Inactivation of Shiga toxin-producing and nonpathogenic Escherichia coli in non-intact steaks cooked in a radio frequency oven

This study evaluated the thermal inactivation of Shiga toxin-producing Escherichia coli (O157:H7, O26:H11 and O111) (STEC) and non-pathogenic E. coli in non-intact beefsteaks (NIBS) cooked by radio frequency (RF). Blade tenderized steaks were inoculated with nalidixic (Nal) acid resistant E. coli strains, vacuum packaged in thermal pouches and cooked using pre-determined cooking times to 60 °C (rare) or 65 °C (medium-rare) inside a RF oven. Log reduction ranged from 0.99, 3.08, 2.85 and 5.0 for O157:H7, O26:H11, O111 and non-pathogenic E. coli respectively at 60 °C and a 5.0 log reduction at 65 °C for all strains. Non-pathogenic E. coli strains selected for the present study did not behave similar to the pathogenic strains being significantly more sensitive; therefore, they were not considered for testing at 63 °C. A second part of the study focused on the extent of thermal inactivation of STEC when NIBS were cooked to 63 °C (minimum safe cooking temperature recommended by USDA-FSIS). There was a 5.0 log reduction for E. coli O157:H7 and E. coli O111; but not for E. coli O26:H11. The results indicated that cooking steaks to 65 °C with a holding time at room temperature of 5 min before refrigeration would be enough to reduce numbers of E. coli O157:H7, O26:H11 and O111 using RF. The cooking protocol developed on the present study, has a practical relevance for the industry since the experiments were carried on a pilot-scale RF oven and also pathogens were tested under realistic processing conditions.     

Ripening conditions: A tool for the control of Listeria monocytogenes in uncooked pressed type cheese

The effect of ripening conditions (two temperatures, 9 °C and 13 °C, and two relative humidities 93% and 97%) on the growth of Listeria monocytogenes and other microbial populations was evaluated in the cores and rinds of uncooked pressed type cheeses prepared with pasteurised milk and inoculated either with Streptococcus thermophilus only or with an anti-listerial consortium. Regardless of temperature and relative humidity (RH), inhibition by the anti-listerial consortium was stronger in the cheese cores than in the rinds. Temperature had no significant effect on L. monocytogenes counts in cores or rinds. However, at the beginning of ripening in the consortium cheese, L. monocytogenes growth was more strongly inhibited at 13 °C than at 9 °C. Regardless of inoculation type and ripening temperature, counts of L. monocytogenes were significantly lower in the cores and rinds of cheeses ripened at 93% than at 97% RH. Lactobacilli counts were higher at 13 °C than at 9 °C and at 93% than at 97% RH. Lactobacilli can help to inhibit L. monocytogenes by catabolising galactose and producing lactate. Further investigations will be needed to evaluate the effect of ripening at 13 °C and 93% RH on the sensorial properties of cheese.     

Adherence of cold-adapted Escherichia coli O157:H7 to stainless steel and glass surfaces

The effects of previous cold-induced cell elongation on adherence of Escherichia coli O157:H7 to glass slides and stainless steel surfaces was evaluated at 4 °C for ≤48 h. Planktonic E. coli O157:H7 with and without cold adaptation were prepared at 15 and 37 °C, respectively, and planktonic E. coli O157:H7 containing elongated (>4 ≤ 10 μm) and filamentous (>10 μm) cells were prepared at 6 °C. Despite morphological differences in planktonic E. coli O157:H7 preparations, all three cell types attached to a greater extent to glass than to the stainless steel surfaces. E. coli O157:H7 cells adapted to growth at 15 °C attached better to both glass and stainless steel surfaces (3.2 and 2.6 log cfu/cm², respectively) than cells of the other treatments at ≥24 h. Cells adapted at 6 °C attached to glass slides and stainless steel coupons at levels of 3.0 and 1.8 log cfu/cm², respectively, while E. coli O157:H7 cells grown at 37 °C attached to these surfaces at levels of 2.0 and 1.7 log cfu/cm², respectively. No further attachment of cells from any of the treatments was noted between 24 and 48 h at 4 °C. These results suggest that E. coli O157:H7 cells adapted at 6 °C–15 °C have greater potential to attach to food contact surfaces than those grown at higher temperature. The enhanced biofilm-forming ability of 6 °C or 15 °C-adapted, elongated and filamentous E. coli O157:H7 cells did not appear to be related to the greater entanglement of longer cells within biofilm matrices.     

The influence of acid stress on the growth of Listeria monocytogenes and Escherichia coli O157:H7 on cooked ham

Acid solutions are increasingly being used for decontaminating meat surfaces. On the surfaces of acid-treated meat, the population of microorganism is reduced due to the low pH of acids, and the subsequent growth of the microorganism is reduced due to the residual acids on meat surfaces. Microbial cells on meat surfaces subjected to acid treatments may cross-contaminate untreated meat surfaces, e.g., microorganisms on the surfaces of acid-treated cooked ham cross contaminate the untreated surfaces during slicing. The objective of this study was to examine this scenario in determining the subsequent growth of acid-treated Listeria monocytogenes and Escherichia coli O157:H7 on the surfaces of untreated meat. Cells of multiple-strain L. monocytogenes or E. coli O157:H7 were exposed to HCl solutions of pH 3, 4, or 5 and deionized water at room temperature for 24 h. The acid or deionized water-treated cells were inoculated separately onto cooked ham. Samples inoculated with L. monocytogenes were stored at 4 and 8 °C and samples inoculated with E. coli O157:H7 were stored at 10 and 12 °C. Populations of the pathogens on ham were enumerated during storage, and the lag phase durations (LPD, h) and growth rates (GR, log CFU/h) of the pathogens were determined. The populations of L. monocytogenes and E. coli O157:H7 in pH 5, 4, and 3 solutions were 1.2–3.1 and 0.6–2.4 log CFU/ml, respectively, lower than those in deionized water, indicating an increased acid stress on both microorganisms at lower pHs. L. monocytogenes subjected to pH 3 and pH 4 stresses and E. coli O157:H7 subjected to pH 3 stress exhibited significantly (p < 0.05) extended LPDs and reduced GRs on cooked ham. The growth of L. monocytogenes on ham was more readily reduced by acid stress than that of E. coli O157:H7. This study showed that acid treatments reduced the viability of L. monocytogenes and E. coli O157:H7and the acid stress reduced their subsequent growth ability on untreated ham. Therefore, cross-contamination of L. monocytogenes or E. coli O157:H7 cells from acid-treated meat surfaces onto untreated meat surfaces may not impose increased risk to the product.     

Pulsed electric field inactivation of E. coli O157:H7 and non-pathogenic surrogate E. coli in strawberry juice as influenced by sodium benzoate,potassium sorbate,and citric acid

Current FDA regulations require that juice processors achieve a 5 log CFU/ml reduction of a target pathogen prior to distributing products. Whereas thermal pasteurization reduces the sensory characteristics of juice, pulsed electric field (PEF) treatments can be conducted at lower temperatures and may preserve sensory characteristics.Escherichia coli O157:H7 (ATCC 43895) and a non-pathogenic E. coli (ATCC 35218), respectively, were inoculated into single-strength strawberry juice with or without 750 ppm sodium benzoate (SB), 350 ppm potassium sorbate (PS), and 2.7% citric acid (CA). Juice was treated at outlet temperatures of 45, 50 and 55 °C at a field strength of 18.6 kV/cm for 150 μs with a laboratory-scale PEF unit. Inactivation of surrogate E. coli at 45, 50, and 55 °C were 2.86, 3.12, and 3.79 log CFU/ml, respectively, in plain juice (pH 3.4), and 2.75, 3.52, and 5.11 with the addition of benzoic and sorbic acids (pH 3.5). Inactivation of E. coli O157:H7 under the same conditions were 3.09, 4.08, and 4.71 log CFU/ml, respectively, and 2.27, 3.29, and 5.40 with antimicrobials. E. coli O157:H7 in juice with antimicrobials and 2.7% CA (pH 2.7) treated with PEF was reduced by 2.60, 4.32 and 6.95 log CFU/ml at 45, 50 and 55 °C while the surrogate E. coli decreased by 3.54, 5.69, and 7.13 log under the same conditions. When juice (pH 2.7) was held for 6 h without PEF treatment, higher numbers of E. coli 35218 (7.17 log CFU/ml) were inactivated than of acid-resistant E. coli O157:H7 (3.89 log). Slightly greater PEF inactivation of E. coli O157:H7 than of the surrogate bacterium indicates that E. coli ATCC 35218 provides a margin of safety when used as a surrogate for O157:H7 in plain strawberry juice or in juice + SB + PS at 45–50 °C, or with SB + PS and CA at 55 °C.     

Antimicrobial activity of ascorbic acid alone or in combination with lactic acid on Escherichia coli O157:H7 in laboratory medium and carrot juice

The antimicrobial effects of ascorbic acid alone and in combination with lactic acid, against Escherichia coli O157:H7 in Brain Heart Infusion (BHI) broth and in carrot juice as a food model was investigated. In control samples, E. coli O157:H7 continued to growth from 3.98 ± 0.2 log CFU and reached to 8.88 ± 0.1 log₁₀ CFU after 8 h at 37° C; however, bacterial population was undetectable level in BHI in the presence of 0.4% ascorbic acid and 0.2% lactic acid. In carrot juice, E. coli O157:H7 continue to grow from initial population count of 4.41 ± 0.9 log₁₀ CFU to 8.75 ± 0.07 log₁₀ CFU at 37 °C for 24 h. Similarly, the bacteria population was undetectable when 0.2 or 0.4% ascorbic acid and 0.2% lactic acid applied. Our findings suggest the application of ascorbic acid, in combination with lactic acid, may have potential as preservative to inhibit the growth of E. coli O157:H7 in food.     

Sublethal injury and recovery of Escherichia coli O157:H7 and K-12 after exposure to lactic acid

Lactic acid can induce sublethal injury of Escherichia coli. When conditions become favorable, injured E. coli can recover physiological function and fully virulence, which is a great concern in the field of food safety. The injury and recovery of E. coli O157:H7 and K-12 by lactic acid were investigated in this study. Sublethally injured E. coli cells widely persisted after a 60-min exposure to lactic acid with different pH values (E. coli O157:H7: pH 3.0–4.6, E. coli K-12: pH 3.4–5.0). The sublethally injured ratio of E. coli O157:H7 and K-12 by lactic acid decreased as incubation temperature decreasing. Both sublethally injured E. coli O157:H7 and K-12 induced by lactic acid could be completely recovered in trypticase soy broth at 37 °C within 60 min. For both E. coli O157:H7 and K-12, sodium pyruvate, Tween 80, or certain cations (Mn, Fe, or Zn) could significantly increase the recovery ratio. The recovery ratios of injured E. coli K-12 and O157:H7 were only 74.2% and 92.6% after 80 min incubation in minA, respectively. But they can both be completely recovered within 80 min in minA with sodium pyruvate, with Tween 80, or with cations (Mn, Fe, or Zn). But Mg and Ca cations did not affect the recovery time. The understanding of injury and recovery of E. coli could contribute to develop effective decontaminating treatment by lactic acid, and develop techniques for detecting sublethally injured E. coli cells.     

How safe is European Internet cheese? A purchase and microbiological investigation

The suitability for consumers of a variety of raw milk cheeses purchased over the Internet was investigated in terms of packaging, labelling, physicochemical parameters and microbiological safety. 108 purchases from seven European countries were examined. The prevalences of Salmonella spp., Listeria monocytogenes, Escherichia coli and coagulase positive staphylococci (SA) were determined. All 108 samples were described on websites as raw milk cheeses and thereby qualified for this study. However, after delivery it was noted that 4.6% (5/108) of cheeses were labelled to be manufactured from heat-treated or pasteurized milk. Delivery duration ranged from 24 h to six days. Immediately upon receipt cheese temperatures were observed to range between 5 and 23 °C, whereas in 61.5% of all cases the temperature was higher than 15 °C. Cheese labelling was examined in respect of EC Guideline 2000/13 and Regulation No. 853/2004. Only 17.6% (19/108) of cheeses were properly labelled and fulfilled all European guideline requirements. In 50.9%, 38.8%, 46.3% and 39.8% of all cases (i) specific storage requirements, (ii) name and address of the manufacturer/packer or seller, (iii) net weight and (iv) shelf life (use by date), were missing. Even the labelling information “made from raw milk” was not apparent on 36% of all cheese items delivered. The major foodborne pathogen L. monocytogenes was detected in 1.9% of all samples, one of which had counts of 9.5 × 10³ CFU/g. None of the 108 investigated cheeses showed a pH ≤ 5.0 and a_w value ≤0.94 which are the limiting values for growth of L. monocytogenes. For two samples (0.9%) and 11 samples (10.2%) the pH and the a_w value was ≤4.4 or ≤0.92, respectively at least at one of three stipulated time points (receipt, mid-shelf-life and at expiry). Salmonella spp. could not be detected in any of the samples. E. coli and SA could be detected in a total of 29.6% (≥10 CFU/g; 32/108) and 8.3% (≥100 CFU/g; 9/108) of samples, respectively, indicating poor conditions of hygiene. Results reveal that labelling and hygiene concerns about the safety of Internet purchased cheeses in Europe are justified.     

Effect of organic acids,hydrogen peroxide and mild heat on inactivation of Escherichia coli O157:H7 on baby spinach

Minimally processed baby spinach contaminated with Escherichia coli O157:H7 has been associated with multiple outbreaks of foodborne illnesses recently. Chlorinated water is widely used to wash vegetables commercially, but this washing procedure has limited efficacy and can lead to the formation of carcinogenic substances. This study was conducted to determine the effects of organic acids and hydrogen peroxide alone and in binary combinations with or without mild heat (40 and 50 °C) on the inactivation of Escherichia coli O157:H7 on baby spinach. Baby spinach leaves were dip-inoculated with E. coli O157:H7 to a level of 6 log CFU/g and stored at 4 °C for 24 h before treatment. Individual washing solutions (1% and 2% lactic acid [LA], citric acid [CA], malic acid [MA], tartaric acid [TA], acetic acid [AA], hydrogen peroxide [H₂O₂] as well as binary combinations of LA, CA, MA and H₂O₂ at final concentrations of 1% were used to decontaminate spinach leaves at 22, 40 or 50 °C for 2–5 min to test their efficacy in reducing E. coli O157:H7. Chlorinated water (200 ppm free chlorine) decreased the population of E. coli O157:H7 on baby spinach by only 1.2–1.6 log CFU/g, which was not significantly different from DI water washing. Washing with 1% LA at 40 °C for 5 min was the most effective treatment achieving a 2.7 log reduction of E. coli O157:H7 which is significantly higher than chlorine washing. Washing with LA + CA or LA + HP at 40 °C for 5 min was equally effective against E. coli O157:H7, resulting in a 2.7 log reduction of E. coli O157:H7. The application of mild heat significantly enhanced the efficacy of washing solutions on the inactivation of E. coli O157:H7. There was, however, no significant difference between treatments at 40 °C for 5 min and 50 °C for 2 min. The results suggested that the use of organic acids in combination with mild heat can be a potential intervention to control E. coli O157:H7 on spinach.     

Antimicrobial activity of essential oils against Escherichia coli O157:H7 in organic soil

Soil can be a significant source of preharvest contamination of produce by pathogens. Demand for natural pesticides such as essential oils for organic farming continues to increase. We examined the antimicrobial activity of several essential oils against Escherichia coli O157:H7 in soil. Two essential oils (cinnamaldehyde and eugenol), two bio-pesticides (Ecotrol and Sporan) containing essential oils, and an organic acid (acetic acid) at 0.5%, 1.0%, 1.5% and 2.0%, were mixed with organic sandy soil and inoculated with five different strains of E. coli O157:H7. Soils were incubated at room temperature (22 °C) and samples obtained at 1, 7 and 28 days were enumerated to determine survival. E. coli O157:H7 populations in soil were reduced by up to 5 log cfu/g after 24 h incubation at room temperature with 2% cinnamanaldehyde, Ecotrol, Sporan or vinegar. Reduction in E. coli O157:H7 by eugenol was not significantly different from control. Overall, E. coli O157:H7 strain 4406 was the most sensitive of all the five strains tested and cinnamaldehyde was superior to other treatments in reducing E. coli O157:H7 in soil. In general, increases in essential oil concentrations corresponded to reduced survival of E. coli O157:H7 with all oils used in this study. The results suggest that oils can reduce potential contamination of fresh organic produce inadvertently contaminated by soil.     

Efficacy of near neutral and alkaline pH electrolyzed oxidizing waters to control Escherichia coli O157:H7 and Salmonella Typhimurium DT 104 from beef hides

In this study efficacy of near neutral and alkaline pH electrolyzed oxidizing waters to reduce aerobic plate counts (AC) and Enterobacteriaceae (EC) from uninoculated fresh cattle hides and Escherichia coli O157:H7 and Salmonella Typhimurium DT 104 from inoculated hides were determined. Fresh hides were cut in to 15 by 20 cm pieces and subjected to a total of eight different treatment solutions; near neutral pH EO water (NEW-pH 6.5 at room temperature, 150 mg/L available chlorine), alkaline pH EO water (AEO-pH 11.6 at room temperature), hot alkaline pH EO water at 43 °C (HAEO-pH 11.60), alkaline pH EO water spray followed by 150 mg/L available chlorine containing near neutral pH EO water spray (A-NEW-both at room temperature), Blitz™ (PAA, pH 3.02 at room temperature), 5% lactic acid (LA, pH 2.04 at room temperature), deionized water (W) and no treatment (Control). For each treatment, 60 ml treatment solution was sprayed on hide using a hand held sprayer. Similar treatment protocol was employed to treat hide pieces inoculated with E. coli O157:H7 and S. Typhimurium DT 104. Five percent lactic acid spray treatment was found to be the most effective treatment and achieved 2.77, 2.74, 2.75 and 2.98 log CFU/cm² of AC, EC, E. coli O157:H7 and S. Typhimurium DT 104 reductions, respectively. All EO water treatments were equally effective in reducing all target microorganisms, except E. coli O157:H7. HAEO and A-NEW treatments yielded significant reduction of E. coli O157:H7 compared to other EO water treatments. These results indicate that various EO water treatments could become viable options to reduce pathogens on hide during slaughter.     

Antimicrobial activity of malic acid against Listeria monocytogenes,Salmonella Enteritidis and Escherichia coli O157:H7 in apple,pear and melon juices

Minimal inhibitory (MIC) and minimal bactericidal (MBC) concentrations of malic acid against Listeria monocytogenes, Salmonella Enteritidis and Escherichia coli O157:H7 inoculated in apple, pear and melon juices stored at 5, 20 and 35 °C were evaluated. MICs and MBCs against L. monocytogenes, S. Enteritidis and E. coli O157:H7 were significantly affected by storage temperature, juice characteristics and type of microorganism. Malic acid was more effective at 35 and 20 °C than at 5 °C in all studied fruit juices. E. coli O157:H7 was more resistant to malic acid than S. Enteritidis and L. monocytogenes. Apple, pear and melon juices without malic acid were inhibitory to E. coli O157:H7, S. Enteritidis and L. monocytogenes at 5 °C, whereas, MBCs of 1.5% (v/v) of malic acid in apple and pear juices, and 2% (v/v) in melon juice at 5 °C were needed to reduce E. coli O157:H7, those concentrations being higher than those required to reduce S. Enteritidis and L. monocytogenes in those fruit juices. In addition, concentrations of 2%, 2.5% and 2.5% (v/v) of malic acid added to apple, pear and melon juices, respectively, were required to inactivate the three pathogens by more than 5 log cycles after 24 h of storage at 5 °C. Transmission electron microscopy showed that malic acid produced damage in the cell cytoplasm of pathogens without apparent changes in the cell membrane.     

Characteristics of coliphage ECP4 and potential use as a sanitizing agent for biocontrol of Escherichia coli O157:H7

Escherichia coli O157:H7 is an important pathogenic bacterium to humans because it produces various toxins, such as shiga-toxin. Coliphage ECP4, which belongs to the Siphoviridae family, was isolated from bovine feces to test its utility as a potential agent for the biocontrol of E. coli O157:H7. The burst size of coliphage ECP4 was about 80 PFU/cell, after a latent period of 30–35 min. Coliphage ECP4 was susceptible to temperatures above 70 °C; however, its stability was slightly reduced to 1–2 log PFU/ml after 30 min in 70% ethanol. In addition, the shiga toxin gene was not detected on coliphage ECP4. Coliphage ECP4 inhibited the growth of E. coli O157:H7 in vegetable juice, and was not detected in cabbage after 5 h. When coliphage ECP4 was applied to biofilm-formed E. coli O157:H7, E. coli O157:H7 was efficiently reduced. The newly identified coliphage ECP4 might effectively reduce E. coli O157:H7 or its biofilmed-form. Therefore, the coliphage ECP4 might be an efficient sanitizer for fresh produce contaminated with E. coli O157:H7 in the biofilm environment.     

Evaluation of a novel antimicrobial solution and its potential for control Escherichia coli O157:H7, non-O157:H7 shiga toxin-producing E. coli,Salmonella spp., and Listeria monocytogenes on beef

The goal of this study was to evaluate the efficacy of a novel antimicrobial solution made with chitosan, lauric arginate ester, and organic acids on Escherichia coli O157:H7, Salmonella spp., Listeria monocytogenes, and non-O157 shiga toxin-producing E. coli cocktails and to test its potential to be used as a marinade for raw beef. Fresh beef top round steaks were surface-inoculated with the pathogen cocktails at approximately 2.5 or 4.5 Log CFU/cm², marinated with the antimicrobial solution (AMS), and then stored at 4 °C for 6, 24, and 48 h. Three commercially available marinades were used for comparison. Results revealed that AMS had the most antimicrobial effect regardless of the type or inoculation level of pathogens (P < 0.05). After 6 h, the AMS marination reduced all pathogens to levels below the limit of detection (<1 Log CFU/cm²), resulting in a 3.5 Log CFU/cm² reduction. When AMS was diluted with autoclaved distilled water by 5 times (AMS 1:5) or 10 times (AMS 1:10), its antimicrobial efficacy was impacted by marination time, the inoculated pathogens, and the inoculation levels. This study demonstrates that the developed antimicrobial solution has a great potential to be used during marination by consumers to ensure better food safety.     

Prevalence of foodborne pathogens in Turkish Van otlu (Herb) cheese

The survey was conducted on 50 unripened Van otlu cheese samples obtained in Van and Hakkari markets at retail level to determine the microbial characteristics with special emphasis on Staphylococcus aureus, Escherichia coli, E. coli O157:H7 and Salmonella spp. The results revealed that S. aureus and E. coli were present in extremely high numbers, with a mean 6.10 and 3.68 log CFU/g, respectively. S. aureus was found in all samples ranging from 2.48 to 7.15 log CFU/g and was present in more than 5.0 × 10⁵ CFU/g in 54% of the samples whereas E. coli was found in 62% of the samples. None of the samples contained E. coli O157:H7; but 3 of the 50 samples had Salmonella spp. The results indicate that Van otlu cheese presents a potential hazard for public health; and the necessary precaution will have to be taken to improve the sanitary practices and cheese manufacturing technique.     

Inhibitory effect of Cinnamomum cassia oil on non-O157 Shiga toxin-producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) have caused numerous foodborne outbreaks. Compared with the most well-known STEC E. coli O157:H7, importance of non-O157 STEC has been underestimated and they have gained far less attention till increasing outbreaks recently. Using natural plant materials as antimicrobial agents is a heated area. Therefore in this study, Cinnamomum cassia, a widely used spice in cuisine, was tested for its antibacterial efficacy on CDC “top six” non-O157 STECs including O26, O45, O103, O111, O121, O145. Gas chromatography-mass spectrometry analysis showed that the major component of C. cassia oil was cinnamaldehyde (59.96%). The disk diffusion assay indicated that 20 μL 4% (v/v) C. cassia oil per disk resulted in inhibition zones of 15.0 mm, 18.5 mm, 15.7 mm, 19.3 mm, 18.8 mm, and 25.3 mm for O26:H11, O45:NM, O103:H2, O111:H2, O121:H19, and O145:NT, respectively. Minimum inhibitory concentration for all tested non-O157 STECs were 0.025% (v/v). Minimum bactericidal concentration was strain dependent, which was 0.05% (v/v) for O26:H11, O121:H19, O145:NT, while 0.1% (v/v) for O45:NM, O103:H2 and O111:H2. Growth kinetics showed that at the low inoculation of approximate 2.5 × 10⁵ CFU/mL, C. cassia oil at the concentration of 0.01875% (v/v) completely inhibited the growth of O26:H11 and O145:NT for at least 24 h, and increased the duration of lag phase of O45:NM, O103:H2, O111:H2, O121:H19 by18, 12, 6, and 16 h, respectively. Including 0.025% (v/v) C. cassia oil completely inhibited the growth of all tested non-O157 STECs for at least 24 h. At high inoculation of 5 × 10⁶ CFU/mL, inhibition effect of C. cassia oil decreased. Death curve showed that including as low as 0.05% (v/v) C. cassia oil could kill non-O157 STECs. 0.1% (v/v) C. cassia oil showed bactericidal effects on all tested non-O157 STECs within 15 min. C. cassia oil at the concentration of 0.15% (v/v) killed all O26:H11, O121:H19 and O145:NT within 30 min, while O45:NM, O103:H2 and O111:H2 at 120, 60, and 60 min, respectively. In conclusion, C. cassia oil can effectively inhibit the growth of non-O157 STECs at concentration as low as 0.025% (v/v). Our data suggest that C. cassia oil has the potential to be used as a natural antibacterial agent in food industry.     

Antimicrobial efficacy of grape seed extract against Escherichia coli O157:H7 growth,motility and Shiga toxin production

Escherichia coli O157:H7 produces Shiga toxin (Stx) which is heat stable and causes Hemolytic Uremic Syndrome (HUS), a serious disease associated with bloody diarrhea and even death. To ensure food safety, both live E. coli O157:H7 and its toxin production in food products need to be controlled. Natural ingredients with inhibitory effects on E. coli O157:H7 growth and toxin production are top choices of antimicrobials for the food industry. The objectives of this study were to evaluate efficacy of grape seed extract (GSE) against the growth, swimming motility and Stx production of E. coli O157:H7. The disc diffusion assay indicated that 3.2 mg GSE per disc resulted in an inhibition zone of 14.8 ± 0.21 mm. The minimal inhibitory concentration of GSE against E. coli O157: H7 was 4.0 mg/ml. At high inoculation level (1 × 10⁷ CFU/ml), including GSE at 0.25–2.0 mg/ml reduced Stx production without inhibiting E. coli O157:H7 growth. At 5 × 10⁵ CFU/ml inoculation level, 2.0 and 4.0 mg/ml GSE effectively inhibited the growth of E. coli O157:H7 for at least 72 h, however, a low level of GSE (0.125–1.0 mg/ml) enhanced E. coli O157:H7 growth and Stx2 production. At 4 mg/ml, GSE completely abolished Stx2 production in addition to it bactericidal effect against E. coli O157:H7. In addition, GSE at concentration as low as 0.125% blocked the swimming motility, which is important for E. coli O157:H7 surface adherence. In conclusion, GSE is effective in inhibiting the motility of E. coli O157:H7, GSE shows potential to be used as a natural antimicrobial to control E. coli O157:H7.     

Inactivation of Escherichia coli O157:H7 in vitro and on the surface of spinach leaves by biobased antimicrobial surfactants

This study was conducted to evaluate the effect of biosurfactants on the populations of Escherichia coli O157:H7 in suspension and on spinach leaves. Eight surfactants including four soybean oil-based biosurfactants, sodium dodecyl sulfate (SDS), polyoxyethylene sorbitan monooleate (Tween 80), sophorolipid (SO) and thiamine dilauryl sulfate (TDS) at concentrations of 0.1%, 0.5% and 1.0% were tested in bacterial suspension, and the most effective biosurfactants were applied on spinach leaves. Results showed that the soybean oil-based biosurfactants, SDS or Tween 80 did not significantly affect E. coli O157:H7 populations. SO and TDS at concentrations of 1.0% were effective in reducing E. coli O157:H7 populations in bacterial suspension. E. coli O157:H7 with an initial population of 7.1 log CFU/mL was not detectable (detection limit: 1 log CFU/mL) after 1 min in 1.0% TDS or after 2 h in 1.0% SO. On spinach leaves, SO at 1% did not significantly affect E. coli when compared to a water wash during 7 days post-treatment storage at 4 °C. However, TDS (1.0%) wash was as effective as 200 ppm chlorine in reducing population of spot inoculated E. coli O157:H7, achieving 3.1 and 2.7 log CFU/per leaf at day 0, and 1.4 and 1.9 log CFU/leaf at day 7 when compared with a water wash. No apparent change in spinach visual quality was observed. None of treatments caused changes in visual quality of spinach. Electron micrographs suggested ultrastructural damage of bacterial cells such as separation of the outer membrane from the cytoplasmic membrane. Overall, our results showed that SO and TDS may be potential sanitizers in inactivating human pathogens such as E. coli O157:H7 in wash water and on fresh produce.     

Growth potential of Listeria monocytogenes in soft,semi-soft and semi-hard artisanal cheeses after post-processing contamination in deli retail establishments

Opportunities for post-processing contamination of cheese may occur in deli retail establishments, either during the further cheese ripening (at maximum 14 °C), during storage and display in the refrigeration cabinet (at maximum 7 °C) or during slicing. A L. monocytogenes post-processing contamination was simulated by inoculation either on the cheese slicing surface or the cheese rind of three soft cheeses (one white-molded raw cow's milk cheese, one pasteurized cow's milk cheese with spicy herbs, one washed rind pasteurized cow and sheep's milk cheese) and two semi-hard cheeses (one smear-ripened raw cow's milk cheese and one natural-ripened raw cow's milk cheese). L. monocytogenes challenge testing was performed on 3 batches of each cheese to assess the growth potential of L. monocytogenes after 14 days storage at either 7 or 14 °C. Substantial growth of L. monocytogenes (>0.5 log CFU/g) was obtained in 79.2% of all individual challenge tests (n = 178) that were performed although huge variation in growth potential was noted among the different cheese types and storage conditions. The growth potential on soft cheeses stored at 7 °C ranged from 1.8 to 4.0 log units and from 3.6 to 5.5 log units upon storage at 14 °C, whilst on semi-hard cheese, this was in general lower, and ranged from 0.1 to 1.4 log units at 7 °C and from 0.0 to 3.0 log units at 14 °C. Overall, increased outgrowth of L. monocytogenes was noted when inoculation was performed on the cheese slicing surface compared to the cheese rind. Thus if occasional post-processing contamination takes place during storage or handling of the cheese, L. monocytogenes has the potential to grow to elevated numbers throughout a reasonably expected storage period of up to 14 days notwithstanding the presence of high numbers of indigenous lactic acid bacteria in these cheeses. Also for a defined cheese type both a considerable inter-batch and intra-batch variability was sometimes noted from the replicate testing, indicating no consistent behavior of L. monocytogenes in these fermented dairy products. As such it is recommended that appropriate hygienic measures are taken to prevent post-processing contamination. Noting the growth potential, absence of L. monocytogenes in 25 g of cheese using a multiple sample subunit approach (n = 5) at the time of production is important to ensure compliance to EU legislation 2073/2005.     

Modelling the interaction of storage temperature,pH, and water activity on the growth behaviour of Listeria monocytogenes in raw and pasteurised semi-soft rind washed milk cheese during storage following ripening

The objective of this study was to investigate the growth of Listeria monocytogenes in semi-soft rind washed cheese made from raw and pasteurised milk at different storage temperatures (4, 10 and 15 °C) over a 28 day period simulating storage following ripening. Changes in water activity (a_w) and pH in cheeses were also monitored during storage. Response surface models were used to model the interaction of storage temperature and time on a_w, pH and L. monocytogenes population. Growth curves were fitted using Baranyi, modified Gompertz and Logistic models at all storage temperatures for both cheeses, and model parameters were statistically analysed. In raw and pasteurised milk cheeses, all models showed a significant (P < 0.05) increase in the specific growth rate (SGR, Day⁻¹) of L. monocytogenes with an increase in storage temperature. A higher SGR was observed for L. monocytogenes in pasteurised milk cheese (0.18–0.85 Day⁻¹) compared to raw milk cheese (0.05–0.37 Day⁻¹) at all storage temperatures studied. Response surface models indicated an increase in the L. monocytogenes population and pH with an increase in storage temperature. However, a decreasing trend in a_w for both cheese types was observed. The predicted regression model parameters for both the raw and pasteurised milk cheese showed a high correlation coefficient R² > 0.87. Overall, the L. monocytogenes population increased up to 3 log₁₀ cfu⁻g⁻¹ for both cheeses during storage following ripening. The fitted models confirmed different L. monocytogenes growth behaviour between raw and pasteurised milk cheeses, which could support the Food Business Operator in predicting growth during storage following ripening.