Detection of Escherichia cole O157 in French food samples using an immunomagnetic separation method and the VIDAS E. coli O157

Validation and verification of LAMP, ISO, and VIDAS UP methods for detection of Escherichia coli O157:H7 in different food matrices

Escherichia coli O157:H7 is one of the most important food-borne pathogens to threaten public health. Cultural methods are used as a gold standard while they are laborious and time-consuming. Loop-mediated isothermal amplification (LAMP) method is an alternative method that became widely used for the detection of food-borne pathogens. The aim of this study was to evaluate the specificity and sensitivity of LAMP method for detection of E. coli O157:H7, also to compare detection performances with VIDAS UP and ISO (International Organization for Standardization) methods in different food matrices (beef meat, minced lamb meat, milk, cheese, apple puree, and soybean sprouts). E. coli O157:H7 were spiked in three different levels (high 4.58; medium 2.32; low 0.30 log₁₀ CFU/g-ml) to food matrices. Although there were no significant differences in terms of the specificity and sensitivity values among the three methods (p ≥ .05), it was determined that the highest specificity and sensitivity values obtained from the LAMP method. Sensitivity and specificity values of LAMP method were found as 0.997 and 0.988, for the ISO method were 0.989 and 0.971, and for the VIDAS UP method were 0.980 and 0.963, respectively. In milk samples, sensitivity and specificity values of the VIDAS UP method were significantly lower than LAMP and ISO methods (p < .05). However, there were no significant differences found for the other food matrices among the three methods (p > .05). It can be summarized from this study that specificity and sensitivity values of the LAMP method are equal or higher and less time-consuming than ISO and VIDAS UP methods. In conclusion, using a simple, fast, and inexpensive detection method, such as LAMP, especially in endemic regions or in an outbreak to control spreading of pathogens, is very important for public health.

Comparison between the Vitek Immunodiagnostic Assay System and PCR for the detection of pathogenic microorganisms in an experimental dry sausage during its curing process

The comparison between the Vitek Immunodiagnostic Assay System (VIDAS) and PCR methods for the detection of the pathogenic microorganisms Salmonella Typhimurium, Listeria monocytogenes, Escherichia coli O157:H7, Clostridium perfringens, and Staphylococcus aureus in salchich6n (a type of Spanish fermented dry sausage) was studied. The automated mini-VIDAS method and the PCR method were used to detect the presence of these microorganisms in 90 samples grouped into six batches (one control batch, and five batches inoculated with Salmonella Typhimurium, L. monocytogenes, E. coli O157:H7, S. aureus, and C. perfringens, respectively). The determinations were carried out on fresh sausage, semicured (14 days), and finished product (28 days of curing). The concordance between both methods was 100%. For salchichón-type sausage samples, both the VIDAS method and PCR (in comparison to traditional methods) permit a significant reduction in the time required for the detection of the pathogens assayed. The absence of Salmonella Typhimurium, E. coli O157:H7, L. monocytogenes, S. aureus, their enterotoxins, and C. perfringens were verified in the control batch. Conversely, for batches inoculated with these pathogens, the results were positive in the fresh sausage up to 14 days of curing and in the finished product; this demonstrated the null or scant influence of the curing conditions and the constitutive lactic acid biota on the pathogens investigated.

Isolation and characterization of Shiga toxin-producing Escherichia coli from precooked sausages (morcillas)

The aim of the study was to establish the microbiological quality of morcillas, typical Argentine sausages, and to investigate the presence of Shiga toxin-producing Escherichia coli (STEC). Between October 2001 and October 2002, a total of 100 morcilla samples were analysed. Several samples showed high levels of total aerobic mesophilic bacteria counts, molds and yeasts. The samples analysed contained Enterobacteriaceae (100%) and fecal coliforms (81%), indicating inadequate application of the thermal treatment and deficient hygiene conditions during the elaboration of the product. STEC strains were isolated from three out of 100 (3%) morcilla samples. Two strains (2%) were characterized as E. coli O157:H7 stx2+stx2vh-a/eae/EHEC-hlyA and one strain (1%), as E. coli O26:H11 stx1/eae/EHEC-hlyA. Considering both the high microbial count in all tested samples and the presence of STEC strains in three of them, morcillas should be considered a food unsafe to consume when inadequately cooked.

Isolation of Escherichia coli O157:H7 from foods in Greece

The presence of Escherichia coli O157:H7 in various foods of animal origin was surveyed in northwestern Greece. Six hundred samples of unpasteurized cows', ewes' and goats' milk, raw minced meat, uncooked frozen beef hamburgers, sandwiches (containing ham or turkey, mixed vegetable salad with mayonnaise and lettuce), fresh traditional Greek pork sausages and swine intestines appropriate for traditional Greek kokoretsi were assayed for E. coli serogroup O157:H7 using the standard cultural method and the immunomagnetic separation technique. The pathogen was detected in 1 out of 100 (1.0%) samples of ewes' milk, 1 out of 75 (1.3%) fresh sausages and 1 out of 50 (2.0%) swine intestines prepared for kokoretsi. The isolated strains were nonsorbitol fermenters, MUG-negative, O157 agglutinating, verotoxin-producing and carried both VT1 and VT2 genes. The three isolated strains were tested for antibiotic resistance and were found to be susceptible to eight antimicrobial agents (ampicillin, chloramphenicol, kanamycin, nalidixic acid, norfloxacin, streptomycin, sulfamethoxazole-trimethoprim and tetracycline).

Methods for the detection and isolation of Shiga toxin-producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) are an important cause of haemorrhagic colitis and the diarrhoea-associated form of the haemolytic uraemic syndrome. Of the numerous serotypes of E. coli that have been shown to produce Shiga toxin (Stx), E. coli O157:H7 and E. coli O157:NM (non-motile) are most frequently implicated in human disease. Early recognition of STEC infections is critical for effective treatment of patients. Furthermore, rapid microbiological diagnosis of individual patients enables the prompt notification of outbreaks and implementation of control measures to prevent more cases. Most human infections caused by STEC have been acquired by the consumption of contaminated foods, especially those of bovine origin such as undercooked ground beef and unpasteurized cows' milk, and by person-to-person contacts. To identify the reservoirs of STEC and the routes of transmission to man, sensitive methods are needed as these pathogens may only be present in food, environmental and faecal samples in small numbers. In addition, sensitive and rapid detection methods are necessary for the food industry to ensure a safe supply of foods. Sensitive methods are also needed for surveillance programmes in risk assessment studies, and for studies on survival and growth of STEC strains. Cultural methods for the enrichment, isolation and confirmation of O157 STEC are still evolving. Several selective enrichment media have been described, of which modified tryptone soy broth with novobiocin and modified E. coli broth with novobiocin, seem to be the most appropriate. These media are minimally-selective broths that give a somewhat limited differential specificity favouring isolation of O157 STEC, as opposed to other Gram-negative bacteria, in the sample. An incubation temperature of 41-42 degrees C further enhances selectivity. The occurrence of heat-, freeze-, acid- or salt-stressed STEC in foods means that it is important to be able to detect cells that are in a stressed state, as STEC generally have a very low infectious dose, and injured cells mostly retain their pathogenic properties. For the isolation of stressed O157 STEC, pre-enrichment in a non-selective broth is necessary. The most widely used plating medium for the isolation of typical sorbitol-non-fermenting strains of STEC of serogroup O157 is sorbitol MacConkey agar with cefixime and tellurite (CT-SMAC). As some STEC strains are sensitive for tellurite and/or are sorbitol-fermenting, the use of a second isolation medium, such as one of the newer chromogenic media, is recommended. Immunomagnetic separation (IMS) following selective enrichment, and subsequent spread-plating of the concentrated target cells onto CT-SMAC agar, appears to be the most sensitive and cost-effective method for the isolation of E. coli O157 from raw foods. IMS increases sensitivity by concentrating E. coli O157 relative to background microflora, which may overgrow or mimic O157 STEC cells on selective agars. While cultural isolation of O157 STEC from foods and faeces is time-consuming, labour-intensive and hence, costly, rapid immunological detection systems have been developed which significantly reduce the analysis time. These methods include enzyme-linked immunosorbent assays (ELISAs), colony immunoblot assays, direct immunofluorescent filter techniques, and several immunocapture techniques. Both polyclonal and monoclonal antibodies specific for the O and H antigens are used for these methods. Many of these test systems are able to detect less than one O157 STEC cell g(-1) of raw meat after overnight enrichment. Presumptive results are available after just one day, but need to be completed with the isolation of the organisms. The primary use of these procedures is therefore to identify food and faecal samples that possibly contain O157 STEC.

Sensitive detection of Escherichia coli O157:H7 in food and water by immunomagnetic separation and solid-phase laser cytometry

Rapid, direct methods are needed to assess active bacterial populations in water and foods. Our objective was to determine the efficiency of bacterial detection by immunomagnetic separation (IMS) and the compatibility of IMS with cyanoditolyl tetrazolium chloride (CTC) incubation to determine respiratory activity, using the pathogen Escherichia coli O157:H7. Counterstaining with a specific fluorescein-conjugated anti-O157 antibody (FAb) following CTC incubation was used to allow confirmation and visualization of bacteria by epifluorescence microscopy. Broth-grown E. coli O157:H7 was used to inoculate fresh ground beef (<17% fat), sterile 0.1% peptone, or water. Inoculated meat was diluted and homogenized in a stomacher and then incubated with paramagnetic beads coated with anti-O157 specific antibody. After IMS, cells with magnetic beads attached were stained with CTC and then an anti-O157 antibody-fluorescein isothiocyanate conjugate and filtered for microscopic enumeration or solid-phase laser cytometry. Enumeration by laser scanning permitted detection of ca. 10 CFU/g of ground beef or <10 CFU/ml of liquid sample. With inoculated meat, the regression results for log-transformed respiring FAb-positive counts of cells recovered on beads versus sorbitol-negative plate counts in the inoculum were as follows: intercept = 1.06, slope = 0.89, and r2 = 0. 95 (n = 13). The corresponding results for inoculated peptone were as follows: intercept = 0.67, slope = 0.88, and r2 = 0.98 (n = 24). Recovery of target bacteria on beads by the IMS-CTC-FAb method, compared with recovery by sorbitol MacConkey agar plating, yielded greater numbers (beef, 6.0 times; peptone, 3.0 times; water, 2.4 times). Thus, within 5 to 7 h, the IMS-CTC-FAb method detected greater numbers of E. coli O157 cells than were detected by plating. The results show that the IMS-CTC-FAb technique with enumeration by either fluorescence microscopy or solid-phase laser scanning cytometry gave results that compared favorably with plating following IMS.