The effects of heat shock on the D(70)-values of Listeria monocytogenes on selected seafood matrices.

摘要

The objective of this research was to determine the effects of heat shocking and subsequent protein production on three strains of Listeria monocytogenes (1/2a, 1/2b, 4c), and Listeria innocua. The three strains of L. monocytogenes utilized were chosen from three lineages; I containing isolates pathogenic to humans, II including isolates pathogenic to both humans and animals, and III strains not linked to disease in humans.; Growth characteristics were examined on standard agar media and on different seafood matrices. The stress protein GroEL was also examined after heat shock conditions. D70-values of heat shocked (HS) and non-heat shocked (NHS) cultures were determined and compared. Results showed D70-values of heat shocked L. monocytogenes 1/2a and 1/2b increased significantly (p 0.05) from 1.04 and 0.94 to 2.07 and 2.24 minutes, respectively. D 70-values of L. monocytogenes 4c and L. innocua , both non-pathogenic, did not show a similar increase.; Studies examining the presence of the stress protein GroEL and other GroEL-like compounds, showed in all cases, the NHS cultures had lower levels as compared to HS cells. Densitometer readings of L. monocytogenes 1/2a showed an approximate 2-fold increase in GroEL levels, presenting 687 ng and 351 ng for HS and NHS cultures, respectively. In tests on HS L. monocytogenes 1/2b, GroEL levels were 913 ng as compared to 209 ng for the NHS strain. Heat shocked L. monocytogenes 4c presented total GroEL levels of 353 ng as compared to 213 ng for the NHS cultures. L. innocua expressed GroEL levels of 659 ng and 273 ng for HS and NHS cultures, respectively.; The two L. monocytogenes cultures, 1/2a and 1/2b, had significantly increased D70-values, but reverted back to a NHS state at the 24-hour sampling interval. The initial D70 -values for HS and NHS L. monocytogenes 1/2a were 2.38 and 1.25 minutes, respectively. The D70-values for Day-1 HS were significantly higher (p 0.05) when compared to NHS culture on both days and to HS cells on days 1 and 2. The same effect was seen for L. monocytogenes 1/2b with initial D70-values of 2.40 and 1.19 for heat and non-heat treated, respectively.