PhD thesis of Papa Abdoulaye Fall (2008-2011)

The bioprotective potential of Lactococcus piscium CNCM I-4031 towards a specific spoilage organism, Brochothrix thermosphacta, was studied on tropical cooked peeled shrimp packed under modified atmosphere. In co-culture, the growth of this spoilage bacterium was inhibited by 4 log ufc/g by L. piscium and the sensory quality of the product was improved during 31 days. The inhibition could not be attributed to lactic acid or nutritional competition for amino acids. The growth modelling of L. piscium and B. thermosphacta was performed in different conditions of temperature, pH and NaCl concentrations. The optimal growth values of these two bacteria are quite similar, however L. piscium seemed to be more psychrotolerant than B. thermosphacta but twice less tolerant to salt. The effect of the inoculation level of L. piscium in combination with temperature and salt on the interaction with B. thermosphacta was modelled. A high concentration (6-7 log ufc/g) was necessary to maintain the number of B. thermosphacta lower than 4 log ufc/g. However sensory improvement of shrimp was obtained with lower inoculation level. Lastly, the interaction between L. piscium and Listeria monocytogenes was studied on cooked peeled shrimp showing a 4 log cfu/g inhibition. The mechanisms involved in this inhibition were studied on a chemically defined medium (MSMA) allowing to obtain a similar inhibition. The inhibitory activity was not linked to glucose consumption, lactic acid production or competition for amino acids, bases or vitamins. However, cellular contact is necessary to obtain the inhibition.