The aim of this paper was to identify the source of microbiological contamination and the bacteriological cause of a unique spoilage pattern, characterized by a distinct blue discoloration of fresh cheese, which occurred in farm cheese from Gran Canaria (Canary Islands, Spain). The genus Pseudomonas are important medically because they areconsidered opportunistic pathogens for humans and animals while others, like phytopathogens, are very important in the agricultural sector (Ridgway and Safarik, 1990). Thus, we studied it using traditional microbiological methods and genotypic tests. These techniques identified Pseudomonas fluorescensbiovar IV, which produces a blue, non-diffusible pigment, as the likely contaminating agent. The isolates collected from contaminated cheese, milk, water and curd were initially identified by the Analytical Profile Index test as P. aeruginosa and later confirmed by 16S rDNA sequencing as Pseudomonas fluorescens. Pulsed field gel electrophoresis with the enzyme XbaI identified Pseudomonas fluorescens in milk and cheese. Pasteurizer installation and a new cleaning-disinfection program allowed us to control Pseudomonas spp. until all the blue coloration in the cheese had been eliminated. Our data suggested that the source of contamination could have been due to the use of non-chlorinated water of uncertain origin in the dairy facilities and on the farm.

Keywords: fresh cheese; Pseudomonas fluorescens; food safety