Attachment of freshwater bacteria to solid surfaces
The initial stages of the permanent attachment of selected freshwater bacteria, particularly Pseudomonas fluorescens, Enterobacter cloacae, a Chromobacterium sp and a Flexibacter sp, to a hydrophobic and a relatively hydrophilic polystyrene surface was investigated. Changes in the nutrient conditions and growth rates of the bacteria caused differences in cell surface characteristics, measured by hydrophobic and electrostatic interaction chromatography and saline contact angle measurements on lawns of cells, and changed attachment levels to the two substrata in both buffer and nutrient media. The effects were different for each species, varying independently with surface, and were, largely, related to changes in predicted physicochemical interactions. However, the Flexibacter sp, a gliding bacterium, showed an increase. in attachment with increased growth rate, i. e. metabolic activity, possibly indicating an active attachment process. Peaks in the attachment of E. cloacae, P. fluorescens and the Chromobacterium sp to both substrata occurred between pH 6 and 7.5 and 20°C and 25°C, while the Flexibacter sp decreased attachment with increasing pH and temperature. These changes in attachment for each species were related to physico-chemical rather than physiological effects. Varying the electrolyte concentration and valency of the attachment solution showed that permanent attachment was independent of diffuse electrical double layer thickness. Similarly, attachment levels could not be predicted thermodynamically using liquid, solid and cell surface tensions, indicating the involvement of electrostatic interactions in the attachment of these species. Estimates of the hydrophobicity of the cell surfaces, by hydrophobic interaction chromatography, showed that hydrophobic interactions did not dominate in bacterial attachment. Thus, attachment of these four species was a multifactorial interaction involving a range of physico-chemical interactions. The attachment of individual species was also found to be affected by the presence of other bacterial species. Thus in natural aquatic systems a variety of biological, environmental and physico-chemical parameters will affect levels of bacterial attachment and hence biofilm composition and development.