ORIGINAL ARTICLE Dual-species relations between Candida tropicalis isolated from apple juice ultrafiltration membranes, with Escherichia coli O157:H7 and Salmonella sp. M.C. Tarifa 1 , J.E. Lozano 1 and L.I. Brugnoni 1,2 1 Pilot Plant of Chemical Engineering (UNS-CONICET), Bah  ıa Blanca, Argentina 2 Department of Biology, Biochemistry and Pharmacy, Universidad Nacional del Sur, Bah  ıa Blanca, Argentina Keywords apple juice, Candida tropicalis, co-cultures, Escherichia coli O157:H7, Salmonella sp., stainless steel. Correspondence Lorena I. Brugnoni, 12 de Octubre 991, 8000 Bah  ıa Blanca, Buenos Aires Province, Argentina. E-mail: brugnoni@uns.edu.ar 2014/1508: received 25 July 2014, revised 11 November 2014 and accepted 11 November 2014 doi:10.1111/jam.12710 Abstract Aims: The objective of this study was to determine the interactions between common spoilage yeast, Candida tropicalis, isolated from ultrafiltration membranes, and Escherichia coli O157:H7 and Salmonella sp. on stainless steel surfaces. Methods and Results: Single and dual-species attachment assays were performed on stainless steel at 25°C using apple juice as culture medium. The growth of Salmonella sp. rose when it was co-cultivated with C. tropicalis in dual biofilms at 16 and 24 h; the same effect was observed for E. coli O157:H7 at 24 h. The colonization of C. tropicalis on stainless steel surfaces was reduced when it was co-cultivated with both pathogenic bacteria, reducing C. tropicalis population by at least 10 log unit. Visualization by SEM demonstrated that E. coli O157:H7 and Salmonella sp. adhere closely to hyphal elements using anchorage structures to attach to the surface and other cells. Conclusions: These results suggest a route for potential increased survival of pathogens in juice processing environments. These support the notion that the species involved interact in mixed yeast–bacteria communities favouring the development of bacteria over yeast. Significance and Impact of the Study: This study support the plausibility that pathogen interactions with strong biofilm forming members of spoilage microbiota, such as C. tropicalis, might play an important role for the survival and dissemination of E. coli O157:H7 and Salmonella sp. in food-processing environments. Introduction Contamination of food by spoilage and pathogenic micro- organisms costs the food industry millions of dollars annually; much of this contamination may be attributed to the presence of biofilms in the processing plants (Brooks and Flint 2008). In food industry, the presence of undesirable biofilms causes serious problems such as impeding the flow of heat across the surface, increase in the fluid frictional resistance at the surface and increase in the corrosion rate at the surface leading to energy and product losses. In addition, the biofilms, including spoilage and pathogenic microflora also offer considerable problems of cross-con- tamination and postprocessing contamination (Kumar and Anand 1998). The risk becomes even more serious, as it has been observed that the antimicrobial resistance of biofilm cells is significantly increased compared to planktonic ones (Bridier et al. 2011; Brugnoni et al. 2012a). As the recognition of biofilms as microbial phenome- non, most investigations have been focused on bacterial biofilms, with little reference to the involvement of yeasts in either single-species phenomena, or as part of mixed- species communities (El-Azizi et al. 2004), even less in the food sector. Journal of Applied Microbiology 118, 431--442 © 2014 The Society for Applied Microbiology 431 Journal of Applied Microbiology ISSN 1364-5072