Acid Tolerance Response of Listeria monocytogenes in Various External pHs with Different Concentrations of Lactic Acid

Abstract

This study evaluated the acid tolerance response (ATR) of two strains of Listeria monocytogenes (serotype 1/2a and 4b) and one strain of Listeria innocua under different mildly acid conditions. Cells were incubated in combinations of three concentrations of lactic acid medium (3, 4.75, and 15 mM) and three external pH's (pH ex 5.0, 6.0, and 6.5), plus, a HCl control, and a blank control (pH 7.4). Results showed that lactic acid induced lower log reduction of all three strains when challenged in severe acid conditions (pH 3.0) after being habituated at a pH ex of 5.5 or 6.0 until the log phase, compared with a pH ex of 6.5 or the two controls. This indicates that when the pH ex was either 5.5 or 6.0 this induced a higher ATR of the strains, which may be caused by the ability of the strains to retain intracellular pH (pH i) homeostasis with pH i maintained in the range of 7.4-7.9. It was also found that a pH ex of 5.5 resulted in the highest pH i of the strains across all incubated conditions, which indicates that the pH i may play an important role in the induction of ATR when Listeria cells are habituated in lactic acid, and if the higher pH i can be maintained, the ATR would be stronger. The concentration of lactic acid per se has no significant effect on ATR, which it is proposed was due to the pH i homeostasis maintained within the cells. However, the difference in ATR among three strains was also significant, which cannot be explained by the stable pH i of all tested strains. Therefore, other underlying mechanisms to mediate ATR under different conditions need to be explored in further studies.

Influence of Hurdle Technology on Foodborne Pathogen Survival in the Human Gastrointestinal Tract

“…The effects of cross-protection and “stress hardening” on L. monocytogenes have been previously illustrated, where the pathogen exhibited an increased acid tolerance to gastric acidity (pH 2.0) after it had been previously habituated to a pH of 5.5 and 6.6 for 15 days [ 46 ]. Moreover, a recent previous study revealed that the habituation of L. monocytogenes to a pH of 5.5 or 6.0 induced a greater ATR that a pH of 6.5 [ 47 ], readily confirming the current findings.…”

Comparative Analysis of Growth, Survival, and Virulence Characteristics of Listeria monocytogenes Isolated from Imported Meat

“…These differences likely stem from the diverse methodological approaches used in different studies. Our results indicate that serotype 1/2a isolates generally exhibited higher acid tolerance compared to the other serotypes, aligning with the findings of Yunge et al (2020) [ 47 ]. Although the observed differences in the population reduction among serotypes under pH 2.5 conditions were not statistically significant, the serotype 1/2a isolates consistently showed the smallest average reduction values.…”

Lactic Acid Bacteria from Gamecock and Goat Originating from Phitsanulok, Thailand: Isolation, Identification, Technological Properties and Probiotic Potential

“…L. monocytogenes growing in a mild acid environment could increase its resistance to the lethal pH condition in the stomach, and this phenomenon has been called acid tolerance response (ATR). With a pH range of about 4.5-6.0 at 30°C or 37°C, lactic and acetic acids can efficiently trigger the ATR of L. monocytogenes [60]. This was in line with Smith et al [61], who stated that L. monocytogenes has a number of processes that enable it to cope with acidic conditions, such as those found in acidic foods and the gastrointestinal tract.…”

“…L. monocytogenes growing in a mild acid environment could increase its resistance to the lethal pH condition in the stomach, and this phenomenon has been called acid tolerance response (ATR). With a pH range of about 4.5-6.0 at 30°C or 37°C, lactic and acetic acids can efficiently trigger the ATR of L. monocytogenes [ 60 ]. This was in line with Smith et al .…”