PhD thesis of Emmanuel Jaffrès (2006-2009)

Cooked and peeled shrimps are considered as fragile foodstuffs from a microbiological point of view and sensitive to the organoleptic spoilage process induced by the growth of certain  bacteria. For a better control of their shelf life, the microbial ecosystem characterisation of this product was carried out by a polyphasic approach combining culture-dependent methods, which allow the phenotypical and molecular identification of bacterial isolates, and culture-independent methods with the PCR-TTGE and DHPLC techniques which generate genetic fingerprintings from a bacterial DNA mixture directly extracted from cooked shrimps matrix. This approach has allowed to highlight the main bacterial genus and species present in the spoilage microbiota of cooked and peeled shrimps. Lactic acid bacteria was the major group with the main genus: Carnobacterium, Vagococcus, Enterococcus. Within the Vagococus genus, a more detailed taxonomic study allowed to discover the novel species Vagococcus penaei .sp .nov. Two other important bacterial groups were found in the shrimp ecosystem: Brochothrix thermosphacta and Serratia liquefaciens-like. The spoilage potential of these bacteria has been investigated by using sensory analysis methods and biochemical techniques to determine the volatile compounds produced by these bacteria, like the gas chromatography coupled to mass spectrometry device. Three species were considered as the most spoiling bacteria in this type of product: Bt. thermosphacta, St. liquefaciens-like and Cb. Maltaromaticum.