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Food spoilage results in a degradation of sensory qualities (smell, taste, texture) which results mainly from the metabolism of so-called spoilage bacteria and which generates considerable food losses for food industry manufacturers. Brochothrix thermosphacta is a bacterium that causes spoilage in many seafood and meat products. This bacterium is a model for the scientists at SECALIM who seek to understand the capacities of this bacterium to settle durably in the industrial environment in particular by its capacity to form biofilms. As part of this study, the aim was to develop a precise and rapid molecular method known as PMA-qPCR, which allows only the viable Brochothrix cells present in cold-smoked salmon to be quantified and responsible for its spoilage. The tools allowing the specific and sensitive detection as well as the quantification of this bacterium in food products are very useful, since the culture method commonly used to quantify viable cells of B. thermosphacta requires several days of incubation and may underestimate cells that would not be immediately cultivable. SECALIM researchers have designed a new set of PCR (Polymerase Chain reaction) primers allowing the amplification and quantification of the rpoC gene present in a single copy in the genome of the bacteria. The efficiency and specificity of this quantitative PCR were compared with those obtained for two other sets of primers already published, targeting the rpoC and rpoB genes. In combination with labeling of bacterial DNA using PMA or PMAxx viability dyes, qPCR has been shown to be specific and effective, regardless of the dye and primer set used, for the selective quantification of DNA from viable cells in pure culture and in cold-smoked salmon samples, thus allowing efficient discrimination between viable and dead cells. PMA / PMAxx-qPCR could represent a relevant tool for smoked salmon producers to rapidly detect and quantify B. thermosphacta in their products early enough before the spoilage is noticeable by the consumer and thus limit food losses. Finally, this method could be adapted to other matrices such as meat products also very sensitive to spoilage due to B. thermosphacta.

Publication: Bouju-Albert, A., Saltaji, S., Dousset, X., Prévost, H., & Jaffrès, E. (2021). Quantification of viable Brochothrix thermosphacta in cold-smoked salmon using PMA/PMAxx-qPCR. Frontiers in Microbiology, 12 (1907). https://doi.org/10.3389/fmicb.2021.654178

Parteners and funding: This study was carried out by UMR INRAE Oniris SECALIM and funded as part of the regional ALTEROBIO project funded by the Pays de la Loire Region.