PhD defense of Taous Saraoui

PhD topic: Mechanisms of interaction between a bioprotective bacterium with a seafood biopreservation interest "Lactococcus piscium CNCMI-4031" and the pathogenic bacterium Listeria monocytogenes

Lactococcus piscium CNCM I-4031, a psychrotrophic lactic acid bacterium isolated from raw salmon, is able to inhibit the growth of Listeria monocytogenes by 3 to 4 log CFU g^-1 in peeled cooked shrimp. To characterize this inhibition a chemically defined medium was developed and the inhibition reproduced. In addition of growth inhibition, L. piscium reduced virulence of L. monocytogenes. Scan electronics microscopy showed that L. monocytogenes cells were elongated and cell walls damaged in presence of L. piscium. The mechanism involved in this inhibition are not due to excretion of antimicrobial molecules nor nutritional competition. It requires cell-to-cell contact and may be controlled by quorum-sensing. To elucidate the mechanism involved in this inhibition, the genome of L. piscium CNCM I-4031 was sequenced and expert annotation was performed. In addition, the gene expression of L. piscium in pure culture and in co-culture with L. monocytogenes using RNA-Seq approach was compared. By compiling the results of both approaches, 13 potential target genes clusters were highlighted. There are involved in biosynthesis and transport of bacteriocins, quorum sensing mechanism, pili formation, bacterial competence and CAZymes. Deeper investigation of the results is necessary to select potential target genes for a functional approach with construction of mutant of L. piscium. At last, applied trial of L. piscium in association with Carnobacterium divergens V41, a bioprotective strain producing a bacteriocin, proved the efficiency of this cocktail to improve the quality and security of peeled cooked shrimp.
Key Words
Lactic acid bacteria, biopreservation, food quality and security, cell-to-cell contact dependent inhibition, cell communication, quorum sensing, Lactococcus piscium genome, RNA-seq