Specific solute effects with special reference to Staphylococcus aureus

Prevalence of sucretolerant bacteria in common soils and their isolation and characterization

Sucretolerant microbes grow in the presence of sugar concentrations high enough to substantially lower water activities. Natural habitats high in sugars are mainly limited to dried fruit, floral nectar, honey, sugarcane, and associated soils. Organisms that tolerate extremes of solute concentration, high enough to lower water activities, might not be expected in common oligoosmotic soils. We report on the isolation of sucretolerant bacteria from common soils using media supplemented with 50% sucrose (a w 0.91) and their physiological characterization and identification by 16S rRNA gene phylogeny. Fifteen of these sucretolerant isolates from common soils were related to four Bacillus spp. A Lysinibacillus and a Microbacterium (actinomycete) also were collected. All grew at 50% sucrose and 13 grew at 60% sucrose. Most probable number counts were used to determine the abundance of sucretolerant microbes in several common soil types, including agricultural, managed turf, and native prairie. Microbial abundance (with fungicides) was about 105 and 103 cells g-1 soil in media containing 50 or 70% sucrose, respectively. The abundances of sucretolerant bacteria in common soils mirror those of halotolerant bacteria that grow at 10 and 20% NaCl. However, there is not a correlation between halotolerance and sucretolerance in our isolates, nor can predictions be made based on taxonomy. Specific solute effects may be at work, rather than biological responses to a single physicochemical parameter such as a w. The occurrence of spore-forming sucretolerant bacteria in common soils has relevance to forward planetary protection and astrobiology. Extraterrestrial habitable regions are defined in part by tolerance to high solute concentrations and osmotolerant soil microbes may contaminate spacecraft.

Bacterial maximum non-inhibitory and minimum inhibitory concentrations of different water activity depressing solutes

The NaCl MNICs (maximum non-inhibitory concentrations) and MICs (minimum inhibitory concentrations) for growth of various strains of six bacterial species were determined and then compared with those obtained for seven other solutes. The influence of prior growth conditions on the MNICs and MICs was also evaluated. No significant changes on the MNICs and MICs were found among the strains studied within each species. Among all factors investigated, only growth phase -for Gram-negatives- and growth at high NaCl concentrations led to a change in the NaCl MNICs. Species could be classified depending on its NaCl MNICs and MICs (in decreasing order) as follows: Staphylococcus aureus, Listeria monocytogenes, Cronobacter sakazakii, Enterococcus faecium, Escherichia coli and Salmonella Typhimurium. Similar results were obtained for KCl, LiCl, and sodium acetate, but not for the remaining solutes investigated (sucrose, glycerol, MgCl2 and CaCl2). Results obtained indicate that, in general, Gram-negatives showed lower MNICs and MICs than Gram-positives for all the solutes, S. aureus being the most solute tolerant microorganism. When compared on a molar basis, glycerol showed the highest MNICs and MICs for all the microorganisms -except for S. aureus- and LiCl the lowest ones. NaCl MNICs and MICs were not significantly different from those of KCl when compared on a molar basis. Therefore, the inhibitory action of NaCl could not be linked to the specific action of Na(+). Results also showed that the Na(+) tolerance of some species was Cl(-) dependent whereas for others it was not, and that factors others than aw-decrease contribute to the inhibitory action of LiCl, CaCl2 and MgCl2.

Implications of salt and sodium reduction on microbial food safety

Excess sodium consumption has been cited as a primary cause of hypertension and cardiovascular diseases. Salt (sodium chloride) is considered the main source of sodium in the human diet, and it is estimated that processed foods and restaurant foods contribute 80% of the daily intake of sodium in most of the Western world. However, ample research demonstrates the efficacy of sodium chloride against pathogenic and spoilage microorganisms in a variety of food systems. Notable examples of the utility and necessity of sodium chloride include the inhibition of growth and toxin production by Clostridium botulinum in processed meats and cheeses. Other sodium salts contributing to the overall sodium consumption are also very important in the prevention of spoilage and/or growth of microorganisms in foods. For example, sodium lactate and sodium diacetate are widely used in conjunction with sodium chloride to prevent the growth of Listeria monocytogenes and lactic acid bacteria in ready-to-eat meats. These and other examples underscore the necessity of sodium salts, particularly sodium chloride, for the production of safe, wholesome foods. Key literature on the antimicrobial properties of sodium chloride in foods is reviewed here to address the impact of salt and sodium reduction or replacement on microbiological food safety and quality.

Evaluation of the pH- and thermal stability of the recombinant green fluorescent protein (GFP) in the presence of sodium chloride

The thermal stability of recombinant green fluorescent protein (GFP) in sodium chloride (NaCl) solutions at different concentrations, pH, and temperatures was evaluated by assaying the loss of fluorescence intensity as a measure of denaturation. GFP, extracted from Escherichia coli cells by the three-phase partitioning method and purified through a butyl hydrophobic interaction chromatography (HIC) column, was diluted in water for injection (WFI) (pH 6.0-7.0) and in 10 mM buffer solutions (acetate, pH 5.0; phosphate, pH 7.0; and Tris-EDTA, pH 8.0) with 0.9-30% NaCl or without and incubated at 80-95 degrees C. The extent of protein denaturation was expressed as a percentage of the calculated decimal reduction time (D-value). In acetate buffer (pH 4.84+/-0.12), the mean D-values for 90% reduction in GFP fluorescence ranged from 2.3 to 3.6 min, independent of NaCl concentration and temperature. GFP thermal stability diluted in WFI (pH 5.94+/-0.60) was half that observed in phosphate buffer (pH 6.08+/-0.60); but in both systems, D-values decreased linearly with increasing NaCl concentration, with D-values (at 80 degrees C) ranging from 3.44, min (WFI) to 6.1 min (phosphate buffer), both with 30% NaCl. However, D-values in Tris-EDTA (pH 7.65+/-0.17) were directly dependent on the NaCl concentration and 5-10 times higher than D-values for GFP in WFI at 80 degrees C. GFP pH- and thermal stability can be easily monitored by the convenient measure of fluorescence intensity and potentially be used as an indicator to monitor that processing times and temperatures were attained.

Humectant permeability influences growth and compatible solute uptake by Staphylococcus aureus subjected to osmotic stress

The effects of different humectants (sodium chloride, sucrose, and glycerol) on the growth of and compatible solute (glycine betaine, proline, and carnitine) uptake by the osmotolerant foodborne pathogen Staphylococcus aureus were investigated. While growth in the presence of the impermeant humectants sodium chloride and sucrose induced the accumulation of proline and glycine betaine by cells, growth in the presence of the permeant humectant glycerol did not. When compatible solutes were omitted from low-water-activity media, growth was very poor in the presence of impermeant humectants. In contrast, the addition of compatible solutes had essentially no effect on growth when cells were grown in low-water-activity media containing glycerol as the humectant. Carnitine was found to accumulate to high intracellular levels in osmotically stressed cells when proline and glycine betaine were absent, making it a potentially important compatible solute for this organism.

Staphylococcus aureus growth boundaries: moving towards mechanistic predictive models based on solute-specific effects

The formulation of shelf-stable intermediate-moisture products is a critical food safety issue. Therefore, knowing the precise boundary for the growth-no-growth interface of Staphylococcus aureus is necessary for food safety risk assessment. This study was designed to examine the effects of various humectants and to produce growth boundary models as tools for risk assessment. The molecular mobility and the effects of various physical properties of humectants, such as their glass transition temperatures, their membrane permeability, and their ionic and nonionic properties, on S. aureus growth were investigated. The effects of relative humidity (RH; 84 to 95%, adjusted by sucrose plus fructose, glycerol, or NaCl), initial pH (4.5 to 7.0, adjusted by HCl), and potassium sorbate concentration (0 or 1,000 ppm) on the growth of S. aureus were determined. Growth was monitored by turbidity over a 24-week period. Toxin production was determined by enterotoxin assay. The 1,792 data points generated were analyzed by LIFEREG procedures (SAS Institute, Inc., Cary, N.C.), which showed that all parameters studied significantly affected the growth responses of S. aureus. Differences were observed in the growth-no-growth boundary when different humectants were used to achieve the desired RH values in both the absence and the presence of potassium sorbate. Sucrose plus fructose was most inhibitory at neutral pH values, while NaCl was most inhibitory at low pH values. The addition of potassium sorbate greatly increased the no-growth regions, particularly when pH was <6.0. Published kinetic growth and survival models were compared with boundary models developed in this work. The effects of solutes and differences in modeling approaches are discussed.

Role of water mobility on mold spore germination

A sugar transport defected strain of Aspergillus nidulans (biA-1 sorA-2) was tested for spore germination in nutrient media containing various water activity (a(w)) values and varying amounts of non-nutritive, nontoxic carbohydrates (L-sorbose and cellulose). Freeze-dried media [containing the same nutrient level but different in sorbose/cellulose (s:c) ratio] were adjusted to 0.75-0.97a(w) at 25 degrees C before inoculation. Minimum a(w) for germination varied with s:c ratio. Because both sorbose and cellulose were not metabolizable and unable to be transported into the cells, the results reflected the molecular mobility of water. (2)H NMR T(2) relaxation time correlated well with spore germination time, and it distinguished the difference between water sorbed to cellulose and water in a solution associated with dissolved sorbose. On the other hand, mold germination time correlated poorly with a(w). It was highly dependent on the s:c ratio. Water mobility was found to correlate better with biological activity than a(w) because it differentiated the availability between water in dissolved sorbose and adsorbed water in cellulose.

Water activity, water glass dynamics, and the control of microbiological growth in foods

Water is probably the single most important factor governing microbial spoilage in foods, and the concept of water activity (a(w)) has been very valuable because measured values generally correlate well with the potential for growth and metabolic activity. Despite some drawbacks (e.g., solute effect), the concept of a(w) has assisted food scientists in their effort to predict the onset of food spoilage as well as to control food-borne disease hazards in food products. In the last decade the concept of a(w) has been challenged. It has been suggested that reduced-moisture food products (e.g., low and intermediate) may be nonequilibrium systems and that most of them are in the amorphous metastable state, which is very sensitive to changes in moisture content and temperature. It has been proposed that the glass transition temperature Tg (temperature at which the glass-rubber transition occurs), is a parameter that can determine many product properties, the safety of foods among them. The concept of water dynamics, originating in a food polymer science approach, has been suggested instead of a(w) to better predict the microbial stability of intermediate-moisture foods. The usage of a(w) to predict microbial safety of foods has been discouraged on the basis that (1) in intermediate-moisture foods the measured water vapor pressure is not an equilibrium one, and because a(w) is a thermodynamic concept, it refers only to equilibrium; and (2) the microbial response may differ at a particular a(w) when the latter is obtained with different solutes. This review analyzes these suggestions on the basis of abundant experimental evidence found in the literature. It is concluded that nonequilibrium effects (e.g., inability of water to diffuse in a semimoist food) appear to be in many cases slow within the time frame (food's shelf life) of the experiments and/or so small that they do not affect seriously the application of the a(w) concept as a predictor of microbial stability in foods. The claims that a food science polymer approach to understanding the behavior of aqueous sugar glasses and concentrated solutions may be used to predict the microbial stability of food systems is not substantiated by experimental evidence. This approach does not offer, at the present time, a better alternative to the concept of a(w) as a predictor of microbial growth in foods. It is also recognized that a(w) has several limitations and should be always used carefully, and this must include precautions regarding the possible influences of nonequilibrium situations. This aspect may be summarized by simply saying that anyone who is going to employ the term water activity must be aware of the implications of its definition.