Physiological response of Enterococcus faecalis JH2-2 to cold shock: growth at low temperatures and freezing/thawing challenge

Solar disinfection - An appropriate water treatment method to inactivate faecal bacteria in cold climates

Solar disinfection (SODIS) is an inexpensive drinking water treatment method applied in tropical and sub-tropical low-income countries. However, it has been unclear whether it functions adequately also in colder climates. To investigate this issue, SODIS experiments were performed in the humid continental climate of Finland by exposing faecally contaminated drinking water to natural solar radiation at different water temperatures (8-23 °C) and UV intensities (12-19 W/m²) in polyethylene (PE) bags. To establish an adequate benchmark, SODIS experiments with the same experimental design were additionally conducted in the Mediterranean climate of Spain in typical conditions of SODIS application (~39 °C and 42 W/m²). Out of all experiments, the highest coliform and enterococci inactivation efficiencies in terms of lowest required doses for 4-log disinfection (25 Wh/m² and 60 Wh/m², respectively) were obtained in humid continental climate at the lowest studied mean water temperature (8-11 °C). Despite the low mean UV irradiance (~19 Wh/m²), 4-log disinfection of coliforms and enterococci were also reached fast in these conditions (1 h 27 min and 3 h 18 min, respectively). Overall, the doses required for disinfection increased as the water temperatures and UV intensities of the experiments rose. Disinfection of 4-logs (> 99.99%) of both bacteria was reached in all SODIS experiments within 6 h, suggesting SODIS could be a sufficient household water treatment method also in colder climates, unlike previously thought. The effects of different water temperatures on bacterial inactivation were also tested in the absence of sunlight. Together the obtained results indicate that while water temperatures below or close to the optima of coliforms and enterococci (~10 °C) alone do not cause inactivation, these temperatures may enhance SODIS performance. This phenomenon is attributed to slower bacterial metabolism and hence slower photorepair induced by the low water temperature.

Storage stability and sourdough acidification kinetic of freeze-dried Lactobacillus curvatus N19 under optimized cryoprotectant formulation

In this study, the response surface methodology was used to optimize the cryoprotective agent (skimmed milk powder, lactose and sucrose) formulation for enhancing the viability of Lactobacillus curvatus N19 during freeze-drying and storage stability of cells freeze-dried by using optimum formulation was evaluated. Our results showed that the most significant cryoprotective agent influencing the viability of L. curvatus N19 to freezing and freeze-drying was sucrose and skim milk, respectively. The optimal formulation of cryoprotective agents was 20 g/100 mL skim milk, 3.57 g/100 mL lactose and 10 g/100 mL sucrose. Using the optimum formulation during freeze-drying, the cell survival was found more than 98%. Under the optimal conditions, although only storage of the cells at 4 °C for 6 month retained the maximum stability (8.85 log cfu/g), the employed protectant matrix showed promising results at 25 °C (7.89 log cfu/g). The storage stability of cells under optimized conditions was predicted by accelerated storage test, which was demonstrated that the inactivation rate constant of the freeze-dried L. curvatus N19 powder was 9.74 × 10^-6 1/d for 4 °C and 2.08 × 10^-3 1/d for 25 °C. The loss of specific acidification activity after the storage at 4 and 25 °C was determined.

Optimization of Production Parameters for Probiotic Lactobacillus Strains as Feed Additive

In animal nutrition, probiotics are considered as desirable alternatives to antibiotic growth promoters. The beneficial effects of probiotics primarily depend on their viability in feed, which demands technical optimization of biomass production, since processing and storage capacities are often strain-specific. In this study, we optimized the production parameters for two broiler-derived probiotic lactobacilli (L. salivarius and L. agilis). Carbohydrate utilization of both strains was determined and preferred substrates that boosted biomass production in lab-scale fermentations were selected. The strains showed good aerobic tolerance, which resulted in easier scale-up production. For the freeze-drying process, the response surface methodology was applied to optimize the composition of cryoprotective media. A quadratic polynomial model was built to study three protective factors (skim milk, sucrose, and trehalose) and to predict the optimal working conditions for maximum viability. The optimal combination of protectants was 0.14g/mL skim milk/ 0.08 g/mL sucrose/ 0.09 g/mL trehalose (L. salivarius) and 0.15g/mL skim milk/ 0.08 g/mL sucrose/ 0.07 g/mL (L. agilis), respectively. Furthermore, the in-feed stabilities of the probiotic strains were evaluated under different conditions. Our results indicate that the chosen protectants exerted an extensive protection on strains during the storage. Although only storage of the strains at 4 °C retained the maximum stability of both Lactobacillus strains, the employed protectant matrix showed promising results at room temperature.

Adaptation to Adversity: the Intermingling of Stress Tolerance and Pathogenesis in Enterococci

Enterococcus is a diverse and rugged genus colonizing the gastrointestinal tract of humans and numerous hosts across the animal kingdom. Enterococci are also a leading cause of multidrug-resistant hospital-acquired infections. In each of these settings, enterococci must contend with changing biophysical landscapes and innate immune responses in order to successfully colonize and transit between hosts. Therefore, it appears that the intrinsic durability that evolved to make enterococci optimally competitive in the host gastrointestinal tract also ideally positioned them to persist in hospitals, despite disinfection protocols, and acquire new antibiotic resistances from other microbes. Here, we discuss the molecular mechanisms and regulation employed by enterococci to tolerate diverse stressors and highlight the role of stress tolerance in the biology of this medically relevant genus.

Cost effectiveness of cryoprotective agents and modified De-man Rogosa Sharpe medium on growth of Lactobacillus acidophilus

The effect of cryoprotective agents (namely, sodium chloride, sucrose, dextran, sorbitol, monosodium glutamate, glycerol, skim milk and skim milk with malt extract) and modified De-Man Rogosa Sharpe (MRS) medium, on the viability and stability of L. acidophilus ATCC 4962, was investigated. The modified MRS medium was not only economical, but it gave a relatively higher yield of L. acidophilus ATCC 4962 than the commercial MRS. Monosodium glutamate, skim milk and skim milk with malt extract provided significantly higher viable counts, with optimum concentration at 0.3%. Nevertheless, at concentration above 0.5%, there was a reduction in cell viability, which could be attributed to cell shrinkage associated with osmotic pressure changes inside the cells. It was also found that L. acidophilus ATCC 4962 was stable at 28 degrees C for eight weeks. Skim milk demonstrated a significant growth of probiotics. Skim milk was the preferred cryoprotective agent, as it is of low cost, easily available and demonstrated a significant growth of probiotics. In conclusion, modified MRS medium with skim milk is suggested for the remarkable growth and yield of L. acidophilus.

Biofilm formation on polystyrene under different temperatures by antibiotic resistant Enterococcus faecalis and Enterococcus faecium isolated from food

The ability of antibiotic resistant E. faecalis and E. faecium isolated from food to form biofilm at different temperatures in the absence or presence of 0.75% glucose was evaluated. A synergistic effect on biofilm at 10 °C, 28 °C, 37 °C and 45 °C and glucose was observed for E. faecalis and E. faecium.

Response to environmental stress as a global phenomenon in biology: the example of microorganisms

Any modification of the environment that leads to a physiological, genetic, or epigenetic adaptive response in microorganisms may be considered as a stress. Historically, forms of stresses affecting biological structures were classified either as non-thermal, such as osmotic, oxidative, or acid stress or as thermal stress, hot or cold. Currently, the classification in biology is as abiotic, including physical and chemical stress, or biotic. The aim of this mini-review is to show, through the example of microorganisms, that the response to stress can be considered, in biology, as a global phenomenon, which can be extended to anthropogenic pressure.

Enhancement of viability of a probiotic Lactobacillus strain for poultry during freeze-drying and storage using the response surface methodology

A rotatable central composite design (CCD) was used to study the effect of cryoprotectants (skim milk, sucrose and lactose) on the survival rate of a probiotic Lactobacillus strain, L. reuteri C10, for poultry, during freeze-drying and storage. Using response surface methodology, a quadratic polynomial equation was obtained for response value by multiple regression analyses: Y = 8.59546-0.01038 X(1)-0.09382 X(2)-0.07771 X(3)-0.054861 X(1)(2)-0.04603 X(3)(2)-0.10938 X(1)X(2). Based on the model predicted, sucrose exerted the strongest effect on the survival rate. At various combinations of cryoprotectants, the viability loss of the cells after freeze-drying was reduced from 1.65 log colony forming units (CFU)/mL to 0.26-0.66 log CFU/mL. The estimated optimum combination for enhancing the survival rate of L. reuteri C10 was 19.5% skim milk, 1% sucrose and 9% lactose. Verification experiments confirmed the validity of the predicted model. The storage life of freeze-dried L. reuteri C10 was markedly improved when cryoprotectants were used. At optimum combination of the cryoprotectants, the survival rates of freeze-dried L. reuteri C10 stored at 4°C and 30°C for 6 months were 96.4% and 73.8%, respectively. Total viability loss of cells which were not protected by cryoprotectants occurred after 12 and 8 weeks of storage at 4°C and 30°C, respectively.

Viability of Bifidobacterium Pseudocatenulatum G4 after Spray-Drying and Freeze-Drying

Viability of Bifidobacterium pseudocatenulatum G4 following spray-drying and freeze-drying in skim milk was evaluated. After spray-drying, the strain experienced over 99% loss in viability regardless of the air outlet temperature (75 and 85 °C) and the heat-adaptation temperature (45 and 65 °C, 30 min). The use of heat-adaptation treatment to improve the thermotolerance of this strain was ineffective. On the other hand, the strain showed a superior survival at 71.65%–82.07% after freeze-drying. Viable populations of 9.319–9.487 log10 cfu/g were obtained when different combinations of skim milk and sugar were used as cryoprotectant. However, the addition of sugars did not result in increased survival during the freeze-drying process. Hence, 10% (w/v) skim milk alone is recommended as a suitable protectant and drying medium for this strain. The residual moisture content obtained was 4.41% ± 0.44%.

Evolutionary Adaptation to Freeze‐Thaw‐Growth Cycles inEscherichia coli

Fifteen populations of Escherichia coli were propagated for 150 freeze-thaw-growth (FTG) cycles in order to study the phenotypic and genetic changes that evolve under these stressful conditions. Here we present the phenotypic differences between the evolved lines and their progenitors as measured by competition experiments and growth curves. Three FTG lines evolved from an ancestral strain that was previously used to start a long-term evolution experiment, while the other 12 FTG lines are derived from clones that had previously evolved for 20,000 generations at constant 37 degrees C. Competition experiments indicate that the former FTG group improved their mean fitness under the FTG regime by about 90% relative to their progenitor, while the latter FTG group gained on average about 60% relative to their own progenitors. These increases in fitness result from both improved survival during freezing and thawing and more rapid recovery to initiate exponential growth after thawing. This shorter lag phase is specific to recovery after freezing and thawing. Future work will seek to identify the mutations responsible for evolutionary adaptation to the FTG environment and use them to explore the physiological mechanisms that allow increased survival and more rapid recovery.

A proteomic analysis of Psychrobacter articus 273-4 adaptation to low temperature and salinity using a 2-D liquid mapping approach

Psychrobacter 273-4 was isolated from a 20,000-40,000-year-old Siberian permafrost core, which is characterized by low temperature, low water activity, and high salinity. To explore how 273-4 survives in the permafrost environment, proteins in four 273-4 samples cultured at 4 and 22 degrees C in media with and without 5% sodium chloride were profiled and comparatively studied using 2-D HPLC and MS. The method used herein involved fractionation via a pH gradient using chromatofocusing followed by nonporous silica (NPS) RP-HPLC and on-line electrospray mass mapping. It was observed that 33 proteins were involved in the adaptation to low temperature in the cells grown in the nonsaline media while there were only 14 proteins involved in the saline media. There were 45 proteins observed differentially expressed in response to salt at 22 degrees C while there were 22 proteins at 4 degrees C. In addition, 5% NaCl and 4 degrees C showed a combination effect on protein expression. A total of 56 proteins involved in the adaptation to low temperature and salt were identified using MS and database searching. The differentially expressed proteins were classified into different functional categories where the response of the regulation system to stress appears to be very elaborate. The evidence shows that the adaptation of 273-4 is based primarily on the control of translation and transcription, the synthesis of proteins (chaperones) to facilitate RNA and protein folding, and the regulation of metabolic pathways.

Effect of low temperature and culture media on the growth and freeze-thawing tolerance of Exiguobacterium strains

Bacteria of the genus Exiguobacterium have been repeatedly isolated from ancient permafrost sediments of the Kolyma lowland of Northeast Eurasia. Here we report that the Siberian permafrost isolates Exiguobacterium sibiricum 255-15, E. sibiricum 7-3, Exiguobacterium undae 190-11 and E. sp. 5138, as well as Exiguobacterium antarcticum DSM 14480, isolated from a microbial mat sample of Lake Fryxell (McMurdo Dry Valleys, Antarctica), were able to grow at temperatures ranging from -6 to 40 degrees C. In comparison to cells grown at 24 degrees C, the cold-grown cells of these strains tended to be longer and wider. We also investigated the effect of growth conditions (broth or surface growth, and temperature) on cryotolerance of the Exiguobacterium strains. Bacteria grown in broth at 4 degrees C showed markedly greater survival following freeze-thawing treatments (20 repeated cycles) than bacteria grown in broth at 24 degrees C. Surprisingly, significant protection to repeated freeze-thawing was also observed when bacteria were grown on agar at either 4 or 24 degrees C.

Increased susceptibility to repeated freeze-thaw cycles in Escherichia coli following long-term evolution in a benign environment

Background

In order to study the dynamics of evolutionary change, 12 populations of E. coli B were serially propagated for 20,000 generations in minimal glucose medium at constant 37°C. Correlated changes in various other traits have been previously associated with the improvement in competitive fitness in the selective environment. This study examines whether these evolved lines changed in their ability to tolerate the stresses of prolonged freezing and repeated freeze-thaw cycles during adaptation to a benign environment.

Results

All 12 lines that evolved in the benign environment for 20,000 generations are more sensitive to freeze-thaw cycles than their ancestor. The evolved lines have an average mortality rate of 54% per daily cycle, compared to the ancestral rate of 34%. By contrast, there was no significant difference between the evolved lines and their ancestor in mortality during prolonged freezing. There was also some variability among the evolved lines in susceptibility to repeated freeze-thaw cycles. Those lines that had evolved higher competitive fitness in the minimal glucose medium at 37°C also had higher mortality during freeze-thaw cycles. This variability was not associated, however, with differences among lines in DNA repair functionality and mutability.

Conclusion

The consistency of the evolutionary declines in freeze-thaw tolerance, the correlation between fitness in glucose medium at 37°C and mortality during freeze-thaw cycles, and the absence of greater declines in freeze-thaw survival among the hypermutable lines all indicate a trade-off between performance in minimal glucose medium at 37°C and the capacity to tolerate this stress. Analyses of the mutations that enhance fitness at 37°C may shed light on the physiological basis of this trade-off.

Optimization of a protective medium for enhancing the viability of freeze-dried Lactobacillus delbrueckii subsp. bulgaricus based on response surface methodology

Response surface methodology (RSM) was used to optimize a protective medium for enhancing the cell viability of Lactobacillus delbrueckii subsp. bulgaricus LB14 during freeze-drying. Using a previous Plackett-Burman design, it was found that sucrose, glycerol, sorbitol and skim milk were the most effective freeze-drying protective agents for L. bulgaricus LB14. A full factorial central composite design was applied to determine the optimum levels of these four protective agents. The experimental data allowed the development of an empirical model (P<0.0001) describing the inter-relationships between the independent and dependent variables. By solving the regression equation, and analyzing the response surface contour and surface plots, the optimal concentrations of the agents were determined as: sucrose 66.40 g/L, glycerol 101.20 g/L, sorbitol 113.00 g/L, and skim milk 130.00 g/L. L. bulgaricus LB14 freeze-dried in this medium obtained a cell viability of up to 86.53%.

Influence of cooling temperature and duration on cold adaptation of Lactobacillus acidophilus RD758

The effect of different cooling temperatures and durations on resistance to freezing and to frozen storage at -20 degrees C in Lactobacillus acidophilus RD758 was studied, by using a central composite rotatable design. A cold adaptation was observed when the cells were maintained at moderate temperature (26 degrees C) for a long time (8h) before being cooled to the final temperature of 15 degrees C. These conditions led to a low rate of loss in acidification activity during frozen storage (0.64 minday(-1)) and a high residual acidification activity after 180 days of frozen storage (1011 min). The experimental design allowed us to determine optimal cooling conditions, which were established at 28 degrees C during 8h. Adaptation to cold temperatures was related to an increase in the unsaturated to saturated fatty acid ratio and in the relative cycC19:0 fatty acid concentration. Moreover, an increased synthesis of four specific proteins was observed as an adaptive response to the optimal cooling conditions. They included the stress protein ATP-dependent ClpP and two cold induced proteins: pyruvate kinase and a putative glycoprotein endopeptidase.

Environmental stress responses in Lactobacillus: a review

Environmental stress responses in Lactobacillus, which have been investigated mainly by proteomics approaches, are reviewed. The physiological and molecular mechanisms of responses to heat, cold, acid, osmotic, oxygen, high pressure and starvation stresses are described. Specific examples of the repercussions of these effects in food processing are given. Molecular mechanisms of stress responses in lactobacilli and other bacteria are compared.

Susceptibility of Vibrio parahaemolyticus to various environmental stresses after cold shock treatment

Vibrio parahaemolyticus was subjected to cold shock treatment at 20 or 15 degrees C for 2 or 4 h. The effect of cold shock on the survival of V. parahaemolyticus subjected to subsequent low temperature (5 and -18 degrees C) and other adverse conditions (47 degrees C, 6 ppm crystal violet, 1000 ppm H(2)O(2), 25 mM acetic acid and 25 mM lactic acid) was investigated. Regardless of the cold shock treatment, survival of V. parahaemolyticus increased when stored at 5 or -18 degrees C, while no increase in survival was noted for cells cold shocked in the presence of chloramphenicol. Cold shock treatment under the conditions tested, in general, enabled V. parahaemolyticus cells to survive better following subsequent challenge by crystal violet, while the cold-shocked organism was more susceptible to high temperature (47 degrees C), H(2)O(2) and organic acids (lactic and acetic acid) than the non-shocked cells. Furthermore, the temperature and time of the cold shock treatment affected the cold shock response of V. parahaemolyticus.

Relationship of critical temperature to macromolecular synthesis and growth yield in Psychrobacter cryopegella

Most microorganisms isolated from low-temperature environments (below 4 degrees C) are eury-, not steno-, psychrophiles. While psychrophiles maximize or maintain growth yield at low temperatures to compensate for low growth rate, the mechanisms involved remain unknown, as does the strategy used by eurypsychrophiles to survive wide ranges of temperatures that include subzero temperatures. Our studies involve the eurypsychrophilic bacterium Psychrobacter cryopegella, which was isolated from a briny water lens within Siberian permafrost, where the temperature is -12 degrees C. P. cryopegella is capable of reproducing from -10 to 28 degrees C, with its maximum growth rate at 22 degrees C. We examined the temperature dependence of growth rate, growth yield, and macromolecular (DNA, RNA, and protein) synthesis rates for P. cryopegella. Below 22 degrees C, the growth of P. cryopegella was separated into two domains at the critical temperature (T(critical) = 4 degrees C). RNA, protein, and DNA synthesis rates decreased exponentially with decreasing temperatures. Only the temperature dependence of the DNA synthesis rate changed at T(critical). When normalized to growth rate, RNA and protein synthesis reached a minimum at T(critical), while DNA synthesis remained constant over the entire temperature range. Growth yield peaked at about T(critical) and declined rapidly as temperature decreased further. Similar to some stenopsychrophiles, P. cryopegella maximized growth yield at low temperatures and did so by streamlining growth processes at T(critical). Identifying the specific processes which result in T(critical) will be vital to understanding both low-temperature growth and growth over a wide range of temperatures.

Improved adaptation to cold-shock, stationary-phase, and freezing stresses in Lactobacillus plantarum overproducing cold-shock proteins

We have investigated the effect of overproducing each of the three cold shock proteins (CspL, CspP, and CspC) in the mesophilic lactic acid bacterium Lactobacillus plantarum NC8. CspL overproduction transiently alleviated the reduction in growth rate triggered by exposing exponentially growing cells to cold shock (8 degrees C), suggesting that CspL is involved in cold adaptation. The strain overproducing CspC resumed growth more rapidly when stationary-phase cultures were diluted into fresh medium, indicating a role in the adaptation and recovery of nutritionally deprived cells. Overproduction of CspP led to an enhanced capacity to survive freezing.

[Inhibition of bacterial growth in thawed cake and rapid evaluation methods for microbiological quality]

The inhibition of bacterial growth in thawed cake kept under refrigeration and rapid evaluation methods for microbiological quality of the cake were investigated. The effects of the freezing temperature and the addition of ethanol or emulsifier on bacterial numbers in a cake model after storage for 72 hr at 10 degrees C following the thawing process were also studied. Bacterial growth in the cake model was inhibited by the additives under various freezing conditions. In addition, rapid evaluation methods for estimating bacterial numbers in the cake model after incubation for 72 hr at 10 degrees C were studied. High correlations were found between bacterial numbers in the cake model incubated for 24 hr at 20 degrees C and for 6 hr at 35 degrees C with tryptic soy broth and that of the cake model incubated for 72 hr at 10 degrees C. This result indicated that rapid evaluation by incubation for 24 hr at 20 degrees C or for 6 hr at 35 degrees C with tryptic soy broth can be used to predict the bacterial numbers in a cake model after incubation for 72 hr at 10 degrees C. Furthermore, the ATP-bioluminescence method was applied to shorten the testing time, because culture on an agar medium was not necessary.

Nystatin and osmotica as chemical enhancers of the phenotypic adaptation to freeze-thaw stress in Geotrichum candidum ATCC 204307

Geotrichum candidum is a yeast-like fungus used as ripening starter in cheese making. The present study focused on chemical stress pretreatments affecting survival of G. candidum ATCC 204307 to freeze-thaw stress. Cryotolerance of G. candidum cells was induced by pretreatment with NaCl, CaCl2, or MgCl2, indicating heterologous phenotypic adaptation to freeze-thaw stress (- 20 to 25 degrees C) by osmotic stress. Furthermore, the nystatin, an antifungal compound, was shown to be a cryotolerance inducer.

Lactic acid bacteria and proteomics: current knowledge and perspectives

Lactic acid bacteria (LAB) are widely used in the agro-food industry. Some of the LAB also participate in the natural flora in humans and animals. We review here proteomic studies concerning LAB. Two methods of research can be distinguished. In the first one, a systematic mapping of proteins is attempted, which will be useful for taxonomy and to function assignment of proteins. The second one focuses particularly on proteins whose synthesis is induced by various environmental situations or stresses. However, both approaches are complementary and will give new insights for the use of bacteria in industry, in human health and in the struggle against bacterial pathogens. Interest in LAB is growing, showing thus an increasing concern of their rational use and one can foresee in the near future an increasing use of proteomics as well as genomics.

Mechanisms involved in the resistance of Enterococcus faecalis to calcium hydroxide

AIM

This study sought to clarify the mechanisms that enable E. faecalis to survive the high pH of calcium hydroxide.

METHODOLOGY

E. faecalis strain JH2-2 was exposed to sublethal concentrations of calcium hydroxide, with and without various pretreatments. Blocking agents were added to determine the role of stress-induced protein synthesis and the cell wall-associated proton pump.

RESULTS

E. faecalis was resistant to calcium hydroxide at a pH of 11.1, but not pH 11.5. Pre-treatment with calcium hydroxide pH 10.3 induced no tolerance to further exposure at pH 11.5. No difference in cell survival was observed when protein synthesis was blocked during stress induction, however, addition of a proton pump inhibitor resulted in a dramatic reduction of cell viability of E. faecalis in calcium hydroxide.

CONCLUSIONS

Survival of E. faecalis in calcium hydroxide appears to be unrelated to stress induced protein synthesis, but a functioning proton pump is critical for survival of E. faecalis at high pH.

Operating conditions that affect the resistance of lactic acid bacteria to freezing and frozen storage

Thermophilic lactic acid bacteria exhibit different survival rates during freezing and frozen storage, depending on the processing conditions. We used a Plackett and Burman experimental design to study the effects of 13 experimental factors, at two levels, on the resistance of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus to freezing and frozen storage. The resistance was evaluated by quantifying the decrease of acidification activity during freezing and throughout 8 weeks of storage. Acidification activity after freezing and frozen storage was affected by 12 experimental factors. Only the thawing temperature did not show any significant effect. S. thermophilus was more resistant than L. bulgaricus and the cryoprotective effect of glycerol during freezing and storage was confirmed. The temperature and duration of the cryoprotection step influenced acidification activity following the freezing step: the lower the temperature and the shorter the duration, the higher the activity. Acidification activity after storage was affected by several experimental factors involved in the fermentation stage: use of NaOH instead of NH4OH for pH control, addition of Tween 80 in the culture medium, and faster cooling led to better cryotolerance. Resistance to freezing and frozen storage was improved by using a high freezing rate and a low storage temperature. Finally, this study revealed that the conditions under which lactic acid bacteria are prepared should be well controlled to improve their preservation and to limit the variability between batches and between species.

Effect of Lipid Composition on the Stability of Cellular Membranes during Freeze–Thawing of Lactobacillus acidophilus Grown at Different Temperatures

Lactobacillus acidophilus CRL 640 grown at the optimal temperature of 37 degrees C (M37) appeared more sensitive to freeze-thawing than when it was grown at 25 degrees C (M25). In the first case, 87% of the cells died, in contrast to 33% for cells grown at 25 degrees C. All the surviving M37 cells showed sensitivity to NaCl. However, among the surviving M25 cells, only 85% were sensitive to NaCl. The rest of the cells were considered uninjured. Freeze-thawing in cells grown at 25 degrees C showed a liberation of nucleic acids and proteins. However, the leakage was higher in M37 cells after freeze-thawing. The greater fraction of damaged cells were observed in M25 culture after freeze-thawing. A relative increase of 81% in cardiolipid (CL), with respect to total phospholipids and 72% triglycosyldiglyceride (TGDG) with respect to the total glycolipids was observed in M37. In addition, a decrease of palmitoyl (C16:0), oleoyl (C18:0) fatty acids at CL, phosphatidylglycerol (PG), and diglicosyldiglyceride (DGDG) fractions and the increase of C19 cyc and C18:0, 10-OH fatty acids in neutral lipid, and CL fractions was also apparent. In M25 cells, the concentration of DGDG and PG was higher than in M37 cells. The difference in cryotolerance between the frozen cultures emphasizes the importance of selecting appropriate conditions of growth of microorganisms for use as dietary adjuncts.

Phenotypic adaptation to freeze-thaw stress of the yeast-like fungus Geotrichum candidum

The effect of cold stress on Geotrichum candidum was investigated at chill and freezing temperatures. Specific growth rates were determined at various temperatures and plotted according to the Ratkowsky and Arrhenius equations. The obtained profiles led to the determination of characteristics including the activation energy and notional minimum temperatures. At temperatures below the optimum single linear slopes were observed. At freezing temperatures, the loss of viability of cell populations was proportional to the number of freezing-thawing cycles. Nevertheless, the ability of G. candidum to survive this challenge depended on the physiological conditions prior to the freezing stress. The loss of viability was growth phase specific. Cells harvested in stationary phase showed a higher resistance compared to those obtained with cells in exponential phase. Furthermore, the cells of G. candidum could be adapted to the freeze-thaw challenge by pre-treatment at chill temperatures. This phenomenon known as cryotolerance was a function of the duration of the preincubation exposure.

Cryoprotectants lead to phenotypic adaptation to freeze-thaw stress in Lactobacillus delbrueckii ssp. bulgaricus CIP 101027T

This study was conducted to investigate the ability of cryoprotective chemicals to induce phenotypic cryoadaptation in Lactobacillus delbrueckii ssp. bulgaricus CIP 101027T. Tolerance to negative temperature stress (freezing at -20 degrees C and thawing at 37 degrees C) was induced by pretreatment with Me(2)SO, glycerol, lactose, sucrose, and trehalose. Interestingly, Me(2)SO has a significantly greater cryoprotective effect than glycerol. Furthermore, lactose, sucrose, and trehalose, often referred to as osmotica, were shown to have greater cryoadaptive than cryoprotective properties. These results suggest that bacteria such as L. delbrueckii ssp. bulgaricus could be phenotypically adapted to freezing and thawing by an osmotic stress applied prior to freeze-thaw stress.

Influence of growth temperature on cryotolerance and lipid composition of Lactobacillus acidophilus

In order to correlate the lipid composition of the membrane of Lactobacillus acidophilus CRL 640 with the freeze-thaw behaviour of the cultures grown at different temperatures, fatty acid methyl esters (FAMEs) from extracts grown at 25, 30, 37 and 40 degrees C were obtained and compared. Cultures grown at 25 degrees C (M25) exhibited more resistance to the freeze-thaw process probably because of an increase in C18:2 and C16:0 fatty acids. This culture also exhibited a lesser amount of phospholipids as shown by the sugar: phosphorus ratio. In all cases, the presence of the uncommon 10-hydroxyoctadecanoic acid was determined. From the extracts of the M25 and M37 cultures, diacylphosphatidylglycerol, cardiolipin, diglycosyldiglycerides, triglycosyldiglycerides and neutral lipids were isolated and identified. The structural elucidation was carried out by FAMEs and sugar analysis and by mass spectrometry using fast atom bombardment ionization. The changes in lipid composition due to different growth temperatures could be indicative of the resistance of the bacteria to freeze-thaw processes.

Analysis of the role of 7 kDa cold-shock proteins of Lactococcus lactis MG1363 in cryoprotection

Low-temperature adaptation and cryoprotection were studied in the lactic acid bacterium Lactococcus lactis MG1363. An approximately 100-fold increased survival after freezing was observed when cells were shocked to 10 degrees C for 4 h compared to mid-exponential-phase cells grown at 30 degrees C, indicating an active protection against freezing. Using two-dimensional gel electrophoresis a group of 7 kDa cold-induced proteins (CSPs) was identified that corresponds to a previously described family of csp genes of L. lactis MG1363 (Wouters et al., 1998, Microbiology 144, 2885-2893). The 7 kDa CSPs appeared to be the most strongly induced proteins upon cold shock to 10 degrees C. Northern blotting and two-dimensional gel electrophoresis showed that the csp genes were maximally expressed at 10 degrees C, while induction was lower at 20 and 4 degrees C. However, pre-incubation at 20 and 4 degrees C, as well as stationary-phase conditions, also induced cryoprotection (approx. 30-, 130- and 20-fold, respectively, compared to 30 degrees C mid-exponential phase). For all treatments leading to an increased freeze survival (exposure to 4, 10 and 20 degrees C and stationary-phase conditions), increased levels of three proteins (26, 43 and 45 kDa) were observed for which a role in cryoprotection might be suggested. Increased freeze survival coincides with increased CSP expression, except for stationary-phase conditions. However, the level of observed freeze protection does not directly correlate with the csp gene expression levels. In addition, for the first time specific overproduction of a CSP in relation to freeze survival was studied. This revealed that L. lactis cells overproducing CspD at 30 degrees C show a 2-10-fold increased survival after freezing compared to control cells. This indicates that the 7 kDa cold-shock protein CspD may enhance the survival capacity after freezing but that other factors supply additional cryoprotection.

Cold shock proteins and low-temperature response of Streptococcus thermophilus CNRZ302

Low-temperature adaptation and cryoprotection were studied in the thermophilic lactic acid bacterium Streptococcus thermophilus CNRZ302. S. thermophilus actively adapts to freezing during a pretreatment at 20 degrees C, resulting in an approximately 1, 000-fold increased survival after four freeze-thaw cycles compared to mid-exponential-phase cells grown at an optimal temperature of 42 degrees C. No adaptation is observed when cells are exposed to a temperature (10 degrees C) below the minimal growth temperature of the strain (just below 15 degrees C). By two-dimensional gel electrophoresis several 7-kDa cold-induced proteins were identified, which are the major induced proteins after a shift to 20 degrees C. These cold shock proteins were maximally expressed at 20 degrees C, while the induction level was low after cold shock to 10 degrees C. To confirm the presence of csp genes in S. thermophilus, a PCR strategy was used which yielded products of different sizes. Sequence analysis revealed csp-like sequences that were up to 95% identical to those of csp genes of S. thermophilus ST1-1, Streptococcus dysgalactiae, Streptococcus pyogenes, and Lactococcus lactis. Northern blot analysis revealed a seven- to ninefold induction of csp mRNA after a temperature shift to 20 degrees C, showing that this thermophilic bacterium indeed contains at least one cold-inducible csp gene and that its regulation takes place at the transcriptional level.

Cold stress responses in mesophilic bacteria

The diversity of the prokaryotes that have been studied, combined with the many different effects of low temperature, has led to an extensive literature concerning cold stress responses in mesophilic bacteria. The aim of this review is to discuss the effects of cold on the behavior of bacteria. The following three responses will be described: (i) biochemical modifications consisting first of membrane fatty acid desaturation and second of the synthesis of cold stress proteins, (ii) physiological responses of the cells to permit growth at low temperatures above 0 degrees C and cryotolerance at lower temperatures, and (iii) control of the cold shock response at a transcriptional and/or translational level. This paper reviews knowledge, most of which has been acquired in the last 10 years, in the field of cold stress responses. It is hoped that these data will help to focus attention on the metabolic responses associated with environmental disturbance.

Differentiation between cold shock proteins and cold acclimation proteins in a mesophilic gram-positive bacterium, Enterococcus faecalis JH2-2

Transfer of Enterococcus faecalis to a cold temperature (8 degrees C for 4 to 30 h) led to increased expression of 11 cold shock proteins (CSPs). Furthermore, this mesophilic prokaryote synthesized 10 cold acclimation proteins, five of them distinct from CSPs, during continuous growth (4 days) at the same temperature (8 degrees C).