Membrane filters provide an excellent way to purify water but the production process is frequently impeded by fouling, with biological fouling or biofouling as the main disrupter. Despite years of research, the fundamental understanding of membrane biofouling remains limited. The lack of culture-dependent studies hampers  understanding of how bacteria interact and respond to the changing conditions on the membrane surface, for example membrane cleaning. Next generation sequencing studies have shown that the bacterial community of many membrane surfaces are comparable in composition and that the Sphingomonadaceae family is an important resident of the membrane microbiota.

Florian and I were at that time both working at Wetsus and this provided us with an advantage because in our research theme also operators of full scale membrane installations were participating. To help us solve our quest, the operators provided us with membranes that were used in full-scale water purification plants. A kind gift because membrane removal leads to downtime, a true nightmare for most membrane operators as they have to ascertain a continuous supply safe drinking water to their customers. Florian harvested twelve membranes that were used at different locations and the obtained biomass was streak plated  on a Sphingomonas selective medium, which yielded around 60 pure strains of Sphingomonads and other pigmented bacteria. Physiological characterization of the Sphingomonad membrane isolates showed that they share many features uncommon for their family, but which must have provided them key advantages for survival at the membrane surface. All membrane isolates were motile and the majority produced a flagellum and pili. Pili are perfect anchors to inhibit washout from the membrane and flagella stimulate membrane colonization. The Sphingomonadaceae family includes few members that are able to grow in a wide range of  pH or under saline conditions, but most of the membrane isolates can. Changes in pH is one of the most common procedure to dissolve biofouling material and consequently to clean membrane surfaces. Our results suggest that membrane surfaces provide an ecological niche with particular environmental conditions, due to the membrane cleaning steps and the inherent accumulation of solutes such as salts at the membrane surface. 

This work has been published under the title: Isolation and characterization of Sphingomonadaceae from fouled membranes