Dissection of spoilage phenotypes of Pseudomonas fluorescens in food matrices using a transcriptomic approach.

The blue pigment produced by several genetically closely related strains belonging to the Pseudomonas fluorescens group was the cause of the blue mozzarella cases. From such spoilage events derived considerable economic losses and recently also lawsuits for several dairy factories in Italy. The identification of the chemical nature and a certain molecular trait related to the blue pigment production could be useful to identify the blue spoiler strains (i.e. the source of the contamination). Additionally, gain understanding the conditions of its production could help to prevent the spoilage event. For these reasons, 4 genetically closely related strains (2 producing and 2 not producing the blue pigment) were studied. The growth curves of the strains demonstrated that the growth trends were similar between the 4 strains and the pigment production was visible when the blue strains reached 107 UFC/ml. The growth of the strains and the pigment production demonstrated that the ability to produce the blue pigment is related to the bacterial cell density and to the presence of oxygen. Therefore, the four strains were analyzed by the complete sequencing of the genomes and the trascriptomes to identify the metabolic pathways probably involved in the production of the blue pigment. The completely sequenced genome of the Pseudomonas fluorescens strains A506, closely related to the four strains, was used as a template for the analysis. These results clearly indicate that the blue pigment is related to iron metabolism and suggest a role as siderophore for iron uptake as several others pigments produced by strains belonging to Pseudomonas genus. The analysis of the two uncolored strains seems to suggest a reduced need of iron for such strains.