Differentiation of viable and dead Escherichia coli O157:H7 cells on and in apple structures and tissues following chlorine treatment

Coating with phenolic branched-chain fatty acid reduces Listeria innocua populations on apple fruit

An antimicrobial coating was produced by mixing phenolic branched-chain fatty acid (PBC-FA) with glycerol and a carboxymethyl cellulose solution (CMC) at pH 7. The resulting PBC-FA-CMC solution formed an emulsion with an average droplet size of 77 nm. The emulsion in the coating solution was stable for at least 30 days at 20 °C. The in vitro antimicrobial activity of the film formed from the PBC-FA emulsion was tested against a mixture of 3 strains of Listeria innocua (7 log CFU/mL). Film with a concentration of 1000 μg/mL of PBC-FA effectively reduced the population of L. innocua below the limit of detection (<1.48 log CFU/mL) in vitro. The effect of the 1000 μg/mL PBC-FA-CMC coating formulation was then evaluated against L. innocua inoculated on "Gala" apples. Results showed that compared with the non-coated control, the coating reduced L. innocua populations by ~2 log CFU/fruit and ~6 log CFU/fruit on the apple when enumerated on tryptic soy agar and selective media (PALCAM), respectively, indicating that PBC-FA applied as a coating on apples resulted in the sub-lethal injury of bacterial cells. When L. innocua was inoculated onto PBC-FA-coated apples, the L. innocua population decreased by ~4 log CFU/fruit during 14 days of shelf-life at 20 °C. The PBC-FA coating lowered the moisture loss but did not affect the color, firmness, or soluble solids content of apples during the 14-day at 20 °C. Overall, this study revealed that there is a potential that PBC-FA can be used as an antimicrobial coating to inactivate Listeria and preserve the quality of apples.

Bio-based phenolic branched-chain fatty acid in wash water reduced populations of Listeria innocua on apple fruit

Phenolic branched-chain fatty acid (PBC-FA) emulsion was produced by dissolving it in ethanol and mixing with water (pH 7). The resulting monodispersed emulsion droplets were approximately 200 nm in diameter. The stability of the emulsion was evaluated by storing it at 4 and 20 °C for 30 days. The antimicrobial activity of the PBC-FA emulsion was tested against Escherichia coli and Listeria innocua (8 log CFU/mL) by determining the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) using a microdilution method. The PBC-FA was effective against L. innocua with MIC and MBC of 14.1 μg/mL and caused membrane permeation as determined with SEM and Live/Dead cell assay, but was not effective against E. coli O157:H7 at the tested concentrations (5-250 μg/mL). We also evaluated PBC-FA emulsion's potential to be used as a wash against L. innocua inoculated on apples. The results showed that the 500 μg/mL PBC-FA emulsion with 5 % ethanol had equivalent antimicrobial activity (2-3 logs reductions) against L. innocua as the 20 μg/mL chlorine solution, a commonly used sanitizer. 500 μg/mL PBC-FA emulsion had better antimicrobial efficacy when organic matter (chemical oxygen demand: 9.0 g/L) was present compared to 20 μg/mL of chlorine. The effect of PBC-FA on the quality of the apples, was determined by measuring changes in color, firmness, and soluble solids content over a 14-day storage period at 20 °C. The quality of the apples was not affected by PBC-FA over the 14-day storage period, suggesting that PBC-FA emulsion can be used as a wash for apples without affecting their quality.

Kitchen-Scale Treatments for Reduction of Listeria monocytogenes in Prepared Produce

ABSTRACT

Listeriosis, a foodborne illness caused by Listeria monocytogenes, has relatively low incidence, but a substantial mortality rate, particularly in immunocompromised populations. Because of the known risk of L. monocytogenes and other pathogens in produce, immunocompromised individuals are often placed on neutropenic diets that exclude fresh produce. Therefore, this study aimed to evaluate several kitchen-scale treatments as potential interventions to reduce L. monocytogenes in prepared produce. Cucumbers, apples, and celery were dip inoculated with a three-strain cocktail of L. monocytogenes and dried for 24 h. Inoculated products were subjected to the following treatments as applicable: commercial sanitizer soak (90 s, with agitation), tap water rinse (15 s), tap water soak (90 s, with agitation), surface blanching (25 s), tap water rinse (15 s) followed by peeling, and surface blanching (25 s) followed by peeling. In addition, inoculum uptake in celery and the impact of two types of peelers (mechanical crank and manual) were assessed. Treated samples were plated on differential media and incubated for 48 h at 37°C. L. monocytogenes populations were then enumerated and compared with the untreated control (in log CFUs per gram). All treatments lacked efficacy for celery, with reductions significantly less (P < 0.05) than in other products, likely because of inoculum internalization. The sanitizer soak, tap water rinse, and tap water soak did not differ in efficacy (P > 0.05), which was low for cucumbers (<1.5 log CFU/g), apples (<1.3 log CFU/g), and celery (<0.7 log CFU/g). The two types of apple peelers did not differ in efficacy (P > 0.05). Surface blanching and surface blanching followed by peeling were the most effective treatments for both cucumbers and apples (P < 0.05), with average reductions of 4.2 to 5.1 and 3.5 to 5.9 log CFU/g, respectively.

HIGHLIGHTS

Why food-poisoning bacteria attached to shredded cabbage are not efficiently disinfected by sodium hypochlorite (NaClO)

The aim of this study was to determine why food poisoning bacteria attached to cut cabbage are not efficiently disinfected by sodium hypochlorite (NaClO). Pretreatment of shredded cabbage with diethyl ether definitely decreased the survival numbers of Escherichia coli O157:H7 and Salmonella spp. after disinfection with 100 ppm of NaClO. The density of E. coli O157:H7 at the cut edge of a cabbage section was larger than that on the surface. The residual ratio of attached bacteria at the cut edge after NaClO disinfection was significantly higher than that on the surface. Microscopical observation indicated that the cut edge of shredded cabbage pretreated with diethyl ether was almost closed, resulting in a decrease in bacterial infiltration. Pretreatment of shredded cabbage with a higher concentration of NaClO to penetrate it more deeply significantly decreased the numbers of surviving bacteria after NaClO disinfection. Based on these results, we concluded that the bacteria attached to cut cabbage were not efficiently disinfected by NaClO, because not enough NaClO deeply infiltrated into the cut edges, and hence not enough came in contact with the bacteria.

Inactivation of Escherichia coli O157:H7 during storage or drying of apple slices pretreated with acidic solutions

Inactivation of Escherichia coli O157:H7 was evaluated on inoculated apple slices without pretreatment or pretreated by immersing in water or acid solutions commonly used to help retain apple color during dehydration, then stored at ambient temperature or dried for 6 h. Half-ring slices (0.6 cm thick) of peeled and cored Gala apples were inoculated by immersion for 30 min in a three-strain composite inoculum of E. coli O157:H7 (7.8-8.0 CFU/g). Inoculated slices received (1) no pre-drying treatment (control); or a 10-min immersion in solutions of (2) sterile water, (3) 2.8% ascorbic acid, (4) 1.7% citric acid, (5) 50% commercial lemon juice, or (6) 50% commercial lemon juice with preservatives. Drained slices were placed in sterile plastic bags and stored at room temperature (25+/-2 degrees C) for up to 6 h or dehydrated (62.8 degrees C) for up to 6 h. Samples were plated on tryptic soy agar (TSA) and sorbitol MacConkey agar (SMAC) for direct enumeration of surviving bacteria at various time intervals. Immersion in sterile water or acidic solutions caused initial bacterial reductions of 0.9-1.3 log CFU/g on apple slices. Between 0 and 6 h of storage at room temperature, slices dipped in acidic solutions showed minor changes in bacterial populations (-0.2 to +0.6 log CFU/g) compared to a 1.1 log CFU/g increase for slices dipped in sterile water. The no treatment samples (control) showed an increase in bacterial populations of 1.3-1.5 CFU/g over the 6-h holding time. For apple slices dried at 62.8 degrees C, bacterial populations were reduced by 2.5 (SMAC) and 3.1 (TSA) log CFU/g in the control (no pre-drying treatment) samples following 6 h dehydration. The slices immersed in sterile water showed a 5.8 (SMAC) and 5.1 (TSA) reduction after 6 h of dehydration. In contrast, after 6 h of dehydration bacterial populations on the four acid-pretreated products were reduced by 6.7-7.3 log CFU/g. The results showed that acidic treatment alone was not effective in destroying E. coli O157:H7 on apple slices but did inhibit growth of the organism during holding before drying. However, pretreatment of the apple slices with common household acidulants enhanced destruction of E. coli O157:H7 during drying compared to slices dried without treatment.

Transmission electron microscopy study of enterohemorrhagic Escherichia coli O157:H7 in apple tissue

We investigated the ability of enterohemorrhagic Escherichia coli O157:H7 to spread in wounded apple tissue by transmission electron microscopy. Red Delicious apples were wounded with an artist knife (7 mm depth) and either inoculated with 10 microl per wound of decimally diluted E. coli O157:H7 or submerged into E. coli O157:H7 suspended in sterile distilled water and then stored at 37 degrees C for 24 h. Transmission electron microscopy showed E. coli O157:H7 formed bacterial aggregates near the apple cell walls, and single cells were in close proximity to the apple cell wall surfaces and to plasma membranes. E. coli O157:H7 presence caused degradation of plasma membranes and release of the cytoplasm contents of the apple cortical cells into the central vacuole. Apple tissue turgor pressure tests showed that the apple cells infected with E. coli O157:H7 isolates were more likely to rupture than the control noninoculated apple cells. E. coli O157:H7 cells grown in apple tissue showed the formation of granules and vesicles within the bacterial cytoplasma and separation of the plasma membranes. Our study shows that E. coli O157:H7 can grow and survive in the apple tissue environment by causing degradation of the apple cellular components.

Enumeration of soil bacteria with the green fluorescent nucleic acid dye Sytox green in the presence of soil particles

Total counts in soils are usually determined using fluorescent dyes, such as DAPI or Sybr green, due to fluorescence enhancement if they are bound to nucleic acids. Unfortunately, these commonly used dyes stain soil particles as well. Therefore, besides fluorescence enhancement, sufficient spectral differentiation is also required. We present a new procedure that overcomes the problems of visualising bacteria on surfaces in soil and avoids the separation of soil particles to a large extent. Spectral differentiation between bacteria and soil matrix is achieved by using Sytox green and a suboptimal excitation wavelength. Bacteria exhibit a bright green fluorescence, while soil particles fluoresce blue or red. Slight homogenisation and sedimentation of the sand and coarse silt that were too big for microscopic investigations were the only separation steps required. We compared the proposed Sytox green staining with Sybr green staining. The recovery of Sybr green-stained cells amounted to 38%, whereas in samples stained by Sytox green 81% of the spiked cells were counted. Sytox green can also be combined with fluorescence in situ hybridisation (FISH) using deep red dyes such as Cy5.

Survival of acid-adapted or nonadapted Escherichia coli O157:H7 in apple wounds and surrounding tissue following chemical treatments and storage

This study evaluated survival/growth of acid-adapted or nonadapted Escherichia coli O157:H7 inoculated (4 log CFU/wound) in wounds (10 mm deepx6 mm diameter) of apples. Wounds were inoculated with a green fluorescent protein (GFP)-expressing derivative of a rifampicin-resistant strain of E. coli O157:H7 ATCC 43895 and allowed to attach (1 h). Apples were dipped (2 min) in solutions (approximately 25 degrees C) of water (W), 5% acetic acid (AA), 5% hydrogen peroxide (HP), 0.02% sodium hypochlorite (SH), or not treated (NT), and stored at 25 degrees C. Survivors were determined in cores (10-mm deep) of the apple wounds (12 mm from center of wound; inner core) and surrounding tissue (18 mm from center of wound; outer core) after homogenizing the samples in Dey-Engley (D/E) neutralizing broth and plating on tryptic soy agar (TSA) and TSA supplemented with 100 microg/ml rifampicin (35 degrees C, 48 h) after 0, 2 and 5 days. Average bacterial populations at day-0 were 4.0 and 2.0 logs in the inner and outer core, respectively. In the inner core of the untreated apples populations increased to 7.0 logs at day-2, while counts did not exceed 3.0 logs in the outer core during storage of all treatments. Previous acid-adaptation of the cultures did not affect survival of the pathogen. Dipping in W, AA and SH did not reduce initial bacterial populations, while at day-2 of storage inner core counts from W, AA and SH reached 7.1, 5.5 and 6.9 logs, respectively. In contrast, HP reduced initial counts in the inner core by approximately 1.5 logs, but they increased to 7.0 logs by day-2. Populations of all treatments reached 6.6-7.2 logs in the inner core by day-5. Thus, sanitizer treatment did not effectively reduce nor inhibit growth of E. coli O157:H7 contamination in apple wounds and surrounding tissue.

Survival and growth of Listeria monocytogenes and enterohemorrhagic Escherichia coli O157:H7 in minimally processed artichokes

The ability of Listeria monocytogenes and Escherichia coli O157:H7 inoculated by immersion (at 4.6 and 5.5 log CFU/ g, respectively) to survive on artichokes during various stages of preparation was determined. Peeling, cutting, and disinfecting operations (immersion in 50 ppm of a free chlorine solution at 4 degrees C for 5 min) reduced populations of L. monocytogenes and E. coli O157:H7 by only 1.6 and 0.8 log units, respectively. An organic acid rinse (0.02% citric acid and 0.2% ascorbic acid) was more effective than a tap water rinse in removing these pathogens. Given the possibility of both pathogens being present on artichokes at the packaging stage, their behavior during the storage of minimally processed artichokes was investigated. For this purpose, batches of artichokes inoculated with L. monocytogenes or E. coli O157:H7 (at 5.5 and 5.2 log CFU/g, respectively) were packaged in P-Plus film bags and stored at 4 degrees C for 16 days. During this period, the equilibrium atmosphere composition and natural background microflora (mesophiles, psychrotrophs, anaerobes, and fecal coliforms) were also analyzed. For the two studied pathogens, the inoculum did not have any effect on the final atmospheric composition (10% O2, 13% CO2) or on the survival of the natural background microflora of the artichokes. L. monocytogenes was able to survive during the entire storage period in the inoculated batches, while the E. coli O157:H7 level increased by 1.5 log units in the inoculated batch during the storage period. The modified atmosphere was unable to control the behavior of either pathogen.