Metabolic and Structural Sites of Damage in Heat- and Sanitizer-Injured Populations of Listeria monocytogenes

Abstract

Two food isolates of Listeria monocytogenes (strains ATCC 51414 and F5027) were sublethally injured by exposure to heat (56°C for 20 min) or to a chlorine sanitizer (Antibac, 100 ppm for 2 min). Percent injury following treatment ranged from 84% to 99%. Injured Listeria were repaired in Listeria repair broth (LRB) at 37°C. Comparison of the repair curves generated by each method indicated that the time for repair was greater for sanitizer-injured cells (14 h) than for heat-injured cells (5 h). Sites of injury were determined by repairing heat- and sanitizer-treated Listeria in LRB supplemented with one of the following inhibitors: rifampicin (10 and 20 μg/ml), chloramphenicol (5 μg/ml), cycloserine D (10 and 20 μg/ml), and carbonyl cyanide m-chlorophenyl-hydrazone(CCCP) (2.5 μg/ml). In both heat- and sanitizer-injured populations, a total inhibition of repair was seen following incubation with rifampicin, chloramphenicol and CCCP. These results clearly indicate a requirement for mRNA, protein synthesis, and oxidative phosphorylation for repair to occur. The cell wall is not a site of damage since cycloserine D had no effect on repair of heat- or sanitizer-injured Listeria. Investigation of damage to the cell membrane showed that stress caused by sublethal heat or sanitizer did not allow proteins or nucleotides to leak into the medium. The recognition of injury and repair in Listeria will lead to improved methods of detection and ultimately to control strategies which prevent outgrowth of this organism in foods.