title: Predicting the minimum liquid surface tension activity of pseudomonads expressing biosurfactants.
abstract: Bacteria produce a variety of biosurfactants capable of significantly reducing liquid (aqueous) surface tension (γ) with a range of biological roles and biotechnological uses. To determine the lowest achievable surface tension (γMin ), we tested a diverse collection of Pseudomonas-like isolates from contaminated soil and activated sludge and identified those expressing biosurfactants by drop-collapse assay. Liquid surface tension-reducing ability was quantitatively determined by tensiometry, with 57 isolates found to significantly lower culture supernatant surface tensions to 24·5-49·1 mN m(-1) . Differences in biosurfactant behaviour determined by foaming, emulsion and oil-displacement assays were also observed amongst isolates producing surface tensions of 25-27 mN m(-1) , suggesting that a range of structurally diverse biosurfactants were being expressed. Individual distribution identification (IDI) analysis was used to identify the theoretical probability distribution that best fitted the surface tension data, which predicted a γMin of 24·24 mN m(-1) . This was in agreement with predictions based on earlier work of published mixed bacterial spp. data, suggesting a fundamental limit to the ability of bacterial biosurfactants to reduce surface tensions in aqueous systems. This implies a biological restriction on the synthesis and export of these agents or a physical-chemical restriction on their functioning once produced.
  
  
  CONCLUSIONS
  Numerous surveys of biosurfactant-producing bacteria have been conducted, but only recently has an attempt been made to predict the minimum liquid surface tension these surface-active agents can achieve. Here, we determine a theoretical minimum of 24 mN m(-1) by statistical analysis of tensiometry data, suggesting a fundamental limit for biosurfactant activity in bacterial cultures incubated under standard growth conditions. This raises a challenge to our understanding of biosurfactant expression, secretion and function, as well as being of interest to biotechnology where they are used in an increasingly wide range of applications.
authors.affiliation: SIMBIOS Centre & School of Science, Engineering and Technology, Abertay University, Dundee, UK.
authors.forename: I U
authors.initials: IU
authors.lastname: Mohammed
authors.forename: Y
authors.initials: Y
authors.lastname: Deeni
authors.forename: S M
authors.initials: SM
authors.lastname: Hapca
authors.forename: K
authors.initials: K
authors.lastname: McLaughlin
authors.forename: A J
authors.initials: AJ
authors.lastname: Spiers
mesh_terms: D001419:Bacteria / Q000302:isolation & purification / Q000378:metabolism; D018407:Bacterial Physiological Phenomena; D011549:Pseudomonas / Q000302:isolation & purification / Q000378:metabolism / Q000502:physiology*; D012722:Sewage / Q000382:microbiology; D012988:Soil Microbiology; D013500:Surface Tension; D013501:Surface-Active Agents / Q000378:metabolism*
chemical_list: D012722:Sewage; D013501:Surface-Active Agents
keywords: Pseudomonas; biosurfactant; liquid surface tension; statistical prediction; surfactant