A rapid molecular-based assay for direct quantification of viable bacteria in slaughterhouses

A rapid test for microbial quantification in carcass and environmental swabs that does not require enrichment and provides results in less than 4 h is described here. Steps in the assay include the rapid concentration of bacteria on sponge swabs by vacuum filtration followed by real-time PCR detection. The assay has been applied for the detection of coliforms, Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes on carcass swabs and environmental samples in a slaughterhouse-processing line. Comparison of this rapid method with standard culture techniques for coliform counts on beef and pork carcass swabs revealed higher numbers of bacteria (2- to 50-fold) by the rapid test compared with the plate counts. This was due to the detection of all bacteria (live, dead, and non-culturable forms) in the rapid assay. To allow detection of only viable bacteria, concentrated samples were treated with ethidium monoazide (EMA) prior to DNA extraction and real-time PCR detection, thereby preventing the amplification of DNA from bacteria with damaged cell walls and allowing only the DNA from bacteria with intact membranes to be detected. EMA treatment resulted in a significant reduction (P < 0.001) in the number of coliforms detected compared to real-time PCR without EMA treatment. In beef swabs, the counts obtained in EMA real-time PCR were not significantly different (P < 0.08) from the culture counts and the correlation coefficient between the two assays was 0.7385. A lower correlation coefficient (0.402) was obtained with pork swabs. The assay described herein has the potential to be applied on a routine basis to slaughterhouse lines for the detection of indicator organisms or specific pathogens.     

Development of an isothermal amplification-based assay for the rapid visual detection of Salmonella bacteria

The efficient and timely detection of pathogens is a major concern worldwide. The aim of this study was to establish a rapid detection method for Salmonella bacteria in food samples to facilitate timely treatment. Widely used detection methods currently include culture-based methods and PCR-based methods. The former are time consuming, requiring 2 to 3 d, whereas the latter have higher accuracy but are typically complicated, requiring expertise and expensive instruments. In this study, a sensitive and rapid approach for the visual and point-of-use detection of Salmonella bacteria based on recombinase polymerase amplification (RPA) and a lateral-flow (LF) nucleic acid strip was established. We designed a pair of primers according to the invA gene of Salmonella bacteria: one was modified with digoxin, and the other was modified with biotin. In the presence of the biotin- and digoxin-modified primers and target DNA, the RPA produced a substantial amount of duplex DNA attached to biotin and digoxin. The products were detected using LF strips through immunoreaction: anti-digoxin antibodies on the gold nanoparticles, digoxin on the duplex, streptavidin on the LF test line, and biotin on the duplex. The developed RPA-LF assay allowed detection of Salmonella genomic DNA in less than 20 min with simple water bath equipment or portable thermal equipment. In addition, the RPA-LF assay was highly sensitive, with a detection limit as low as 20 fg of target DNA or 1.05 × 10¹ cfu of bacteria in pure culture, and highly specific, exhibiting no cross-reaction with Staphylococcus aureus, Escherichia coli, Listeria monocytogenes, Shigella, Enterobacter aerogenes, or Campylobacter jejuni. Importantly, Salmonella could be detected in milk and chicken breast at concentrations as low as 1.05 × 10⁰ cfu/mL or 1.05 × 10⁰ cfu/g after enrichment for 2 h and in eggs at 1.05 × 10⁰ cfu/g after enrichment for 4 h. Furthermore, RPA was more sensitive than PCR, which requires a thermal cycling device. In summary, this study describes a sensitive, simple, and point-of-use detection method for Salmonella bacteria.     

PMA-qPCR方法快速检测Lactobacillus paracasei N1115活菌的初步研究

目的:建立快速准确的副干酪乳杆菌N1115（Lactobacillus Paracasei N1115）活菌的荧光定量检测方法并应用于菌粉的活菌计数。方法:根据副干酪乳杆菌N1115的苯丙氨酰-tRNA合成酶α亚基基因（phe S）设计特异性引物;使用叠氮溴化丙锭（Propidium monoazide PMA）抑制死菌DNA扩增,然后通过q PCR的方法检测菌粉中的N1115的活菌数。结果:经验证得到一对N1115特异引物:C15F、C15R;副干酪乳杆菌N1115在90℃水浴条件下热损伤时间为8 min;PMA对于N1115死菌DNA的扩增有明显的抑制效果;PMA-q PCR能够快速准确的测得菌粉中N1115活菌数。结论:建立了一种快速、准确的副干酪乳杆菌N1115活菌的荧光定量检测方法。      

Evaluation of a rapid detection method for Escherichia coli in foods using fluorogenic assay

The fluorogenic assay with the 4-methylumbelliferone glucuronide (MUG) incorporated in lauryl tryptose broth (LTB) was compared with the conventional FDA's Bacteriological Analytical Manual MPN Method for the detection and confirmation of E. coli in foods. One-hundred and one food samples comprising froglegs, oysters, mussels, shrimps, red meats, fish, dried prawns and dried beef were tested. Ninety one per cent of the LTB-MUG tubes were positive for E. coli using the fluorogenic assay and 81% using the MPN method. Analysis time was reduced from 5 days for the MPN method to only 2 days for the fluorogenic assay. In spite of problems posed by false positive and negative reactions, the fluorogenic assay remains a much more sensitive and rapid method.     

A sandwich enzyme-linked immunosorbent assay for the detection of almonds in foods

An enzyme-linked immunosorbent assay was developed to detect almonds as potential allergenic contaminants in food. Polyclonal antibodies directed against roasted almonds were partially purified from immunized sheep and rabbits and used as capture and secondary antibodies, respectively, in a sandwich-type, 96-well plate format. Food samples and almond-spiked samples were extracted 1:10 in phosphate-buffered saline at 60 degrees C for 2 h, centrifuged, and applied to wells coated with sheep anti-almond antibody. After incubation, washing, and the addition of rabbit anti-almond antibody, the amount of almond present was detected with the subsequent addition of goat anti-rabbit immunoglobulin G-alkaline phosphatase conjugate and p-nitrophenyl phosphate substrate. Plate absorbances were read at 410 nm, and standard curves were developed in all matrices to quantify unknowns. Antibodies developed were specific for almond; however, some cross-reactivity was observed with extracts of some tree nuts and sesame seeds. Sodium dodecylsulfate-polyacrylamide gel electrophoresis and Western immunoblotting indicated that sheep anti-almond antibody recognized proteins extracted from black walnuts, Brazil nuts, cashews, hazelnuts, macadamia nuts, pistachios, and sesame seeds in addition to those from almond. The assay was optimized to detect less than 1 ppm of almond and was used successfully to determine almond residues in cereal and chocolate without cross-reacting interferences. A retail survey of 20 brands of cereal demonstrated that the assay produced statistically consistent results. This assay provides a useful quality control tool for the food industry for the protection of consumers allergic to almonds.     

Molecular methods for the detection of biogenic amine-producing bacteria on foods

Biogenic amines are low molecular weight organic bases that can be detected in raw and processed foods. Several toxicological problems resulting from the ingestion of food containing biogenic amines have been described. Biogenic amines are mainly produced by the decarboxylation of certain amino acids by microbial action. Since the ability of microorganisms to decarboxylate amino acid is highly variable, being in most cases strain-specific, the detection of bacteria possessing amino acid decarboxylase activity is important to estimate the risk of biogenic amine food content and to prevent biogenic amine accumulation in food products. Molecular methods for the early and rapid detection of these producer bacteria are becoming an alternative to traditional culture methods. PCR methods offer the advantages of speed, sensitivity, simplicity and specific detection of amino acid decarboxylase genes. Moreover, these molecular methods detect potential biogenic amine risk formation in food before the amine is produced. The aim of the present review is to give a complete overview of the molecular methods proposed in the literature for the detection of biogenic amine-producing bacteria. These genetic procedures allow the introduction of early control measures to avoid the development of these bacteria.     

一种快速检测呕吐毒素的胶体金试纸条

利用胶体金免疫层析法,研制了一种能够快速检测黄豆中呕吐毒素含量的试纸条。通过试验,胶体金试纸条对呕吐毒素的检测限是1μg/g,对玉米赤霉烯酮毒素、黄曲霉毒素B1、赭曲毒素A等无交叉反应,假阳性率和假阴性率均为0,检测时间为10min。试纸条能够准确、可靠、简便、快速地检测黄豆中残留的呕吐毒素,适合大量样品的现场检测。     

Development of a PCR assay for detection of Enterobacteriaceae in foods

A broad-range PCR assay for the detection of bacteria belonging to the Enterobacteriaceae family was developed in this study. Primers targeting the bacterial 16S rRNA gene were newly designed and used in this PCR assay. To determine the specificity of the assay, 72 different bacterial species (of 49 genera), 2 fungi, 3 animals, and 4 plants were tested. Results were positive for every tested Vibrioaceae or Enterobacteriaceae strain except Proteus mirabilis. For all other bacterial strains and eukaryotes tested, results were negative. Bacterial DNA for PCR was prepared by a simple procedure with the use of Chelex 100 resin from culture after growth in brain heart infusion medium. To test this PCR assay for the monitoring of the Enterobacteriaceae family, either Escherichia coli or Salmonella Enteritidis was inoculated into various foods as an indicator. Prior to the PCR, the inoculation of 10 to 40 CFU of bacteria per g of food was followed by a 5-h enrichment culture step, and the PCR assay allowed the detection of bacterial cells. When actual examinations of the contamination of 15 noodle foods with Enterobacteriaceae by this PCR assay were conducted, 33% (5 of 15) of the samples tested positive. These results agreed with those of the Petrifilm Enterobacteriaceae Count Plate assay. Including the enrichment culture step, the entire PCR detection process can be completed within 7 h.     

A rapid method for detection of histamine-producing bacteria

A method for detecting histamine-producing bacteria is described. The method is based on automated conductance measurements in a histidine-containing medium (HDB) incubated at 25°C, with a large and distinct increase in the conductance being indicative of the presence of histamine-producing bacteria. An initial low pH (5.5) is optimal for measuring the histidine decarboxylating activity of Morganella morganii, but clear results are obtained at higher pH values (6.0–7.0) as well. It is shown that the histidine decarboxylating activity of M. morganii is unaffected by the presence of non-histamine producing bacteria. Using this new method in the examination of mackerel spoiled at high temperatures, the results obtained indicated the presence of large amounts of histamine-producing organisms. These were subsequently isolated and identified as belonging to the family Enterobacteriaceae.     

微波消解-石墨炉原子吸收光谱法测定面制食品中的铝

目的 根据以往实际应用中发现的分光光度法操作繁复、干扰因素多,电感耦合等离子体质谱法仪器昂贵、运行成本较高等问题有针对性地建立微波消解石墨炉原子吸收法测定面制食品中的铝.方法 试样经微波消解后导入原子吸收分光光度计石墨炉中,原子化后吸收257.4 nm共振线,测得试样吸光度与标准系列比较定量.结果 铝在0 - 500 μg/L范围内线性关系良好,相关系数大于0.999;方法检出限为2 mg/kg;定量限为6 mg/kg.结论 该方法测定面制食品中的铝快速、简便、结果准确.     

食品病原微生物分子检测技术研究进展

传统食品病原微生物检验方法费时、敏感性低、特异性差;而新的生物分子技术能更快、灵敏、特异性检测。该文介绍几种分子技术,如生物传感器、微阵列、电子鼻和纳米装置用于食品中有害微生物检测。     

嗜冷菌快速检测方法研究进展

本文对原料乳中嗜冷菌的几种快速检测技术的发展情况进行了讨论,涉及的检测技术主要有直接外荧光过滤(DEFT)、流式细胞计数(FCM)、实时PCR检测(RT-PCR)、荧光原位杂交探针(FISH)、瞬时温度梯度凝胶电泳(TTGE)、ATP生物发光等技术,并分析了各种方法的优缺点,期望能为乳品加工业发展提供帮助。     

The development of an efficient and rapid enzyme linked fluorescent assay method for the detection of Listeria spp. from foods

Conventional isolation methods, including the Health Products and Food Branch (HPFB), Health Canada method used for the isolation and identification of Listeria species and Listeria monocytogenes from foods and environmental samples, can take a week or more to complete and are usually labor-intensive. This has led to the development of various rapid methods which attempt to generate results comparable to standard methods but in a reduced time-frame with less hands-on operation. Our previous work with rapid detection systems indicated that the recommended enrichment protocols failed to grow the Listeria to detectable levels in a reliable and consistent manner.In the present study, a novel enrichment protocol is described and consists of samples being pre-enriched in Palcam broth (incubated at 35 degrees C for 26 h), enriched in UVM 2 (30 degrees C for 26 h) and then plated and analysed by a rapid detection kit, with results being generated after 52 h of incubation.In total, 200 naturally contaminated samples were analysed by both the HPFB standard method and the Palcam method. The results showed that the Palcam method is comparable to the HPFB method. Further analysis involved a rapid detection system, which applies ELISA techniques and automation in an enzyme-linked fluorescent assay (ELFA) system. This system, referred to as the Vitek Immuno Diagnostic Assay System or VIDAS, can identify Listeria to the genus or species (L. monocytogenes) level.In this comparison, an additional 324 naturally contaminated samples were analysed by both the Palcam and ELFA methods. The sensitivity and specificity of the ELFA method were 98.1% and 97.0%, respectively, while the efficiency was 97.5%. False-negative and false-positive rates were 1.9% and 3.0%, respectively. These results show that the ELFA method (when using the Palcam method for pre- and secondary enrichment) was efficient and gave reliable results after 52 h of incubation, and met Health Canada's criteria for approval as a rapid method.     

A two-site monoclonal antibody immunochromatography assay for rapid detection of peanut allergen Ara h1 in Chinese imported and exported foods

Food allergen labeling has not yet been implemented in China. Therefore, a gold immunochromatography assay (GICA) was developed using two monoclonal antibodies (mAb) against the peanut allergen Ara h1. The GICA was specific for standard peanut samples with a sensitivity of 10 ng/ml. Peanut protein traces extracted from 124 food products imported and exported by China Customs were easily and rapidly detected by GICA. 68 food samples originally labeled as containing peanuts were positive for Ara h1 and 54 food samples labeled as not containing peanuts were negative for Ara h1, indicating that the labels from the manufacturers were accurate. However, 2 food samples labeled as not containing peanuts tested positive for Ara h1. The present GICA provides a fast, simple, semi-quantitative method for the determination of peanut allergens in foods. This detection system can be used to ensure the safety of food imported and exported by China Customs.     

多重PCR快速检测三种食品病原菌

建立一种快速、经济、实用的可以同步检测食品中金黄色葡萄球菌、沙门氏菌、小肠结肠炎耶尔森氏菌的多重聚合酶链式反应(polymerase chain reaction,PCR)的方法。根据金黄色葡萄球菌的nuc基因,沙门氏菌的invA基因,小肠结肠炎耶尔森氏菌的ail基因,分别设计了三对引物,对单个基因PCR和单管多重PCR扩增进行特异性、灵敏性实验以及优化反应体系。三对引物能特异性扩增出236、475、127bp的目的条带。建立的多重PCR同时对3种食源性致病菌进行检测具有较高的灵敏度,灵敏度金黄色葡萄球菌为102CFU/mL、沙门氏菌为102CFU/mL、小肠结肠炎耶尔森氏菌为102CFU/mL。初步建立能同步、简便、快速、灵敏地检测食品中金黄色葡萄球菌、沙门氏菌和小肠结肠炎耶尔森氏菌的三重PCR方法。     

多重PCR快速检测婴幼儿奶粉中的病原菌

为了建立同时检测婴幼儿奶粉中阪崎肠杆菌、金黄色葡萄球菌、蜡样芽孢杆菌的多重PCR方法,根据阪崎肠杆菌ompA基因、金黄色葡萄球菌nuc基因、蜡样芽孢杆菌hblA基因分别设计3对引物进行多重PCR扩增,并对反应条件进行优化。结果多重PCR扩增出长度为514、156、235bp的特异性目的条带。不增菌的情况下,多重PCR同时检测3种病原菌的灵敏度是103cfu/mL,3种病原菌在奶粉中的检出限是104cfu/g。建立的多重PCR反应准确、快速、高效,为同时检测婴幼儿奶粉中的阪崎肠杆菌、金黄色葡萄球菌、蜡样芽孢杆菌提供了新方法。     

一种喹诺酮类药物的酶联免疫快速检测试剂盒的研制

利用酶联免疫技术研制了一种快速检测鱼肉中的喹诺酮类药物试剂盒,经过测试,该试剂盒对鱼肉样本中氧氟沙星、环丙沙星、氨氟沙星等3种喹诺酮类药物的检测限为1μg/kg,批内、批间相对标准偏差均小于10%;稳定性测试结果表明,试剂盒能够在4℃条件下保存12个月,37℃条件下保存7d。喹诺酮类药物单克隆抗体与氧氟沙星、环丙沙星、氨氟沙星的交叉反应率均为100%。该方法准确,可靠,使用简便,适合大量样品的现场检测。