactants are identified through surveys of bacteria recovered from a variety of environments. In this work, environmental Pseudomonas spp. were screened for biosurfactant production and behaviours determined in order to investigate the limits of biosurfactant activity and potential structural diversity within a phylogenetically related group of bacteria.
A total of 355 pseudomonads and Pseudomonas-like isolates were isolated from activated sludge wastewater and potentially petroleum-contaminated soils from road side drainage (SUDS) site. These were phenotypically characterized using a number of growth and behaviour assays, including air-liquid interface biofilm formation in static microcosms and in a column bead system, and shown to be a diverse collection of isolates with a minimal level of biological replication (i.e. little evidence of identical strains recovered more than once in or between samples). Of these, 57 isolates were found to express biosurfactants in vitro by drop-collapse assay and confirmed by quantitative tensiometry. The surface tension of cell-free culture supernatants produced by these isolates was between 24.5 – 49.1 mN m-1, with a minimum theoretical surface tension (Min) of 24.2 mN m-1. This is in agreement with earlier predictions, suggesting a fundamental limit to the ability of bacterial biosurfactants to reduce surface tensions in aqueous systems. This finding suggests that further effort to isolate stronger active surfactants are likely to be wasted, and poses the interesting question of what biological or physical factors limit the production of stronger biosurfactants by bacteria.
Differences in biosurfactant behaviour determined by foaming, emulsion and oil-displacement assays were also observed amongst select isolates producing surface tensions of 25 – 27 mN m-1, suggesting structural diversity in the biosurfactants produced. These findings provide a system for selecting biosurfactants for further chemical-structural analyses and future testing for various biotechnology applications where low surface activity, but varied behaviour is required.
|Date of Award||Apr 2016|
|Sponsors||Petroleum Technology Development Fund (PTDF)|
|Supervisor||Andrew Spiers (Supervisor) & Yusuf Deeni (Supervisor)|