Shewanella putrefaciens adhesion and biofilm formation on food processing surfaces

Laboratory model systems were developed for studying Shewanella putrefacien s adhesion and biofilm formation under batch and flow conditions. S. putref aciens plays a major role in food spoilage and may cause microbially induce d corrosion on steel surfaces. S. putrefaciens bacteria suspended in buffer adhered readily to stainless steel surfaces. Maximum numbers of adherent b acteria per square centimeter were reached in 8 h at 25 degreesC and reflec ted the cell density in suspension. Numbers of adhering bacteria from a sus pension containing 10(8) CFU/ml were much lower in a laminar flow system (m odified Robbins device) (reaching 10(2) CFU/cm(2)) than in a batch system ( reaching 10(7) CFU/cm(2)), and maximum numbers were reached after 24 h. Whe n nutrients were supplied, S. putrefaciens grew in biofilms with layers of bacteria. The rate of biofilm formation and the thickness of the film were not dependent on the availability of carbohydrate (lactate or glucose) or o n iron starvation. The number of S. putrefaciens bacteria on the surface wa s partly influenced by the presence of other bacteria (Pseudomonas fluoresc ens) which reduced the numbers of S. putrefaciens bacteria in the biofilm. Numbers of bacteria on the surface must be quantified to evaluate the influ ence of environmental factors on adhesion and biofilm formation. We used a combination of fluorescence microscopy (4',6'-diamidino-2-phenylindole stai ning and in situ hybridization, for mixed-culture studies), ultrasonic remo val of bacteria from surfaces, and indirect conductometry and found this co mbination sufficient to quantify bacteria on surfaces.