Effects of acid adaptation of Escherichia coli O157:H7 on efficacy of acetic acid spray washes to decontaminate beef carcass tissue

Exposure to low pH and organic acids in the bovine gastrointestinal tract may result in the induced acid resistance of Escherichia coli O157:H7 and other pathogens that may subsequently contaminate beef carcasses. The effect of acid adaptation of E. coli O157:H7 on the ability of acetic acid spray washing to reduce populations of this organism on beef carcass tissue was examined. Stationary-phase acid resistance and the ability to induce acid tolerance were determined for a collection of E. coli O157:H7 strains by testing the survival of acid-adapted and unadapted cells in HCl-acidified tryptic soy broth (pH 2.5). Three E. coli O157:H7 strains that were categorized as acid resistant (ATCC 43895) or acid sensitive (ATCC 43890) or that demonstrated inducible acid tolerance (ATCC 43889) were used in spray wash studies. Prerigor beef carcass surface tissue was inoculated with bovine feces containing either acid-adapted or unadapted E. coli O157:H7. The beef tissue was subjected to spray washing treatments with water or 2% acetic acid or left untreated. For strains ATCC 43895 and 43889, larger populations of acid-adapted cells than of unadapted cells remained on beef tissue following 2% acetic acid treatments and these differences remained throughout 14 days of 4 degrees C storage. For both strains, numbers of acid-adapted cells remaining on tissue following 2% acetic acid treatments were similar to numbers of both acid-adapted and unadapted cells remaining on tissue following water treatments. For strain ATCC 43890, there was no difference between populations of acid-adapted and unadapted cells remaining on beef tissue immediately following 2% acetic acid treatments. These data indicate that adaptation to acidic conditions by E. coli O157:H7 can negatively influence the effectiveness of 2% acetic acid spray washing in reducing the numbers of this organism on carcasses.     

Induced thermotolerance under nonisothermal treatments of a heat sensitive and a resistant strain of Staphylococcus aureus in media of different pH

AIMS: The aim was to assess the induced thermotolerance under nonisothermal treatments of two strains of Staphylococcus aureus in media of different pH. METHODS AND RESULTS: Staphylococcus aureus ATCC 25923 was more heat resistant than S. aureus ATCC 13565 at any pH investigated under isothermal conditions. At pH 7.4, the D58 value of the resistant strain was approx. 30 times greater. Both strains showed a higher heat resistance at pH 4.0 than at pH 7.4. In contrast, under nonisothermal treatments (0.5-2 degrees C min(-1)), both strains were more heat resistant when treated at pH 7.4 than at pH 4.0 due to heat adaptation at the higher pH. At the slowest heating up rate tested at pH 7.4, the initially heat-sensitive strain nearly reached the thermotolerance of the heat-resistant strain. CONCLUSIONS: The induced thermotolerance under nonisothermal treatments depended on the treatment medium pH and the microbial strain tested. The induced thermotolerance in a sensitive strain can be greater than in a heat-resistant strain, showing similar resistance under nonisothermal conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: This work shows data of interest about mechanisms of microbial resistance and adaptation to heat. Moreover, it contributes to the development of more adequate combined processes for food preservation.     

Enhancing the antimicrobial effects of bovine lactoferrin against Escherichia coli O157:H7 by cation chelation, NaCl and temperature

AIM: To evaluate the effect of NaCl, growth medium and temperature on the antimicrobial activity of bovine lactoferrin (LF) against Escherichia coli O157:H7 in the presence of different chelating agents. METHODS AND RESULTS: LF (32 mg ml(-1)) was tested against E. coli O157:H7 strain 3081 in Luria broth (LB) and All Purpose Tween (APT) broth with metal ion chelators sodium bicarbonate (SB), sodium lactate (SL), sodium hexametaphosphate (SHMP), ethylene diamine tetraacetic acid (EDTA) or quercetin at 0.5 and 2.5% NaCl at 10 and 37 degrees C. LF and the chelators were tested against four other E. coli O157:H7 strains in LB at 2.5% NaCl and 10 degrees C. LF alone was bacteriostatic against strains 3081 and LCDC 7283 but other strains grew. Antimicrobial effectiveness of LF was reduced in APT broth but enhanced by SB at 2.5% NaCl and 10 degrees C where 4.0 log(10) CFU ml(-1) inoculated cells were killed. EDTA enhanced antimicrobial action of the LF-SB combination. SL alone was effective against E. coli O157:H7 but a reduction in its activity at 2.5% NaCl and 10 degrees C was reversed by LF. The combinations LF-SHMP and LF-quercetin were more effective at 37 degrees C and NaCl effects varied. CONCLUSIONS: LF plus SB or SL were bactericidal toward the same 3/5 E. coli O157:H7 strains and inhibited growth of the others at 2.5% NaCl and 10 degrees C. SIGNIFICANCE AND IMPACT OF THE STUDY: The combination of LF with either SL or SB shows potential for reducing viability of E. coli O157:H7 in food systems containing NaCl at reduced, but growth permissive temperature.     

Survival strategies of plasmid-carrier and plasmidless Escherichia coli strains under illuminated and non-illuminated conditions, in a fresh water ecosystem.

A comparative study, in illuminated and non-illuminated systems, was made to determine the survival strategies of plasmid-carrier and plasmidless bacteria in sterile river water. Two strains of Escherichia coli from river water were selected: one plasmidless, EC1, and one antibiotic-resistant strain, EC7, which showed plasmid bands. By matings with EC7 as donor and E. coli K12 strain J62 as recipient, transconjugants were generated, the J62(7) strain, which showed both antibiotic resistance and plasmid bands. Ethidium bromide curing of the EC7 strain generated the EC7(2) strain which showed a partial loss of resistance and a reorganization of plasmid bands. Under non-illuminated conditions the total number of cells detected by direct count and the number of culturable cells (injured and non-injured cells) remained practically constant throughout the period of incubation. In the illuminated systems, however, the number of cfu decreased in four of the five strains studied. The greatest decreases are those of the J62 strain, followed by those of the J62(7), EC1, EC7(2) and EC7 strains. Differences in survival strategies as a consequence of the presence or absence of plasmids are discussed.     

Compatible solutes contribute to heat resistance and ribosome stability in Escherichia coli AW1.7

This study investigated the mechanisms of heat resistance in Escherichia coli AW1.7 by quantification of cytoplasmic solutes, determination of ribosome denaturation, and by determination of protein denaturation. To assess the contribution of heat shock proteins and compatible solutes, experiments were conducted after exposure to sublethal heat shock, and with cultures grown at NaCl concentrations ranging from 0 to 6%. Heat resistance of E. coli AW1.7 was compared to the heat sensitive E. coli GGG10 and a plasmid-cured, heat sensitive derivative of E. coli AW1.7 named E. coli AW1.7ΔpHR1. Sublethal heat shock improved survival at 60°C of E. coli GGG10 and AW1.7ΔpHR1 but not of E. coli AW1.7. Addition of NaCl increased the heat resistance of all three strains, but only E. coli AW1.7 exhibited high heat resistance when grown in NaCl concentrations ranging from 2 to 6%. E. coli AW1.7 and GGG10 accumulated 16.1±0.8 and 8.8±0.8mmolL(-1) amino acids when grown at 0% NaCl, and 1.47±0.07 and 0.78±0.06mmolL(-1) carbohydrates when grown at 6% NaCl, respectively. Ribosome denaturation was determined by differential scanning calorimetry. After growth in the presence of 0% NaCl, the 30S subunit denatured at 63.7±0.8°C and 60.7±0.3°C in E. coli AW1.7 and GGG10, respectively. Fourier-transformed-infrared-spectroscopy did not indicate differences in protein denaturation between the strains during heating. In conclusion, heat resistance in E. coli AW1.7 correlates to ribosome stability at 60°C and is dependent on accumulation of cytoplasmic solutes.     

Survival strategies of plasmid-carrier and plasmidless Escherichia coli strains under illuminated and non-illuminated conditions, in a fresh water ecosystem

A Comparative study, in illuminated and non-illuminated systems, was made to determine the survival strategies of plasmid-carrier and plasmidless bacteria in sterile river water. Two strains of Escherichia coli from river water were selected: one plasmidless, EC1, and one antibiotic-resistant strain, EC7, which showed plasmid bands. By matings with EC7 as donor and E. coli K12 strain J62 as recipient, transconjugants were generated, the J62₇ strain, which showed both antibiotic resistance and plasmid bands. Ethidium bromide curing of the EC7 strain generated the EC7₂ strain which showed a partial loss of resistance and a reorganization of plasmid bands. Under non-illuminated conditions the total number of cells detected by direct count and the number of culturable cells (injured and non-injured cells) remained practically constant throughout the period of incubation. In the illuminated systems, however, the number of cfu decreased in four of the five strains studied. The greatest decreases are those of the J62 strain, followed by those of the J62₇, EC1, EC7₂ and EC7 strains. Differences in survival strategies as a consequence of the presence or absence of plasmids are discussed.     

Adaptation to sublethal environmental stresses protects Listeria monocytogenes against lethal preservation factors.

A sublethal dose of ethanol (5%, vol/vol), acid (HCl, pH 4.5 to 5.0), H2O2 (500 ppm), or NaCl (7%, wt/vol) was added to a Listeria monocytogenes culture at the exponential phase, and the cells were allowed to grow for 1 h. Exponential-phase cells also were heat shocked at 45 degrees C for 1 h. The stress-adapted cells were then subjected to the following factors at the indicated lethal levels--NaCl (25%, wt/vol), ethanol (17.5%, vol/vol), hydrogen peroxide (0.1%, wt/vol), acid (pH 3.5), and starvation on 0.1 M phosphate buffer at pH 7.0 (up to 300 h). Viable counts of the pathogen, after the treatment, were determined on Trypticase soy agar-yeast extract, and survivor plots were constructed. The area (h.log10 CFU/ml) between the control and treatment curves was calculated to represent the protective effect resulting from adaptation to the sublethal stress factor. Adaptation to pH 4.5 to 5.0 or 5% ethanol significantly (P < 0.05) increased the resistance of L. monocytogenes to lethal doses of acid, ethanol, and H2O2. Adaptation to ethanol significantly (P < 0.05) increased the resistance to 25% NaCl. When L. monocytogenes was adapted to 500 ppm of H2O2, 7% NaCl, or heat, resistance of the pathogen to 1% hydrogen peroxide increased significantly (P < 0.05). Heat shock significantly (P < 0.05) increased the resistance to ethanol and NaCl. Therefore, the occurrence of stress protection after adaptation of L. monocytogenes to environmental stresses depends on the type of stress encountered and the lethal factor applied. This "stress hardening" should be considered when current food processing technologies are modified or new ones are developed.     

The Effect of Nacl on Campylobacter Jejuni/Coli

The growth of 2 strains of Campylobacter jejuni/coli was investigated in 0–2.0 % NaCl in Brucella broth at 35° G and 30° C. Both strains tolerated more NaCl in the growth medium at 35° C than at 30° C. 2 % NaCl was bacteriocidic at both temperatures. The strains also grew in the medium without added NaCl. At 35° C, low concentrations of NaCl stimulated the growth of strain 5616, but not the growth of strain B33. At 30° C, strain 5616 grew in NaCl concentrations up to 1.0 % and strain B33 in 0 % and at the control concentration (0.5 % NaCl). The survival of 22 C. jejuni/coli strains in 2.0 % NaCl at 4° C and 35° C was also investigated. Human strains showed significantly greater tolerance to 2.0 % NaCl at both temperatures than did the strains isolated from animals. These findings suggest that the salting of food can be effective in preventing the growth or survival of C. jejuni/coli.     

Role of sigma(B) in heat, ethanol, acid, and oxidative stress resistance and during carbon starvation in Listeria monocytogenes

To determine the contribution of sigma B (sigma(B)) to survival of stationary-phase Listeria monocytogenes cells following exposure to environmental stresses, we compared the viability of strain 10403S with that of an isogenic nonpolar sigB null mutant strain after exposure to heat (50 degrees C), ethanol (16.5%), or acid (pH 2.5). Strain viabilities were also determined under the same conditions in cultures that had been previously exposed to sublethal levels of the same stresses (45 degrees C, 5% ethanol, or pH 4.5). The DeltasigB and wild-type strains had similar viabilities following exposure to ethanol and heat, but the DeltasigB strain was almost 10,000-fold more susceptible to lethal acid stress than its parent strain. However, a 1-h preexposure to pH 4.5 yielded a 1,000-fold improvement in viability for the DeltasigB strain. These results suggest the existence in L. monocytogenes of both a sigma(B)-dependent mechanism and a pH-dependent mechanism for acid resistance in the stationary phase. sigma(B) contributed to resistance to both oxidative stress and carbon starvation in L. monocytogenes. The DeltasigB strain was 100-fold more sensitive to 13.8 mM cumene hydroperoxide than the wild-type strain. Following glucose depletion, the DeltasigB strain lost viability more rapidly than the parent strain. sigma(B) contributions to viability during carbon starvation and to acid resistance and oxidative stress resistance support the hypothesis that sigma(B) plays a role in protecting L. monocytogenes against environmental adversities.     

Isolation of Dunaliella spp. from a hypersaline lake and their ability to accumulate glycerol

The purpose of the present work was to study the potential biotechnological use of Dunaliella species isolated from a hypersaline lake in Turkey. Dunaliella spp. grown in Johnson's medium were isolated and their glycerol production was studied in a batch system in order to determine the optimal conditions required for the highest glycerol accumulation. In the experiments performed with four newly isolated Dunaliella spp., the maximum glycerol accumulation was obtained at 20% NaCl concentration, and pH 6 (for strains T1 and T2) and pH 9 (for strains T3 and T4). Biomass production by strain T2 was significantly higher that by the other strains but the highest glycerol production in broth was obtained by strain T1 followed by strain T2. Strain T1 showed high glycerol production, i.e. 452.57microg/ml of culture broth at 20% NaCl concentration. The highest glycerol accumulation on both dry weight and cell basis was obtained with strain T1, followed by strains T3 and T4 (55.01, 50.16, and 40.23microg/10(6) cells (or pg/cell), respectively) at 25% NaCl concentration. When the high initial inoculum concentration was used at 25% NaCl concentration, strain T1 had the shortest (approximately 10-15days) lag period. This study shows that the isolated strains T1 and T2 can be used for glycerol production because of their high productivity.     

Adaptation of Escherichia coli O157:H7 to pH alters membrane lipid composition, verotoxin secretion, and resistance to simulated gastric fluid acid

The influence of adaptation to pH (from pH 5.0 to 9.0) on membrane lipid composition, verotoxin concentration, and resistance to acidic conditions in simulated gastric fluid (SGF) (pH 1.5, 37 degrees C) was determined for Escherichia coli O157:H7 (HEC, ATCC 43895), an rpoS-deficient mutant of ATCC 43895 (HEC-RM, FRIK 816-3), and nonpathogenic E. coli (NPEC, ATCC 25922). Regardless of the strain, D values (in SGF) of acid-adapted cells were higher than those of non-acid-adapted cells, with HEC adapted at pH 5.0 having the greatest D value, i.e., 25.6 min. Acid adaptation increased the amounts of palmitic acid (C16:0) and decreased cis-vaccenic acid (C18:1 omega 7c) in the membrane lipids of all strains. The ratio of cis-vaccenic acid to palmitic acid increased at acidic pH, causing a decrease in membrane fluidity. HEC adapted to pH 8.3 and HEC-RM adapted to pH 7.3 exhibited the greatest verotoxin concentrations (2,470 and 1,460 ng/ml, respectively) at approximately 10(8) CFU/ml. In addition, the ratio of extracellular to intracellular verotoxin concentration decreased at acidic pH, possibly due to the decrease of membrane fluidity. These results suggest that while the rpoS gene does not influence acid resistance in acid-adapted cells it does confer decreased membrane fluidity, which may increase acid resistance and decrease verotoxin secretion.     

Acid and base resistance in Escherichia coli and Shigella flexneri: role of rpoS and growth pH.

Escherichia coli K-12 strains and Shigella flexneri grown to stationary phase can survive several hours at pH 2 to 3, which is considerably lower than the acid limit for growth (about pH 4.5). A 1.3-kb fragment cloned from S. flexneri conferred acid resistance on acid-sensitive E. coli HB101; sequence data identified the fragment as a homolog of rpoS, the growth phase-dependent sigma factor sigma 38. The clone also conferred acid resistance on S. flexneri rpoS::Tn10 but not on Salmonella typhimurium. E. coli and S. flexneri strains containing wild-type rpoS maintained greater internal pH in the face of a low external pH than strains lacking functional rpoS, but the ability to survive at low pH did not require maintenance of a high transmembrane pH difference. Aerobic stationary-phase cultures of E. coli MC4100 and S. flexneri 3136, grown initially at an external pH range of 5 to 8, were 100% acid resistant (surviving 2 h at pH 2.5). Aerobic log-phase cultures grown at pH 5.0 were acid resistant; survival decreased 10- to 100-fold as the pH of growth was increased to pH 8.0. Extended growth in log phase also decreased acid resistance substantially. Strains containing rpoS::Tn10 showed partial acid resistance when grown at pH 5 to stationary phase; log-phase cultures showed < 0.01% acid resistance. When grown anaerobically at low pH, however, the rpoS::Tn10 strains were acid resistant. E. coli MC4100 also showed resistance at alkaline pH outside the growth range (base resistance). Significant base resistance was observed up to pH 10.2. Base resistance was diminished by rpoS::Tn10 and by the presence of Na+. Base resistance was increased by an order of magnitude for stationary-phase cultures grown in moderate base (pH 8) compared with those grown in moderate acid (pH 5). Anaerobic growth partly restored base resistance in cultures grown at pH 5 but not in those grown at pH 8. Thus, both acid resistance and base resistance show dependence on growth pH and are regulated by rpoS under certain conditions. For acid resistance, and in part for base resistance, the rpoS requirement can be overcome by anaerobic growth in moderate acid.     

Cell density dependent acid sensitivity in stationary phase cultures of enterohemorrhagic Escherichia coli O157:H7

Escherichia coli O157:H7, the causative agent of hemorrhagic colitis and hemolytic uremic syndrome, can survive in a highly acidic environment. The acid resistance of this organism, as measured by its ability to survive in low pH, depended on the density of the cells present during the assay. At low cell densities (相似文献   

Mutagenic action of nitroso compounds on Escherichia coli cells]

An attempt to induce some forward and back mutations in two Escherichia coli strains (his- and HfrH requiring thiamine) under the action of the carcinogenic nitrosamines--dimethylnitrosamine (DMN) and diethylnitrosamine (DEN)--is described. For this purpose the cells of E. coli were treated with 5% DMN or 1% DEN for 1 hour at 37 degrees C in 0.14 M NaCl. It was shown that the sensitivity of both strains to both nitrose compounds was not the same. DEN was 5-fold as toxic as DMN for the E. coli cells. DMN and DEN induced neither mutations of resistance to 10(-3) M valine, nor reversions in histidine-dependent strain. These mutations were obtained after the cells were treated with 0.1 M NaNO2. Lethal effects of DMN increased more than in 5 times and the toxicity of DEN did not change in hydroxylating mixture, in which nitrosamines derived to active compounds. Under these conditions both carcinogenes showed a mutagenic activity. DEN proved to be about twice as strong mutagenically as DMN. Thus, in our experiments we could see that DMN and DEN could induce both forward and back mutations in E. coli.     

Transfer of class 1 integron-mediated antibiotic resistance genes from shiga toxin-producing Escherichia coli to a susceptible E. coli K-12 strain in storm water and bovine feces

Transfer of class 1 integron-mediated antibiotic resistance genes has been demonstrated under laboratory conditions. However, there is no information concerning the transfer of these genes in an agricultural environment. The present study sought to determine if integron-mediated streptomycin and sulfisoxazole resistance genes could be transferred from Shiga toxin-producing Escherichia coli (STEC) strains 6-20 (O157:H7) and 7-63 (O111:H8) to the susceptible strain E. coli K-12 MG1655 in bovine feces (pH 5.5, 6.0, or 6.5) and storm water (pH 5, 6, 7, or 8) at 4, 15, and 28 degrees C, which are average seasonal temperatures for winter, spring-fall, and summer, respectively, in the Griffin, GA, area. The results indicated that at 28 degrees C, the integron-mediated antibiotic resistance genes were transferred from both of the STEC donors in bovine feces. Higher conjugation efficiencies were, however, observed in the conjugation experiments involving STEC strain 6-20. In storm water, the resistance genes were transferred only from STEC strain 6-20. Greater numbers of transconjugants were recovered in the conjugation experiments performed with pH 6.5 bovine feces and with pH 7 storm water. Antibiotic susceptibility tests confirmed the transfer of integron-mediated streptomycin resistance and sulfisoxazole resistance, as well as the transfer of non-integron-mediated oxytetracycline resistance and tetracycline resistance in the transconjugant cells. These results suggest that the antibiotic resistance genes in STEC could serve as a source of antibiotic resistance genes disseminated via conjugation to susceptible cells of other E. coli strains in an agricultural environment.     

Combined effect of growth medium,age of cells and phase of sporulation on heat resistance and recovery of Hansenula anomala

Effects of two growth media, age of cells and phase of sporulation on heat resistance of Hansenula anomala were determined. Cells were grown on two solid media, McClary's acetate and V8 juice agars, at 21 ° C for 16 days. Heat resistance of cells was determined in 0.06 M potassium phosphate buffer at 48 ° C. Heat-stressed cells were plated on four recovery media: yeast extract-malt extract-peptone-glucose (YMPG), pH 7.0; YMPG, pH 3.5; YMPG containing 6% NaCl, pH 7.0; and YMPG containing 20% sucrose, pH 7.0. The composition of sporulation medium influenced the extent of sporulation and the relative heat resistance of sporulating cells. One-day-old cells were the most sensitive to heat. The heat resistance of cells was generally increased as the incubation time was extended to 16 days. Heat treatment caused a greater increase in sensitivity to NaCl than to sucrose or acid pH in recovery media. Young cells were more sensitive to NaCl than were older cells.     

Boophilus microplus: strain differences of the cholinesterase system.

Cholinesterase (EC 31.1.7 and EC 31.1.8) activity was determined for homogenates of two Argentinian strains of larval ticks, Boophilus microplus. One strain (strain A) was sensitive to organophosphate acaricides. The other strain (strain G) was insensitive to coumaphos and to a lesser degree to other acaricides. For both strains maximal activity was found at 8 × 10^?3M substrate, showing inhibition by excess substrate. Maximal activity was recorded at pH 6.7 and 7.8, the latter peak being more evident for strain G. Values of Q₁₀ of 1.15 and 1.25 were obtained for strains A and G, respectively. Centrifugation at 27,000g resulted in the separation of the total cholinesterase activity into two fractions, one soluble and one particulate; the soluble fraction in strain G showed marked resistance to inhibition by an organophosphate ester and to heat inactivation.     

A glutamate-dependent acid resistance gene in Escherichia coli.

Stationary-phase cultures of Escherichia coli can survive several hours or exposure to extreme acid (pH 2 to 3), a level well below the pH range for growth (pH 4.5 to 9). To identify the genes needed for survival in extreme acid, a microliter screening procedure was devised. Colonies from a Tn10 transposon pool in E. coli MC4100 were inoculated into buffered Luria broth, pH 7.0, in microtiter wells, grown overnight, and then diluted in Luria broth, pH 2.5, at 37 degrees C for 2 h. From 3,000 isolates screened, 3 Tet(r) strains were identified as extremely acid sensitive (<0.1% survival at pH 2.5 for 2 h). Flanking sequences of the Tn10 inserts were amplified by inverse PCR. The sequences encoded a hydrophobic partial peptide of 88 residues. A random-primer-generated probe hybridized to Kohara clones 279 and 280 at 32 min (33.7 min on the revised genomic map EcoMap7) near gadB (encoding glutamate decarboxylase). The gene was designated xasA for extreme acid sensitive. xasA::Tn10 strains grown at pH 7 to 8 showed 100-fold-less survival in acid than the parent strain. Growth in mild acid (pH 5 to 6) restored acid resistance; anaerobiosis was not required, as it is for acid resistance in rpoS strains. xasA::Tn10 eliminated enhancement of acid resistance by glutamic acid. xasA was found to be a homolog of gadC recently sequenced in Shigella flexneri, in which it appears to encode a permease for the decarboxylated product of GadB. These results suggest that GadC (XasA) participates in a glutamate decarboxylase alkalinization cycle to protect E. coli from cytoplasmic acidification. The role of the glutamate cycle is particularly important for cultures grown at neutral pH before exposure to extreme acid.     

The effect of some environmental parameters on the growth of yeasts originating from sugar containing foods

In this study, 65 yeast strains originating from sugar containing foods were tested for growth under different environmental conditions. In the experiments performed with glucose and NaCl, inhibition effect was observed at reduced a_w levels. Most of the strains were able to grow at both 4 and 37°C. Of the tested pH values, the most adverse effect on growth of the yeasts was observed at pH 2.5. All of the strains were able to grow at both 5 and 20% CO₂ levels. Acetic and propionic acids were found to be the most effective preservatives for inhibition of the strains. All of the yeasts, except aCandida glucosophila strain, failed to grow in the presence of 0.05% (w/v) sodium benzoate.