Initial Inoculum Level Research Articles

Bacterial dynamics in model cheese systems, aiming at safety and quality of Portuguese-style traditional ewe's cheeses.

An experiment using model ewe's milk cheeses was designed to characterize microbial interactions that arise in actual raw milk cheese environments. These model cheeses were manufactured according to Portuguese artisanal practices, except that the microbial load and biodiversity were fully controlled: single potential pathogens and spoilage bacteria, or a combination thereof, were combined at various initial inoculum levels in sterilized raw ewe's milk with several lactic acid bacteria (LAB) normally found in traditional cheeses. Viable microbial counts were monitored throughout a 60-day ripening period. Two alternative mathematical approaches were used to fit the experimental data generated in terms of population dynamics: percent of inhibition and D-values. These were able to explain the complex competitive interactions between the contaminant microorganisms and the LAB adventitious populations. In general, the tested LAB were less able to inhibit contaminants present in combination and in higher concentrations. Lactococcus lactis, with its strong acidifying potential, was the most effective factor in controlling the unwanted bacterial population, especially single Staphylococcus aureus. The two lactobacilli studied, especially Lactobacillus brevis, were shown to be less effective; Escherichia coli and Listeria innocua were the contaminants least inhibited by the LAB.

Effect of high pressure homogenization on Saccharomyces cerevisiae inactivation and physico-chemical features in apricot and carrot juices

This experimental work aimed to evaluate the potential of high pressure homogenization (HPH) to inactivate Saccharomyces cerevisiae 635 inoculated both in apricot and carrot juices. The sensitivity of the yeast was evaluated both in relation to its initial inoculum level (about 3.0 and 6.0 Log 10 cfu/ml) and number of passes applied at 100 MPa. Moreover, the effects of repeated pressure treatments at 100 MPa were evaluated for pH, water activity and viscosity of carrot and apricot juices. Data obtained showed that repeated high pressure homogenization passes at 100 MPa allowed a significant inactivation of the spoilage yeast inoculated in both juices. However, as expected, the inactivation of the considered strain was greatly affected by the food matrix. In fact, a higher viability loss of S. cerevisiae 635 was observed in carrot juice than in apricot juice. Concerning the recovery, data obtained showed that the decrease of the inoculum level to 3.0 Log 10 cfu/ml prevented (at least for 6 days) cell proliferation in the samples of apricot and carrot juices treated with more than four and seven passes, respectively. Also the refrigeration of the treated samples prevented cell recovery and, in some cases, induced a further decrease in cell viability also in the samples inoculated with 6.0 Log 10 cfu/ml allowing a further increase in the juice shelf-life. An interesting and promising result was the enhanced viscosity of apricot juices treated up to five passes at 100 MPa.

Mathematical modeling the cross-contamination of Escherichia coli O157:H7 on the surface of ready-to-eat meat product while slicing

Microbial cross-contamination either at home or production site is one of the major factors of causing contamination of foods and leading to the foodborne illness. The knowledge regarding Escherichia coli O157:H7 surface transfer on ready-to-eat (RTE) deli meat and the slicer used for slicing different RTE products are needed to ensure RTE food safety. The objectives of this study were to investigate and to model the surface cross-contamination of E. coli O157:H7 during slicing operation. A five-strain cocktail of E. coli O157:H7 was inoculated directly onto a slicer's round blade rim area at an initial level of ca. 4, 5, 6, 7 or 8 log CFU/blade (ca. 3, 4, 5, 6 or 7 log CFU/cm 2 of the blade edge area), and then the RTE deli meat (ham) was sliced to a thickness of 1–2 mm. For another cross-contamination scenario, a clean blade was initially used to slice ham which was pre-surface-inoculated with E. coli O157:H7 (ca. 4, 5, 6, 7 or 8 log CFU/100 cm 2 area), then, followed by slicing un-inoculated ham. Results showed that the developed empirical models were reasonably accurate in describing the transfer trend/pattern of E. coli O157:H7 between the blade and ham slices when the total inoculum level was ≥5 log CFU on the ham or blade. With an initial inoculum level at ≤4 log CFU, the experimental data showed a rather random microbial surface transfer pattern. The models, i.e., a power equation for direct-blade-surface-inoculation, and an exponential equation for ham-surface-inoculation are microbial load and sequential slice index dependent. The surface cross-contamination prediction of E. coli O157:H7 for sliced deli meat (ham) using the developed models were demonstrated. The empirical models may provide a useful tool in developing the RTE meat risk assessment.

Burkholderia spp. are the most competitive symbionts of Mimosa, particularly under N‐limited conditions

Bacteria isolated from Mimosa nodules in Taiwan, Papua New Guinea, Mexico and Puerto Rico were identified as belonging to either the alpha- or beta-proteobacteria. The beta-proteobacterial Burkholderia and Cupriavidus strains formed effective symbioses with the common invasive species Mimosa diplotricha, M. pigra and M. pudica, but the alpha-proteobacterial Rhizobium etli and R. tropici strains produced a range of symbiotic phenotypes from no nodulation through ineffective to effective nodulation, depending on Mimosa species. Competition studies were performed between three of the alpha-proteobacteria (R. etli TJ167, R. tropici NGR181 and UPRM8021) and two of the beta-rhizobial symbionts (Burkholderia mimosarum PAS44 and Cupriavidus taiwanensis LMG19424) for nodulation of these invasive Mimosa species. Under flooded conditions, B. mimosarum PAS44 out-competed LMG19424 and all three alpha-proteobacteria to the point of exclusion. This advantage was not explained by initial inoculum levels, rates of bacterial growth, rhizobia-rhizobia growth inhibition or individual nodulation rate. However, the competitive domination of PAS44 over LMG19424 was reduced in the presence of nitrate for all three plant hosts. The largest significant effect was for M. pudica, in which LMG19424 formed 57% of the nodules in the presence of 0.5 mM potassium nitrate. In this host, ammonium also had a similar, but lesser, effect. Comparable results were also found using an N-containing soil mixture, and environmental N levels are therefore suggested as a factor in the competitive success of the bacterial symbiont in vivo.

The Role of Different Inoculum Levels of Meloidogyne javanica Juveniles on Nematode Reproduction and Host Response of Peanut Plant

A pot experiment was conducted to determine the influence of three of inoculum levels (1000, 2000 and 3000 J2 pot(-1)) of Meloidogyne javanica on nematode reproduction and host response of peanut plant cv. Giza 4 under greenhouse conditions at 30 +/- 5 degrees C. In general, nematode reproduction and host damage were both affected by the initial inoculum levels. The greater reduction percentage of plant fresh (57.7%), shoot dry (38.82) and pods weights (52.59%) and nodules numbers (73.33%) were recorded at inoculum level 2000 J2/peanut plant, when rate of nematode build-up reached the maximum value of 1.64. Regression analysis of Pi vs. rate of nematode build-up on peanut plants gave value of R2 amounted to 0.3193. On the other hand, when the initial inoculum level added increased up to 3000 J2/peanut plant, the percentage reduction of whole plant fresh weight (47.07%) and other growth parameters as well as nematode build-up (0.8) also obviously decreased.

Differential Structural Responses of Ginseng Root Tissues to Different Initial Inoculum Levels of Paenibacillus polymyxa GBR-1

Root discs of 4-year-old ginseng, Panax ginseng C. A. Meyer, were inoculated with the higher(<TEX>$10^8$</TEX> colonyforming units(CFU)/ml) and lower(<TEX>$10^6\;or\;10^5$</TEX> CFU/ml) initial inoculum levels of a plant-growth promoting rhizobacterium(PGPR), Paenibacillus polymyxa GBR-1 to examine rot symptom development and bacterial population changes on the root discs. At the higher inoculum level, brown rot symptoms developed and expanded on the whole root discs in which the bacterial population increased continuously up to 4 days after inoculation. In light and electron microscopy, ginseng root cells on the inoculation sites were extensively decayed, which were characterized by dissolved cell walls and destructed cytoplasmic contents. However, no rot symptoms were developed and the bacterial population increased only during the initial two days of inoculation at the lower inoculum level(<TEX>$10^6$</TEX> CFU/ml) of P. polymyxa GBR-1. At the lower inoculum level(<TEX>$10^5$</TEX> CFU/ml), boundary layers with parallel periclinal cell divisions, structurally similar to wound periderm, were formed internal to the inoculation sites, beneath which the cells were intact containing numerous normal-looking starch granules and no disorganized cell organelles, suggesting that these structural features may be related to the suppression of symptom development, a histological defense mechanism.

A Rapid and Simple DNA Extraction Procedure to Detect Salmonella spp. and Listeria monocytogenes from Fresh Produce Using Real-time PCR

DNA isolation procedures significantly influence the outcome of PCR-based detection of human pathogens. Unlike clinical samples, DNA isolation from food samples, particularly from fresh and fresh-cut produce has remained a formidable task and has hampered the sensitivity and accuracy of molecular methods. We utilized a commercially available filter-based DNA isolation method (FTA) in conjunction with real-time PCR-based detection of Salmonella spp. and Listeria monocytogenes. The protocol uses filter paper discs impregnated with a patented chemical formulation that lyses cells, immobilizes DNA, and protects it from degradation. Use of the FTA method in conjunction with real-time PCR for the detection of Salmonella spp. and L. monocytogenes was compared with two commercially available DNA isolation procedures that are commonly used for high throughput real-time PCR pathogen detection systems. Both pathogens were successfully detected from artificially inoculated fresh and fresh-cut produce such as alfalfa sprouts, cilantro, green onion, broccoli, prepacked mixed salad, and spinach at low cell numbers (four to seven colony forming units per 25 g initial inoculum level before enrichment). The FTA protocol had distinct advantages of simplicity, biosafety, and compatibility for high throughput screening. This DNA preparation protocol was rapid, sensitive, required minimal handling, and reduced interference from produce-associated inhibitors of real-time PCR.

Use of mild-heat treatment following high-pressure processing to prevent recovery of pressure-injured Listeria monocytogenes in milk

This study examined the inactivation of Listeria monocytogenes in milk by high-pressure processing (HPP) and bacterial recovery during storage after HPP. We developed a technique to inhibit the bacterial recovery during storage after HPP (550 MPa for 5 min) using a mild-heat treatment (30–50 °C). Various mild-heat treatments were conducted following HPP to investigate the condition on which the bacterial recovery was prevented. Immediately after HPP of 550 MPa at 25 °C for 5 min, no L. monocytogenes cells were detected in milk regardless of the inoculum levels (3, 5, and 7 log 10 CFU/ml). However, the number of L. monocytogenes cells increased by >8 log 10 CFU/ml regardless of the inoculum levels after 28 days of storage at 4 °C. Significant recovery was observed during storage at 25 °C; the bacterial number increased by >8 log 10 CFU/ml after 3 days of storage in the case of an initial inoculum level of 7 and 5 log 10 CFU/ml. Even in the case of an initial inoculum level of 3 log 10 CFU/ml, the bacterial number reached the level of 8 log 10 CFU/ml after 7 days of storage. No bacterial recovery was observed with storage at 37 °C for 28 days. Milk samples were treated by various mild-heat treatments (30–50 °C for 5–240 min) following HPP of 550 MPa at 25 °C for 5 min, and then stored at 25 °C for 70 days. The mild-heat treatment (e.g., 37 °C for 240 min or 50 °C for 10 min) inhibited the recovery of L. monocytogenes in milk after HPP. No recovery of L. monocytogenes in milk was observed during 70-day storage at 25 °C in samples that received mild-heat treatments such as mentioned above following HPP (550 MPa for 5 min). Moreover, the mild-heat treatment conditions (temperature and holding time) required to inhibit the recovery of L. monocytogenes in milk was modelled using a logistic regression procedure. The predicted interface of recovery/no recovery can be used to calculate the mild-heat treatment condition to control bacterial recovery during storage at 25 °C after HPP (550 MPa for 5 min). The results in this study would contribute to enhance the safety of high-pressure-processed milk.

Predictive modelling of the recovery of Listeria monocytogenes on sliced cooked ham after high pressure processing

This study examined bacterial recovery on sliced cooked ham that was inoculated with Listeria monocytogenes, treated by high pressure processing (HPP) and then stored at 10 °C for 70 days. The number of L. monocytogenes on the ham inoculated with 5 log 10 CFU/g was initially reduced by HPP at 500 MPa for 10 min to below the detectable level (10 CFU/g). However, the bacterial count gradually increased during storage, and exceeded the initial inoculum level at the end of the 70-day period, having risen by 7–8 log 10 CFU/g. A novel predictive model was therefore developed to estimate the recovery of L. monocytogenes during storage after HPP. Recovery of L. monocytogenes was defined as the detection of > 10 2 CFU/g bacteria, in view of the relevant food safety objectives of L. monocytogenes. At each 14-day sampling session, the ham was scored as either 1 or 0 indicating bacterial recovery or no bacterial recovery, respectively. The data were then subjected to a simple linear logistic regression model, which provided a good fit as indicated by the performance statistics. Using this model, we estimated the minimum HPP conditions necessary for the required storage periods. Additionally, as the developed model was based on logistic regression, the probability of the recovery of L. monocytogenes during storage after HPP was estimated. Our model not only calculated the appropriate shelf life and process conditions, but also provided a method for evaluating the risk of the recovery of pathogenic bacteria during storage.

Selection of probiotics by various inhibition test methods for use in the culture of western king prawns, Penaeus latisulcatus (Kishinouye)

The growth inhibition ability of nine probiotic strains were investigated against 15 Vibrio spp. isolated from western king prawns, Penaeus latisulcatus (Kishinouye), and eight pathogenic Vibrio strains from other aquatic animals. Five different inhibition test methods such as bacteriocin-like inhibitory substance (BLIS), modified BLIS, disc-diffusion, well-diffusion and co-culture method were compared in order to select the most suitable probiotics for use in the culture of western king prawns. The results showed that the modified BLIS method was the most effective one for the selection of probiotics. Pseudomonas synxantha and P. aeruginosa were promising probiotics as they caused a significantly higher (P<0.05) growth inhibition of all the Vibrio spp. tested, with Pseudomonas aeruginosa being more effective than P. synxantha. The growth inhibition zones were significantly greater (P<0.05) when probiotics were grown on Marine Salt Agar (MSA) for three days before inoculating the plate with the Vibrio spp., but were not significantly different (P>0.05) when the probiotics and the Vibrio spp. were inoculated onto the plate at the same time. In the co-culture method, probiotics at a concentration of 103 CFU/mL allowed the Vibrio spp. (103 CFU/mL) to grow, but the Vibrio spp. cell densities never reached beyond their initial inoculum levels during the culture. At higher concentrations (105–107 CFU/mL), the probiotics dominated the growth of the Vibrio spp. Therefore, administering a suitable concentration of probiotics and allowing growth and production of antimicrobial compounds before the addition of Vibrio spp. produced the best inhibition results.

The role of eggs inoculum level of Meloidogyne incognita on their reproduction and host reaction

Two pot experiments were separately conducted to determine the influence of four or three inoculum levels (0, 250, 500 and 1000 or 0, 1000 and 2000 nematode eggs/ 850 g soil/pot) ofMeloidogyne incognita on population density of the nematode and host reaction of two solanacious plants viz tomato cv. Castle rock or pepper cv. Anaheim under partly controlled greenhouse at 23 ± 4oC. Nematode reproduction and host damage were both affected by the initial inoculum levels and revealed a reduction in total the fresh weight of the plants as the inoculum level increased from 250 to 1000 eggs/ tomato plant with values of 18.6 and 43.9%. The rate of build up of M. incognita on tomato increased from 1.14 to 1.48, respectively. When the initial inoculum (Pi) level was increased up to 2000 eggs per pepper plant, the percentage reduction of whole plant fresh weight (73.2%) and shoot dry weight (55.3%) as well as rate of nematode build-up (1.49) also obviously increased. Galls and egg masses/root system increased as Pi was increased on both host plants. Regression analysis of Pi vs rate of nematode build-up either on tomato or pepper plants gave values of R2 amounted to 0.6904 or 0.8149, respectively. This means the susceptibility of tomato cv. Castle Rock to M. incognita infection was more than did pepper cv. Anaheim under greenhouse condition. Key words: Population density, pepper, tomato, inoculum level, Meloidogyne incognita.

Modeling the growth of Listeria monocytogenes based on a time to detect model in culture media and frankfurters

In the current study, temperature dependent growth of Listeria monocytogenes strain H7776, with an initial inoculum level of approximately 0.1 CFU/mL or g, was modeled based on “time to detect” (TTD) using the Arrhenius equation. The activation energies ( E a) obtained from specific growth rate and lag phase duration in tryptic soy broth (TSB) and frankfurters were compared. At 4 °C, the TTD for L. monocytogenes was 217 h in TSB and 48 h in portions of frankfurters. The TTD decreased as temperature increased from 4 to 36 °C in both culture media and frankfurters, and this relationship was modeled using the Arrhenius equation. Based on this model, the E a values for the TTD in TSB and frankfurters averaged 22.7 and 18.7 kcal/mol, respectively, and were not significantly different ( p < 0.05). Linear regression was performed on the exponential part of the growth curve to evaluate specific growth rate constants at each temperature using the Monod model. The E a values were also calculated based on the specific growth rate and the lag phase of L. monocytogenes in TSB and frankfurters incubated in the same range of temperatures. The average E a values for the specific growth rates and the lag phase durations in TSB cultures were 19 and 21 kcal/mol, respectively. In frankfurters, the average E a values were slightly greater for both specific growth rate and lag phase duration (29 and 35 kcal/mol, respectively), but these values were not significantly different from the E a calculated for the TTD in each medium. These results indicate that the TTD concept can be used to develop and validate safety-based shelf life models.

In vitro fermentation of sugar beet arabinan and arabino-oligosaccharides by the human gut microflora

To determine the fermentation profiles by human gut bacteria of arabino-oligosaccharides of varying degree of polymerization. Sugar beet arabinan was hydrolyzed with a commercial pectinase and eight fractions, of varying molecular weight, were isolated by gel-filtration chromatography. Hydrolysis fractions, arabinose, arabinan and fructo-oligosaccharides were fermented anaerobically by gut bacteria. Total bacteria, bifidobacteria, bacteroides, lactobacilli and the Clostridium perfringens/histolyticum sub. grp. were enumerated using fluorescent in situ hybridization. Bifidobacteria were stimulated to different extents depending on molecular weight, i.e. maximum increase in bifidobacteria after 48 h was seen on the lower molecular weight fractions. Lactobacilli fluctuated depending on the initial inoculum levels. Bacteroides numbers varied according to fraction; arabinan, arabinose and higher oligosaccharides (degree of polymerization, dp > 8) resulted in significant increases at 24 h. Only carbohydrate mixtures with dp of 1-2 resulted in significant increases at 48 h (log 8.77 +/- 0.23). Clostridia decreased on all substrates. Arabino-oligosaccharides can be considered as potential prebiotics. Arabinan is widely available as it is a component of sugar beet pulp, a co-product from the sugar beet industry. Generation of prebiotic functionality from arabinan would represent significant added value to a renewable resource.

Application of caffeine, 1,3,7-trimethylxanthine, to control Escherichia coli O157:H7

The objective of this research was to determine the effectiveness of caffeine on inactivation of Escherichia coli O157:H7 in brain heart infusion (BHI) broth. Overnight samples of five E. coli O157:H7 strains of (E0019, F4546, H1730, 944 and Cider) were used in this study. These strains were individually inoculated at an initial inoculum level of 2 log CFU/ml into BHI broth containing caffeine at different concentrations (0.00%, 0.25%, 0.50%, 0.75%, 1.00%, 1.25%, 1.50%, 1.75%, and 2.00%). Samples were then incubated at 37 °C for 24 h. Bacterial growth was monitored at different time intervals by measuring turbidity at 610 nm using a spectrophotometer. Results revealed that the addition of caffeine inhibited the growth of E. coli O157:H7. Significant growth inhibition was observed with concentration levels of 0.50% and higher. These results indicate that caffeine has potential as an antimicrobial agent for the treatment of E. coli O157:H7 infection and should be investigated further as a food additive to increase biosafety of consumable food products.

Fate of Salmonella typhimurium DT 104 during the Fermentation of ‘Siljo’, a Traditional Ethiopian Fermented Legume Condiment, and during Product Storage at Ambient and Refrigeration Temperatures

The survival of Salmonella spp. in fermenting foods and their elimination during prolonged fermentation is documented. This prompted the study to evaluate the fate of Salmonella typhimurium DT 104 during the fermentation of ‘Siljo’, a traditional fermented legume condiment, and during the storage of the fermented product at ambient and cold temperatures. ‘Siljo‘ was made to ferment naturally and the count of lactic acid bacteria reached 9.9 log c.f.u./ml on day 5. The pH dropped from an initial value of 5.8–4.65 during this time. The lactic acid flora was dominated by Leuconostoc spp. At ambient temperature storage (18–22 °C), the product spoiled on day 16. The spoilage was caused by Bacillus spp. At refrigerated storage (4 °C), however, the count of Bacillus spp. was below detectable limits (<1 log c.f.u./ml) until the end of experiment on day 16. When Salmonella typhimurium DT 104 was inoculated into the fermenting gruel at low initial levels (2.8 log c.f.u./ml), the count decreased steadily and the test strain was not detected by enrichment on day 5. At higher initial inoculum level (5.5 log c.f.u./ml), complete elimination was observed on day 7. In a non-fermenting control gruel, count of the test strain increased by about 3 log units on day 7. In another experiment, the fermented product was inoculated with Salmonella typhimurium DT 104 at low or high inoculum levels and stored at ambient and refrigeration temperatures. Under ambient temperature storage, the test strain was not detectable by enrichment on days 3 and 6 at low and high initial inoculum levels, respectively. The count of the test strain did not decrease markedly under refrigerated storage. The results indicated that, at normal contamination levels in the kitchen environment, fermentation for five days would completely eliminate Salmonella typhimurium DT 104 from the fermenting gruel; ambient temperature storage of the product would completely eliminate the pathogen after three days; and its survival is markedly prolonged by cold storage.

Fate of Escherichia coli O157:H7 during the processing and storage of Ergo and Ayib, traditional Ethiopian dairy products

The long-term survival of E. coli O157:H7 in acid foods is well documented. This prompted us to evaluate the organism's survival during the making of Ergo, a traditional Ethiopian sour milk and Ayib, a traditional Ethiopian cottage cheese and during storage of these products at both ambient and refrigeration temperatures. E. coli O157:H7 test strains were separately inoculated into milk at initial levels of log 10 3 cfu/ml. Levels of E. coli O157:H7 in the absence of lactic acid bacteria (LAB) reached log 10 8.4 cfu/ml at 24 h as the pH dropped to 5.6±1.0. In milk inoculated with LAB (log 10 6.4 cfu/ml) and E. coli O157:H7 (log 10 3 cfu/ml), levels of LAB were log 10 9.6 cfu/ml and 9.4 cfu/ml at 24 h and 72 h, respectively, and the pH values were 3.5 and 3.9, respectively. In the presence of LAB, E. coli O157:H7 grew to log 10 6.5 cfu/ml at 24 h, with the levels decreasing to log 10 3.2 cfu/ml at 72 h. Post-fermentation inoculation of E. coli O157:H7 in Ergo at an initial level of log 10 3 cfu/ml, resulted in complete elimination of test organisms at 6 h at ambient temperature storage, but they were recovered until 72 h at refrigerated storage. At a higher initial inoculum level (log 10 6 cfu/ml) of the E. coli O157:H7, the counts decreased by 4 logs within 12 h (pH 4.2) at ambient temperature storage, and complete elimination was observed at 36 h (pH 4.0). At refrigeration temperature, counts at 72 h were between 2.2 and 3.5 log cfu/ml for the different strains. During Ayib processing, E. coli O157:H7 in souring milk increased by 3 logs in 24 h, with a slight reduction being observed at 36 h. The pH dropped to 4.3 during this time. The numbers of E. coli O157:H7, immediately after curd cooking, were below detectable levels, but could be recovered by enrichment. Complete inactivation was observed on 24 h after curd cooking. When E. coli O157:H7 was inoculated into steam-treated Ayib (pH 4.2–4.3) at levels of log 10 6.0–6.7 cfu/g and maintained at ambient temperatures, there was a gradual decrease in numbers to 3.7–4.3 logs by day 3. After day 7, pH values were 3.8–3.9 and E. coli O157:H7 was only detectable after enrichment. E. coli O157:H7 strain MF-1847 was completely inactivated by day 9, but the other two strains could still be recovered by enrichment at this time. At refrigeration storage, decrease in count of the test strains was gradual and counts at day 8 and 9 were >4 log cfu/g. As Ergo is preferably consumed soon after (24 h) fermentation, traditional fermentation of Ergo would not guarantee that E. coli O157:H7 can be controlled and, therefore, Ergo can be a potential health hazard if prepared from milk contaminated with E. coli O157:H7.

Behavior of Escherichia coli Cells and Bacillus cereus Spores on Poplar Wood Crates by Impedance Measurements

Dry poplar wood crates and glass surfaces (bottoms of Pyrex flasks) were contaminated with aqueous suspensions of Escherichia coli Collection de l'Institut Pasteur (CIP) 54.8 cells or Bacillus cereus CIP 78.3 spores. After different periods of storage at 25°C, the number of cells still present on both surfaces was determined by impedance microbiology. The physical and chemical properties of the wood greatly and rapidly decreased the number of cells. After 4 h of contact time, B. cereus residual metabolic activity corresponded to less than 10% of the initial inoculum level. With a low inoculum level of 2.4 × 102E. coli cells, sterility of the wood was obtained after a contact time of 24 h. With a higher inoculum level of 106E. coli cells, only a few viable bacteria, corresponding to the metabolic activity of four cells, could be recovered after a prolonged contact time of 145 h. Conversely, under the same conditions of storage, when bacteria were deposited on inert and nonporous materials such as glass surfaces, growth occurred. After 24 h of contact time at 25°C, bacterial populations were about 109 cells for E. coli and 107 cells for B. cereus. Under our experimental conditions after a prolonged contact time, wood exhibited growth-inhibiting properties and cells were no longer metabolically active. These results indicate that the potential for cross-contamination of foods stored directly in contact with previously contaminated poplar wood crates is low under our experimental conditions.

Resistance of Soybean Cultivars to Field Populations of Heterodera glycines in North Carolina.

The soybean cyst nematode (SCN), Heterodera glycines, is the most important pathogen of soybean, Glycine max, in North Carolina. Cultural practices are the most effective means of managing this pathogen because a majority of cultivars are susceptible to the races of this nematode that predominate in the state. Resistant and susceptible cultivars were evaluated in 14 H. glycines-infested fields from 1992 to 1999. Resistance in cvs. Hartwig and Delsoy 5710, and line S92-1603 derived from plant introduction (PI) 437654, was highly effective against all populations of H. glycines evaluated in these experiments. Numbers of cysts (cysts and white females) per three plants 28 days after planting and final egg population densities (Pf) were lower than on other cultivars evaluated. Cultivars with SCN resistance derived from PI 90763 were moderately resistant in many of the test fields, but cultivars with Peking-derived resistance were effective at only two locations. Some cultivars with resistance derived from PI 88788 were highly to moderately resistant to races 9 or 14 of SCN, but were not consistently effective against other populations. Hartwig and Delsoy 5710 had low SCN reproductive factors (Rf = egg density at harvest/mean egg density at planting for site) of 0.16 and 0.23 compared with an Rf of 1.9 and 2.19 on the susceptible cvs. Essex and Hutcheson, respectively. In contrast, the Rf on cultivars derived from Peking generally was greater than on susceptible cultivars. Resistant cvs. Hartwig and Delsoy 5710 generally yielded more than susceptible cultivars or cultivars derived from other sources of resistance. The initial inoculum level (Pi) was negatively correlated with soybean seed yield, but cysts 28 days after planting proved to be better at predicting seed yield than Pi. Due to the genetic diversity of H. glycines populations with regard to the ability to parasitize resistant cultivars, cultivars with resistance derived from PI 437654 or other genotypes are needed to manage this nematode in North Carolina.

Behaviour of Escherichia coli O157:H7 during the fermentation of Datta and Awaze, traditional Ethiopian fermented condiments, and during product storage at ambient and refrigeration temperatures

The survival of E. coli O157:H7 in fermenting foods and its prolonged survival in refrigerated fermented foods is documented. This prompted the study to evaluate survival of E. coli O157:H7 during the fermentation of Datta and Awaze, traditional Ethiopian condiments. Datta was prepared by wet milling a variety of spices along with green or red chilli and fermenting it by lactic acid bacteria. Awaze is a slurry made of red pepper, garlic and ginger to which various other spices were added and fermented by lactic acid bacteria (LAB) and yeasts. The Datta or Awaze slurry was separately inoculated with three strains of E. coli O157:H7 and the fermentation was allowed to proceed at ambient (20–25°C) temperatures for 7 days. When fermenting Datta or Awaze was initially inoculated at low inoculum level (3 log cfu/g), the test strains were not recovered after 24 h of fermentation. At higher initial inoculum level (6 log cfu/g), however, counts of the test strains in Datta at day 7 were less by about 1.5 log unit than the initial inoculum level. In fermenting Awaze, all test strains were completely eliminated in 7 days. The pH of the fermenting green and red Datta was reduced from 5.2 to 4.4 and that of Awaze dropped from 4.9 to 3.8 during this time. In another experiment, the fermented products were separately inoculated with the E. coli O157:H7 test strains at levels of 6 log cfu/g and incubated at ambient and refrigeration (4°C) temperatures for 7 days. In fermented Datta, two of the three strains were not recovered by enrichment after 6 days of storage at ambient temperatures. In fermented Awaze, all strains were below countable levels at day 5, but could still be recovered by enrichment at day 7. At refrigeration storage, counts of the test strains in Datta and Awaze products were <3 log cfu/g at day 7. The inhibition of our E. coli O157:H7 test strains in Datta and Awaze may be due to the antimicrobial activity of spices and other metabolites produced by LAB which may be effective at low pH.

Survival of Escherichia coli O157:H7, Salmonella Enteritidis, Staphylococcus aureus, and Listeria monocytogenes in Kimchi

The survival of gram-positive and gram-negative foodborne pathogens in both commercial and laboratory-prepared kimchi (a traditional fermented food widely consumed in Japan) was investigated. It was found that Escherichia coli O157:H7, Salmonella Enteritidis, Staphylococcus aureus, and Listeria monocytogenes could survive in both commercial and laboratory-prepared kimchi inoculated with these pathogens and incubated at 10°C for 7 days. However, when incubation was prolonged, the S. aureus level decreased rapidly from the initial inoculum level to the minimum detectable level within 12 days, whereas Salmonella Enteritidis and L. monocytogenes took 16 days to reach similar levels in commercial kimchi. On the other hand, E. coli O157:H7 remained at high levels throughout the incubation period. For laboratory-prepared kimchi, the S. aureus level decreased rapidly from the initial inoculum level to the minimum detectable level within 12 days, and L. monocytogenes took 20 days to reach a similar level. E. coli O157:H7 and Salmonella Enteritidis remained at high levels throughout the incubation period. The results of this study suggest that the contamination of kimchi with E. coli O157:H7, Salmonella Enteritidis, S. aureus, or L. monocytogenes at any stage of production or marketing could pose a potential risk.