Physiological aspects of Listeria monocytogenes during inactivation accelerated by mild temperatures and otherwise non-growth permissive acidic and hyperosmotic conditions

Application of simultaneous low pH (pH 3.5) and low water activity (aw = 0.9; 2.5 M NaCl) conditions to Listeria monocytogenes strains ScottA and FW03/0035, and growth permissive temperatures from 25 °C up to 45 °C result in increasingly accelerated inactivation rates. This phenomenon was related to i) increased cell permeability as suggested by ethidium homodimer-1 uptake and ii) de-energization as indicated by rapidly reduced ATP basal levels. Enrichment-based recovery experiments indicated that the stress conditions eventually lead to complete loss of reproductive capacity, possibly corresponding to an irreversible collapse of pH homeostasis. Transcriptomic analyses were used to obtain further insights into the physiology of the inactivation process occurring at 25 °C where inactivation times were more prolonged. QPCR, mRNA decay and microarray experiments revealed transcripts of tufA and other genes become substantially more stable during inactivation resulting from exposure to combined low pH/aw and from non-growth permissive temperature exposure. Genes that appear to be important for initial survival of combined low pH/aw were delineated by K-means clustering of expression data and included an overrepresentation of SigB-activated genes, the overall response of which fades with increasing time of inactivation exposure.