Quantification of contamination of lettuce by GFP-expressing Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium

The primary objective of this study was to determine the possibility of internalization of GFP-expressing Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium (S. Typhimurium) strains MAE 110 (multi-cellular morphology) and 119 (wild type morphology) into lettuce seedlings (Lactuca sativa cv. Tamburo) grown in an inoculated hydroponic and soil system. The second aim was to quantify the level of contamination with the use of a proper surface sterilization method. Silver nitrate was superior in reducing the number of viable bacteria on leave surfaces compared to sodium hypochlorite and ethanol. With the hydroponic system internal colonization of lettuce only occurred at high densities with S. Typhimurium MAE 119. With the soil system E. coli O157:H7, S. Typhimurium 110 and S. Typhimurium 119 were found at considerable densities in sterilized leaf samples (respectively, 3.95, 2.57 and 2.37 log cfu/g on average) with prevalences of 0.29, 0.23 and 0.15, respectively. No statistical differences were observed between the Salmonella strains. A negative correlation was observed between shoot weight and leaf contamination. The observed presence of the pathogens in lettuce, after thorough surface sterilization, demonstrates the possible presence of human pathogens in locations were they are unlikely to be removed by the actions of consumer washing and therefore pose a serious threat when occurring in field situations.     

Antimicrobial activity of plant extracts against Salmonella Typhimurium, Escherichia coli O157:H7, and Listeria monocytogenes on fresh lettuce

Plant extracts have been found to be effective in reducing microorganisms. This study evaluated antimicrobial activity of 12 plant extracts against Salmonella Typhimurium, Escherichia coli O157:H7, and Listeria monocytogenes by using a disk diffusion assay, and Syzygium aromaticum (clove) showed the highest inhibitory effect. To investigate the efficacy of clove extract that inactivates pathogens on lettuce, inoculated lettuce with S. Typhimurium, E. coli O157:H7, and L. monocytogenes was treated with diluted clove extracts or distilled water for 0, 1, 3, 5, and 10 min. Clove extract treatment significantly reduced populations of the 3 tested pathogens from the surface of lettuce. Practical Application: This result indicated that clove extract is a useful antimicrobial agent to reduce the microbial level of foodborne pathogens on fresh lettuce. It also might be a natural antimicrobial for reducing or replacing chemical sanitizers in food preservation.     

Survival of Escherichia coli O157:H7 and Salmonella enterica serovars Typhimurium in iceberg lettuce and the antimicrobial effect of rice vinegar against E. coli O157:H7

The microbiological safety of fresh produce is a significant concern of consumers and industry. After applying at an inoculated level (about 10(6) CFUg(-1)) of E. coli O157:H7 and Salmonella enterica serovars Typhimurium on shredded iceberg lettuce and water samples individually, they were stored at 4 degrees C for 14 days and 22 degrees C for 7 days to monitor the growth and survival of pathogens. The results showed that at the end of 4 degrees C storage, populations of two pathogens in lettuce and water decreased approximately 1 log CFUg(-1). However, microbial levels on shredded lettuce increased 3 logs within 3 days at 22 degrees C. Vinegar (acetic acid) had been used to reduce populations of foodborne pathogens in foods; hence, the antimicrobial effect of rice vinegar on the survival of E. coli O157:H7 in inoculated lettuce (10(4) and 10(7) CFUg(-1)) is examined in this study. Results were observed that the treatment of inoculated lettuce (10(7) CFUg(-1)) with commercial vinegar containing 5% acetic acid (pH 3.0) for 5 min would reduce 3 logs population at 25 degrees C. Less than a 1-log decrease in bacterial numbers was recovered during 5 min exposure to 0.5% (pH 3.26) acetic acid.     

Use of organic acids to inactivate Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on organic fresh apples and lettuce

Abstract: This study was undertaken to investigate the antimicrobial effect of organic acids against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on whole red organic apples and lettuce. Several studies have been conducted to evaluate organic acids as sanitizers. However, no studies have compared antimicrobial effects of various organic acids on organic fresh produce, including evaluation of color changes of produce. Apples and lettuce were inoculated with a cocktail of 3 strains each of 3 foodborne pathogens provided above and treated with 1% and 2% organic acids (propionic, acetic, lactic, malic, and citric acid) for 0, 0.5, 1, 5, and 10 min. With increasing treatment time and acid concentration, organic acid treatments showed significant reduction compared to the control treatment (distilled water), and differences in antimicrobial effects between organic acids were observed. After 10 min of treatment with 1% and 2% organic acids in apples, propionic (0.92 to 2.75 log reduction), acetic (0.52 to 2.78 log reduction), lactic (1.69 to >3.42 log reduction), malic (1.48 to >3.42 log reduction), and citric acid (1.52 to >3.42 log reduction) exhibited significant (P < 0.05) antibacterial effects against 3 foodborne pathogens compared to the control treatment. In lettuce, propionic (0.93 to 1.52 log reduction), acetic (1.13 to 1.74 log reduction), lactic (1.87 to 2.54 log reduction), malic (2.32 to 2.98 log reduction), and citric acid (1.85 to 2.86 log reduction) showed significant (P < 0.05) effects compared to the control treatment. Changes in sample color subjected to organic acids treatment were not significant during storage. Practical Application: It is suggested that organic acids have a potential as sanitizers for organic fresh produce. These data may help the organic produce industry provide safe fresh produce for consumers.     

Survival characteristics of Salmonella Typhimurium and Escherichia coli O157:H7 in minimally processed lettuce during storage at different temperatures

Salmonella Typhimurium and Escherichia coli O157:H7 strains were inoculated in minimally processed lettuce samples and survival characteristics of these pathogens during storage at two different temperatures. One group of samples was kept at refrigerator at +4?°C, another group was kept at ambient temperature (+25?°C). According to data obtained, the factor ??temperature?? significantly affected the pathogen bacteria counts of the samples (P?<?0.05). Counts of these two pathogen bacteria were decreased in minimally processed lettuce during storage at +4?°C.     

Transfer of Escherichia coli O157:H7 to romaine lettuce due to contact water from melting ice

Ice can be used to chill romaine lettuce and maintain relative humidity during transportation. Escherichia coli O157:H7 may contaminate water used for ice. The objective of this study was to determine the potential for E. coli O157:H7 contamination of romaine lettuce from either ice contaminated with the pathogen or by transfer from lettuce surfaces via melting ice. In experiment 1, lettuce was spot inoculated with E. coli O157:H7 and chilled with ice prepared from uncontaminated tap water. In experiment 2, water inoculated with this pathogen was frozen and used to ice lettuce. Three heads of lettuce were stacked in each container and stored at 4 or 20 degrees C. After the ice melted, E. coli O157:H7 attachment to and recovery from the lettuce leaves were determined. For experiment 1, the population of E. coli O157:H7 attached to inoculated sites averaged 3.8 and 5.5 CFU/cm2 at 4 and 20 degrees C, respectively. Most of the uninoculated sites became contaminated with the pathogen due to ice melt. For experiment 2, 3.5 to 3.8 log CFU E. coli O157:H7 per cm2 was attached to the top leaf on the first head. After rinsing with chlorinated water (200 microg/ml), E. coli O157:H7 remained on the surface of the top head (1.8 to 2.0 log CFU/cm2). There was no difference in numbers of E. coli O157:H7 recovered from each sampling site at 4 and 20 degrees C. Results show that E. coli O157:H7 can be transferred onto other produce layers in shipping containers from melted ice made of contaminated water and from contaminated to uncontaminated leaf surfaces.     

Inactivation kinetics of inoculated Escherichia coli O157:H7 and Salmonella enterica on lettuce by chlorine dioxide gas

The purpose of this investigation was to study inactivation kinetics of inoculated Escherichia coli O157:H7 and Salmonella enterica on lettuce leaves by ClO(2) gas at different concentrations (0.5, 1.0, 1.5, 3.0, and 5.0 mg l(-1)) for 10 min and to determine the effect of ClO(2) gas on the quality and shelf life of lettuce during storage at 4 degrees C for 7 days. One hundred microliters of each targeted organism was separately spot-inoculated onto the surface (5 cm(2)) of lettuce (approximately 8-9 log CFU ml(-1)), air-dried, and treated with ClO(2) gas at 22 degrees C and 90-95% relative humidity for 10 min. Surviving bacterial populations on lettuce were determined using a membrane transferring method, which included a non-selective medium followed by a selective medium. The inactivation kinetics of E. coli O157:H7 and S. enterica was determined using first-order kinetics to establish D-values and z-values. The D-values of E. coli and S. enterica were 2.9+/-0.1 and 3.8+/-0.5 min, respectively, at 5.0 mg l(-1) ClO(2) gas. The z-values of E. coli and S. enterica were 16.2+/-2.4 and 21.4+/-0.5 mg l(-1), respectively. A 5 log CFU reduction (recommended by the United States Food and Drug Administration) for E. coli and S. enterica could be achieved with 5.0 mg l(-1) ClO(2) gas for 14.5 and 19.0 min, respectively. Treatment with ClO(2) gas significantly reduced inherent microflora on lettuce and microbial counts remained significantly (p<0.05) lower than the uninoculated control during storage at 4 degrees C for 7 days. However, treatment with ClO(2) gas had a significantly (p<0.05) negative impact on visual leaf quality. These results showed that treatment with ClO(2) gas significantly reduced selected pathogens and inherent microorganisms on lettuce; however, the processing conditions would likely need to be altered for consumer acceptance.     

Antimicrobial activity of epsilon-polylysine against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes in various food extracts

ABSTRACT:  This study compared the antimicrobial effects of ɛ-polylysine (ɛ-PL) against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes in 6 food extracts and in broth. The food extracts (10% (w/w) in distilled water) evaluated were fat-free and whole fat milk, beef, bologna, rice, and vegetables (50:50 ratio of broccoli and cauliflower). ɛ-PL was tested at 0.005% and 0.02% (w/v) against E. coli O157:H7 and L. monocytogenes , and 0.02% and 0.04% (w/v) against S. Typhimurium . The substrates were inoculated (5 log CFU/mL) and periodically analyzed for surviving populations during storage at 12 °C for 6 d. In general, all 3 pathogens reached 7 to 9 log CFU/mL within 2 d in control substrates (no ɛ-PL). Immediate bactericidal effects ( P < 0.05) following exposure to ɛ-PL were obtained in the rice (all pathogens) and vegetable ( E. coli O157:H7 and S. Typhimurium ) extracts. During storage, antimicrobial effects of ɛ-PL were more pronounced in the food extracts than in the broth medium. The greatest antimicrobial activity for all 3 pathogens was obtained in the rice and vegetable extracts, where counts were reduced ( P < 0.05) to below the detection limit (0.0 log CFU/mL) by one or both ɛ-PL concentrations tested. In the other food extracts (fat-free milk, whole fat milk, beef, and bologna), both ɛ-PL concentrations tested generally resulted in lower ( P < 0.05) pathogen levels at the end of storage compared to initial counts, with better bactericidal effects exerted by the higher of the 2 ɛ-PL concentrations. Additional research is needed to explore the potential antimicrobial effects of ɛ-PL in real food systems.     

Comparative examination of Escherichia coli O157:H7 survival on romaine lettuce and in soil at two independent experimental sites

Little is known about the influence of abiotic factors such as climate and soil chemistry on the survival of Escherichia coli O157:H7 in field lettuce. We applied a nalidixic acid-resistant derivative of strain ATCC 700728 to field-grown romaine lettuce in two regions in Canada characterized by large variances in soil type and climate. Surviving populations in soil and on lettuce leaves were estimated on sorbitol MacConkey agar supplemented with nalidixic acid. Data were fitted with the Weibull decline function to permit comparison of decay rates in the two experimental sites. E. coli O157:H7 populations fell from 10? to <102 CFU/g on leaves, and <103 CFU/g in soil within 7 days after inoculation. Analysis revealed there was no significant difference between decay rates at the two experimental sites in either environment. The results of this study suggest that the inherent ecological fitness of E. coli O157:H7 ATCC 700728 determines the extent of survival in the production environment.     

Irradiation compared with chlorination for elimination of Escherichia coli O157:H7 internalized in lettuce leaves: influence of lettuce variety

ABSTRACT:  Pathogenic bacteria internalized in leaf tissues are not effectively removed by surface treatments. Irradiation has been shown to inactivate leaf-internalized bacteria, but many aspects of targeting these protected pathogens remain unknown. Bacterial cells of a cocktail mixture of 3 isolates of Escherichia coli O157:H7 were drawn into the leaves of iceberg, Boston, green leaf, and red leaf lettuce using vacuum perfusion. The inoculated leaves were treated with a 3-min wash with sodium hypochlorite solution (0, 300, or 600 ppm) or various doses of ionizing radiation (0.25 to 1.5 kGy). Leaves were stomached to recover the internalized cells and survivors enumerated. Washes with 0 ppm (water), 300 ppm, and 600 ppm chlorine solutions each gave reductions of less than 1 log. These reductions were statistically significant only in the case of green leaf lettuce. In contrast, irradiation effectively reduced E. coli O157:H7 on all varieties examined, with all doses tested being significantly reduced from the untreated control. The specific variety influenced the efficacy of irradiation. The greatest reduction obtained was 5 logs on iceberg lettuce treated with 1.5 kGy. The D ₁₀ values (the dose necessary to achieve a 1 log reduction) were significantly ( P < 0.05) different among the varieties of lettuce tested, and ranged from 0.30 kGy (iceberg) to 0.45 kGy (Boston). These values were observed to be notably higher than previous irradiation D ₁₀ values for E. coli O157:H7 surface inoculated onto these 4 lettuce varieties. This study has shown that irradiation is able to effectively reduce viable E. coli O157:H7 cells internalized in lettuce, and that the variety of lettuce influences the specific response.     

Efficacy of adding detergents to sanitizer solutions for inactivation of Escherichia coli O157:H7 on Romaine lettuce

Numerous Escherichia coli O157:H7 outbreaks have been linked to consumption of fresh lettuce. The development of effective and easily implemented wash treatment could reduce such incidents. The purpose of this study was to evaluate the addition of food-grade detergents to sanitizer solutions for inactivation of E. coli O157:H7 on Romaine lettuce. Freshly-cut leaves of Romaine lettuce were dip-inoculated to achieve a final cell concentration of 7.8 ± 0.2 log CFU/g, air-dried for 2 h, and stored overnight at 4 °C. Leaves were then washed for 2 min in an experimental short chain fatty acid formulation (SCFA) or in one of the following solutions with or without 0.2% dodecylbenzenesulfonic acid or 0.2% sodium 2-ethyl hexyl sulfate: 1) deionized water; 2) 100 ppm chlorine dioxide; 3) 100 ppm chlorine; and 4) 200 ppm chlorine. Following wash treatment, samples were blended in neutralizing buffer (1:3) and surface plated on the selective media CT-SMAC. The efficacy of wash treatments, with or without the detergents, in inactivating E. coli O157:H7 cells on lettuce leaves were not significantly different. The most effective wash solution was SCFA, which was capable of reducing E. coli O157:H7 populations by more than 5 log CFU/g. The rest of the wash treatments resulted in a population reduction of less than 1 log CFU/g. The effectiveness of SCFA surpasses that of other sanitizer treatments tested in this study and requires further research to optimize treatments to preserve lettuce quality. Conventional detergents did not enhance the efficacy of any of the wash treatments tested during this study.     

Sanitation and design of lettuce coring knives for minimizing Escherichia coli O157:H7 contamination

This study was undertaken to examine the effect of ultrasound in combination with chlorine on the reduction of Escherichia coli O157:H7 populations on lettuce coring knives. Two new coring devices designed to mitigate pathogen attachment were fabricated and evaluated. The coring rings of the knives were dip inoculated with soil slurry containing 10? E. coli cells and treated with chlorinated water with and without ultrasonication for 30, 60, and 120 s. The rough welding joints on currently used in-field lettuce coring knives provided a site conducive to bacterial attachment and resistant to cell removal during sanitation treatment. The two modified coring knives harbored significantly fewer E. coli cells than did the currently used commercial model, and the efficacy of the disinfection treatment was high (P < 0.05). Ultrasound treatment reduced the E. coli O157:H7 counts to below the detection limit of 1.10 log CFU/cm2 at both the coring ring blade and welding joint within 30 s in 1 ppm of chlorinated water. The redesigned coring knives and an ultrasound plus chlorine combination treatment may provide practical options for minimizing the microbial safety hazards of lettuce processed by core-in-field operations.     

Fate of Escherichia coli O157:H7 in field-inoculated lettuce

Impact of drip and overhead sprinkler irrigation on the persistence of attenuated Escherichia coli O157:H7 in the lettuce phyllosphere was investigated using a split-plot design in four field trials conducted in the Salinas Valley, California, between summer 2007 and fall 2009. Rifampicin-resistant attenuated E. coli O157:H7 ATCC 700728 (BLS1) was inoculated onto the soil beds after seeding with a backpack sprayer or onto 2- or 4-week-old lettuce plant foliage with a spray bottle at a level of 7 log CFU ml⁻¹. When E. coli O157:H7 was inoculated onto 2-week-old plants, the organism was recovered by enrichment in 1 of 120 or 0 of 240 plants at 21 or 28 days post-inoculation, respectively. For the four trials where inoculum was applied to 4-week-old plants, the population size of E. coli O157:H7 declined rapidly and by day 7, counts were near or below the limit of detection (10 cells per plant) for 82% or more of the samples. However, in 3 out 4 field trials E. coli O157:H7 was still detected in lettuce plants by enrichment 4-weeks post-inoculation. Neither drip nor overhead sprinkler irrigation consistently influenced the survival of E. coli O157:H7 on lettuce.     

Transfer and internalisation of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium in cabbage cultivated on contaminated manure-amended soil under tropical field conditions in Sub-Saharan Africa

Surface contamination and internalisation of Escherichia coli O157:H7 and Salmonella Typhimurium in cabbage leaf tissues at harvest (120 days post-transplantation) following amendment of contaminated bovine manure to soil at different times during crop cultivation were investigated under tropical field conditions in the Central Agro-Ecological Zone of Uganda. Fresh bovine manure inoculated with rifampicin-resistant derivatives of non-virulent strains of E. coli O157:H7 and S. Typhimurium was incorporated into the soil to achieve inoculum concentrations of 4 and 7 log CFU/g at the point of transplantation, 56 or 105 days post-transplantation of cabbage seedlings. Frequent sampling of the soil enabled the accurate identification of the survival kinetics in soil, which could be described by the Double Weibull model in all but one of the cases. The persistence of 4 log CFU/g E. coli O157:H7 and S. Typhimurium in the soil was limited, i.e. only inocula applied 105 days post-transplantation were still present at harvest. Moreover, no internalisation in cabbage leaf tissues was observed. In contrast, at the 7 log CFU/g inoculum level, E. coli O157:H7 and S. Typhimurium survived in the soil throughout the cultivation period. All plants (18/18) examined for leaf contamination were positive for E. coli O157:H7 at harvest irrespective of the time of manure application. A similar incidence of leaf contamination was found for S. Typhimurium. On the other hand, only plants (18/18) cultivated on soil amended with contaminated manure at the point of transplantation showed internalised E. coli O157:H7 and S. Typhimurium at harvest. These results demonstrate that under tropical field conditions, the risk of surface contamination and internalisation of E. coli O157:H7 and S. Typhimurium in cabbage leaf tissues at harvest depend on the inoculum concentration and the time of manure application. Moreover, the internalisation of E. coli O157:H7 and S. Typhimurium in cabbage leaf tissues at harvest seems to be limited to the worst case situation, i.e., when highly contaminated manure is introduced into the soil at the time of transplantation of cabbage seedlings.     

Effect of xylitol on adhesion of Salmonella Typhimurium and Escherichia coli O157:H7 to beef carcass surfaces

Effects of 10% xylitol (a five-carbon sugar alcohol) on adhesion of Escherichia coli O157:H7 and Salmonella Typhimurium to meat surfaces were examined with three approaches. First, beef outside round was inoculated with rifampin-resistant E. coli O157:H7 and Salmonella Typhimurium dispersed in xylitol or peptone solution. Samples were rinsed with water or not rinsed in a 2 x 2 factorial arrangement. No interaction existed between inoculum and rinsing treatments (P > 0.84). Incubation in xylitol had minimal impact on pathogen adhesion (P > 0.76); however, rinsing reduced pathogen cell counts (P < 0.01). Second, meat samples were treated with water, xylitol, or no rinse; inoculated with pathogens dispersed in peptone solution (8.6 log CFU/ml for each pathogen); and then treated with water, xylitol, or no rinse in a 3 x 3 factorial arrangement. No interactions were observed (P > 0.50). Postinoculation rinsing reduced pathogen loads (P < 0.01) without difference between water and xylitol (P > 0.64). Third, carcass surfaces inoculated with pathogens (5.5 log CFU/cm2) were treated with 35 degrees C water wash, 2.5% L-lactic acid spray, 10% xylitol spray, lactic acid plus xylitol, or hot water plus xylitol. Lactic acid treatments reduced Salmonella Typhimurium at 0 h (P < 0.01) and 24 h (P < 0.02). Hot water treatments tended to reduce Salmonella Typhimurium at 0 h (P < 0.07). Xylitol did not reduce pathogens (P > 0.62) or increase effectiveness of other treatments. Xylitol does not influence E. coli O157:H7 and Salmonella Typhimurium adhesion to meat surfaces.     

Reduction of Escherichia coli O157:H7 and Salmonella enterica serovar Enteritidis in chicken manure by larvae of the black soldier fly

Green fluorescent protein-labeled Escherichia coli O157:H7 and Salmonella enterica serovar Enteritidis were inoculated at 10(7) CFU/g into cow, hog, or chicken manure. Ten- or 11-day-old soldier fly larvae (Hermetia illucens L.) (7 to 10 g) were added to the manure and held at 23, 27, or 32 degrees C for 3 to 6 days. Soldier fly larvae accelerated inactivation of E. coli O157:H7 in chicken manure but had no effect in cow manure and enhanced survival in hog manure. The initial pH values of the hog and chicken manure were 6.0 to 6.2 and 7.4 to 8.2, respectively, and it is surmised that these conditions affected the stability of the larval antimicrobial system. Reductions of E. coli O157:H7 populations in chicken manure by larvae were affected by storage temperature, with greater reductions in samples held for 3 days at 27 or 32 degrees C than at 23 degrees C. Pathogen inactivation in chicken manure by larvae was not affected by the indigenous microflora of chicken manure, because Salmonella Enteritidis populations in larvae-treated samples were approximately 2.5 log lower than control samples without larvae when either autoclaved or nonautoclaved chicken manure was used as the contaminated medium during 3 days of storage. Extending the storage time to 6 days, larvae again accelerated the reduction in Salmonella Enteritidis populations in chicken manure during the first 4 days of storage; however, larvae became contaminated with the pathogen. After 2 days of feeding on contaminated manure, Salmonella Enteritidis populations in larvae averaged 3.3 log CFU/g. Populations decreased to 1.9 log CFU/g after 6 days of exposure to contaminated chicken manure; however, the absence of feeding activity by the maggots in later stages of storage may be responsible for the continued presence of Salmonella Enteritidis in larvae. Transfer of contaminated larvae to fresh chicken manure restored feeding activity but led to cross-contamination of the fresh manure.     

Radio-frequency heating to inactivate Salmonella Typhimurium and Escherichia coli O157:H7 on black and red pepper spice

The efficacy of radio-frequency (RF) heating to inactivate Salmonella Typhimurium and Escherichia coli O157:H7 on black and red pepper spice was investigated. A 27.12 MHz RF heating system consisted of two parallel-plate electrodes was used, with the sample being placed between them. Black peppers (whole and ground) and red peppers (+ 16 mesh, -16 + 25 mesh, and -25 mesh) inoculated with S. Typhimurium and E. coli O157:H7 were treated with RF energy during 50s for black peppers and 40s for red peppers, and color change of samples was evaluated after treatment. RF heating for 50s resulted in 2.80 to 4.29 log CFU/g reductions of S. Typhimurium and E. coli O157:H7 in black peppers and RF heating of red peppers for 40s reduced pathogens by 3.38 log CFU/g to more than 5 log CFU/g (below the detection limit) without affecting the color quality change. The results suggest that RF heating has the potential for novel thermal process to control foodborne pathogens in spice.     

Response of Salmonella and Escherichia coli O157:H7 to UV energy

To determine the efficacy of a UV light treatment at 253.7 nm (UVC light) on microbial growth, plates containing tryptic soy agar plus 50 ppm of nalidixic acid (TSAN) were inoculated with known concentrations of five-strain cocktails of Salmonella and Escherichia coli O157:H7 and subjected to different UVC treatments. The concentration of the cocktail inoculum was determined with TSAN prior to inoculation. Serial dilutions were carried out, and inoculation levels of 10(0) to 10(8) CFU/ ml were tested for each pathogen. Multiple replications of doses of UV light ranging from 1.5 to 30 mW/cm2 were applied to different cocktail concentrations, and doses of > 8.4 mW/cm2 resulted in a 5-log reduction of Escherichia coli O157:H7, while a 5-log reduction of Salmonella was observed with doses of > 14.5 mW/cm2. Results for both organisms yielded sigmoidal inactivation curves. UVC light is effective in reducing microbial populations of pathogens on agar surfaces.     

Antimicrobial Wine Formulations Active Against the Foodborne Pathogens Escherichia coli O157: H7 and Salmonella enterica

ABSTRACT:  We developed wine formulations containing plant essential oils and oil compounds effective against foodborne pathogenic bacteria Escherichia coli O157:H7 and Salmonella enterica. HPLC was used to determine maximum solubility of antimicrobials in wines as well as amounts of antimicrobials extracted by wines from commercial oregano and thyme leaves. Activity of essential oils (cinnamon, lemongrass, oregano, and thyme) and oil compounds (carvacrol, cinnamaldehyde, citral, and thymol) in wines were evaluated in terms of the percentage of the sample that resulted in a 50% decrease in the number of bacteria (BA₅₀). The ranges of activities in wines (30 min BA₅₀ values) against S. enterica/E. coli were carvacrol, 0.0059 to 0.010/0.011 to 0.021; oregano oils, 0.0079 to 0.014/0.022 to 0.031; cinnamaldehyde, 0.030 to 0.051/0.098 to 0.13; citral, 0.033 to 0.038/0.060 to 0.070; lemongrass oil, 0.053 to 0.066/0.059 to 0.071; cinnamon oil 0.038 to 0.057/0.066 to 0.098; thymol, 0.0086 to 0.010/0.016 to 0.022; and thyme oil, 0.0097 to 0.011/0.033 to 0.039. Detailed studies with carvacrol, the main component of oregano oil, showed that antibacterial activity was greater against S. enterica than against E. coli and that wine formulations exhibited high activities at low concentrations of added antimicrobials. The results suggest that wines containing essential oils/oil compounds, added or extracted from oregano or thyme leaves, could be used to reduce pathogens in food and other environments.     

Combined effect of ultrasound and organic acids to reduce Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on organic fresh lettuce

This study was performed to compare the effectiveness of individual treatments (ultrasound and organic acids) and their combination on reducing foodborne pathogens on organic fresh lettuce. Lettuce leaves were inoculated with a cocktail of three strains each of Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes and treated with ultrasound (40 kHz) alone, organic acids (0.3, 0.5, 0.7, 1.0, and 2.0% — malic acid, lactic acid, and citric acid) alone and combined with ultrasound and organic acids for 5 min. For all 3 pathogens, the combined treatment of ultrasound and organic acids resulted in additional 0.8 to 1.0 log reduction compared to individual treatments, without causing significant quality change (color and texture) on lettuce during 7 day storage. The maximum reductions of E. coli O157:H7, S. Typhimurium, and L. monocytogenes were 2.75, 3.18, and 2.87 log CFU/g observed after combined treatment with ultrasound and 2% organic acid for 5 min, respectively. Our results suggest that the combined treatment of ultrasound with organic acids was effective at increasing pathogen reduction compared to individual treatments without significantly affecting quality, and demonstrates its potential as a novel method to increase the microbial safety on organic fresh lettuce.