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Characterization of a novel air-liquid interface biofilm of Pseudomonas fluorescens SBW25

¹ SIMBIOS Centre;
² SCRI;
³ AgResearch Limited, NZ

ABSTRACT

Pseudomonads are able to form a variety of biofilms that colonise the air-liquid (A-L) interface of static liquid microcosms, and differ in matrix composition, strength, resilience, and degrees of attachment to the microcosm walls. From Pseudomonas fluorescens SBW25, mutants have evolved during prolonged adaptation-evolution experiments which produce robust, physically cohesive-class biofilms at the A-L interface, which have been well-characterised. In this study we describe a novel iron-induced A-L interface biofilm produced by SBW25, which is categorised as a viscous mass (VM)-class biofilm. Iron was demonstrated to induce SBW25 to express cellulose, which provided the matrix of the biofilm, a weak structure that was readily destroyed by physical disturbance. This was confirmed in situ by a low 0.023-0.047 g maximum deformation mass and relatively poor attachment as measured by Crystal violet staining. Biofilm strength increased with increasing iron concentrations, in contrast to attachment levels, which decreased with increasing iron. Furthermore, iron added to mature biofilms significantly increased strength, suggesting that iron may also promote interactions between cellulose fibres that increase matrix interconnectivity. Whilst weak attachment is important in maintaining the biofilm at the A-L interface, surface interaction-effects involving cellulose, which reduced surface tension by ~3.8 mN m^-1, may also contribute towards this localisation. The fragility and viscoelastic nature of the biofilm was confirmed from controlled-stress amplitude sweep tests to characterise critical rheological parameters, including a shear modulus of 0.75 Pa, zero shear viscosity of 0.24 Pa s^-1 and a flow point of 0.028 Pa. Growth and morphological data thus far support a physiological, rather than mutational origin for production of the SBW25 VM biofilm, which is an example of the versatility of bacteria to inhabit optimal niches within their environment.

⁴ E-mail: a.spiers{at}abertay.ac.uk

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