Changes in heat tolerance of Escherichia coli O157:H7 after exposure to acidic environments

Acid-adapted bacterial cells are known to have enhanced tolerance to various secondary stresses. However, a comparison of heat tolerance of acid-adapted and acid-shocked cells of Escherichia coli O157:H7 has not been reported. D - and z -values of acid-adapted, acid-shocked, and control cells of an unusually heat-resistant strain (E0139) of E. coli O157:H7, as well as two other strains of E. coli O157:H7, were determined based upon the number of cells surviving heat treatment at 52, 54 or 56°C in tryptic soy broth (pH 7·2) for 0, 10, 20 or 30 min. The unusual heat tolerance of E. coli O157:H7 strain E0139 was confirmed. D -values for cells from 24-h cultures were 100·2, 28·3, and 6·1 min at 52, 54 and 56°C, respectively, with a z -value of 3·3°C. The highest D -values of other E. coli O157:H7 strains were 13·6 and 9·2 min at 52 and 54°C, respectively, whereas highest D -values of non-O157:H7 strains were 78·3 and 29·7 min at 52 and 54°C. D -values of acid-adapted cells were significantly higher than those of unadapted and acid-shocked cells at all temperatures tested. In a previous study, we observed that both acid-adapted cells and acid-shocked cells of strain E0139 had enhanced acid tolerance. This suggests that different mechanisms protect acid-adapted and acid-shocked cells against subsequent exposure to heat or an acidic environment. The two types of cells should be considered separately when evaluating survival and growth characteristics upon subsequent exposure to different secondary stress conditions.     

Inactivation ofEscherichia coliO157:H7 by combinations of GRAS chemicals and temperature

Reported outbreaks of foodborne illness involvingEscherichia coliO157:H7 have increased in the United States during the last decade, with a variety of food products being implicated as vehicles of infection. Studies were carried out to determine the efficacy of combinations of various GRAS chemicals and moderate temperatures to killE. coliO157:H7. A five-strain mixture ofE. coliO157:H7 of approximately 10⁸cfu ml⁻¹was inoculated into 0·1% peptone solutions containing 1·0 or 1·5% lactic acid plus 0·1% hydrogen peroxide, 0·1% sodium benzoate or 0·005% glycerol monolaurate. The solutions were incubated at 8°C for 0, 15 and 30 min; at 22°C for 0, 10 and 20 min; or at 40°C for 0, 10 and 15 min; populations ofE. coliO157:H7 were determined at each sampling time. At 40°C, the pathogen was inactivated to undetectable levels within 10 min of incubation in the presence of 1·0 or 1·5% lactic acid plus hydrogen peroxide, and within 15 min of incubation in the presence of 1·5% lactic acid plus sodium benzoate or glycerol monolaurate. At 22°C, complete inactivation ofE. coliO157:H7 was observed after 20 min of exposure to 1·5% lactic acid plus 0·1% hydrogen peroxide, whereas a reduction of 5 log₁₀cfu ml⁻¹was observed with a treatment of 1·5% lactic acid plus glycerol monolaurate. None of the treatments resulted in total inactivation of the pathogen at 8°C. The aforementioned treatments could potentially be used to inactivate or reduceE. coliO157:H7 populations on raw produce.     

Effect of pH-dependent,stationary phase acid resistance on the thermal tolerance of Escherichia coli O157:H7

The ability of pH-dependent, stationary phase acid resistance to cross-protect Escherichia coli O157:H7 against a subsequent lethal thermal stress was evaluated using microbiological media and three liquid foods. Three strains were grown for 18 h at 37°C in acidogenic (TSB+G, final pH 4·6–4·7) and non-acidogenic (TSB-G, final pH 7·0–7·2) media to provide stationary phase cells with and without induction of pH-dependent acid resistance. The cells were then heated in BHI broth (pH 6·0) at 58°C, using a submerged coil apparatus. The TSB+G grown strains had greatly increased heat resistance, with the heating time needed to achieve a five-log inactivation, being increased two- to four-fold. The z -values of TSB+G and TSB-G grown cells were 4·7°C and 4·3°C, respectively. Increases in heat resistance with TSB+G-grown E. coli O157:H7 were also observed using milk and chicken broth, but not with apple juice. However, cross-protection was restored if the pH of the apple juice was increased from 3·5 to 4·5. The data indicate that pH-dependent acid resistance provides E. coli O157:H7 with cross-protection against heat treatments, and that this factor must be considered to estimate this pathogen's thermal tolerance accurately.     

Growth and processing conditions affecting acid tolerance inEscherichia coli O157:H7

The impact of growth conditions (anaerobiosis, growth phase, NaCl, pH, and temperature) on the development of acid tolerance in Escherichia coli O157:H7 was investigated directly (D_pH1.15) and indirectly by monitoring the specific activity of acid phosphatase. Anaerobic growth of O157:H7 strain 43895 in synthetic rumen fluid resulted in earlier development of acid tolerance than aerobic growth. However, stationary-phase cells of both aerobic and anaerobic cultures had an equivalent degree of acid tolerance that was greater than that achieved in log-phase cultures grown anaerobically. These results are consistent with the growth-phase regulation of acid tolerance by the stationary-phase sigma factors³⁸. The addition of NaCl (1%) also enhanced acid tolerance of log-phase but not stationary-phase cells of strain 43895. Growth temperature influenced the acid tolerance with progressively greater D_pH1.15values obtained at 15, 25, and 37°1C, in both log and stationary phase. Therefore, the influence of temperature on the subsequent survival and acid tolerance of E. coli O157:H7 strain 43895 in ground beef was evaluated. Numbers of strain 43895 decreased c. 1.14 log₁₀cfu g^-1in inoculated ground beef stored at 4°1C, whereas numbers remained essentially unchanged during storage at -20°C. While pre-incubation at 15°1C for 4 h prior to storage at 4 or -20°C did not influence survival, the acid tolerance of E. coli O157:H7 survivors was significantly decreased (P<0.10001). These results indicate that the processing temperature can influence acid tolerance in E. coli O157:H7.     

Determination of the effect of sodium lactate on the survival and heat resistance of Escherichia coli O157:H7 in two commercial beef patty formulations

The effect of 4% sodium lactate (NaL) in beefburger patty formulations on the survival and heat resistance of Escherichia coli O157:H7 was investigated. Fresh beef trimmings were inoculated with E. coli O157:H7 to a concentration of 6·0–7·0 log₁₀ cfu g⁻¹ and subjected to the processing stages of beefburger patty production. Two commercial beefburger patty formulations were produced: a ‘quality’ patty (100% beef) and an ‘economy’ patty (70% beef, 30% other ingredients, including onion, water, salt, seasoning, rusk and soya concentrate). Sodium lactate (4% w/v) was added to the beefburger patties during mincing and the formed patties were frozen and stored for 1 month. Beefburger patties without added NaL were used as controls. After frozen storage for 1 month, patties were examined for E. coli O157:H7 counts. There was a synergistic effect between freezing and NaL, which resulted in a small but significant reduction (P<0·05) (approximately 0·5 log₁₀ cfu g⁻¹) in E. coli O157:H7 numbers. The frozen beefburger patties were also heat-treated at 50, 55 and 60°C and the data analysed to derive D -values for E. coli O157:H7 cells. At each temperature treatment, theD -values of the quality and economy beefburger patties with 4% NaL were significantly lower (P<0·001) than the D -values of the patty formulations without NaL. The study demonstrates that the presence of 4% NaL in beefburger patty formulations can reduce the overall risks posed to consumers by the presence ofE. coli O157:H7 by, first; reducing pathogen survival during freezing and frozen storage of the uncooked product; and, second, by increasing the susceptibility of the pathogen to heat during normal cooking processes.     

Thermal Inactivation of Escherichia coli O157:H7 in Cooked Turkey Products

Survival of Escherichia coli O157:H7 when heated in commercial-type turkey products was determined. Thermal death times (TDT) were determined at 52–60°C in ground turkey with no additives, 3% fat; ground turkey with no additives, 11% fat; turkey ham batter, 11% fat; turkey frank batter, 17% fat; and turkey sausage batter, 31% fat. Mean D₅₂-values ranged from 44.9 to 116 min; D₅₅-values from 6.63 to 39.4 min; D₅₇-values from 2.20 to 11.7 min; D₆₀-values from 0.68 to 5.86 min. At all temperatures, survival of E. coli O157:H7 was greater in formulated products than in turkey meat with no additives. Greatest survival occurred in the turkey frank batter. Using our z-value data, times to provide a 5 D kill of E. coli O157:H7 in turkey franks cooked at 60°C, 65.6°C, or 71°C would be 26, 3.1, or 0.37 min, respectively.     

Recovery of heat-stressed E. coli O157:H7 from ground beef and survival of E. coli O157:H7 in refrigerated and frozen ground beef and in fermented sausage kept at 7°C and 22°C

A study was undertaken to obtain information on survival of Escherichia coli O157:H7 in ground beef subjected to heat treatment, refrigeration and freezing and on survival of E. coli O157:H7 in fermented sausage kept at 7°C and 22°C. For the challenge test, a mixture of E. coli O157:H7 strains (EH 321, EH 385, EH 302) was used and enumeration was performed on an isolation medium suitable for recovery of stressed organisms: modified Levine's eosin methylene blue agar (mEMB). Heat resistance of E. coli O157:H7 decreased after pre-incubation at a reduced temperature.Escherichia coli O157:H7 was more susceptible to heat inactivation after storage at 7°C and die-off was even more enhanced if cultures were frozen prior to heat inactivation. The enhanced reduction of the pathogen at 56°C after prior storage under refrigeration was confirmed in a test with inoculated ground beef.Escherichia coli O157:H7 was able to survive in ground beef at 7°C for 11 days and at −18°C for 35 days showing maximal one log reduction during the storage period. Thus, ground beef contaminated with E. coli O157:H7 will remain a hazard even if the ground beef is held at low or freezing temperatures. At both 7°C and 22°C, a gradual reduction of E. coli O157:H7 was noticed in fermented sausage over the 35 days storage period resulting in a 2 log decrease of the high inoculum (10⁶cfu 25 g⁻¹). For the low inoculum (10³cfu 25 g⁻¹) a 2·5 log reduction was obtained in 7 and 28 days storage at respectively 22 and 7°C. Application of good hygienic practices and implementation of HACCP in the beef industry are important tools in the control of E. coli O157:H7.     

The fate of Escherichia coli O157 : H7 in Myzithra,Anthotyros, and Manouri whey cheeses during storage at 2 and 12°C

Myzithra, Anthotyros and Manouri whey cheeses were inoculated the day after production withEscherichia coli O157 : H7 at concentrations of approx. 1·8×10⁶cfu g⁻¹, and stored at 2 and 12°C for 30 and 20 days, respectively. The pH of the whey cheeses decreased from an initial value of approx. 6·20 to 5·83 or 5·60 (Myzithra) 5·75 or 5·20 (Anthotyros) and 5·80 or 5·30 (Manouri) by the end of the corresponding storage periods at 2 and 12°C, respectively. Escherichia coli O157 : H7 populations in the whey cheeses at the end of the 12°C storage period, had grown with an increase of approx. 1·3 log₁₀cfu g⁻¹. E. coli O157 : H7 populations in whey cheeses at the end of the 2°C storage period did not grow and decreased, with an approx. 2·5 log₁₀cfu g⁻¹reduction. Results showed that E. coli O157 : H7 can grow at 12°C and survive at 2°C storage in Myzithra, Anthotyros and Manouri whey cheeses, and therefore post-manufacturing contamination with this pathogen must be avoided by employing hygienic control programmes such as HACCP.     

Evaluation of a novel enrichment procedure for the isolation of Escherichia coli O157 from naturally contaminated raw beef,lamb and mixed meat products

The aim of this study was to compare enrichment at 41·5°C for 24 h in a prototype enrichment broth developed by Oxoid, UK (sTSB), with enrichment at 37°C for 6 h in buffered peptone water supplemented with vancomycin, cefixime and cefsulodin (BPW-VCC) for isolation of Escherichia coli O157 from naturally contaminated raw beef, lamb and mixed meat products.A total of 120 meat samples from previous surveillance studies, 91 of which were previously positive and 29 previously negative for E. coli O157, were used for the study. All had been stored at −70°C for between 19 and 58 months since first examination. Of the 91 previously positive samples, 67 were positive after enrichment in sTSB and 42 were positive after enrichment in BPW-VCC;E.coli O157 was not recovered from 20 previously positive samples by either method. Of the 29 previously negative samples, three were positive by enrichment in sTSB and two were positive by enrichment in BPW-VCC;E.coli O157 was not recovered from 25 previously negative samples by either method.Of the 71 isolates obtained from the meat samples, only 53 had the same toxin genotype and plasmid profile as the E.coli O157 isolated from the sample originally. On four occasions, the strain isolated originally, the strain isolated from re-culture in BPW-VCC and the strain isolated from sTSB were all different. The only feasible explanation for these differences is that two or more different strains of E.coli O157 were present in the original sample. Results of strain typing ofE.coli O157 isolated from food samples, particularly during an outbreak investigation, should therefore be interpreted with caution.     

Fate of Escherichia coliO157: H7 in Chinese-style sausage subjected to different packaging and storage conditions

In this study, Chinese-style sausages were subjected to air, vacuum or nitrogen packaging and stored at either 5 or 25°C. The survival characteristics of Escherichia coli O157: H7 during the storage period were determined. Results revealed that, when stored at 5°C, the number of viable E coli O157: H7 in sausages decreased slowly as the storage period extended, regardless of packaging methods. E coli O157: H7 in sausages decreased from an initial population of ca 5·97 log CFU g⁻¹ to ca 4·42–4·81 log CFU g⁻¹ after 40 days of storage at 5°C. It was also found that viable cells of E coli O157: H7 declined more rapidly in sausage stored at 25°C than at 5°C. No viable E coli O157: H7 was detected in either vacuum-packed or nitrogen-packed sausage after 40 days of storage at 25°C. On the other hand, the population of E coli O157: H7 reduced to non-detectable levels in air-packed sausages after 20 days of storage. Refrigerated storage and vacuum or nitrogen packaging provided conditions that slowed down the death rate of E coli O157: H7 in sausage. Furthermore, it was noted that, among the curing agents tested, NaCl exerted the most significant lethal effect on E coli O157: H7 in sausage during the storage period. © 1998 Society of Chemical Industry.     

Fate of Escherichia coli O157: H7 in Chinese-Style Sausage During the Drying Step of the Manufacturing Process as Affected by the Drying Condition and Curing Agent

In this study, Chinese-style sausage was subjected to three different air-blast drying conditions commonly employed during the manufacturing process. The fate of Escherichia coli O157: H7 during the drying period was determined and compared. The effect of curing agents on the survival of E coli O157: H7 was also identified. Results showed that populations of E coli O157: H7 decreased ca 1.51 Log CFU g^-1 in sausage containing curing agents after a 6-h drying period at 50°C. However, the number of viable cells of E coli O157: H7 increased slightly in sausage without curing agents. When subjected to air-blast drying at 55°C for 6 h or at 55°C for 2·5 h and then 60°C for 3·5 h, a reduction in the number of viable cells of E coli O157: H7 was observed in sausage with or without curing agents. The reduction was more significant in sausage containing curing agents than in those without curing agents. No viable E coli O157: H7 was detected after 6 h of drying in samples containing curing agents, while the control samples still contained a viable E coli O157: H7 population of ca 2·65–4·36 Log CFU g^-1. After drying the sausage at 55°C for 4 h, inactivation of E coli O157: H7 increased in the presence of 30·00 g kg^-1 sodium chloride or 1·50 g kg^-1 sodium sorbate. On the other hand, the presence of 0·07–0·15 g kg^-1 sodium nitrite did not increase the inactivation of E coli O157: H7 compared to that in the control. © 1997 SCI.     

Thermal Inactivation of Escherichia coli O157:H7 in Strawberry Puree and Its Effect on Anthocyanins and Color

Raw whole strawberries, if contaminated with pathogens, such as Escherichia coli O157:H7, must be pasteurized prior to consumption. Therefore, the objective of this research was to investigate the thermal inactivation kinetics of E. coli O157:H7 in strawberry puree (SP), and evaluate the changes in anthocyanins and color, and the survival of yeasts and molds (YM) after thermal processing. Inoculated with a 5‐strain cocktail, fresh SP, with or without added sugar (20 and 40 °Brix), was heated at 50, 52, 54, 57.5, 60, and 62.5 °C to determine the thermal resistance of E. coli O157:H7. In raw SP, the average D‐values of E. coli O157:H7 were 909.1, 454.6, 212.8, 46.1, and 20.2 s at 50, 52, 54, 57.5, and 60 °C, respectively, with a z‐value of 5.9 °C. While linearly decreasing with temperature, the log D‐values of E. coli O157:H7 increased slightly with sugar concentration. The log degradation rates of anthocyanins increased linearly with temperature, but decreased slightly with sugar concentrations. These results suggest that sugar may provide some protection to both E. coli O157: H7 and anthocyanins in SP. The browning index was not affected by heating at 50 and 52 ºC at low sugar concentrations, but increased by an average of 1.28%, 2.21%, and 10.1% per min when SP was exposed to heating at 54, 57.5, and 60 °C, respectively. YM was also inactivated by heating. This study demonstrated that properly designed thermal processes can effectively inactivate E. coli O157:H7 and YM in contaminated SP, while minimizing the changes in anthocyanins and color.     

Heat resistance in Escherichia coli and its implications on ground beef cooking recommendations in Canada

This study assessed the adequacy of the current cooking recommendations in relation to heat resistant Escherichia coli by evaluating eight potentially heat resistant E. coli strains (four generic and four E. coli O157:H7) along with AW1.7. The D_60°C-values for these strains varied from 1.3 to 9.0 min, with J3 and AW1.7 being the least and most heat resistant strains, respectively. The D_60°C-values for E. coli 62 and 68 were similar and were not affected by growth medium, while the heat resistance of C37, J3, and AW1.7 varied with the growth medium. When heated in extra lean ground beef (100 g) in vacuum pouches, the mean D_54°C, D_57°C, and D_60°C-values were 44.8, 18.6, and 2.9 min for C37, 13.8, 6.9, and 0.9 min for J3, and 40.5, 9.1, and 6.1 min for AW1.7. Burger temperatures continued to rise after being removed from heat when the target temperature was reached, by 3–5°C, and resting of 1 min would result in a destruction of 133, 374 and 14 log C37, J3 and AW1.7. These findings along with the very low occurrence of heat resistant E. coli expected in ground beef show that cooking ground beef to 71°C should be adequate.     

Injury,Inhibition and Inactivation ofEscherichia coli O157:H7 by Potassium Sorbate and Sodium Nitrite as Affected by pH and Temperature

The effect of potassium sorbate (0–2 g litre⁻¹) and sodium nitrite (0–1 g litre⁻¹) on the growth of four strains of Escherichia coli O157: H7 in tryptic soya broth at various pH levels (pH 4·0–7·0 for sorbate, pH 5·0–8·0 for nitrite) were determined at 37°C and 4°C. Among the pH levels tested, sorbate and nitrite exhibited the highest antimicrobial activity at pH 4·0 and 5·0, respectively. At pH 5·0 and 37°C, the presence of 500 mg litre⁻¹ sorbate or 200 mg litre⁻¹ nitrite completely inhibited the growth of E coli O157: H7. While at higher pH levels, 2 g litre⁻¹ sorbate or 1 g litre⁻¹, nitrite, the highest concentration tested, did not show significant antimicrobial action against the test organisms. At 4°C and pH 5·0, the inoculated test organisms did not showed any significant growth in preservative-free control media. Different degree of inactivation and injury was observed when E coli O157: H7 strain 933 was stored in TSB (pH 5·0) containing 1 g litre⁻¹ sorbate or nitrite at 37°C. At 4°C, inactivation and injury of E coli O157: H7 cells was not observed in the medium containing sorbate or nitrite throughout the 24 h experimental period.     

The influence of lactoperoxidase,heat and low pH on survival of acid-adapted and non-adapted Escherichia coli O157:H7 in goat milk

In hot climates where quality of milk is difficult to control, a lactoperoxidase (LP) system can be applied in combination with conventional preservation treatments at sub-lethal levels to inhibit pathogenic microbes. This study investigated the effect of combined heat treatments (55 °C, 60 °C and 72 °C) and milk acidification (pH 5.0) on survival of acid-adapted and non-adapted Escherichia coli O157:H7 strains UP10 and 1062 in activated LP goat milk. Heat treatment at 72 °C eliminated E. coli O157:H7. Acid-adapted strains UP10 and 1062 cells showed resistance to combined LP and heat at 60 °C in fresh milk. The inhibition of acid-adapted and non-adapted E. coli O157:H7 in milk following combined LP-activation, heat (60 °C) and milk acidification (pH 5.0) suggests that these treatments can be applied to reduce E. coli O157:H7 cells in milk when they occur at low numbers (<5 log₁₀ cfu mL^?1) but does not eliminate E. coli O157:H7 to produce a safe product.     

Lethality of heat to Escherichia coli O157:H7: D- and z-value determinations in turkey,lamb and pork

Thermal inactivation of a four-strain mixture of E. coli O157:H7 was determined in lean ground turkey, lamb and pork. Inoculated meat was packaged in bags completely immersed in a circulating water bath and held at 55, 57.5, 60, 62.5, and 65°C for predetermined lengths of time. The surviving cell population was enumerated by spiral plating meat samples on tryptic soy agar overlaid with Sorbitol MacConkey agar. D-values, determined by linear regression, in turkey were 11.51, 3.59, 1.89, 0.81 and 0.29 min at 55, 57.5, 60, 62.5 and 65°C, respectively (z=6.5°C). When a survival model was fitted to the non-linear survival curves, D-values in turkey ranged from 11.26 min at 55°C to 0.23 min at 65°C (z=6°C). When the E. coli O157:H7 four-strain cocktail was heated in ground pork or lamb, D-values calculated by both approaches were similar at all temperatures. Thermal-death-times from this study will assist the retail food industry to design cooking regimes that ensure safety of ground muscle foods contaminated with E. coli O157:H7.     

Growth patterns of Escherichia coli O157:H7 in ground beef treated with nisin,chelators, organic acids and their combinations immobilized in calcium alginate gels

The effects of antimicrobial substances including nisin, acetic acid, lactic acid, potassium sorbate and chelators (disodium ethylenediamine tetraacetic acid [EDTA] and sodium hexametaphosphate [HMP]), alone or in combination and, with or without immobilization in calcium alginate gels, on the growth of Escherichia coli O157:H7 in ground beef were investigated. Results showed that acetic acid and potassium sorbate could inhibit the growth of E. coli O157:H7 effectively at 10°C and at 30°C. Both EDTA and HMP did not halt the growth of E. coli O157:H7. In an antimicrobial system immobilized with calcium alginate, most of the antimicrobials could not inhibit the growth of E. coli O157:H7 in ground beef at 10°C and at 30°C, with the exception of acetic acid and lactic acid. Immobilization did not enhance the effectiveness of acetic acid against E. coli O157:H7 in ground beef at 10°C and at 30°C (P>0.05) but it did enhance the effectiveness of lactic acid at 10°C. In a system combining different antimicrobials, treatment with nisin /EDTA or nisin/potassium sorbate at 10°C revealed a significantly lower population change of E. coli O157:H7 compared to samples treated with nisin, EDTA or potassium sorbate alone. The use of calcium alginate immobilization further enhanced the effectiveness of the combination system of nisin/EDTA, nisin/acetic acid and nisin/potassium sorbate on the growth of E. coli O157:H7 in ground beef at 10°C but it was not effective at 30°C.     

The effect of a competitive microflora,pH and temperature on the growth kinetics of Escherichia coli O157:H7

Growth curves were generated for Escherichia coli O157:H7 in brain–heart infusion broth incubated at 37 or 15°C in the presence of individual and combinations of competing microflora. Broths were inoculated withE. coli O157:H7 (log₁₀3·00 cfu ml⁻¹) and competitors (log₁₀4·00 cfu ml⁻¹) and the initial pH of the broth was either neutral (7·0) or adjusted to 5·8 and then sequentially reduced to 4·8 over 10 h to simulate fermentation conditions. Growth curves were also generated for the competitors in these cultures, including Pseudomonas fragi, Hafnia alvei, Pediococcus acidilactici (pepperoni starter culture) and Brochothrix thermosphacta . Gompertz equations were fitted to the data and growth kinetics including lag phase duration, exponential growth rates and maximum population densities (MPD) calculated. In pure culture, the growth parameters for E. coli O157:H7 in neutral pH broths were significantly different from those recorded in simulated fermentation broths (P<0·05). The presence of competitors in the broth also had a significant effect on the growth kinetics of the pathogen. H. alvei significantly inhibited the growth (lag phase, growth rate and MPD) of E. coli O157:H7 at 37°C, neutral pH and outgrew the pathogen under these conditions. In neutral pH cultures, two other competitors, B. thermosphacta and P. acidilactici also inhibited the lag phase of the pathogen but had no effect on the other growth parameters. In simulated fermentation broths, the growth rate of E. coli O157:H7 was consistently slower and the MPD lower in the presence of a competitive microflora than when grown individually. At 15°C, only one competitor, P. fragi significantly inhibited the lag phase of the pathogen. The implications of these findings for food safety are discussed.     

The effects of washing and chlorine dioxide gas on survival and attachment of Escherichia coli O157: H7 to green pepper surfaces

The effects of washing and chlorine dioxide (ClO₂) gas treatment on survivability and attachment of Escherichia coli O157: H7 C7927 to uninjured and injured green pepper surfaces were investigated using scanning electron microscopy and colony enumeration methods. Escherichia coli O157: H7 preferentially attached to coarse and porous intact surfaces and injured surfaces. The bacterial attachment to injured green pepper surfaces may be determined mainly by the hydrophilic properties of the injured surfaces and might not be assisted by the exocellular polymers of the bacteria. Injuries to the wax layer, the cuticle and underlying tissues increased bacterial adhesion, growth, and resistance to washing and disinfecting treatments. No significant growth of E. coli O157: H7 was found on uninjured surfaces after inoculation and incubation for 24 h at 37°C, whereas significant growth and multiplication were found on injured surfaces (P<0·05). ClO₂gas treatment was more effective as a disinfecting method than water washing. Using a membrane-plating method for resuscitation and enumeration of ClO₂-treated E. coli O157: H7 on surface-injured green peppers, 3·03±0·02 and 6·45 ±0·02 log reductions were obtained after treatments by 0·62 and 1·24 mg l⁻¹ClO₂, respectively, for 30 min at 22°C and 90–95% relative humidity. In contrast, water washing achieved log reductions of 1·5±0·05–1·67±0·10 on injured surfaces and 2·44±0·04 on uninjured surfaces.     

Increased acid tolerance of Escherichia coli O157:H7 as affected by acid adaptation time and conditions of acid challenge

Three strains of Escherichia coli O157:H7, ATCC 43889, 43895 and 933 were subjected to acid adaptation in Tryptic Soy Broth (pH 5.0) for 1, 2, 3, 4 and 6 h. Acid tolerance of the adapted cells was determined in subsequent acid challenge at pH 3.0, 4.0 and 5.0 (acidified with HCl) and in the presence of lactic, acetic or propionic acid. It was found that acid adaptation increased acid tolerance of the E. coli O157:H7 strains tested and was dependent on strain, acid adaptation time and pH of the challenge. Among the acid adaptation times tested, 4 h of adaptation enabled the test organism, regardless of strains, to exhibit the most pronounced acid adaptation response which was most marked at pH 3.0, followed by pH 4.0 and 5.0. The extent of increased acid tolerance varied with the strains of E. coli O157:H7 and challenge of organic acid. The 4-h acid-adapted cells of ATCC 43889 and 933 showed an increase in acid tolerance in the presence of lactic, acetic and propionic acids. An increase in tolerance was also noted with ATCC 43895 in the presence of acetic and lactic acid, but not in the presence of propionic acid.