Frozen stored murine hybridoma cells can be used to determine the virulence of Listeria monocytogenes

Mammalian Cell-Based Immunoassay for Detection of Viable Bacterial Pathogens

Rapid detection of live pathogens is of paramount importance to ensure food safety. At present, nucleic acid-based polymerase chain reaction and antibody-based lateral flow assays are the primary methods of choice for rapid detection, but these are prone to interference from inhibitors, and resident microbes. Moreover, the positive results may neither assure virulence potential nor viability of the analyte. In contrast, the mammalian cell-based assay detects pathogen interaction with the host cells and is responsive to only live pathogens, but the short shelf-life of the mammalian cells is the major impediment for its widespread application. An innovative approach to prolong the shelf-life of mammalian cells by using formalin was undertaken. Formalin (4% formaldehyde)-fixed human ileocecal adenocarcinoma cell line, HCT-8 on 24-well tissue culture plates was used for the capture of viable pathogens while an antibody was used for specific detection. The specificity of the Mammalian Cell-based ImmunoAssay (MaCIA) was validated with Salmonella enterica serovar Enteritidis and Typhimurium as model pathogens and further confirmed against a panel of 15 S. Enteritidis strains, 8 S. Typhimurium, 11 other Salmonella serovars, and 14 non-Salmonella spp. The total detection time (sample-to-result) of MaCIA with artificially inoculated ground chicken, eggs, milk, and cake mix at 1-10 CFU/25 g was 16-21 h using a traditional enrichment set up but the detection time was shortened to 10-12 h using direct on-cell (MaCIA) enrichment. Formalin-fixed stable cell monolayers in MaCIA provide longer shelf-life (at least 14 weeks) for possible point-of-need deployment and multi-sample testing on a single plate.

Mammalian cell-based sensor system

Use of living cells or cellular components in biosensors is receiving increased attention and opens a whole new area of functional diagnostics. The term "mammalian cell-based biosensor" is designated to biosensors utilizing mammalian cells as the biorecognition element. Cell-based assays, such as high-throughput screening (HTS) or cytotoxicity testing, have already emerged as dependable and promising approaches to measure the functionality or toxicity of a compound (in case of HTS); or to probe the presence of pathogenic or toxigenic entities in clinical, environmental, or food samples. External stimuli or changes in cellular microenvironment sometimes perturb the "normal" physiological activities of mammalian cells, thus allowing CBBs to screen, monitor, and measure the analyte-induced changes. The advantage of CBBs is that they can report the presence or absence of active components, such as live pathogens or active toxins. In some cases, mammalian cells or plasma membranes are used as electrical capacitors and cell-cell and cell-substrate contact is measured via conductivity or electrical impedance. In addition, cytopathogenicity or cytotoxicity induced by pathogens or toxins resulting in apoptosis or necrosis could be measured via optical devices using fluorescence or luminescence. This chapter focuses mainly on the type and applications of different mammalian cell-based sensor systems.

A novel and simple cell-based detection system with a collagen-encapsulated B-lymphocyte cell line as a biosensor for rapid detection of pathogens and toxins

Cell-based biosensors (CBBs) are becoming important tools for biosecurity applications and rapid diagnostics in food microbiology for their unique capability of detecting physiologically hazardous materials. A multi-well plate-based biosensor containing B-cell hybridoma, Ped-2E9, encapsulated in type I collagen matrix, was developed for rapid detection of viable cells of pathogenic Listeria, the toxin listeriolysin O, and the enterotoxin from Bacillus species. This sensor measures the alkaline phosphatase release from infected Ped-2E9 cells colorimetrically. Pathogenic L. monocytogenes cells and toxin preparations from L. monocytogenes or B. cereus showed cytotoxicity ranging from 24 to 98% at 3-6 h postinfection. In contrast, nonpathogenic L. innocua (F4247) and B. subtilis induced minimal cytotoxicity, ranging only 0.4-7.6%. Laser scanning cytometry and cryo-nano scanning electron microscopy confirmed the live or dead status of the infected Ped-2E9 cells in gel matrix. This paper presents the first example of a cell-based sensing system using collagen-encapsulated mammalian cells for rapid detection of pathogenic bacteria or toxin, and demonstrates a potential for onsite use as a portable detection system.

Biosensors and bio-based methods for the separation and detection of foodborne pathogens

The safety of our food supply is always a major concern to consumers, food producers, and regulatory agencies. A safer food supply improves consumer confidence and brings economic stability. The safety of foods from farm-to-fork through the supply chain continuum must be established to protect consumers from debilitating, sometimes fatal episodes of pathogen outbreaks. The implementation of preventive strategies like hazard analysis critical control points (HACCP) assures safety but its full utility will not be realized unless supportive tools are fully developed. Rapid, sensitive, and accurate detection methods are such essential tools that, when integrated with HACCP, will improve safety of products. Traditional microbiological methods are powerful, error-proof, and dependable but these lengthy, cumbersome methods are often ineffective because they are not compatible with the speed at which the products are manufactured and the short shelf life of products. Automation in detection methods is highly desirable, but is not achievable with traditional methods. Therefore, biosensor-based tools offer the most promising solutions and address some of the modern-day needs for fast and sensitive detection of pathogens in real time or near real time. The application of several biosensor tools belonging to the categories of optical, electrochemical, and mass-based tools for detection of foodborne pathogens is reviewed in this chapter. Ironically, geometric growth in biosensor technology is fueled by the imminent threat of bioterrorism through food, water, and air and by the funding through various governmental agencies.

Hybridoma Ped-2E9 cells cultured under modified conditions can sensitively detect Listeria monocytogenes and Bacillus cereus

Lymphocyte origin hybridoma Ped-2E9 cell-based cytotoxicity assay can detect virulent Listeria or Bacillus species, and its application in a cell-based biosensor for onsite use would be very attractive. However, maintaining enough viable cells on a sensor platform for a prolonged duration is a challenging task. In this study, key factors affecting the survival and growth of Ped-2E9 cells under modified conditions were investigated. When the Ped-2E9 cells were grown in media containing 5% fetal bovine serum in sealed tubes without any replenishment of nutrients or exogenous CO(2) supply, a large portion of the cells remained viable for 6 to 7 days and cells entered into G0/G1 resting phase. The media pH change was negligible and no cell death was observed in the first 4 days, then cells sequentially underwent apoptotic (fourth day onward) phase until day 7 after which a majority was dead. Subsequent cytotoxicity testing of 3- to 7-day stored Ped-2E9 cells sensitively detected virulent Listeria and Bacillus species. These data strongly suggest that Ped-2E9 cells can be maintained in viable state for 6 days in a sealed tube mimicking the environment in a potential sensor device for onsite use without the need for expensive cell culture facilities.

Rapid Ped-2E9 cell-based cytotoxicity analysis and genotyping of Bacillus species

Bacillus species causing food-borne disease produce multiple toxins eliciting gastroenteritis. Toxin assays with mammalian cell cultures are reliable but may take 24 to 72 h to complete and also lack sensitivity. Here, a sensitive and rapid assay was developed using a murine hybridoma Ped-2E9 cell model. Bacillus culture supernatants containing toxins were added to a Ped-2E9 cell line and analyzed for cytotoxicity with an alkaline phosphatase release assay. Most Bacillus cereus strains produced positive cytotoxicity results within 1 h, and data were comparable to those obtained with the standard Chinese hamster ovary (CHO)-based cytotoxicity assay, which took about 72 h to complete. Moreover, the Ped-2E9 cell assay had 25- to 58-fold-higher sensitivity than the CHO assay. Enterotoxin-producing Bacillus thuringiensis also gave positive results with Ped-2E9 cells, while several other Bacillus species were negative. Eight isolates from food suspected of Bacillus contamination were also tested, and only one strain, which was later confirmed as B. cereus, gave a positive result. In comparison with two commercial diarrheal toxin assay kits (BDE-VIA and BCET-RPLA), the Ped-2E9 assay performed more reliably. Toxin fractions of >30 kDa showed the highest degree of cytotoxicity effects, and heat treatment significantly reduced the toxin activity, indicating the involvement of a heat-labile high-molecular-weight component in Ped-2E9 cytotoxicity. PCR results, in most cases, were in agreement with the cytotoxic potential of each strain. Ribotyping was used to identify cultures and indicated differences for several previously reported isolates. This Ped-2E9 cell assay could be used as a rapid (approximately 1-h) alternative to current methods for sensitive detection of enterotoxins from Bacillus species.

The role of dietary vitamin E in experimental Listeria monocytogenes infections in turkeys

The current study was designed to determine if dietary vitamin E influenced either the gut clearance or levels of peripheral blood CD4+ and CD8+ T lymphocytes in adult turkeys experimentally infected with Listeria monocytogenes. Turkeys were fed vitamin E (0, 100, or 200 IU) from day of hatch to time of necropsy. After 6 wk on the experimental diet, turkeys were orally inoculated with L. monocytogenes (approximately 10(9) cfu). To monitor infection status, cloacal swabs were taken on selected days post-inoculation (DPI). At necropsy, samples of viscera, including liver, spleen, cecum, duodenum, ileum, and colon were collected and cultured for L. monocytogenes. In experiments 1 and 2, recovery of L. monocytogenes from cloacal swabs, tissues, and intestines from turkeys fed vitamin E was generally lower than that from turkeys fed the control diet, although these differences were not statistically significant. When data from both trials were combined, L. monocytogenes was cultured less frequently from cloacal swabs of the vitamin E-treated group (200 IU) on 2 and 3 DPI, when compared to controls (0 IU, P < 0.01). There were no changes in virulence characteristics of L. monocytogenes cells, as measured by in vitro killing of Ped-2E9 cells, recovered from cloacal swabs or tissues of experimentally infected turkeys fed the control or a vitamin E treatment diet. Flow cytometric analysis indicated that CD4+ and CD8+ peripheral blood T lymphocytes were elevated at 6 and 8 DPI in infected turkeys given 200 IU vitamin E.

Dithiothreitol enhances Listeria monocytogenes mediated cell cytotoxicity

The hybridoma Ped-2E9 based cytotoxicity assay was developed to distinguish virulent from avirulent Listeria species in 6 hr. The cytotoxicity effect on Ped-2E9 was reported to be primarily due the cytolytic action of listeriolysin O (LLO), produced by L. monocytogenes. In this study, the effect of a reducing agent, dithiothreitol (DTT, 0-2 mM) that is known to activate LLO was investigated to make the Ped-2E9 based cytotoxicity assay an even more sensitive and rapid. Also, we examined the effect of fetal bovine serum (FBS, 0-50%), a common ingredient of tissue culture media on cytotoxicity. A DTT concentration of 0.5 mM gave an optimum cytotoxicity effect, which could be measured by both alkaline phosphatase (AP) and lactate dehydrogenase (LDH) assays in just 1.5-2 hr. FBS, at levels between 10 to 50%, significantly inhibited Listeria-mediated cytotoxicity. Concentrated culture filtrates from L. monocytogenes or LLO producing recombinant L. innocua (prfA+ hlyA+) strain also caused cytotoxicity effects, which were observed by scanning electron microscopy or a cytotoxicity assay in 2-3 hr. Interestingly, addition of DTT to culture filtrates produced 100% cell cytotoxicity in just 15 min. This indicated that LLO activity, which is responsible for Ped-2E9 cytotoxicity, was augmented several folds with the addition of a reducing agent. Examination of Listeria isolates belonging to different serogroups from clinical sources or naturally contaminated meat products with DTT gave cytotoxicity results in 2 hr, which were comparable to the 5-hr assay analyzed concurrently without DTT. These results indicated that DTT, which activated the LLO, could be used in the cytotoxicity assay to enhance Listeria-mediated Ped-2E9 cell cytotoxicity. This knowledge will greatly assist us to develop a user-friendly rapid assay to screen cytopathogenic properties of Listeria species.