Prodigiosin-Producing Serratia marcescens as the Causal Agent of a Red Colour Defect in a Blue Cheese

Abstract

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 Serratia marcescens 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 S. marcescens 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 S. marcescens 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 S. marscescens 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 Enterobacteriaceae, and the delayed development of Penicillium roqueforti during cheesemaking. The association between S. marcescens 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 S. marcescens in milk, the red defect the bacterium causes in cheese, and its associated economic losses.

The Microbial Diversity on the Surface of Smear-Ripened Cheeses and Its Impact on Cheese Quality and Safety

“…However, in another type of cheese from Spain called Cabrales, a traditional, blue-veined Spanish cheese made from raw milk, a prodigiosinproducing strain of Serratia marcescens was identified to be responsible for the development of this defect. Although the source of S. marcescens was not undoubtedly identified, the requirement of high numbers of bacterial cells for the red defect to develop in experimental cheeses and the occasional presence of prodigiosin-producing S. marcescens strains in the milk destined for Cabrales manufacture strongly suggested that the cheese milk is the source for this bacterium [188]. Furthermore, pink and red discoloration defects, as well as brown pigments, are also associated with the growth of certain yeast species, such as Rhodotorula spp.…”

“…The predominant mold identified as spoiling microorganisms on the surface of smear-cheese are Penicillium species like P. commune, P. glabrum, and P. nalgiovense, whereas P. verrucosum, P. solitum, P. roqueforti, P. discolor, P. crustosum, and P. palitans together with Aspergillus versicolor were only frequently reported [175,193,194]. Epicoccum spp., which are widely recognized as plant pathogens, have been recently identified as the causative agent for reddish-brown spot defects on cheese [188]. Epicoccum sp.…”

“…Epicoccum sp. were found to colonize the whole cheesemaking environment, thus revealing a difficult task for the elimination as a spoilage source in order to render a hygienic production environment [188]. Besides poor factory hygiene, mold defects can also develop due to poor sanitation of ripening areas, airborne mold spores, movement of personnel, aerosol generation, mold-contaminated brine, poor smear development, 'old-young smearing' application, and inadequate humidity or surface treatment [175].…”