Effect of sodium chloride on the intracellular solute pools of Listeria monocytogenes

Abstract

The concentrations of intracellular solutes in Listeria nwnocytogenes were examined in cells grown at various concentrations of NaCl. At 5% NaCl, cells contained elevated concentrations of potassium and glycine betaine compared with concentrations in cells grown without NaCl. At 7.5% NaCl, cells contained increased concentrations of K+, glycine betaine, glycine, alanine, and proline. Only glycine betaine, choline, or glycine promoted growth on a solidified defined medium containing 4% NaCl; there was no growth at higher concentrations of NaCl in the defined medium. Listenia monocytogenes is a gram-positive, rod-shaped bacterium of widespread distribution in the environment. It is able to live as an intracellular pathogen in humans and animals (6, 7, 10, 14). It has been isolated from a range of food samples, and there have been major outbreaks of listeriosis associated with contaminated foods. The organism is of concern to the food industry, particularly as it can survive and grow at refrigeration temperatures and high salt concentrations, e.g., up to 10% (18). This tolerance to osmotic stress suggests an adaptation of the organism to the osmotic environment. The adaptation of bacteria to a high-salt environment by the intracellular accumulation of compatible solutes has been extensively studied in gram-negative bacteria. Solutes commonly accumulated include potassium ions, amino acids, imino acids, quaternary ammonium compounds, and carbohydrates. The genetics and control of the uptake mechanism have been well characterized in Escherichia coli and Salmonella typhimurium, showing an integrated response involving accumulation of K+, glutamate, betaine, and trehalose, dependent on the nature of the environment and the degree of upshock (5). The physiological response of gram-positive bacteria to external osmotic potential has received considerably less attention. Streptomyces species accumulate proline, glutamine, and alanine because of increased synthesis when grown at enhanced salt concentrations (11). Staphylococcus aureus accumulates glutamine, proline, and betaine (1, 13, 19) in saline conditions. Proline accumulation has also been observed in Bacillus subtilis, but when betaine was present, this was accumulated in preference to proline (2, 21, 24). This paper describes the intracellular solutes accumulated in L. monocytogenes in response to increased salinity and osmotic stress. MATERIALS AND METHODS Growth of bacteria. L. monocytogenes was grown in coryneform broth containing (liter-') 10 g of tryptone (Dif

Osmoprotectants and cryoprotectants for Listeria monocytogenes

“…Accumulated compounds may include potassium ions, amino acids and their derivatives, and carbohydrates (Galinski 1995). Glycine betaine and carnitine are osmoprotectants (compounds that increase growth rates at high osmolarity) for L. monocytogenes (Patchett et al 1992;Ko et al 1994;Call and Miller 1997). Several studies have shown that these compounds have little or no growth-promoting effect at temperatures from 30°C to 37°C in the absence of osmotic stress (Beumer et al 1994;Ko et al 1994;Smith 1996).…”

Comparative assessment of acid, alkali and salt tolerance inListeria monocytogenesvirulent and avirulent strains

“…From a food safety point of view, an obvious implication from this and other studies [1][2][3][4][5] is that use of acid, alkali and/or salt in food product processing may not be sufficient to eliminate all L. monocytogenes bacteria, as a certain number of this species will survive at very extreme pH and salt conditions. This puts good manufacturing practice to the forefront to ensure that processed, ready-to-eat food products are free of L. monocytogenes contamination.…”

“…The ability of L. monocytogenes virulent strains to tolerate and survive under harsh environmental conditions such as extreme pH (<4.0 or >9.0) and high concentration of salt (around 10%) has been well documented [1][2][3][4][5]. These external stresses often result in inhibition of L. monocytogenes growth, and once the stress factors are removed, this bacterium will resume its rapid multiplication.…”

“…The fact that this bacterium is ubiquitously present in all environments and tolerant to extreme environmental stresses may account for its prevalence. It is well known that L. monocytogenes bacteria can sustain wide ranges of pH (4.0-9.5) and temperature (<1-45°C) and are able to survive in the presence of high salt concentrations (up to 10% NaCl) [1][2][3][4][5]. This renders many of the conventional food manufacturing processes ineffective in eliminating the bacterium.…”

Solute Transport Enzymes Related to Stress Tolerance in Listeria Monocytogenes: A Review

“…The amount of osmolyte in an extract was calculated by comparison of peak intensity to that of an internal standard (50 mM alanine) whose a-carbon resonance is shown at 5 1.5 ppm. Internal concentrations were calculated by normalizing the amount of solute to cellular protein, then dividing by the cytoplasmic volume per unit protein (Patchett et al 1992). stress. lntracellular levels of glycine betaine were lower than those observed under salt stress, which is appropriate, because high levels would lead to unacceptably large turgor pressure.…”