Occurrence,serotypes and virulence genes of non-O157 Shiga toxin-producing Escherichia coli in fresh beef,ground beef,and beef burger

This study determined the prevalence, serotypes and virulence genes distribution of non-O157 Shiga toxin-producing Escherichia coli in meat products collected from butchers shops and supermarkets in Mansoura city, Egypt. We have characterized 18 non-O157 STEC strains among the identified 100 E. coli isolates recovered from the examined 87 meat product samples. The prevalence of non-O157 STEC strains in fresh beef, ground beef and beef burger samples were 11.1% (3/27), 16.7% (5/30), and 33.3% (10/30), respectively. The eighteen non-O157 STEC isolated strains were serotyped into seven (38.9%) O111:H8, six (33.3%) O26:H11, two (11.1%) O111:H–, and one (5.56%) for each of O55:H7, O126:H5 and O128:H2. PCR assays for different virulence genes showed that nine (50%), eleven (61.1%), and nine (50%) strains carry stx1, stx2, and eae genes, respectively. The distribution of shiga toxin genes among the isolated strains indicated that seven (38.9%) strains harbored stx1 only, nine (50%) strains harbored stx2 only, and two (11.1%) strains harbored both stx1 and stx2. The eae gene was present in association with five (27.8%), three (16.7%), and one (5.6%) strains that harbored stx1 only, stx2 only, and both stx1 and stx2, respectively. This study concluded that the examined meat products, particularly beef burger, consumed in Egypt are considerably contaminated with a variety of non-O157 STEC serotypes, and hence consumption of such products may constitute a potential health risk for consumers.     

Molecular screening and characterization of Shiga toxin-producing Escherichia coli in retail foods

Shiga toxin-producing Escherichia coli (STEC) strains are one of the most important recently emerged groups of food borne pathogens. This study investigated the prevalence of molecular markers for STEC and characteristics of E. coli O157 isolates from foods sold at retail markets in Wuhan, China. A total of 489 samples (350 meat products and 139 raw vegetables) were purchased from 22 large scale markets between July of 2011 and September of 2013. The meat samples consisted of frozen chicken products, raw pork, raw beef, frozen fish products and processed duck products. The raw vegetable samples consisted of lettuce, bok choy, radish, spinach, cucumber, and tomato. Shiga toxin genes (stx1 and stx2) and an O-group marker of the seven main pathogenic STEC serogroups (O157, O26, O45, O103, O111, O121, and O145) were detected in the samples by using PCR. 100% agreement was obtained between the results of the PCR targeting for wzy_O157 and the PCR targeting for rfbE_O157 gene. The result demonstrated that PCR assay targeting for wzy_O157 gene can be employed as an effective screening method for E. coli O157 in food sample. In the study, E. coli O157 and non-O157 STEC were detected in 55 (11.2%) and 75 (15.3%) samples by PCR screening, respectively. There was significant difference in the occurrence of STEC contamination between supermarkets (19/127, 15.0%) and open markets (111/362, 30.7%) (P < 0.05). Out of 489 samples, 5 samples carried O45, 1 sample carried O145 and 1 sample carried O111. Markers for O103, O26 and O121 were not detected. This result differed from other reports. Immunomagnetic separation based cultivation technique was used to isolate E. coli O157 from 27 food samples collected in 2013. Finally 7 E. coli O157 isolates were obtained. Among the 7 isolates, the prevalent stx genotype was stx_1a and stx_2a. Four E. coli O157 strains exhibited toxic effects on Vero cells, while 3 isolates had no detectable cytotoxicity effects even though they contained stx genes. All E. coli O157 isolates were sensitive to the 12 antimicrobials tested except for roxithromycin. There are some inconsistencies between the PCR screening and culture results. Characteristics of STEC isolates should be evaluated and considered for monitoring STEC contamination in foods.     

Prevalence and characterization of Escherichia coli O157:H7 from samples along the production line in Chinese beef-processing plants

This work investigated the prevalence of Escherichia coli O157:H7 and the antibiotic susceptibility, genetic diversity and virulence genes of isolated strains from four beef slaughter plants in China. A total of 510 samples (feces, hide, carcasses and raw meat) were tested for E. coli O157:H7 using enrichment, immunomagnetic separation, and plating on selective media. The prevalence of E. coli O157:H7 in the feces, hide, pre-evisceration carcass, post-evisceration carcass, post-washing carcass, chilled carcass, and raw meat samples was 1.43%, 1.43%, 0%, 0%, 0%, 1.25%, and 0%, respectively. Multiplex PCR assays were used for serotype confirmation and virulence gene detection. stx₂, eaeA and EHEC-hlyA genes were present in all six of the strains, and the stx₁ gene was not present. Fluorescence amplified fragment length polymorphism (FAFLP) was used to determine the genetic diversity of E. coli O157:H7 and revealed a high similarity between the strains isolated from feces and those isolated from carcasses. None of the isolated strains were found to be resistant to sixteen commonly used antimicrobial agents. The results of this study indicate that although E. coli O157:H7 contamination in the Chinese beef industry is sporadic and not as common as reported in other counties, all of the isolates contained three major virulence genes, presenting a high risk of disease for humans. The current research provides baseline information on E. coli O157:H7 prevalence and character profiles in Chinese beef-processing plants that can be used for future studies.     

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.     

Shiga toxin-producing Escherichia coli in food: Incidence,ecology, and detection strategies

Commensal Escherichia coli are commonly utilized for investigating the genetic and biochemical requirements of microorganisms, and have served in a wide variety of applications. Pathogenic E. coli known as Shiga toxin (Stx)-producing E. coli (STEC) are associated with various food products including ground beef. These pathogens are present in a wide range of environments, and have caused numerous foodborne outbreaks and recalls. These outbreaks and the increased awareness of STEC have led to certain STEC serotypes to be declared adulterants in non-intact raw meat. Various STEC detection methods have been investigated, and numerous cultural and molecular-based detection methods continue to be modified to meet regulatory requirements. However, STEC serotypes may possess certain characteristics that lead to bias in the likelihood of a certain serotype being detected in an assay. Understanding the characteristics of these STEC serotypes will provide means for optimizing the detection platforms, and as a result limit foodborne illness and recalls caused by STEC due to enhanced cultural and molecular detection capabilities.     

Bacterial quality of raw milk investigated by Escherichia coli and isolates analysis for specific virulence-gene markers

The bacterial quality and safety of raw milk sources in Taif region (Western Saudi Arabia) were analyzed for the natural contamination of fecal coliform and Escherichia coli by standard most probable number method. The E. coli isolates were identified to species identity using API 20E and screened for markers of STEC (Stx1, Stx2), ETEC (ST, LT), (EaeA) and ExPEC (CNF1, CNF2, SfaS, PapA, CdtB, IutA, FyuA, TraT) using PCR assays. Thirty-three E. coli strains were recovered from raw milk sample sources, which were contaminated by fecal coliform. None of the investigated isolates reacted positively with the PapA, SfaS and CdtB, ST and LT-specific primers. Markers of STEC and EPEC were each detected in three strains. NTEC was detected in four strains. The most frequent virulence markers were TraT (17 strains), IutA (11 strains) and FyuA (8 strains). Results suggested a possibility of potential public health threat of E. coli originating from raw milk sources.     

Comparison of effects of mild heat combined with lactic acid on Shiga toxin producing Escherichia coli O157:H7, O103, O111, O145 and O26 inoculated to spinach and soybean sprout

The aim of the present study was to investigate the effect of lactic acid against Shiga toxin producing Escherichia coli (O157:H7 and non-O157 serogroups including O103, O111, O145 and O26) at different conditions. Soybean sprouts and spinach leaves inoculated with each serogroup of E. coli (∼7.00 + 1.00 log₁₀ cfu/g) were treated with the lactic acid solutions at different concentrations (0% (control), 1.5%, 2.0%, or 2.5%) and at different temperatures (20, 40, or 50 °C) for 3 min. Results indicated that regardless of the treatment temperature, no significant reduction in the numbers of any serogroup occurred in the control group (0%) (p > 0.05). However, lactic acid at concentration of 1.5%, 2% and 2.5% was found to be effective against all organisms tested. There was no significant difference (p > 0.05) between E. coli O157:H7 and non-O157 STEC serogroups at any treatment group. The highest reductions (ca. 4.00 log₁₀ cfu/g) of all serotypes in both produces were observed after immersing into 2.5% lactic acid at 50 °C. The results of this study showed that decontamination of fresh produces such as spinach and soybean sprout with lactic acid solutions prepared at mild temperatures (40 °C and 50 °C) might be an effective safety measure in preventing public health risks associated with these products contaminated with STEC.     

Presence of indicator bacteria,Salmonella and diarrheagenic Escherichia coli pathotypes on mung bean sprouts from public markets in Pachuca,Mexico

Coliform bacteria (CB), fecal coliforms (FC), Escherichia coli, diarrheagenic E. coli pathotypes (DEP) and Salmonella frequencies were determined for mung bean (Vigna radiata) sprouts. One hundred sprout samples were collected from markets in Pachuca, Hidalgo state, Mexico. Of these samples, 100% were positive for CB, 98% for FC, 95% for E. coli, 10% for DEP and 5% for Salmonella. Identified DEP included enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC) and Shiga toxin-producing E. coli (STEC). The ETEC and EIEC were each isolated from 2% of samples, and the STEC from 6% of samples. No E. coli O157:H7 were detected in any STEC-positive samples. Positive correlations were observed between FC and E. coli and between E. coli and DEP. A negative correlation occurred between CB and DEP, and between CB and Salmonella. Neither FC nor E. coli correlated with Salmonella presence in the sprout samples. This is the first report of ETEC, EIEC and STEC isolated from sprouts in Mexico and the first report of Salmonella isolation from mung bean in Mexico. Mung bean sprouts are very probably an important factor contributing to the endemicity of ETEC, EIEC and STEC and Salmonella-related gastroenteritis in Mexico.     

Incidence of Listeria monocytogenes and Escherichia coli O157:H7 in two Kasar Cheese processing environments

This study was designed to determine the presences of two environmental pathogens in two dairy factories in Sakarya, Turkey. A total of 264 environmental samples, raw milk and cheese samples were taken at four different seasons. According to the results, Listeria monocytogenes or Escherichia coli O157:H7 was isolated from 26 or 2.7% of the samples collected from both factories, respectively. None of the cheese or curd samples were found to be positive for Listeria or E. coli O157:H7. However, 50% of raw milk samples contained Listeria innocua. Listeria was mostly isolated from the swap samples taken from the drains or the floors in processing or packaging areas. However, E. coli was also isolated from the swap samples taken from the workers’ hands and gloves as well as the drains and the floor. Only one raw milk sample contained E. coli O157:H7. A higher prevalence of both pathogens was observed in the summer months than in the other months.     

Mechanism of the combined anti-bacterial effect of green tea extract and NaCl against Staphylococcus aureus and Escherichia coli O157:H7

The mechanism of the combined anti-bacterial effect of green tea extract (GTE) and NaCl against Staphylococcus aureus NBRC 13276 and Escherichia coli O157:H7 was investigated. After treatment for 1 h, GTE was more effective against S. aureus than E. coli O157:H7, and combined GTE/NaCl treatment caused greater cellular damage in S. aureus NBRC 13276, where it was bactericidal, than E. coli O157:H7. Compared to treatment with 1.0 mg/mL GTE, which had no effect on the survival rate of E. coli O157:H7 after 48 h, treatment with 4% NaCl alone caused greater cellular damage. Moreover, bacteria pretreated with NaCl showed delayed growth in the presence of GTE. It is therefore likely that susceptibility of E. coli O157:H7 to GTE was increased by exposure to NaCl. E. coli O157:H7 pretreated with GTE and NaCl did not multiply in the presence of GTE. Visualization of the catechin components of GTE-treated bacteria using an electron microscope and SDS-PAGE analysis of cell proteins showed that GTE attached to proteins on the surface of the bacteria to form high-molecular weight complexes, suggesting the possibility that GTE inhibits the uptake and secretion of substrates and inhibits enzyme activity. Notably, after the GTE treatment for 1 h, both bacterial strains suffered injury but recovered by cultivation in rich medium. The damage and aggregation of proteins caused by GTE treatment were repaired upon treatment with LP diluent.     

Role of cellulose in protecting Shiga toxin producing Escherichia coli against oxidative and acidic stress

This study assessed the role of cellulose in protecting Shiga toxin-producing Escherichia coli (STEC) against environmental stress. Cellulose producing STEC strain 19B (O5:H-) and 49B (O103:H2) as well as their cellulose deficient derivates (19D and 49D) were subjected to oxidative (10, 20 and 30 mM H₂O₂) and acidic (pH 3.0, 3.5 and 4.5) stress. Approximately 4.25, 2.46 or 1.74 log CFU ml⁻¹ of the 19B cells survived a 2.5-h treatment with 10, 20 and 30 mM H₂O₂, respectively, while none of the 19D cells (8.02 logs) survived the same treatments. The population of 49D decreased from 7.70 to <0.60 log CFU ml⁻¹, whereas that of 49B decreased from 7.70 to 2.47 log CFU ml⁻¹ by a 2-h treatment with 30 mM H₂O₂. Approximately 2.63 log CFU ml⁻¹ of the 19B cells survived at pH 3.0 for 1 h and 4.75 log CFU ml⁻¹ of the 49B cells survived at pH 3.0 for 1.5 h, but the 19D and 49D cells did not survive these treatments. Results indicated that cellulose protected the selected STEC strains against oxidative treatments with 10-30 mM of H₂O₂ and acidic treatments at pH 3.0-3.5.     

Detection of verocytotoxin-producing Escherichia coli (VTEC) in minced beef and raw milk by colony blot hybridization

Verocytotoxin-producing Escherichia coli (VTEC) are foodborne pathogens that cause outbreaks linked to consumption of meat and raw milk. In this note the authors report results obtained from a survey conducted on minced beef and raw bovine milk samples using a Multiplex PCR (M-PCR) for the detection of eae, stx₁, stx₂ and hlyA genes as a screening step followed by a colony blot hybridization (CBH) technique for the isolation of the VTEC. Of 100 minced beef and 123 raw milk samples, 13 (13%) and 7 (5.7%) were positive in the M-PCR and among these 9 and 3 strains were isolated using CBH, respectively. All isolates showed the presence of the stx₂ gene, single or in association with the other investigated genes. None of the isolates belonged to the O157, O26, O91, O103, O111 and O145 serogroups. The study showed that the use of M-PCR for the screening of samples coupled with a sensitive and specific detection technique, could improve the possibility of detection of VTEC strains in foods. Moreover, the presence of VTEC in minced beef and in raw milk confirms their important role as putative vehicles of infection to humans. Stringent control of these foodstuffs is essential for food safety purposes.     

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.     

Development of a controlling method for Escherichia coli O157:H7 and Salmonella spp. in fresh market beef by using polylysine and modified atmosphere packaging

Escherichia coli O157:H7 and Salmonella spp. often contaminate fresh beef. In Japan, an E coli outbreak caused by raw beef made 181 people ill and 5 individuals dead in 2011. Responding to this outbreak, an effective sterilization method for fresh beef is expected to be developed. In this study, ε-polylysine combined with CO₂-packaging method was examined for controlling these pathogens in fresh beef. At an incubation temperature of 4 °C, approximately 4.3 log and 2.4 log reduction in bacterial numbers were observed after 7-day incubation for E. coli O157:H7 and Salmonella, respectively, in ε-polylysine-added beef. When effectiveness of CO₂-packaging combined with ε-polylysine was investigated, CO₂ did not have additional inhibiting effect on bacterial growth compared to only-ε-polylysine-treated samples when incubated at 4 °C. However, effectiveness of CO₂ was observed when incubated at 10 °C where approximately 2.9 log and 4.4 log reduction in E. coli cell numbers were observed in only-ε-polylysine-treated samples and polylysine- and CO₂-treated group, respectively, and approximately 1.7 log and 3.5 log reduction in Salmonella cell numbers were observed in only-ε-polylysine-treated samples and polylysine and CO₂-treated group, respectively. This study confirmed that ε-polylysine or ε-polylysine combined with CO₂ packaging are effective in preventing foodborne diseases caused by raw beef.     

Detection and CRISPR subtyping of Salmonella spp. isolated from whole raw chickens in Yangzhou from China

This study was undertaken to acquire data on Salmonella contamination of whole raw chickens, eggs, and vegetables available to consumers in Yangzhou city, China, between April 2011 and March 2012. In total, 240 chicken carcasses were tested, and the overall contamination rate for Salmonella was 33.8%. While the prevalence of Salmonella in 100 eggs and 155 vegetable samples was 7.0% and 3.2%, respectively. The 84 isolated strains were identified in 19 different serotypes with Salmonella enterica serovar Indiana (S. Indiana) (25.0%), S. Typhimurium (21.4%) and S. Enteritidis (17.9%) as the predominant serovars. Moreover, the median load of the contaminated chicken samples reached 6.4 MPN/100 g with 3.6 MPN/100 g as the 25th percentile and 15.0 MPN/100 g as the 75th percentile. Chicken carcasses collected in October had not only the highest prevalence of Salmonella (70%), but also the highest median load (33 MPN/100 g) and 75th percentile load (460 MPN/100 g), while the lowest prevalence (10%) was in April. The clustered regularly interspaced short palindromic repeats (CRISPR) subtyping method was then used to identify serotypes of Salmonella and distinguish strains from the same Salmonella serotypes. We found that 85.7% of strains were distributed in 11 serotypes speculated by CRISPR typing, which corresponded to the identified serotypes by O and H antiserum. The speculated serotypes of 7.1% of the strains by CRISPR typing are very close to the identified ones, as they belong to the same O group with a small difference in the O or H antigen. All of the above findings implied that CRISPR typing could be applied to serotyping of Salmonella. In addition, CRISPR typing method could be used to subtype different strains from the same serotype, specifically S. Hadar.     

Combination of bioprotective cultures with EDTA to reduce Escherichia coli O157:H7 in frozen ground-beef patties

The effectiveness of bacteriocin-producing Lactobacillus curvatus CRL705 and Lactococcus lactis CRL1109 in combination with Na₂EDTA on frozen ground-beef patties contaminated with Escherichia coli O157:H7, was investigated under temperature abuse conditions (5 °C during 9 days). The presence of the bioprotective cultures (ca. 10⁷ CFU/g) and chelator (48 mM) resulted in one log CFU/g reduction for E. coli strain, compared to the control on day 0. Similarly, a significant decline for indigenous coliforms in ground-beef patties was also observed in the presence of bacteriocinogenic strains and chelator. However, in the absence of Na₂EDTA, neither E. coli nor coliforms were inhibited by the bioprotective cultures, the pathogen reaching similar counts than control samples (5.22 and 3.60 log CFU/g, respectively) at 9 days. When the growth of bacteriocinogenic strains on patties was evaluated, they were able to increase their population producing bacteriocins after 48 h up to the end of incubation period while a near neutral pH in the presence of Na₂EDTA was detected. Non substantial effect on of ground-beef patties color was produced in the presence of bioprotective cultures, while a darker color developed in those added with the chelator. The simultaneous treatment with bioprotective cultures and Na₂EDTA may be of value for the control of E. coli O157:H7 in temperature abused ground-beef patties.     

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 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.     

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.