Impact of Moderate Heat, Carvacrol, and Thymol Treatments on the Viability, Injury, and Stress Response of Listeria monocytogenes

The microbial safety and stability of minimally processed foods are based on the application of combined preservative factors. Since microorganisms are able to develop adaptive networks to survive under conditions of stress, food safety may be affected, and therefore understanding of stress adaptive mechanisms plays a key role in designing safe food processing conditions. In the present study, the viability and the sublethal injury of Listeria monocytogenes exposed to moderate heat (55 °C) and/or essential oil compounds (carvacrol and thymol, 0.3 mM) treatments were studied. Synergistic effects were obtained when combining mild heat (55 °C) with one or both essential oil compounds, leading to inactivation kinetics values three to four times lower than when using heat alone. All the treatments applied caused some injury in the population. The injury levels ranged from around 20% of the surviving population under the mildest conditions to more than 99.99% under the most stringent conditions. Protein extracts of cells exposed to these treatments were analysed by two-dimensional gel electrophoresis. The results obtained revealed that stressed cells exhibited differential protein expression to control cells. The proteins upregulated under these stressing conditions were implicated, among other functions, in stress response, metabolism, and protein refolding.

Comparative proteomic analysis of Listeria monocytogenes exposed to enterocin AS-48 in planktonic and sessile states

Enterocin AS-48 is a cyclic peptide of great interest for application in food preservation and sanitation. In the present study, the proteome response of Listeria monocytogenes to purified enterocin AS-48 was studied under two different conditions: planktonic cells and sessile cells grown on polystyrene plates. Ten different proteins were differentially expressed in planktonic L. monocytogenes cells treated with 0.1 μg/ml enterocin AS-48 compared to the untreated controls. Overexpressed proteins were related to stress response (DnaK) or carbohydrate transport and metabolism, while underexpressed and unexpressed proteins were related to metabolism (such as glyceraldehyde-3-phosphate dehydrogenase, pyruvate oxidase, glutamate dehydrogenase or glutamate decarboxylase) or stress (GroEL). In the sessile state, L. monocytogenes cells tolerated up to 10 μg/ml bacteriocin, and the treated biofilm cells overexpressed a set of 11 proteins, some of which could be related to stress response (DnaK, GroEL), protein synthesis and carbohydrate metabolism, while glyceraldehyde-3-phosphate dehydrogenase was the only unexpressed protein. Some of the overexpressed proteins (such as elongation factor Tu and GroEL) could also be implicated in cell adhesion. These results suggest different cell responses of L. monocytogenes to enterocin AS-48 in the planktonic and in the sessile state, including stress response and cell metabolism proteins. While in the planktonic state the bacterium may tend to compensate for the cytoplasmic cell permeability changes induced by AS-48 by reinforcing carbohydrate transport and metabolism, sessile cells seem to respond by shifting carbohydrate metabolism and reinforcing protein synthesis. Stress response proteins also seem to be important in the response to AS-48, but the stress response seems to be different in planktonic and in sessile cells.

Listeria monocytogenes dairy isolates show a different proteome response to sequential exposure to gastric and intestinal fluids

The gastrointestinal system poses different stresses to the foodborne pathogen, Listeria monocytogenes, including the low pH of the stomach and the presence of bile and the high osmolality of the intestinal fluid. The present study evaluated how previous exposure of three L. monocytogenes dairy isolates (C882 and T8, serovar 4b isolates and A9 serovar 1/2a or 3b isolate) to a cheese-simulated medium (p H5.5 and 3.5% NaCl [w/v], adapted cultures) affected subsequent survival in a simulated gastrointestinal system. Listerial cultures exposed to the cheese-simulated medium at pH7.0, with no added NaCl, were considered non-adapted. To investigate the main events involved in listerial survival during the gastric and intestinal subsequent challenge, a proteomic approach was used. All L. monocytogenes strains were able to survive the deleterious effects of the gastrointestinal fluids and no significant differences were observed between adapted and non-adapted cells. However the L. monocytogenes strains showed a different protein pattern in response to the gastrointestinal stress. Data indicated that synthesis of stress related proteins is more pronounced in non-adapted cells. Although, a significant number of enzymes involved in glycolysis and energy production were also consistently over-produced by the three strains. These findings provided new insights into the means used by L. monocytogenes to overcome the gastrointestinal system and allow the pathogen to move to the next phase of the infectious process.