Antimicrobial activity of nisin incorporated in pectin and polylactic acid composite films against Listeria monocytogenes

An extruded composite food packaging film containing pectin, polylactic acids (PLAs) and nisin was developed to inhibit Listeria monocytogenes . The mechanical properties and surface structure of the film were also examined. Cells of L. monocytogenes were reduced by 2.1, 4.5 and 3.7 log units mL⁻¹ by the pectin plus PLA (pectin/PLA) film containing nisin (1000 IU mL⁻¹ of tested liquid) in Brain Heart Infusion (BHI) broth, liquid egg white and orange juice, respectively, after 48 h at 24 °C. Pectin played an important roll in embedding nisin into the film. The pectin/PLA film had a similar stiffness but lower tensile strength, elongation and fracture energy than the pure PLA film. These data suggested that nisin incorporated into the pectin/PLA film was an effective approach to reducing L. monocytogenes in a typical growth medium (e.g. BHI broth) as well as in foods (e.g. orange juice and liquid egg).     

Inhibition of Listeria monocytogenes by Nisin Combined with Grape Seed Extract or Green Tea Extract in Soy Protein Film Coated on Turkey Frankfurters

The objective of this study was to evaluate the inhibitory effect of grape seed extract (GSE), green tea extract (GTE), nisin and their combinations (nisin with either GSE or GTE) against Listeria monocytogenes. The inhibitory effect of these natural compounds was evaluated in phosphate buffer solution (PBS) medium containing approximately 10⁹ colony‐forming units (CFU/mL) of L. monocytogenes. The effectiveness of these compounds in a meat model system was evaluated by surface inoculation (approximately 10⁶ CFU/g) of L. monocytogenes onto turkey frankfurters. The inoculated frankfurters were dipped into soy protein film‐forming solutions with and without the addition of antimicrobial agents (GSE 1% or GTE 1% or nisin 10000 IU or combinations). Samples were stored at either 4 °C or 10 °C. The inhibitory effects of edible coatings were evaluated on a weekly basis for 28 d. The greatest inhibitory effect was observed in the PBS medium containing GSE (1%) and nisin (10000 IU/mL), which caused a 9‐log cycle reduction of L. monocytogenes population after 3 h incubation at 37 °C. In the meat system, the L. monocytogenes population (7.1 CFU/g) was decreased by more than 2 log cycle after 28 d at 4 °C and 10 °C, in the samples containing nisin (10000 IU) combined with either GSE (1%) or GTE (1%). This research has demonstrated that the use of an edible film coating containing both nisin and natural extracts is a promising means of controlling the growth and recontamination of L. monocytogenes on ready‐to‐eat meat products.     

Occurrence and Antimicrobial Susceptibility of Listeria monocytogenes Isolated from Brined White Cheese in Jordan

Listeria monocytogenes is a serious foodborne pathogen that has been isolated from different dairy food products. Several foodborne outbreaks of listeriosis have been associated with consumption of cheese. The aims of this study were to determine the occurrence of L. monocytogenes and Listeria spp. in brined white cheese (BWC) sold in Jordan, and to determine the susceptibility of isolated L. monocytogenes to antimicrobials. Three hundred and fifty samples of 5 different types of BWC (akkawi, boiled, halloumi, pasteurized, and shellal) were collected from a local market in Jordan. The ISO (11290-1) procedure was followed for isolation and identification of Listeria spp. from cheese samples and a polymerase chain reaction (PCR) technique was used for confirmation of L. monocytogenes isolates. The VITEK2 automated system was used for testing antimicrobial susceptibility of L. monocytogenes isolates. The overall prevalence of Listeria spp. in cheese sample was 27.1%. L. monocytogenes was isolated from 39 (11.1%) samples. Other isolated species were L. grayi (6.9%), L. innocua (2%), L. ivanovii (4%), L. seeligeri (2%), and L. welshimeri (0.3%). The pH values and salt concentrations of L. monocytogenes positive cheese samples ranged from 5.10 to 6.32 and 5.64 to 13.16, respectively. L. monocytogenes isolates were sensitive or intermediate susceptible to imipenem, gentamicin, linezolid, teicoplanin, vancomycin, fusidic acid, trimethoprim/sulfamethoxazole, benzylpenicillin, ciprofloxacin, erythromycin, tetracycline, and rifampicin, but resistant to fosfomycin, oxacillin, and clindamycin.     

Potassium Sorbate Does Not Increase Control of Listeria monocytogenes when Added to Zein Coatings with Nisin on the Surface of Full Fat Turkey Frankfurter Pieces in a Model System at 4°C

ABSTRACT: The antimicrobial effect of zein coatings containing nisin and potassium sorbate on turkey frankfurters against Listeria monocytogenes strain V7 at 4°C was determined. Our objectives were (1) to determine whether zein coatings with nisin and potassium sorbate alone or in combinations would reduce the growth of L. monocytogenes on turkey frankfurters at 4°C; (2) to determine the effect of zein, nisin, or potassium sorbate on L. monocytogenes after being challenged with high or low initial inoculum counts (log 6 or log 4); and (3) to determine whether potassium sorbate had any synergistic effect on the activity of nisin. Initial counts decreased for all the treatments containing nisin. Over 28 d, the nisin-alone treatment counts were lower than the control by 6.1 logs for the high inoculum. No cells were detected for the low inoculum test by day 21. The solvent controls (ethanol-glycerol or propylene glycol), yielded mean counts similar to those for zein-ethanol-glycerol or zein-propylene-glycol, giving 4 to 5 log lower counts versus the untreated controls at 28 d. Therefore zein per se had no antimicrobial activity. Use of 0.4% potassium sorbate did not significantly inhibit growth compared with the control or solvent-only controls. No significantly lower counts of L. monocytogenes were observed for zein-nisin coating treatments with sorbate versus without sorbate. Therefore, treatments using nisin alone or in combination with zein, ethanol-glycerol, or propylene glycol if approved for use on ready-to-eat foods, show promise for use as barriers against the growth of recontaminating, L. monocytogenes cells on this food substrate at 4°C.     

实时RT-PCR检测存活于乳中的单核细胞增多性李斯特菌

单增李斯特菌是一种可在低温下生长、能引起人畜共患病的食源性致病菌.为克服传统PCR检测的假阳性问题,有效的检测单增李斯特活菌,本研究提取单增李斯特菌的总RNA并进行反转录,以单增李斯特菌的必要毒力基因hlyA为靶基因,设计特异性引物和TaqMan探针进行实时PCR检测,研究了检测的特异性和灵敏度,考查了对人工污染牛乳进行检测的应用性.结果表明,所用引物和探针能较好的扩增目的基因,而对其他食源性致病菌无交叉反应.单增李斯特菌经1h增菌,可检出3×102CFU/ml.而经3h增菌,活菌检测限可达30CFU/ml.人工污染牛乳样品经6h增菌,检测限为17CFU/ml.此方法可应用于乳中单增李斯特菌的快速检测和污染状况调查.     

Utilization of Zein Coating and Sorbic Acid to Reduce Listeria monocytogenes Growth on Cooked Sweet Corn

Several proteins, lipids and waxes were tested as edible coatings on sweet corn. Only zein, a natural constituent of corn, gave a continuous adhesive and stable coating with satisfactory sensory properties. After 8 days at 10 °C, the population of L. monocytogenes was 10‐fold lower on coated sweet corn than on non‐coated sweet corn indicating a barrier effect of zein coating. Sorbic acid was incorporated in the coating at a concentration required to inhibit L. monocytogenes growth (approximately 1 mg sorbic acid/g of sweet corn). The inhibitory concentration was the same for both coated and non‐coated sweet corn. Zein coating therefore did not improve the preservative effect of sorbic acid.     

Control of Listeria monocytogenes on the Surface of Refrigerated, Ready-to-eat Chicken Coated with Edible Zein Film Coatings Containing Nisin and/or Calcium Propionate

ABSTRACT: This study investigated the effect of nisin added to zein film coatings (Z) coated onto ready-to-eat chicken against L. monocytogenes. L. monocytogenes inoculated chicken samples were dipped into Z dissolved in propylene glycol (ZP) or ethanol (ZE), with and without added nisin (N) (1000 IU/g) and/or 1% calcium propionate (CP) then stored at 4 °C or 8 °C for 24 d. After 16 d at 4 °C the growth of L. monocytogenes (6.8 log CFU/g) was suppressed by 4.5 to 5 log CFU/g and at 2.7 log CFU/g counts were maintained at a nondetectable level from day 0 to day 24 with ZEN, ZPNCP, or ZENCP. Zein film coatings with nisin can prevent the growth of L. monocytogenes on ready-to-eat chicken.     

Inactivation of Escherichia coli O157:H7 and Salmonella enteritidis in Liquid Egg White Using Pulsed Electric Field

ABSTRACT: The effects of temperature and pulsed electric field (PEF) intensity on inactivation of pathogens such as Escherichia coli O157:H7 and Salmonella enteritidis in egg white was investigated. Liquid egg white inoculated with 10⁸ colony-forming units (CFU)/mL of each pathogen was treated with up to 60 pulses (each of 2 JAS width) at electric field intensities of 20 and 30 kV/cm. The processing temperatures were 10°C, 20°C, and 30°C. After treatment, uninjured and total viable cells were enumerated in selective and nonselective agars, respectively. Maximum inactivations of 3.7 and 2.9 log units were obtained for S. enteritidis and E. coli O157:H7, respectively, while injured cells accounted for 0.5 and 0.9 logs for E. coli O157:H7 and S. enteritidis , respectively. For both bacteria, increasing treatment temperature tended to increase the inactivation rate. There was synergy between electric field intensity and processing temperature. The inactivation rate constant k _T values for E. coli O157:H7 on both selective and nonselective agars were 8.2 × 10^-3 and 6.6 × 10^-3/μS, whereas the values for S. enteritidis were 16.2 × 10^-3 and 12.6 × 10^-3/μS, respectively. The results suggest that E. coli O157:H7 was more resistant to heat-PEF treatment compared with S. enteritidis.     

高效液相色谱法测定乳粉及液态奶中牛磺酸含量

建立高效液相色谱法测定乳粉和液态奶中牛磺酸含量。样品用水溶解,超声提取10 min,提取液经亚铁氰化钾和乙酸锌沉淀蛋白后,离心,上清液用异硫氰酸苯酯和三乙胺衍生化,室温衍生1 h,加入正己烷为衍生化终止剂,静置分层,下层溶液用水定容后过0.22μm微孔滤膜,过滤液经Diamonsil AAA氨基酸分析柱(4.6×250 mm,5μm)分离,以0.05 mol/L乙酸钠水溶液(pH值=6.50)和甲醇/乙腈(1/1,V:V)为流动相进行梯度洗脱。采用外标法定量,在5.0～40μg/mL线性范围内相关系数(r)大于0.999,线性关系良好,定量限为5 mg/100 g(S/N≥10),添加水平为5、10、25 mg/100 g时,回收率范围在90%～108%之间,相对标准偏差范围为1.16%～2.24%。结果显示,本方法准确、可靠,满足乳粉和液态奶中牛磺酸含量的有效确定,特别对于特膳奶粉比GB方法更适合样品中牛磺酸含量的有效检测。     

抗菌肽zp37抑制果汁中单核细胞增生李斯特菌的活性及其作用机制

单核细胞增生李斯特菌（简称单增李斯特菌）作为一种常见的食源性致病菌,是影响果汁食用安全性的主要威胁因子。本实验设计并获得一种抗菌肽zp37,通过测定抗菌肽zp37最小抑菌浓度（minimum inhibitory concentration,MIC）,分析抗菌肽zp37处理后单增李斯特菌的生长曲线、膜电位、膜完整性、细胞微观结构、细胞聚集率、DNA结合情况以及胞内活性氧等,研究抗菌肽zp37抑制果汁中单增李斯特菌的活性及其作用机制。结果表明,抗菌肽zp37对单增李斯特菌的MIC为16μg/mL,当其质量浓度大于4MIC时,单增李斯特菌生长受到明显抑制。同时,8MIC的抗菌肽zp37处理1h能杀死绝大部分的单增李斯特菌,且在7 d贮藏期内,2MIC的抗菌肽zp37能将草莓汁、猕猴桃汁和苹果汁中的单增李斯特菌控制到检测限以下。抗菌肽zp37处理后,单增李斯特菌细胞膜离子通透性增加、膜电位消失,细胞膜完整性被破坏,细胞出现明显的凹陷变形,细胞间发生聚集。荧光标记示踪法结果显示,抗菌肽zp37同时作用于单增李斯特菌的细胞膜和细胞质。在细胞质中,当抗菌肽zp37质量浓度大于DNA质量浓度的1/4...     

Reduction of Listeria monocytogenes in Raw Catfish Fillets by Essential Oils and Phenolic Constituent Carvacrol

The antimicrobial activity of various essential oils and carvacrol was determined on fresh raw catfish fillets against a 4-strain Listeria monocytogenes mixture representing serotypes 1/2b, 3b, 4b, and 4c that were predominantly isolated from catfish processing environments. Thyme oil, oregano oil and carvacrol exhibited concentration and time dependent responses in broth against L. monocytogenes; for example 0.5% concentrations resulted in 4 log CFU/mL reduction within 30 min whereas 0.1% concentrations required more than 24 h for the same level of reduction. Lemon, orange, and tangerine oils, at 0.5% showed listeriostatic effect in which 4 log CFU/mL of the initial L. monocytogenes load was unchanged at 4 °C in 10 d whereas 1% concentrations were listericidal in a time dependent manner. Apart from carvacrol, efficacy of tested essential oils in reducing L. monocytogenes and total microbial load from catfish fillet was very limited. Dipping treatment of catfish fillets in 2% carvacrol solution for 30 min at 4 °C reduced L. monocytogenes to an undetectable level from their initial load of 5 log CFU/g and reduced total microbial load from catfish fillets by approximately 5 log CFU/g. In sensory analysis trained panelist preferred control samples over 2% carvacrol treated samples implying potential limitation in applicability of carvacrol for fillet treatments.     

Comparative Study of Thermal Inactivation of Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes in Ground Pork

ABSTRACT: Thermal inactivation of Escherichia coli O157:H7, Salmonella , and Listeria monocytogenes in ground pork was compared. The D (decimal reduction time at a certain heating temperature) values of E. coli O157:H7, Salmonella , and L. monocytogenes at 55 to 70°C were 33.44 to 0.048 min, 45.87 to 0.083 min, and 47.17 to 0.085 min, respectively. The z (temperature rise for 1 log10 reduction of D) value of E. coli O157:H7, Salmonella , and L. monocytogenes in ground pork was 4.94°C, 5.89°C, and 5.92°C, respectively. Significant difference was found on the D and z values between E. coli O157:H7 and Salmonella or between E. coli O157:H7 and L. monocytogenes . The D and z values of Salmonella in ground pork were not significantly different from L. monocytogenes .     

Listeria monocytogenes在经高氧化还原电位酸性水处理的鲜食莴苣上预测模型的建立

采用高氧化还原电位酸性水(EOW)对接种过单核细胞增生李斯特菌的鲜食莴苣进行处理,研究残留菌在不同温度下的保存期限。建立Gompertz,Logistic和Baranyi初级模型,描述单增李斯特菌在不同温度下的生长情况,对比结果表明,Gompertz模型的判定系数R2=0.9913,能够更好地拟合李斯特菌在各个温度下的生长状况,并得到李斯特菌生长的Gompertz模型生长参数(SGR,LT,MPD)。利用平方根模型对其的最大比生长速率的平方根-温度进行拟合,得到莴苣上单核细胞增生李斯特菌生长的二级模型:SGR=0.015T+0.069。使用判定系数(R2)、均方误差(MSE)、偏差因子(BF)和准确因子(AF)对模型进行验证,结果表明,本研究得出的二级模型能够很好地预测单核细胞增生李斯特菌在相应环境下的生长状况。     

Efficacy of High Hydrostatic Pressure Treatment in Reducing Escherichia coli O157 and Listeria monocytogenes in Alfalfa Seeds

ABSTRACT: The application of high hydrostatic pressure (HHP) technology as a seed decontamination technique was evaluated. Alfalfa seeds inoculated with Escherichia coli O157 and Listeria monocytogenes were air-dried and subjected to independent HHP treatments of 275 to 575 MPa for 2 min or at 475 MPa for 2 to 8 min (40°C). There were 1.4-log and 2.0-log reductions in E. coli O157 populations at 575 MPa (2 min) and 475 MPa (8 min), respectively. However, these treatments caused only 0.8-log and 1.1-log reductions in L. monocytogenes counts. Treated seeds took longer to germinate, achieving germination rate of up to 34%, whereas 95% of the control germinated. Results suggest that L. monocytogenes is more resistant to the bactericidal effects of HHP than E. coli O157. Although HHP treatments achieved a greater reduction in E. coli O157, it was at the expense of seed germination. Overall, our results indicate that although HHP treatments reduced the populations of E. coli O157 and L. monocytogenes in alfalfa seeds, they did not completely eliminate these microorganisms.     

超高效液相色谱串联质谱法快速测定乳及乳制品中三聚氰胺

建立快速、经济测定乳及乳制品中三聚氰胺的超高效液相色谱-串联质谱(UPLC-MS/MS)分析方法。乳及乳制品样品中加入乙腈-水溶液,通过超声波提取和二氯甲烷液液萃取除去蛋白质、脂肪等杂质,进一步稀释后直接进样。经超高效液相色谱(UPLC)快速分离,采用电喷雾正离子(ESI+)多反应监测模式(MRM),实现对三聚氰胺的定性和外标法定量。乳及乳制品样品在1.2 min内完成分析,且具有良好的线性关系(r=0.999 1);方法检出限和定量限(LOQ)分别为0.005 mg/kg、0.017 mg/kg;在0.5～100μg/L的基质添加浓度范围内,平均回收率为91.3%～97.1%,相对标准偏差(RSD)小于9.03%。     

高效液相色谱法测定母乳及牛乳中总核苷酸含量

建立高效液相色谱测定母乳和牛乳中总核苷酸含量和组成的分析方法。总核苷酸包括游离核苷酸、游离核苷、核苷聚合物及加合物等不同来源的可利用核苷总量。利用酶解法释放出母乳中的核苷,以硼化聚丙烯酰胺凝胶（Affi-Gel 601）作为固相萃取介质进行样品净化,用反相高效液相色谱仪检测,内标法定量,检测结果以核苷酸计。且针对母乳样品的特性,增加高温灭活步骤进行样品前处理,以检测母乳中核苷酸的天然含量及组成。对所建方法进行全面验证,结果表明其具有良好的线性,相关系数（r2）均在0.999以上;母乳中各核苷酸的检出限和定量限分别在0.18～0.45 mg/L和0.60～1.51 mg/L之间;母乳基质中3 种不同加标水平下总核苷酸的回收率均在99%～108%之间,连续3 d 6 次检测结果的相对标准偏差在0.8%～7.8%之间,显示了良好的准确性及精密度。然后进一步利用所建检测方法,评估母乳及5 种市面常见的牛乳中的核苷酸总量及组成用以验证该方法的适用性。该方法的灵敏度、准确性及精密度均可满足对母乳和牛乳样品中天然的核苷酸含量及组成的测定要求。本研究为今后大样本量调查母乳及牛乳样品中核苷酸的含量和组成提供了更加科学准确的方法。