EFFECT OF IONIZATION ON LISTERIA-MONOCYTOGENES IN CONTAMINATED SHRIMPS

Listeria monocytogenes is a highly virulent pathogenic bacteria that commonly occurs in various types of foods. Its elimination is problematic since it may resist usual levels of pasteurization or other preservation treatments (heat, cold, pH, ionization...). Also, its isolation requires enrichment procedures. Our aim was to determine how the ionization of L. monocytogenes in artificially contaminated shrimps would affect its survival and how efficient were some current isolation methods to detect these stressed bacteria. L. monocytogenes has increasing resistance to ionization, as expressed by the Decimal Reduction Dose (D10) when it is treated in saline solution (NaCl 0.85%), thawed shrimps, tryptic soy broth-yeast extract (0.6%) and frozen shrimps respectively. Among the factors that influenced the calculation of L. monocytogenes's radioresistance, were the recovery growth media and the specific strain used. Although the difference was never significant (P < 0,05), tryptic soy agar-yeast extract (0.6%) always provided higher survival rates (0-13%) than Oxford media. Strain ATCC 19115 was significantly (P < 0.05) more resistant than strain FDA-1A1, by approximately 15 to 45%. A cold enrichment procedure also improved the recovery of L. monocytogenes from irradiated shrimp samples. L. monocytogenes (ATCC 19115) was still present in frozen shrimp samples exposed to 5 kGy when the initial contamination level reached 8.5 x 10(4) colony forming units/g. The United States Department of Agriculture method was the next most efficient method while the Food and Drug Administration method was less efficient at high dose levels.