| Summary: | Technological defects in the organoleptic characteristics of cheese (odour, colour, texture, and flavour) reduce quality and consumer acceptance. A red colour defect in Cabrales cheese (a traditional, blue-veined, Spanish cheese made from raw milk) occurs infrequently but can have a notable economic impact on family-owned, artisanal cheesemaking businesses. This work reports the culture-based determination of <i>Serratia marcescens</i> as the microbe involved in the appearance of red spots on the surface and nearby inner areas of such cheese. Sequencing and analysis of the genome of one <i>S. marcescens</i> isolate, RO1, revealed a cluster of 16 genes involved in the production of prodigiosin, a tripyrrole red pigment. HPLC analysis confirmed the presence of prodigiosin in methanol extracts of <i>S. marcescens</i> RO1 cultures. The same was also observed in extracts from red areas of affected cheeses. The strain showed low survival rates under acidic conditions but was not affected by concentrations of up to 5% NaCl (the usual value for blue cheese). The optimal conditions for prodigiosin production by <i>S. marscescens</i> RO1 on agar plates were 32 °C and aerobic conditions. Prodigiosin has been reported to possess antimicrobial activity, which agrees with the here-observed inhibitory effect of RO1 supernatants on different bacteria, the inhibition of <i>Enterobacteriaceae</i>, and the delayed development of <i>Penicillium roqueforti</i> during cheesemaking. The association between <i>S. marcescens</i> and the red colour defect was strengthened by recreating the fault in experimental cheeses inoculated with RO1. The data gathered in this study point towards the starting milk as the origin of this bacterium in cheese. These findings should help in the development of strategies that minimize the incidence of pigmenting <i>S. marcescens</i> in milk, the red defect the bacterium causes in cheese, and its associated economic losses.
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