Survival of Campylobacter jejuni and Salmonella enterica Typhimurium in vacuum-packed, moisture-enhanced pork

The abilities of Campylobacter jejuni and Salmonella enterica Typhimurium to survive in vacuum-packaged, moisture-enhanced pork stored at 4 or 10°C were examined. Pork loins were surface inoculated with either C. jejuni or Salmonella Typhimurium and then moisture enhanced to a target of 10 or 20%. The enhanced pork loins were sliced 1 cm thick and vacuum packaged. A pork loin without moisture enhancement was sliced and vacuum packaged as a control. Samples were collected, plated, and the numbers of surviving organisms were determined periodically during storage at 4 and 10°C. The numbers of C. jejuni or Salmonella Typhimurium in samples with different moisture enhancement levels were similar (P > 0.05). No significant differences (P > 0.05) in C. jejuni counts were observed between samples at 10°C and those at 4°C. In contrast, the numbers of Salmonella Typhimurium in samples at 10°C had significantly (P < 0.05) increased (0.41 log CFU/g) from those at the refrigerated temperature of 4°C. Vacuum storage at 4 and 10°C for 28 days did not result in dramatic reductions in the mean numbers of C. jejuni and Salmonella Typhimurium. Our findings indicate that vacuum packaging under chilled conditions will not add substantially to safety for moisture-enhanced pork. Strict hygienic practices or the implementation of decontamination technologies is recommended.     

Survival characteristics of Salmonella enterica serovar Newport in the dairy farm environment

Multi-drug resistant (MDR) Salmonella enterica serovar Newport (S. Newport) has established a reservoir in dairy cattle. Infected herds suffer significant mortality in both adult and young animals, posing a considerable economic loss to producers. Land application of manure from infected animals may further spread the pathogen into the agroecosystem, causing public health concerns. Previous work by our group demonstrated that the organism persisted in manure and manured soil for 6 to 10 mo under laboratory conditions. In the present study, we determined the survival characteristics of MDR S. Newport in a dairy lagoon, compost pile, and soil of a grass field under natural conditions using environmental sentinel chambers with an initial concentration of S. Newport around 7 log₁₀ per gram. In the static compost pile at 64°C, S. Newport was eliminated within 18 h. In the dairy effluent lagoon, the pathogen survived for >137 d, whereas in the field soil, the organisms persisted for over 276 d. The survival of MDR S. Newport in both the lagoon and field soil followed a pattern of (1) an increase or plateau for a few days, (2) log-linear decline for 6 to 13 wk, and (3) a long tailing phase at low and variable concentration for 4 to 9 mo. Log reduction times (days required for 90% decrease in concentration) based on the log-linear decline phase were 7 d in the lagoon and 14 to 20 d in the soil. Conditions leading to faster inactivation during the initial phase do not necessarily translate into a quicker elimination of the pathogen. Regression models of the log-linear phase may be inaccurate for estimating complete pathogen elimination.     

Viability of Listeria monocytogenes and Salmonella Typhimurium after isochoric freezing

Isochoric freezing, different from isobaric (conventional) freezing, allows for storage below freezing temperatures without significant damage from ice formation. While several types of tissues have been successfully stored in sub-zero isochoric conditions, it is unknown how isochoric freezing affects pathogenic microorganisms. Thus, the objective of this study was to investigate the survival of Salmonella Typhimurium and Listeria monocytogenes at below freezing storage (<0°C) in isochoric conditions. Tested conditions included storage at −4, −7, and −15°C for 24 hr and at −15°C for 1, 2, 3, 6, 12, and 24 hr. A comparison of bacterial survival during isobaric freezing was included with every trial. Additionally, bacterial cells were examined for morphological damage using transmission electron and field-emission scanning electron microscopes. Isochoric freezing at −15°C for 24 hr reduced both species of bacteria down to unrecoverable levels and maximum efficacy achieved after the 6 hr timepoint for L. monocytogenes and the 12 hr timepoint for S. Typhimurium. When viewed using electron microscopy, S. Typhimurium cells were noticeably disfigured with regions of cytosol separated from the cell wall. The results of this study demonstrate that isochoric freezing is capable of substantial levels of pathogen reduction. Unlike conventional nonthermal interventions, isochoric freezing does not require additional devices such as elevated pressure machines or pulsed electric fields and can be achieved with simple, inexpensive, rigid closed volume containers such as household freezers or commercial cold storage facilities.     

Survival of Salmonella in spices and growth in cooked food

Contamination of spices with pathogens has been reported worldwide, and Salmonella might result in foodborne infections. In this study, we investigated the survival of Salmonella in black pepper and red pepper, and the growth of the surviving Salmonella in cooked food. Salmonella Enteritidis, Salmonella Weltevreden and Salmonella Senftenberg were inoculated into spices, and their survival during storage was examined. In black pepper, S. Enteritidis was no longer viable after storage for 28 days, but S. Weltevreden and S. Senftenberg remained viable. In red pepper, S. Weltevreden and S. Senftenberg survived for 28 days although S. Enteritidis was not viable after 7 days. Salmonella Weltevreden and Salmonella Senftenberg were inoculated into cooked food, and their survival during storage was determined. In potato salad, egg salad, namul and kimchi as cooked foods, both pathogens grew at 30 degrees C, but not at 10 degrees C. Our results indicate that cooked food should be stored at low temperature after addition of spices, such as black pepper and red pepper, following the cooking.     

Survival, elongation, and elevated tolerance of Salmonella enterica serovar Enteritidis at reduced water activity

Growing microorganisms on dry surfaces, which results in exposure to low water activity (a(w)), may change their normal morphology and physiological activity. In this study, the morphological changes and cell viability of Salmonella enterica serovar Enteritidis challenged to low a(w) were analyzed. The results indicated that exposure to reduced a(w) induced filamentation of the cells. The amount of filamentous cells at a(w) 0.94 was up to 90% of the total number of cells. Surviving filamentous cells maintained their membrane integrity after exposure to low a(w) for 21 days. Furthermore, cells prechallenged to low a(w), obtained with an ionic humectant, demonstrated higher resistance to sodium hypochlorite than control cells. These resistant cells are able to survive disinfection more efficiently and can therefore cause contamination of foods coming in contact with surfaces. This points to the need for increased attention to cleaning of surfaces in household environments and disinfection procedures in processing plants.     

Survival of mouse embryos after freezing in medium containing dimethylsulphoxide and yolk extract

The ability of the cryoprotective solution containing dimethylsulphoxide in combination with yolk extract (DYG 550) to protect mouse embryos at ultra low temperatures is reported. Of the embryos of 46.1% frozen to -196 C at various rates formed blastocysts in vitro compared to 41.2% and 23.2% survivals protected by 1M dimethylsulphoxide and 1M glycerol.     

鼠伤寒沙门菌对抗生素和消毒剂耐药性研究

目的检测302株食品源、动物源和人源鼠伤寒沙门菌(Salmonella enterica Typhimurium)对抗生素和消毒剂的耐药表型,测定消毒剂耐药基因型,并分析鼠伤寒沙门菌对抗生素和消毒剂耐药的相关性。方法纸片扩散法测定鼠伤寒沙门菌对16种抗生素的耐药性,微量肉汤稀释法测定4种消毒剂的最小抑菌浓度(minimal inhibitory concentrations, MICs),聚合酶链式反应(polymerase chain reaction, PCR)检测消毒剂的耐药基因。结果鼠伤寒沙门菌分离株中96.03%(290/302)至少对一种抗生素有耐药性,79.80%(241/302)是多重耐药(multidrug resistant, MDR)菌株。分离株对链霉素的耐药率最高(78.81%,238/302),其次是磺胺复合物(78.15%,236/302)、四环素(75.50%,228/302)、氨苄青霉素(71.85%,217/302),所有分离株均对头孢吡肟和亚胺培南敏感。食品源鼠伤寒沙门菌对磺胺复合物、四环素、氨苄西林、萘啶酸和庆大霉素的耐药率均高于动物源和人源鼠伤寒沙门菌,差异有统计学意义(P0.05);动物源鼠伤寒沙门菌对环丙沙星和氧氟沙星的耐药率高于食品源和人源鼠伤寒沙门菌,差异有统计学意义(P0.05);人源鼠伤寒沙门菌对头孢噻肟和头孢他啶的耐药率高于食品源和动物源分离株。消毒剂苯扎氯铵、三氯生、三氯异氰尿酸和聚维酮碘对鼠伤寒沙门菌分离株的MICs范围分别为2～64、0.031 25～1、32～1 024和256～1 024 mg/L。食品源与动物源鼠伤寒沙门菌对苯扎氯铵、三氯生和聚维酮碘的耐药率高于人源鼠伤寒沙门菌,差异有统计学意义(P0.05)。分离株中消毒剂耐药基因qacEΔ1、sugE(p)和qacE检出率分别为56.95%(172/302)、20.53%(62/302)和2.65%(8/302),qacEΔ1基因与鼠伤寒沙门菌对β-内酰胺类、氨基糖苷类、四环素类、氟喹诺酮类、磺胺类、氯霉素类和喹诺酮类的耐药性显著相关(P0.01)。结论鼠伤寒沙门菌对抗生素耐药普遍,多重耐药率较高,并表现对消毒剂的耐药,鼠伤寒沙门菌对抗生素和消毒剂耐药存在相关性。     

Effect of the growth environment on the strain variability of Salmonella enterica kinetic behavior

Intra-species variability of microbial growth kinetic behavior is an event with important implications for food safety research. Aiming at the evaluation of the growth variability among Salmonella enterica strains as affected by the growth environment, the kinetic behavior of 60 isolates of the pathogen was assessed at 37 °C in tryptone soy broth of different pH values (4.3-7.0) and NaCl concentrations (0.5-6.0%). Maximum specific growth rate (μ_max) values corresponding to each strain and growth condition were estimated by means of absorbance detection times of serially decimally diluted cultures using the automated turbidimetric system Bioscreen C. A total of 9600 optical density curves were generated for the strains and the growth conditions tested. The variability of μ_max among the S. enterica strains was important and greater than that observed within the strains (i.e. among replicates). Moreover, strain variability increased as the growth conditions became more stressful both in terms of pH and NaCl. The coefficient of variation of μ_max among the tested strains at pH 7.0-0.5% NaCl was 6.1%, while at pH 4.3-0.5% NaCl and pH 7.0-6.0% NaCl was 11.8% and 23.5%, respectively. Beyond the scientific interest in understanding strain variability, the findings of this study should be useful in strain selection for exploitation in food safety challenge studies as well as in incorporating strain variability in predictive microbiology and microbial risk assessment.     

Population changes and growth modeling of Salmonella enterica during alfalfa seed germination and early sprout development

Food Science and Biotechnology - This study examined the effects of alfalfa seed germination on growth of Salmonella enterica. We investigated the population changes of S. enterica during early...     

Survival of Vibrio parahaemolyticus during freezing

The ability of Vibrio parahaemolyticus to survive on frozen rock lobster tails was investigated. Rock lobster tails were artificially inoculated with the organism and were stored at ? 18°C. Vibrio died off within 1 week at ?18°C in lightly contaminated tails but a heavy infection survived for a few months at this temperature. Recovery depended on the degree of infection, the presence of other bacteria and the selectivity of the medium.     

On the different growth conditions affecting silver antimicrobial efficacy on Listeria monocytogenes and Salmonella enterica

Silver is known to inhibit microorganisms and therefore it is an ideal candidate for its incorporation in a wide variety of materials for food applications. However, there is still a need for understanding how silver prolonged exposure to bacterial contamination affects the bioavailability of the active silver species. In the present study, growth curves of Listeria monocytogenes and Salmonella enterica were performed for 3-5days in Tryptic Soy Broth (TSB) and M9 minimal medium (M9) in the presence of silver ions and silver solutions previously in contact with the growth media. The cultivability of the bacteria under these conditions was correlated with the viability of the bacterial populations as measured by flow cytometry analysis (FC) using a LIVE/DEAD BacLight kit. It was found that, after a period where viable counts were not detected, bacterial populations recovered and were able to proliferate in most cases. The resuscitation of the cultures was explained by both the existence of a resilient fraction of bacteria in a compromised state and the parallel inactivation of the silver species. This inactivation was found to be highly influenced by time dependant chemical reactions taking place in the environment of exposure, producing differences of at least 3 fold between results for nutrient rich environments and results for limiting environments. This study points out the need for understanding these chemical interactions and bacterial mechanisms of adaptation and may have relevance in the design of silver-based antimicrobial systems for food-related applications.     

Recovery of Salmonella enterica serovars Typhimurium and Tennessee in peanut butter after electron beam exposure

The effect of electron beam (e-beam) radiation on the recovery of Salmonella serotypes Tennessee (ATCC 10722) and Typhimurium (ATCC 14028) in creamy peanut butter over a 14-d storage period at 22 °C was studied. Each Salmonella type was independently inoculated into peanut butter and subjected to e-beam doses that ranged from 0 to 3.1 kGy, confirmed by film dosimetry. After 2-, 4-, 6-, 8-, and 14-d of storage, microbial analyses were conducted. Survivors were recovered on growth and selective media using standard spread-plating methods. Microbial counts (CFU/g) were log-converted and differences were determined by ANOVA and Tukey's Honestly Significant Differences test. When samples were not e-beam-treated, there were no significant changes (P > 0.05) in microbial numbers over time. In e-beamed samples, microbial numbers decreased over time; however, reductions were not always significant. Initial recovery rates (R-rates) 2 d after e-beam treatment were significantly different for the 2 strains of Salmonella and between recovery media (P < 0.05); however, these differences did not persist for the remainder of the storage period (P > 0.05) indicating that injured cells were not able to survive in the high-fat, low-water activity peanut butter environment. R-rates for both strains of Salmonella were maintained until day 14 when there were significant reductions in Salmonella Typhimurium (P < 0.05). These results indicate that Salmonella Tennessee and Salmonella Typhimurium will survive in peanut butter when exposed to nonlethal doses of e-beam irradiation. PRACTICAL APPLICATION: Electron beam (e-beam) irradiation is an alternative to thermal processing; this technique inactivates microorganisms and insects that might be present in a food by generating radiation by accelerated electrons that inactivate organisms directly because of interaction with cell components and indirectly by producing free radicals that disrupt integrity of the cell membrane. E-beam radiation will reduce the number of probable microbiological hazards that could be present while the food remains generally unaffected in texture, taste, and nutritional value. A recent study showed e-beam irradiation to be effective at reducing both Salmonella Tennessee and Typhimurium in peanut butter by one log after exposure to less than 1 kGy, highlighting the need to explore this process further.     

Salmonella enterica serovars from pigs on farms and after slaughter and validity of using bacteriologic data to define herd Salmonella status

The primary objective of this study was to evaluate the validity of using data obtained from slaughtered pigs for farm-level epidemiologic studies of Salmonella. The study involved groups of pigs from five farms. Salmonella isolates were obtained from on-farm samples, and a total of 370 on-farm and an additional 486 isolates from samples collected after commercial slaughter were subsequently tested. Preharvest samples included feces of individual animals from defined groups of nursery and finishing pigs on commercial farms and swabs from trucks. Postslaughter samples were cecal contents and mesenteric lymph node samples. The concordance between Salmonella serovars isolated from on-farm samples and those serovars isolated after slaughter varied widely among farms. Results of paired lymph node and cecal cultures were strongly associated (odds ratio, 7.0), but the agreement between on-farm and postslaughter results at the pig level was poor (kappa = 0.34). The results support recent findings that risk of exposure to Salmonella during transport and lairage remains a concern under contemporary industry conditions. The findings further imply that slaughter plant studies based on phenotyping of Salmonella alone (such as serovars) may not reliably indicate the Salmonella status of commercial swine farms.     

Survival of Bifidobacterium longum 1941 microencapsulated with proteins and sugars after freezing and freeze drying

Five types of proteins and three types of sugars were examined for their effectiveness in protecting B. longum after freeze drying, including their acid and bile tolerance, surface hydrophobicity, retention of β-glucosidase, lactate dehydrogenase and adenosine triphosphatase. Sodium caseinate 12%, whey protein concentrate 12%, sodium caseinate:whey protein concentrate 6%:6%, skim milk 12%, or soy protein isolate 12% was combined with glycerol (3% w/v), mannitol (3% w/v) or maltodextrin (3% w/v). Fifteen emulsion systems containing sugars were obtained. Concentrated B. longum 1941 was incorporated into each emulsion system at a ratio of 1:4 (bacteria:emulsion). All the mixtures were then freeze dried. Water activity (a_w) of freeze dried microcapsules was in the range of 0.30 to 0.35. WPC–CAS GLY provided high stability of bacteria (99.2%) during freezing, while high stability of cells after freeze drying and during exposure to acid and bile environment was achieved when CAS–MAN was applied (97.4%, 81.6% and 99.3%, respectively). High retention of β-glu of freeze-dried bacteria was achieved using SM–MAN as protectant (94.6%). ATPase and LDH were successfully retained by SM–GLY (94.9 and 83.6%, respectively) but there was no significant difference in protection effect using CAS–MAN (93.8 and 82.6%, respectively). Overall, milk proteins were superior to SPI and sugar alcohols provided more protection than MD.     

Survival of E. coli and Salmonella after Chilling and Freezing in Liquid Media

Predictive microbiology requires a sound basis of observed results, and factors that contribute to death of bacterial cells, before accurate equations can be developed. The effect of chilling and freezing strains of Escherichia coli and salmonella serotypes in nutrient broth, a noninhi-bitory liquid medium, was investigated. The phase of growth, small changes in composition of test medium, and sub-cultures made after primary isolation, influenced survival. Therefore, such influences must be considered when attempting to extrapolate results Erom pure cultures on laboratory media, to predict behavior of similar organisms in foods during chilling and freezing.     

Interaction of Salmonella enterica subspecies enterica serovar Typhimurium and mung bean (Phaseolus aureus) plants

The effect of Salmonella enterica subspecies enterica serovar Typhimurium, a zoonotic serovar, on mung bean (Phaseolus aureus) cultivar Pant Mung-3 plants was studied. Inoculation of mung bean seeds with Salmonella Typhimurium (7.2 x 10(5) CFU/ml) reduced sprouting rate (P < 0.07). This effect was more pronounced at higher levels of contamination. In the soil inoculated with Salmonella Typhimurium (7.2 x 10(6) CFU/g), germination was retarded and the number of defective sprouts was also significantly higher (P < 0.002). Salmonella Typhimurium grew inside germinating seeds and plant tissues and persisted in seedlings, adult plants, and harvested seedlings dried and stored at room temperature (30 degrees C) up to 45 days. Phaseolus aureus plants grown in sterile soil was resistant to Salmonella Typhimurium infection at 15 days of age and cleared Salmonella from all the aerial parts within 3 h of infection. However, Salmonella Typhimurium could be reisolated from the basal area of the stem and from soil even after 45 days of exposure to the pathogen.     

Modeling the growth rate and lag time of different strains of Salmonella enterica and Listeria monocytogenes in ready-to-eat lettuce

The growth parameters (growth rate, μ and lag time, λ) of three different strains each of Salmonella enterica and Listeria monocytogenes in minimally processed lettuce (MPL) and their changes as a function of temperature were modeled. MPL were packed under modified atmosphere (5% O₂, 15% CO₂ and 80% N₂), stored at 7–30 °C and samples collected at different time intervals were enumerated for S. enterica and L. monocytogenes. Growth curves and equations describing the relationship between μ and λ as a function of temperature were constructed using the DMFit Excel add-in and through linear regression, respectively. The predicted growth parameters for the pathogens observed in this study were compared to ComBase, Pathogen modeling program (PMP) and data from the literature. High R² values (0.97 and 0.93) were observed for average growth curves of different strains of pathogens grown on MPL. Secondary models of μ and λ for both pathogens followed a linear trend with high R² values (>0.90). Root mean square error (RMSE) showed that the models obtained are accurate and suitable for modeling the growth of S. enterica and L. monocytogenes in MP lettuce. The current study provides growth models for these foodborne pathogens that can be used in microbial risk assessment.     

Survival of Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes on inoculated walnut kernels during storage

The survival of single strains or cocktails of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes was evaluated on walnut kernels. Kernels were separately inoculated with an aqueous preparation of the pathogens at 3 to 10 log CFU/g, dried for 7 days, and then stored at 23°C for 3 weeks to more than 1 year. A rapid decrease of 1 to greater than 4 log CFU/g was observed as the inoculum dried. In some cases, the time of storage at 23°C did not influence bacterial levels, and in other cases the calculated rates of decline for Salmonella (0.05 to 0.35 log CFU/g per month) and E. coli O157:H7 (0.21 to 0.86 log CFU/g per month) overlapped and were both lower than the range of calculated declines for L. monocytogenes (1.1 to 1.3 log CFU/g per month). In a separate study, kernels were inoculated with Salmonella Enteritidis PT 30 at 4.2 log CFU/g, dried (final level, 1.9 log CFU/g), and stored at -20, 4, and 23°C for 1 year. Salmonella Enteritidis PT 30 declined at a rate of 0.10 log CFU/g per month at 23°C; storage time did not significantly affect levels on kernels stored at -20 or 4°C. These results indicate the long-term viability of Salmonella, E. coli O157:H7, and L. monocytogenes on walnut kernels and support inclusion of these organisms in hazard assessments.     

Survival of Salmonella Newport in oysters

Salmonella enterica is the leading cause of laboratory-confirmed foodborne illness in the United States and raw shellfish consumption is a commonly implicated source of gastrointestinal pathogens. A 2005 epidemiological study done in our laboratory by Brands et al., showed that oysters in the United States are contaminated with Salmonella, and in particular, a specific strain of the Newport serovar. This work sought to further investigate the host-microbe interactions between Salmonella Newport and oysters. A procedure was developed to reliably and repeatedly expose oysters to enteric bacteria and quantify the subsequent levels of bacterial survival. The results show that 10 days after an exposure to Salmonella Newport, an average concentration of 3.7 × 10³ CFU/g remains within the oyster meat, and even after 60 days there still can be more than 10² CFU/g remaining. However, the strain of Newport that predominated in the market survey done by Brands et al. does not survive within oysters or the estuarine environment better than any other strains of Salmonella we tested. Using this same methodology, we compared Salmonella Newport's ability to survive within oysters to a non-pathogenic strain of E. coli and found that after 10 days the concentration of Salmonella was 200-times greater than that of E. coli. We also compared those same strains of Salmonella and E. coli in a depuration process to determine if a constant 120 L/h flux of clean seawater could significantly reduce the concentration of bacteria within oysters and found that after 3 days the oysters retained over 10⁴ CFU/g of Salmonella while the oysters exposed to the non-pathogenic strain of E. coli contained 100-times less bacteria. Overall, the results of this study demonstrate that any of the clinically relevant serovars of Salmonella can survive within oysters for significant periods of time after just one exposure event. Based on the drastic differences in survivability between Salmonella and a non-pathogenic relative, the results of this study also suggest that unidentified virulence factors may play a role in Salmonella's interactions with oysters.