Biopreservation in modified atmosphere stored mungbean sprouts: the use of vegetable-associated bacteriocinogenic lactic acid bacteria to control the growth of Listeria monocytogenes

Two bacteriocinogenic strains of Pediococcus parvulus and one bacteriocinogenic Enterococcus mundtii strain were evaluated for their potential to control the growth of Listeria monocytogenes on refrigerated, modified atmosphere (MA) stored mungbean sprouts. These three strains, which were isolated from minimally-processed vegetables, were shown to grow in culture broth at 4, 8, 15 and 30 degrees C. However, only Ent. mundtii was capable of bacteriocin production at 4-8 degrees C. Examination of the growth of these strains on agar under 1.5% O2 in combination with 0, 5, 20 or 50% CO2 revealed significantly higher maximum specific growth rates for Ent. mundtii than for Pediococcus parvulus at CO2 concentrations below 20%, which are relevant for MA-storage of vegetables. Enterococcus mundtii was subsequently evaluated for its ability to control the growth of L. monocytogenes on vegetable agar and fresh mungbean sprouts under 1.5% O2/20% CO2/78.5% N2 at 8 degrees C. The growth of L. monocytogenes was inhibited by bacteriocinogenic Ent. mundtii on sterile vegetable-medium but not on fresh produce. However, mundticin, the bacteriocin produced by Ent. mundtii, was found to have potential as a biopreservative agent for MA-stored mungbean sprouts when used in a washing step or a coating procedure.     

Hatching of common carp (Cyprinus carpio L.) embryos stored at 4 and -2 degrees C in different concentrations of methanol and sucrose

The hatching performance of common carp (Cyprinus carpio L.) embryos was examined after 12-72-h storage at 4 and -2 degrees C using different concentrations of sucrose (0.1, 0.25, 0.5 and 1.0 M or 3.42, 8.55, 17.10 and 34.2%), methanol (MeOH) (0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 M or 1.6, 3.2, 4.8, 6.4, 8.0, 9.6 and 11.2%), or varying concentrations of methanol in 0.5 M (17.10%) sucrose. For sucrose, 0.5 M (17.10%) showed the maximum survival (41+/-1% (12 h) to 11+/-1.5% (72 h)) at 4 degrees C. No survival was observed at -2 degrees C with any concentration of sucrose. At both temperatures employed, hatching was higher with mixed combination of methanol (1.5 M or 4.8%) and 0.5 M (17.10%) sucrose (4 degrees C: 41+/-1.5% (12 h), 38+/-1.2% (72 h); -2 degrees C: 33+/-1.7% (12 h), 28+/-1.2% (72 h)) compared to methanol alone (4 degrees C: 38+/-1.5% (12 h), 35+/-2.5% (72 h); -2 degrees C: 31+/-2.5% (12 h), 25+/-2% (72 h)). The combination of 1.5 M (4.8%) methanol and 0.5 M (17.10%) sucrose produced the best results among all the concentrations tested at both temperatures.     

Shelf life and safety aspects of chilled cooked and peeled shrimps (Pandalus borealis) in modified atmosphere packaging

AIMS: To evaluate the growth of Listeria monocytogenes and shelf life of cooked and peeled shrimps in modified atmosphere packaging (MAP). METHODS AND RESULTS: Storage trials with naturally contaminated cooked and peeled MAP shrimps (Pandalus borealis) were carried out at 2, 5 and 8 degrees C. Challenge tests at the same conditions were performed after inoculation with Listeria monocytogenes. Both storage trials and challenge tests were repeated after 4 months of frozen storage (-22 degrees C). Brochothrix thermosphacta and Carnobacterium maltaromaticum were responsible for sensory spoilage of cooked and peeled MAP shrimps. In challenge tests, growth of L. monocytogenes was observed at all of the storage temperatures studied. At 5 and 8 degrees C the concentration of L. monocytogenes increased more than a 1000-fold before the product became sensory spoiled whereas this was not observed at 2 degrees C. Frozen storage had only a minor inhibiting effect on growth of L. monocytogenes in the thawed product. CONCLUSIONS: To prevent L. monocytogenes becoming a safety problem, cooked and peeled MAP shrimps should be distributed at 2 degrees C and with a maximum shelf life of 20-21 d. At higher temperatures shelf life is significantly reduced. SIGNIFICANCE AND IMPACT OF THE STUDY: Information is provided to establish shelf life of cooked and peeled MAP shrimps.     

Transformation of mono- and dichlorinated phenoxybenzoates by phenoxybenzoate-dioxygenase in pseudomonas pseudoalcaligenes POB310 and a modified diarylether-metabolizing bacterium

Pseudomonas pseudoalcaligenes POB310 contains genes that encode phenoxybenzoate dioxygenase. The enzyme transforms mono- and dichlorinated phenoxybenzoates to yield protocatechuate that is used as a growth substrate and chlorophenols that are nonmetabolizable. Mass spectral analysis of (18)O metabolites obtained from the protocatechuate 3,4-dioxygenase-deficient mutant, POB310-B1, suggested that the reaction mechanism is a regioselective angular dioxygenation. A cloning vector containing reaction relevant genes (pD30.9) was transferred into Pseudomonas sp. strain B13 containing a modified ortho-cleavage pathway for aromatic compounds. The resultant Pseudomonas sp. strain B13-D5 (pD30.9) completely metabolized 3-(4-chlorophenoxy)benzoate. During growth on 3-phenoxybenzoate, strain B13-D5 (pD30.9) (K(s) = 0.70+/-0.04 mM, mu(max) = 0.45+/-0.03 h(-1), t(d) = 1.5 h, Y = 0.45+/-0.03 g bio- mass x g substrate(-1)) was better adapted to low substrate concentrations, had a faster rate of growth, and a greater yield than POB310 (K(s) = 1.13+/-0.06 mM, mu(max) = 0.31+/-0.02 h(-1), t(d) = 2.2 h, Y = 0.39+/-0.02 g biomass. g substrate(-1)).     

Effects of gaseous environment and temperature on the storage behaviour of Listeria monocytogenes on chicken breast meat

Portions of skinless chicken breast meat (pH 5.8) were inoculated with a strain of Listeria monocytogenes and stored at 1, 6 or 15 degrees C in (1) aerobic conditions; (2) 30% CO2 + air; (3) 30% CO2 + N2; and (4) 100% CO2. When samples were held at 1 degree C the organism failed to grow under any of the test conditions, despite marked differences between treatments in spoilage rate and ultimate microflora. At 6 degrees C counts of L. monocytogenes increased ca 10-fold in aerobic conditions before spoilage of the meat, but only when the inoculum culture was incubated at 1 degree C rather than 37 degrees C. In CO2 atmospheres growth of L. monocytogenes was inhibited on meat held at 6 degrees C, especially under 100% CO2. By contrast, storage at 15 degrees C led to spoilage of the meat within 2 d, in all gaseous environments, and listeria levels increased up to 100-fold. Differences in the behaviour of L. monocytogenes on poultry and red meats are discussed.     

Mechanisms of monoclonal antibody-mediated protection against virulent Semliki Forest virus.

Both neutralizing and nonneutralizing immunoglobulin G2a monoclonal antibodies (MAs) directed against the E2 glycoprotein of Semliki Forest virus (SFV) protected mice prophylactically and therapeutically against virulent SFV infection. The neutralizing MAs, however, conferred protection to mice at lower doses than did nonneutralizing MAs. The antibody-dependent, complement-mediated cytolysis of SFV-infected L cells was effectuated by both kinds of antibodies, but again neutralizing MAs were more effective. Removal of the Fc part of the neutralizing MA UM 5.1 by pepsin digestion resulted in a 100-fold reduction of the neutralization titer (10(4) versus 10(6)) and a complete loss of its capacity to mediate antibody-dependent, complement-mediated cytolysis. Passive protection of infected mice occurred only after administration of relatively high doses of F(ab')2 of MA UM 5.1 (30.0 micrograms versus 0.1 microgram). F(ab')2 fragments prepared from the nonneutralizing MA UM 4.2 had lost their protective capacity completely. Surprisingly, the nonneutralizing MA UM 4.2 retarded virus growth in mouse fibroblasts (L cells), although inhibition was at much higher doses than with the neutralizing MA UM 5.1. Furthermore, both MAs promoted the uptake of virulent SFV in the Fc receptor-bearing WEHI-3 cells. The results suggest that nonneutralizing MAs protect mice not only by antibody-dependent, complement-mediated cytolysis but also by growth inhibition and enhanced uptake of SFV in the nonpermissive macrophages of BALB/c mice. This hypothesis is supported by the absence of viremia in recipients of nonneutralizing MA UM 4.2 at 24 h after infection.     

Growth characteristics of bakers' yeast in ethanol

The influence of temperature (15 degrees -40 degrees C) and pH (2.5-6.0) on the continuous growth of bakers' yeast (Saccharomyces cerevisiae) at steady state in 1% ethanol was investigated. Optimal temperature and pH were 30 degrees C and 4.5, respectively. The short-term effect of ethanol concentration (0.1-10.0%) on the yeast growth was assessed in batch culture. Up to 1% of ethanol, the yeast growth increased in function of the ethanol concentration in the medium. The biomass reached a maximum within the interval of 1-4% of ethanol (7.9 and 31.6 g/L, respectively) and decreased at higher concentrations. The residual ethanol concentration in the medium increased rapidly when the initial ethanol concentration exceeded 4%. The best-fit model obtained for growth inhibition as a function of ethanol concentrations was that of Tseng and Wayman: mu(m)S/)K + S( - i (S - S(theta)). With this model, the specific growth rate (mu) decreased linearly as the ethanol concentration increased between the threshold value (S(theta)) of 11.26 g/L to be fully inhibited at 70.00 g/L (S;) an inhibition constant (i) of 0.0048 g L(-1) h(-1), a maximum specific growth rate (mu(m)) of 0.284 h(-1), and a saturation constant (K) of 0.611 g/L were obtained.     

Effect of culture pH on erythropoietin production by Chinese hamster ovary cells grown in suspension at 32.5 and 37.0 degrees C

To investigate the effect of culture pH in the range of 6.85-7.80 on cell growth and erythropoietin (EPO) production at 32.5 and 37.0 degrees C, serum-free suspension cultures of recombinant CHO cells (rCHO) were performed in a bioreactor with pH control. Lowering culture temperature from 37.0 to 32.5 degrees C suppressed cell growth, but cell viability remained high for a longer culture period. Regardless of culture temperature, the highest specific growth rate (mu) and maximum viable cell concentration were obtained at pH values of 7.00 and 7.20, respectively. Like mu, the specific consumption rates of glucose and glutamine decreased at 32.5 degrees C compared to 37.0 degrees C. In addition, they increased with increasing culture pH. Culture pH at 32.5 degrees C affected specific EPO productivity (q(EPO)) in a different fashion from that at 37 degrees C. At 37 degrees C, the q(EPO) was fairly constant in the pH range of 6.85-7.80, while at 32.5 degrees C, the q(EPO) was significantly influenced by culture pH. The highest q(EPO) was obtained at pH 7.00 and 32.5 degrees C, and its value was approximately 1.5-fold higher than that at pH 7.00 and 37.0 degrees C. The proportion of acidic EPO isoforms, which is a critical factor for high in vivo biological activity of EPO, was highest in the stationary phase of growth, regardless of culture temperature and pH. Although cell viability rapidly decreased in death phase at both 32.5 and 37.0 degrees C, the significant degradation of produced EPO, probably by the action of proteases released from lysed cells, was observed only at 37.0 degrees C. Taken together, through the optimization of culture temperature and pH, a 3-fold increase in maximum EPO concentration and a 1.4-fold increase in volumetric productivity were obtained at pH 7.00 and 32.5 degrees C when compared with those at 37.0 degrees C. These results demonstrate the importance of optimization of culture temperature and pH for enhancing EPO production in serum-free, suspension culture of rCHO cells.     

Ethylene removal at low temperatures under biofilter and batch conditions

Removal of the plant hormone ethylene (C(2)H(4)) is often required by horticultural storage facilities, which are operated at temperatures below 10 degrees C. The aim of this study was to demonstrate an efficient, biological C(2)H(4) removal under such low-temperature conditions. Peat-soil, acclimated to degradation of C(2)H(4), was packed in a biofilter (687 cm(3)) and subjected to an airflow ( approximately 73 ml min(-1)) with 2 ppm (microl liter(-1)) C(2)H(4). The C(2)H(4) removal efficiencies achieved at 20, 10, and 5 degrees C, respectively, were 99.0, 98.8, and 98.4%. This corresponded to C(2)H(4) levels of 0.022 to 0.032 ppm in the biofilter outlet air. At 2 degrees C, the average C(2)H(4) removal efficiency dropped to 83%. The detailed temperature response of C(2)H(4) removal was tested under batch conditions by incubation of 1-g soil samples in a temperature gradient ranging from 0 to 29 degrees C with increments of 1 degrees C. The C(2)H(4) removal rate was highest at 26 degrees C (0.85 microg of C(2)H(4) g [dry weight](-1) h(-1)), but remained at levels of 0.14 to 0.28 microg of C(2)H(4) g (dry weight)(-1) h(-1) at 0 to 10 degrees C. At 35 to 40 degrees C, the C(2)H(4) removal rate was negligible (0.02 to 0.06 microg of C(2)H(4) g [dry weight](-1) h(-1)). The Q(10) (i.e., the ratio of rates 10 degrees C apart) for C(2)H(4) removal was 1.9 for the interval 0 to 10 degrees C. In conclusion, the present results demonstrated microbial C(2)H(4) removal, which proceeded at 0 to 2 degrees C and produced a moderately psychrophilic temperature response.     

Nisin and ALTATM 2341 inhibit the growth of Listeria monocytogenes on smoked salmon packaged under vacuum or 100% CO2

The effects of nisin and ALTA 2341 on the growth of Listeria monocytogenes were assessed on smoked salmon packaged under vacuum or 100% CO2. Smoked salmon slices (pH 6.3) were inoculated with a cocktail of seven L. monocytogenes isolates at a level of approximately 2.5 log10 colony forming units (cfu) g-1. After inoculation, the surface of the smoked salmon slices was treated with either nisin (400 or 1250 IU g-1) or ALTA 2341 (0.1 or 1%). The smoked salmon was packaged and stored at 4 degrees C (28 d) or 10 degrees C (9 d). On untreated vacuum-packaged smoked salmon, L. monocytogenes grew by 3.8 log10 cfu g-1 at 4 degrees C and 5.1 log10 cfu g-1 at 10 degrees C. Growth was reduced on nisin- and ALTA 2341-treated vacuum-packaged smoked salmon. On the nisin-treated samples, L. monocytogenes increased by 2.5 (400 IU g-1) and 1.5 (1250 IU g-1) log10 cfu g-1 at 4 degrees C, and by 4.3 (400 IU g-1) and 2.7 (1250 IU g-1) log10 cfu g-1 at 10 degrees C. With the ALTA 2341-treated samples, L. monocytogenes increased by 2.8 (0.1%) or 1.6 (1.0%) log10 cfu g-1 at 4 degrees C, and 3.3 (0.1%) or 3.6 (1.0%) log10 cfu g-1 at 10 degrees C. The growth of L. monocytogenes was retarded by packaging the smoked salmon in 100% CO2. On untreated smoked salmon, only a 0.8 log10 cycle increase was observed at 10 degrees C. Under all the other conditions tested with 100% CO2, L. monocytogenes was detected but growth was prevented.     

Water and heat balance during flight in the rose-colored starling (Sturnus roseus)

Water imbalance during flight is considered to be a potentially limiting factor for flight ranges in migrating birds, but empirical data are scarce. We studied flights under controlled ambient conditions with rose-colored starlings in a wind tunnel. In one experiment, we measured water fluxes with stable isotopes at a range of flight speeds (9-14 m s(-1)) at constant temperature (15 degrees C). In a second experiment, we measured evaporation rates at variable ambient temperatures (Ta = 5 deg -27 deg C) but constant speed (12 m s(-1)). During all flights, the birds experienced a net water loss. On average, water influx was 0.98 g h(-1) (SD = 0.16; n = 8), and water efflux was 1.29 g h(-1) (SD = 0.14; n = 8), irrespective of flight speed. Evaporation was related to temperature in a biphasic pattern. At temperatures below 18.2 degrees C, net evaporation was constant at 0.36 g h(-1) (SD = 0.18; n = 10), rising at higher temperatures with a slope of 0.11 per degree to about 1.5 g h(-1) at 27 degrees C. We calculated the relative proportion of dry and evaporative heat loss during flight. Evaporative heat loss at Ta < 18.2 deg C was 14% of total heat production during flight, and dry heat loss accounted for 84%. At higher temperatures, evaporative heat loss increased linearly with T(a) to about 25% at 27 degrees C. Our data suggest that for prolonged flights, rose-colored starlings should adopt behavioral water-saving strategies and that they cannot complete their annual migration without stopovers to replenish their water reserves.     

The effect of the dilution rate on CHO cell physiology and recombinant interferon-gamma production in glucose-limited chemostat culture

The physiology of a recombinant Chinese hamster ovary cell line in glucose-limited chemostat culture was studied over a range of dilution rates (D = 0.008 to 0.20 h(-1)). The specific growth rate (mu) deviated from D at low dilution rates due to an increased specific death rate. Extrapolation of these data suggested a minimum specific growth rate of 0.011 h(-1) (mu(max) = 0.025 h(-1)) The metabolism at each steady state was characterized by determining the metabolic quotients for glucose, lactate, ammonia, amino acids, and interferon-gamma (IFN-gamma). The specific rate of glucose uptake increased linearly with mu, and the saturation constant for glucose (K(s)) was calculated to be 59.6 muM. There was a linear increase in the rate of lactate production with a higher yield of lactate from glucose at high growth rates. The decline in the rate of production of lactate, alanine, and serine at low growth rate was consistent with the limitation of the glycolytic pathway by glucose. The specific rate of IFN-gamma production increased with mu in a manner indicative of a growth-related product. Despite changes in the IFN-gamma production rate and cell physiology, the pattern of IFN-gamma glycosylation was similar at all except the lowest growth rates where there was increased production of nonglycosylated IFN-gamma. (c) 1993 John Wiley & Sons, Inc.     

Effect of cell growth rate on the performance of a two-stage continuous culture system in a recombinant Escherichia coli fermentation

As part of the process optimization of a two-stage continuous culture system, the effect of growth rate mu(2) (app) on the performance of the second stage (production stage) was studied in a recombinant Escherichia coli K12 (DeltaH1Deltatrp/pPLc23trpA1). Important parameters considered were specific gene expression rate, plasmid content, and plasmid stability, all of which were closely related to the cell growth rate and the production rate of the cloned gene product (trpalpha). When operating conditions were maintained constant (T(1) = 35 degrees C, D(1) = 0.9 h(-1), T(2) = 40 degrees C, and D(2) = 0.7 h(-1)) and mu(2) (app) was varied, plasmid content in the second stage showed its maximum at mu(2) (app) = 0.4 h(-1) and decreased thereafter. Specific gene expression rate linearly increased with increasing mu(2) (app), while plasmid stability decreased. Optimum cell growth rate giving the maximum value in overall productivity was observed at around mu(2) (app) = 0.4 h(-1). The contribution or role of the three parameters, specific gene expression rate, plasmid content, and plasmid stability in exhibiting the maximum productivity at the optimal mu(2) (app) is discussed.     

Effect of operating parameters on specific production rate of a cloned-gene product and performance of recombinant fermentation process

A two-stage continuous system in combination with a temperature-sensitive expression system were used as model systems to maximize the productivity of a cloned gene and minimize the problem associated with the plasmid instability for a high-expression recombinant. In order to optimize the two-stage fermentation process, the effects of such operational variables as temperature and dilution rate on productivity of cloned gene were studied using the model systems and a recombinant, Escherichia coli K12 DeltaH1 Deltatrp/pPLc23trp A1. When the expression of cloned gene is induced by raising the operating temperature above 38 degrees C, a significant decrease in the colony-forming-units (CFU) of the plasmid-harboring cell was observed, and the decrease was related to the product concentration. In order to describe this phenomenon, a new kinetic parameter related to the metabolic stress (metabolic stress factor) was introduced. It is defined as the ratio of the rate of change of pheno-type from colony-forming to non-colony-forming cells to the product accumulation per unit cell mass. At a fixed temperature of 40 degrees C, the varying dilution rate D in the range of 0.35-0.90 h(-1) did not affect the metabolic stress factor significantly. At a fixed dilution rate of D = 0.35 h(-1), this factor remained practically constant up to 41 degrees C but increased rapidly beyond 41 degrees C. The effects of temperature and dilution rate in the second stage on the specific production rate were also studied while maintaining the apparent specific growth rate (mu(2) (app)) of the second stage constant at or near mu(2) (app) = 0.26 h(-1). Under a constant dilution rate, D(2) = 0.35 h(-1), the maximum specific production rate obtained was about q(p, max) = 38 units TrpA/mg cell/h at 41 degrees C. At a constant temperature, T(2) = 40 degrees C, specific production rate increased with decreasing dilution rate with in the dilution rate range of D(2) = 0.35-0.90 h(-1). Based on the results of our study, the optimal operating conditions found were dilution rate D(2) = 0.35 h(-1) and operating temperature T(2) = 41 degrees C at the apparent specific growth rate of 0.26 h(-1). Under the optimal operating conditions, about threefold increase in productivity was achieved compared to the best batch culture result. In addition, the fermentation period could be extended for more than 100 h.     

Effects of light regime, temperature, and plant age on uptake of arsenic by Spartina pectinata and Carex stricta

We report here on efforts to show that a combination of native wetland plant species might perform better than a monoculture in wetlands designed for arsenic remediation by supplementing weaknesses. Carex stricta and Spartina pectinata were used in hydroponic experiments. (i) Arsenic uptake was first assessed at two ages via exposure to control or arsenic-laden solutions (0 or 1.5 mg As L(-1) as Na2HAsO4) for two weeks. Age had no significant effect on arsenic concentrations in roots, but translocation factors were greater in older plants of C. stricta and S. pectinata (0.45 and 0.07, respectively) than in younger plants (0.10 and 0.01, respectively). (ii) Seasonal effects were assessed by determining uptake kinetics for both species in conditions representative of spring temperatures (15/5 degrees C) and light regimes (1050 micromol m(-2) s(-1), 13 h day(-1)) and summer temperatures (28/17 degrees C) and light regimes (1300 micromol m(-2) s(-1), 15 h day(-1)). Both species had comparable rates of arsenic uptake into roots in summer conditions (44.0 and 46.5 mg As kg(-1) dry wt. h(-1) in C. stricta and S. pectinata, respectively), but C. stricta had a higher maximum net influx rate in spring conditions (24.5 versus 10.4 mg As kg(-1) dry wt. h(-1)).     

Control of Listeria monocytogenes in a biofilm by competitive-exclusion microorganisms

Biofilms from drains in food processing facilities with a recent history of no detectable Listeria monocytogenes in floor drains were cultured for microorganisms producing antilisterial metabolites. A total of 413 microbial isolates were obtained from 12 drain biofilm samples and were assayed at 15 and 37 degrees C for activities that were bactericidal or inhibitory to L. monocytogenes, by two agar plate assays. Twenty-one of 257 bacterial isolates and 3 of 156 yeast isolates had antilisterial activity. All 24 isolates which produced metabolites inhibitory to L. monocytogenes were assayed for antilisterial activity in coinoculated broth cultures containing tryptic soy broth with yeast extract (TSB-YE). A five-strain mixture of 10(3) CFU of L. monocytogenes/ml and 10(5) CFU of the candidate competitive-exclusion microorganism/ml was combined in TSB-YE and incubated at 37 degrees C for 24 h, 15 degrees C for 14 days, 8 degrees C for 21 days, and 4 degrees C for 28 days. Substantial inhibition of L. monocytogenes growth (4 to 5 log CFU/ml) was observed for nine bacterial isolates at 37 degrees C, two at 15 and 8 degrees C, and three at 4 degrees C. The inhibitory isolates were identified as Enterococcus durans (six isolates), Lactococcus lactis subsp. lactis (two isolates), and Lactobacillus plantarum (one isolate). The anti-L. monocytogenes activity of these isolates was evaluated in biofilms of L. monocytogenes on stainless steel coupons at 37, 15, 8, and 4 degrees C. Results revealed that two isolates (E. durans strain 152 and L. lactis subsp. lactis strain C-1-92) were highly inhibitory to L. monocytogenes (growth inhibition of >5 log(10) CFU of L. monocytogenes/cm(2)). These two bacterial isolates appear to be excellent competitive-exclusion candidates to control L. monocytogenes in biofilms at environmental temperatures of 4 to 37 degrees C.     

Physiological characterization of Dunaliella sp. (Chlorophyta, Volvocales) from Yucatan, Mexico

Physiological responses of Dunaliella salina and Dunaliella viridis, isolated from solar saltworks on the Yucatan Peninsula, were studied. Optimal growth temperature for D. salina was 22 degrees C (3.06 x 10(6) cells mL(-1)) and 26 degrees C for D. viridis (4.04 x 10(6)cells mL(-1)). Total carotenoid content in D. salina increased with temperature to a maximum of 35.14 pg cell(-1) at 38 degrees C. Dunaliella salina alpha-carotene and beta-carotene content was 0.083+/-0.003 and 0.598+/-0.020 mg 100g dry wt(-1) respectively, whereas lower values were found in D. viridis cultured under same experimental conditions (0.018+/-0.002 and 0.136+/-0.012 mg 100g dry wt(-1) respectively). The highest specific growth rate in D. salina was obtained at 10% NaCl (0.28 d(-1)), while its cell volume increased from 524 to 2066.93 microm(3) when cultured from 10% to 35% NaCl. Maximum photosynthetic rates were attained when increasing from optimal growing temperature to 30 degrees C for D. viridis (108 n mol O(2)microg chl alpha h(-1)) and D. salina (139 n mol O(2)microg chl alpha h(-1)). Photosynthetic responses to temperature variations indicated physiological adjustments in both species, with higher acclimation in D. salina. Evaluation of physiological attributes of these species will be used for to carry out mass cultivation.     

Simple method to observe the adaptive response of Listeria monocytogenes in food

A simple, novel method for determining stress-adaptive response of Listeria monocytogenes in food systems is presented. The method involves plating samples on Listeria-selective agar (LSA) acidified to pH 5.25 with incubation at 36 degrees C for 60 h to detect acid adaptation and plating on LSA with 70 gl-1 NaCl and incubation at 7 degrees C for 7 d to detect cold-osmotic adaptation. Adapted cells produced larger colonies (> 1 mm) under these conditions than unadapted cells. Scot A (97%) and Brie-1 (100%) cells incubated in milk at pH 5 for 3 h manifested the acid-adapted colony type compared with 6% and 21% of viable cells in the unstressed control population. After a 5-d adaptation period at 4 degrees C in milk with 80 gl-1 salt, 29% of Scot A and 91% of Brie-1 viable cells exhibited the adapted colony type compared with < 1% of the unstressed control population. Stress-adapted L. monocytogenes were isolated from soft cheese held for 42 d at 10 C.     

The effect of pH, salt concentration and temperature on the survival and growth of Listeria monocytogenes

Factorially designed experiments have been used to study the growth and survival of Listeria monocytogenes in different combinations of pH and salt concentrations at ambient and chill temperatures. Survival at low pH and high salt concentration was strongly temperature dependent. The minimum pH values that allowed survival after 4 weeks from an initial 10(4) cells were 4.66 at 30 degrees C, 4.36 at 10 degrees C and 4.19 at 5 degrees C. These limits were salt dependent, low (4-6%) salt concentrations improved and higher concentrations reduced survival at limiting pH values. The lowest pH that allowed a 100-fold increase in cell numbers within 60 d was 4.66 at 30 degrees C but this was increased to 4.83 at 10 degrees C. At 5 degrees C growth occurred at pH 7.0 but not at pH 5.13. Simple predictive models describing the effect of hydrogen-ion and salt concentration on the time for at least a 100-fold increase in numbers at 10 degrees C and 30 degrees C were constructed after analysis of the results for a least squares fit to a quadratic model. The interactions between salt and hydrogen-ion concentration on growth were found to be purely additive.     

Inhibition of Listeria monocytogenes by Lactobacillus bavaricus MN in beef systems at refrigeration temperatures.

The ability of Lactobacillus bavaricus, a meat isolate, to inhibit the growth of three Listeria monocytogenes strains was examined in three beef systems: beef cubes, beef cubes in gravy, and beef cubes in gravy containing glucose. The beef was minimally heat treated, inoculated with L. bavaricus at 10(5) or 10(3) CFU/g and L. monocytogenes at 10(2) CFU/g, vacuum sealed, and stored at 4 or 10 degrees C. The meat samples were monitored for microbial growth, pH, and bacteriocin production. The pathogen was inhibited by L. bavaricus MN. At 4 degrees C, L. monocytogenes was inhibited or killed depending on the initial inoculum level of L. bavaricus. At 10 degrees C, at least a 10-fold reduction of the pathogen occurred, except in the beef without gravy. This system showed a transient inhibition of the pathogen during the first week of storage followed by growth to control levels by the end of the incubation period. Bacteriocin was detected in the samples, and inhibition could not be attributed to acidification. Low refrigeration temperatures significantly (P < or = 0.05) enhanced L. monocytogenes inhibition. Moreover, the addition of glucose-containing gravy and the higher inoculum level of L. bavaricus were significantly (P < or = 0.05) more effective in reducing L. monocytogenes populations in most of the systems studied.