Validating the use of green fluorescent-marked Escherichia coli O157:H7 for assessing the organism behaviour in foods

Rapid loss of a green fluorescent plasmid in Escherichia coli O157:H7

Plasmids encoding green fluorescent protein (GFP) are frequently used to label bacteria, allowing the identification and differentiation from background flora during experimental studies. Because of its common use in survival studies of the foodborne pathogen Escherichia coli O157:H7, it is important to know the extent to which the plasmid is retained in this host system. Herein, the stability of a pGFPuv (Clontech Laboratories Inc) plasmid in six Escherichia coli O157:H7 isolates was assessed in an oligotrophic environment (phosphate buffered saline, PBS) without antibiotic selective pressure. The six test isolates were recovered from a variety of animal and human sources (cattle, sheep, starlings, water buffalo, and human feces). GFP labeling of the bacteria was accomplished via transfer electroporation. The stability of the GFP plasmid in the different E. coli O157:H7 isolates was variable: in one strain, GFP plasmid loss was rapid, as early as one day and complete plasmid loss was exhibited by four of the six strains within 19 days. In one of the two isolates retaining the GFP plasmid beyond 19 days, counts of GFP-labeled E. coli O157:H7 were significantly lower than the total cell population (P < 0.001). In contrast, in the other isolate after 19 days, total E. coli O157:H7 counts and GFP-labeled E. coli counts were equivalent. These results demonstrate strain-to-strain variability in plasmid stability. Consequently the use of GFP-labeled E.coli O157:H7 in prolonged survival studies may result in the underestimation of survival time due to plasmid loss.

Characterization and survival of environmental Escherichia coli O26 isolates in ground beef and environmental samples

In addition to Escherichia coli O157:H7, shiga toxin-producing E. coli (STEC) O26 was added to the zero-tolerance adulterant list together with other 5 non-O157 STEC serogroups in 2012. Four farm O26 isolates were used in this study; they were obtained from a on-farm survey study conducted in Alabama. The presence of 3 major pathogenic genes (stx1, stx2, and eaeA) was determined through multiplex polymerase chain reaction (PCR). Two major pathogenic gene profiles were observed: 3 of the farm isolates contain only the eaeA gene whereas 1 farm isolate has both the eaeA and the stx1 genes. No significant difference was seen among the 4 farm isolates in the antibiotic resistance tests. To test their survival in ground beef and environmental samples, 2 inoculums were prepared and inoculated at various concentrations into samples of ground beef, bovine feces, bedding materials, and trough water. One inoculum was made of 3 farm isolates containing only the eaeA gene and another inoculum contained the isolate with both the eaeA and stx1 genes. Inoculated beef samples were stored at 4 °C for 10 d and the inoculated environmental samples were stored at ambient temperature for 30 d. Results showed that virulence gene profiles do not have an impact on O26's ability to survive in ground beef and in environment (P > 0.05). The inoculation levels, sample types as well as the storage times are the major factors that impact O26 survival (P < 0.05).

Decontamination of green onions and baby spinach by vaporized ethyl pyruvate

Foodborne illnesses associated with fresh produce continue to be a major concern as consumer demand for healthier and nonthermally processed food increases. The objective of this study was to evaluate vaporized ethyl pyruvate (EP; CAS 617-35-6) as a safe alternative antimicrobial agent for the decontamination of Escherichia coli O157:H7 on green onions and spinach. Baby spinach leaves and green onions were inoculated with a five-strain cocktail of E. coli O157:H7 (pGFP) by the dipping method. Samples were treated with concentrations of 0, 42, 105, and 420 mg/liter vaporized EP in a 2.6-liter enclosed container. The efficacy of EP vapors for reducing E. coli O157:H7((GFP)) populations on green onions and baby spinach at 4 and 10°C was monitored for 7 and 5 days, respectively. The lowest EP concentration (42 mg/liter) resulted in a 1.7-log reduction of E. coli O157:H7((GFP)) on green onions after 7 days at 4°C and a 1.9-log reduction after 5 days at 10°C (P < 0.05). In baby spinach, the same concentration resulted in 0.9-log and 1.4-log reductions (P < 0.05) of E. coli O157:H7((GFP)) after 7 days at 4°C and 5 days at 10°C, respectively. On green onions, the highest concentration of EP (420 mg/liter) reduced the population of E. coli O157:H7((GFP)) by >4.7 log CFU/g after 7 days at 4°C and 5 days at 10°C. The same concentration was also effective for reducing E. coli O157:H7((GFP)) populations in baby spinach by 4.3 log CFU/g after 7 days at 4°C and by >6.5 log CFU/g after 3 days at 10°C. Although the successful EP treatments minimally affected the sensory attributes of green onions, the treatments resulted in significant changes in the sensory attributes of baby spinach samples stored at 4 and 10°C. These results indicate that EP is an effective antimicrobial that could be used to enhance the safety of fresh produce depending on the sensory characteristics of the product.

Transfer of enteric pathogens to successive habitats as part of microbial cycles

Escherichia coli O157:H7 gfp and Salmonella enterica Typhimurium gfp passed through six successive habitats within a microbial cycle. Pathogen cultures were introduced into cow dung or fodder. Microscopically observed cells and CFUs were monitored in fodder, dung, dung-soil mix, rhizosphere and phyllosphere of cress or oat plants grown in infested dung-soil mix, and in excrements of snails or mice fed with contaminated cress or oat shoots. Both methods were sensitive enough to monitor cells and CFUs throughout the chain. There was a positive correlation between cells and CFUs. Both pathogens declined through the successive habitats, but with unexpected increased densities on plants compared to dung-soil mix. Pathogen densities were higher in the phyllosphere than the rhizosphere of cress, but for oat plants this was reverse. Survival in dung was better after passage through the digestive tract of cows than after introduction of cultures into dung. Positive correlations between pathogens and copiotrophic bacteria (CB) and dissolved organic carbon (DOC) were observed in dung and dung-soil mixtures, but at low DOC contents CB densities were higher than pathogen densities. Thus, the pathogens are able to cycle through different habitats, surviving or growing better at high DOC concentrations, but maintaining population densities that are sufficiently high to cause disease in humans.

The effect of the native bacterial community structure on the predictability of E. coli O157:H7 survival in manure-amended soil

AIMS

The survival capability of pathogens like Escherichia coli O157:H7 in manure-amended soil is considered to be an important factor for the likelihood of crop contamination. The aim of this study was to reveal the effects of the diversity and composition of soil bacterial community structure on the survival time (ttd) and stability (irregularity, defined as the intensity of irregular dynamic changes in a population over time) of an introduced E. coli O157:H7 gfp-strain were investigated for 36 different soils by means of bacterial PCR-DGGE fingerprints.

METHODS AND RESULTS

Bacterial PCR-DGGE fingerprints made with DNA extracts from the different soils using bacterial 16S-rRNA-gene-based primers were grouped by cluster analysis into two clusters consisting of six and 29 soils and one single soil at a cross-correlation level of 16% among samples per cluster. Average irregularity values for E. coli O157:H7 survival in the same soils differed significantly between clusters (P = 0.05), whereas no significant difference was found for the corresponding average ttd values (P = 0.20). The irregularity was higher for cluster 1, which consisted primarily of soils that had received liquid manure and artificial fertilizer and had a significant higher bacterial diversity and evenness values (P < 0.001).

CONCLUSIONS

Bacterial PCR-DGGE fingerprints of 36 manure-amended soils revealed two clusters which differed significantly in the stability (irregularity) of E. coli O157 decline. The cluster with the higher irregularity was characterized by higher bacterial diversity and evenness.

SIGNIFICANCE AND IMPACT OF THE STUDY

The consequence of a high temporal irregularity is a lower accuracy of predictions of population behaviour, which results in higher levels of uncertainty associated with the estimates of model parameters when modelling the behaviour of E. coli O157:H7 in the framework of risk assessments. Soil community structure parameters like species diversity and evenness can be indicative for the reliability of predictive models describing the fate of pathogens in (agricultural) soil ecosystems.

Fluorescent protein-marked Escherichia coli biotype I strains as surrogates for enteric pathogens in validation of beef carcass interventions

The efficacy of antimicrobial interventions implemented in slaughter establishments to reduce enteric pathogens on beef carcasses should optimally be validated under commercial operation conditions. This study was conducted to identify surrogate organisms for enteric pathogens that could be used to validate beef carcass interventions. The growth, resistance, and attachment properties of nonpathogenic fluorescent protein-marked Escherichia coli strains were compared with those of E. coli O157:H7 and Salmonella strains. Growth curves were obtained based on growth in tryptic soy broth at 37 degrees C. In general, growth parameters were not different among potential surrogates and target pathogens (P > 0.05). Thermal resistance was compared in phosphate-buffered saline (pH 7.4) at 55, 60, and 65 degrees C, and D-values of potential surrogates were not different (P > 0.05) or were higher (P < 0.05) than those of the target pathogens. Acid resistance was tested in phosphate-buffered saline acidified with L-lactic acid at pH 2.5, 3.0, and 3.5, and log reductions (CFU per milliliter) were not different (P > 0.05) among potential surrogates and E. coli O157:H7 strains; however, some Salmonella serotypes were less acid resistant than were surrogates (P < 0.05). The cell surface hydrophobicity was different (P < 0.05) among surrogates and some E. coli O157:H7 strains, but the strength of attachment to beef carcasses was not different (P > 0.05) among all microorganisms. Log reductions (CFU per square centimeter) after application of hot water washes and 2% L-lactic acid sprays on beef carcasses were not different (P > 0.05) among surrogates and pathogens. The nonpathogenic E. coli strains evaluated in this study could be used as surrogates for E. coli O157:H7 and Salmonella to validate hot water and lactic acid interventions on beef carcasses.

Estimating the stability of Escherichia coli O157:H7 survival in manure-amended soils with different management histories

The objective of this study is to describe survival of Escherichia coli O157:H7 populations in manure-amended soils in terms of population stability, i.e. the temporal variation around the decline curve, in relation to soil characteristics indicative of soil health. Cow manure inoculated with E. coli O157:H7 was mixed with 18 pairs of organically and conventionally managed soils (10% of manure, kg kg(-1)). For four of the soil pairs, also three different manure densities (5%, 10% and 20%) were compared. All soil-manure mixtures were incubated for 2 months, and population densities of E. coli O157:H7 were quantified weekly. De-trending of survival data was done by modified logistic regression. The residual values were used to assess variation in the changes of E. coli O157:H7 populations by performing the approximate entropy (ApEn) procedure. The term irregularity is used to describe this variation in ApEn literature. On average, the decline of E. coli O157:H7 was more irregular in conventional and loamy soils than in organic and sandy soils (P < 0.05). Multiple regression analysis of irregularity of E. coli O157:H7 survival on 13 soil characteristics showed a positive relation with the ratio of copiotrophic/oligotrophic bacteria, suggesting greater instability at higher available substrate concentrations. Incremental rates of manure application significantly changed the irregularity for conventional soils only. Estimation of temporal variation of enteropathogen populations by the ApEn procedure can increase the accuracy of predicted survival time and may form an important indication for soil health.

Monitoring biosensor capture efficiencies: development of a model using GFP-expressing Escherichia coli O157:H7

One of the known limitations for biosensor assays is the high limit of detection for target cells within complex samples (e.g., Escherichia coli at 10(4) to 10(5) CFU/mL) due to poor capture efficiencies. Currently, researchers can only estimate the cell capture efficiency necessary to produce a positive signal for any type of biosensor using either cumbersome techniques or regression modeling. To solve this problem, green fluorescent protein (GFP) transformed E. coli O157:H7 was used to develop a novel method for directly and easily measuring the cell capture efficiency of any given biosensor platform. For demonstration purposes, E. coli-GFP was assayed on both fiber optic and planar waveguide biosensor platforms. Cells were enumerated using an epifluorescent microscope and digital camera to determine the number of cells captured on the surfaces. Conversion algorithms were used with these digital images to determine the cell density of entire waveguide surface areas. For E. coli-GFP, the range of cell capture efficiency was between 0.4 and 1.2%. This indicates that although the developed model works for calculating cell capture, there is still need for significant improvements in capture methods themselves, to increase the capture efficiency and thereby lower detection limits. The use of GFP-transformed target cells and cell capture efficiency calculations can facilitate the development and optimization processes by allowing direct enumeration of new biosensor design configurations and sample processing strategies.

Persistence of Shiga toxin-producing Escherichia coli O26 in cow slurry

AIMS

The main objective of this study was to evaluate the growth and survival of Shiga toxin-producing Escherichia coli (STEC) O26 in cow slurry; this serogroup is regarded as an important cause of STEC-associated diseases.

METHODS AND RESULTS

Four STEC were examined by polymerase chain reaction (PCR) to determine whether they harbour key virulence determinants and also by pulsed-field gel electrophoresis (PFGE) to obtain overview fingerprints of their genomes. They were transformed with the pGFPuv plasmid and were separately inoculated at a level of 10(6) CFU ml(-1) in 15 l of cow slurry. All STEC O26 strains could be detected for at least 3 months in cow slurry without any genetic changes. The moisture content of the slurry decreased over time to reach a final value of 75% while the pH increased from 8.5 to 9.5 units during the last 50 days.

CONCLUSION

STEC O26 strains were able to survive in cow slurry for an extended period.

SIGNIFICANCE AND IMPACT OF THE STUDY

Long-term storage of waste slurry should be required to reduce the pathogen load and to limit environmental contamination by STEC O26.

Growth and survival of non-O157:H7 Shiga-toxin-producing Escherichia coli in cow manure

AIMS

The main objective of this study was to evaluate the behaviour of non-O157:H7 Shiga-toxin-producing Escherichia coli (STEC) strains in cow manure.

METHODS AND RESULTS

A mixture of eight green-fluorescent-protein-labelled STEC strains was inoculated around 10(6)-10(7) CFU g(-1) into four manure heaps. Two heaps were regularly turned and the two others remained unturned. STEC counts and physical parameters (temperature, pH, moisture content and oxido-reduction potential) were monitored for 1000 manure samples. The highest mean pH values were obtained near the surface at the base of all manure heaps. At the surface, the moisture content decreased from 76.5% to 42% in turned heaps. Temperatures reached 65 degrees C near the main body of all manure heaps, and only 35 degrees C near the superficial parts located at the base of them. These two sites (the centre and the base) were associated with D values for the STEC counts of 0.48 and 2.39 days, respectively. We were able to detect STEC strains during 42 days in turned manure heaps and during at least 90 days in unturned ones.

CONCLUSIONS

These results emphasize the long-term survival of non-O157:H7 STEC in cow manure.

SIGNIFICANCE AND IMPACT OF THE STUDY

Good management practices (e.g. turning) should be respected in order to minimize the risk of environmental contamination by STEC.

Quantification of total and bioavailable lysine in feed protein sources by a whole-cell green fluorescent protein growth-based Escherichia coli biosensor

Using a fluorescent whole-cell Escherichia coli biosensor previously developed in our laboratory, we determined total and bioavailable lysine in four feed ingredients (soybean, cottonseed, meat and bone meal, and sorghum) and three complete feeds (chick starter and finisher, and swine starter). The same feed sources were analyzed for total lysine by high performance liquid chromatography (HPLC) and bioavailable lysine by chick bioassay. No significant differences were found between bioavailable lysine estimates for soybean, cottonseed, meat and bone meal, chick starter and finisher, and swine starter obtained by the fluorescent E. coli biosensor and chick bioassay. Except for sorghum, the E. coli biosensor estimates for total lysine were highly comparable to those obtained by HPLC. Comparisons were also conducted between conventionally performed optical density-based and the newly developed fluorescence-based lysine assay. The lack of significant differences in data obtained for total and bioavailable lysine by both detection modes indicated reliance and accuracy of the fluorescent E. coli biosensor. Overall results suggest that the microbial assay based on green fluorescent protein fluorescence represents a promising alternative method for lysine quantification.

Inactivation of escherichia coli O157:H7 in cattle drinking water by sodium caprylate

Escherichia coli O157:H7 is an important foodborne pathogen. Cattle serve as one of the major reservoirs of E. coli O157:H7, excreting the pathogen in feces. Environmental persistence of E. coli O157:H7 is critical in its epidemiology on farms, and the pathogen has been isolated from cattle water troughs. Thus, there is a need for an effective method for killing E. coli O157:H7 in cattle drinking water. In this study, the efficacy of sodium caprylate for killing E. coli O157:H7 in cattle drinking water was investigated. A four-strain mixture of E. coli O157:H7 was inoculated (6.0 log CFU/ml) into 100-ml samples of well water containing 0, 75, 100, or 120 mM sodium caprylate. Water samples containing 1% (wt/vol) bovine feces or feed also were included. The samples were incubated at 21 or 8 degrees C for 21 days. Water samples were analyzed for viable E. coli O157:H7 on days 0, 1, 3, 5, and 7 and weekly thereafter. Triplicate samples of each treatment and control were included, and the study was repeated twice. The magnitude of E. coli O157:H7 inactivation in water significantly increased (P < 0.01) with increases in caprylate concentration and storage temperature. At 120 mM, sodium caprylate completely inactivated E. coli O157:H7 in all the samples after 1 to 20 days, depending on the treatments. The presence of feces or feed also had a significant effect (P < 0.01) on the antibacterial property of caprylate; the presence of feces decreased the antibacterial effect, whereas addition of feed enhanced the effect. These results indicate that sodium caprylate is effective in killing E. coli O157:H7 in cattle drinking water, but detailed cattle palatability studies of water containing caprylate are necessary.

Application of an Escherichia coli green fluorescent protein-based lysine biosensor under nonsterile conditions and autofluorescence background

AIMS

To examine the utility of an Escherichia coli green fluorescent protein (GFP) containing biosensor for quantification of bioavailable lysine in selected feed samples under nonsterile conditions and to estimate the background fluorescence of analyzed feed samples and evaluate the risk of confounding GFP emission from the lysine assay organism.

METHODS AND RESULTS

Escherichia coli lysine auxotroph GFP based biosensor was used to determine the percentage of bioavailable lysine in two samples of soybean-, cottonseed-, and meat and bone meal under nonsterile conditions. The fluorescence emitted by GFP was successfully measured using a spectrofluorimeter to monitor bacterial growth response to protein-derived lysine and lysine containing small peptides. The autofluorescence of analyzed feed samples at different concentrations could also be estimated.

CONCLUSIONS

When feed protein concentrations are decreased, autofluorescence interference can be avoided.

SIGNIFICANCE

The E. coli lysine auxotroph GFP-based biosensor can successfully be used for the determination of bioavailable lysine in these selected animal feed proteins under nonsterile conditions.

IMPACT OF THE STUDY

E. coli GFP biosensor for lysine has potential for routine application in animal feeds.

Transformation of Escherichia coli K-12 with a high-copy plasmid encoding the green fluorescent protein reduces growth: implications for predictive microbiology

The green fluorescent protein (GFP) of the jellyfish Aequorea victoria has been widely used as a biomarker and has potential for use in developing predictive models for growth of pathogens on naturally contaminated food. However, constitutive production of GFP can reduce growth of transformed strains. Consequently, a high-copy plasmid with gfp under the control of a tetracycline-inducible promoter (pTGP) was constructed. The plasmid was first introduced into a tetracycline-resistant strain of Escherichia coli K-12 to propagate it for subsequent transformation of tetracycline-resistant strains of Salmonella. In contrast to transformed E. coli K-12, which only fluoresced in response to tetracycline, transformed Salmonella fluoresced maximally without tetracycline induction of gfp. Although pTGP did not function as intended in Salmonella, growth of parent and GFP E. coli K-12 was compared to test the hypothesis that induction of GFP production reduced growth. Although GFP production was not induced during growth on sterile chicken in the absence of tetracycline, maximum specific growth rate (mumax) of GFP E. coli K-12 was reduced 40 to 50% (P < 0.05) at 10, 25, and 40 degrees C compared with the parent strain. When growth of parent and GFP strains of E. coli K-12 was compared in sterile broth at 40 degrees C, mumax and maximum population density of the GFP strain were reduced (P < 0.05) to the same extent (50 to 60%) in the absence and presence of tetracycline. These results indicated that transformation reduced growth of E. coli K-12 independent of gfp induction. Thus, use of a low-copy plasmid or insertion of gfp into the chromosome may be required to construct valid strains for development of predictive models for growth of pathogens on naturally contaminated food.

Survival of salmonella transformed to express green fluorescent protein on Italian parsley as affected by processing and storage

To study the effect of processing and storage parameters on the survival of Salmonella on fresh Italian parsley, parsley bunches were dipped for 3 or 15 min in suspensions that were preequilibrated to 5, 25, or 35 degrees C and inoculated with Salmonella transformed to express enhanced green fluorescent protein. Loosely attached and/or associated, strongly attached and/or associated, and internalized and/or entrapped Salmonella cells were enumerated over 0, 1, and 7 days of storage at 25 degrees C and over 0, 1, 7, 14, and 30 days of storage at 4 degrees C using surface-plating procedures. Leaf sections obtained from samples after 0, 1, and 7 days of storage were examined using confocal scanning laser microscopy. Temperature of the dip suspension had little effect on the attachment and survival of Salmonella cells on parsley. Regardless of the temperature or duration of dip, Salmonella was internalized. Immersion for longer times resulted in higher numbers of attached and internalized cells. Microscopic observations supported these results and revealed Salmonella cells near the stomata and within cracks in the cuticle. Storage temperature had the greatest impact on the survival of Salmonella cells on parsley. When stored at 25 degrees C, parsley had a shelf life of 7 days, and Salmonella populations significantly increased over the 7 days of storage. For parsley stored at 4 degrees C, numbers of Salmonella cells decreased over days 0, 1, and 7. After 7 days of storage, there were no viable internalized Salmonella cells detected. Storage temperature represents an important control point for the safety of fresh parsley.