Improvement of the immunomagnetic separation method selective for Escherichia coli O157 strains

Use of DNA aptamer for sandwich type detection of Listeria monocytogenes

A single stranded (ss) DNA aptamer, specific to members of Listeria genus, was used to develop a two-site binding sandwich assay for capture and detection of L. monocytogenes. Antibody-immobilized immunomagnetic beads were used to capture L. monocytogenes, followed by their exposure to the aptamer detector. Detection was achieved by amplification of cell-bound aptamers by qPCR. The lower limit of detection for the combined assay was 2.5 CFU L. monocytogenes in 500 μl buffer. This is juxtaposed to a detection limit of 2.4 log₁₀ CFU in 500 μl buffer for immunomagnetic separation coupled with qPCR detection of L. monocytogenes targeting the hly gene. When applied to turkey deli meat, subjected to 24 h of non-selective enrichment, the two-site binding sandwich assay showed positive results at initial inoculum levels of 1-2 log₁₀ CFU per 25 g sample. Because of its lower limit of detection, the assay reported here could be useful for detection of L. monocytogenes in foods and environmental samples.

Pretreatment with propidium monoazide/sodium lauroyl sarcosinate improves discrimination of infectious waterborne virus by RT-qPCR combined with magnetic separation

RT-qPCR allows sensitive detection of viral particles of both infectious and noninfectious viruses in water environments, but cannot discriminate non-infectious from infectious viruses. In this study, we aimed to optimize RT-qPCR-based detection of chlorine-inactivated human norovirus (NoV) and pepper mild mottle virus (PMMoV) in suspension by pretreatment with an optimal combination of a monoazide and a detergent that can efficiently penetrate damaged viral capsids. Four methods were compared to determine the efficacy of chlorine disinfection (at 1, 3, and 5 min mg/L): (A) RT-qPCR alone, (B) RT-qPCR assay preceded by magnetic bead separation for enrichment of viral particles (MBS-RT-qPCR), (C) MBS-RT-qPCR assay with pretreatment with propidium monoazide (PMA-MBS-RT-qPCR), and (D) PMA-MBS-RT-qPCR assay with pretreatment with sodium lauroyl sarcosinate (INCI-PMA-MBS-RT-qPCR). On the basis of a PMA optimization assay, 200 and 300 μM PMA were used in subsequent experiments for NoV GII.4 and PMMoV, respectively. Optimal INCI concentrations, having minimal influence on NoV GII.4 and PMMoV, were found to be 0.5% and 0.2% INCI, respectively. For NoV GII.4, there were significant differences (P < 0.05) in log₁₀ genome copies between the PMA-treated and the INCI + PMA-treated samples (log₁₀ genome copies differed by 1.11 and 0.59 log₁₀ for 3 and 5 min mg/L of chlorine, respectively). For PMMoV, INCI induced differences in log₁₀ genome copies of 0.92, 1.18, and 1.86, for 1, 3, and 5 min mg/L of chlorine, respectively. Overall, the results of this study indicate that an optimal combination of PMA and INCI could be very useful for evaluating disinfection methods in water treatment strategies.

Generation and characterization of nucleic acid aptamers targeting the capsid P domain of a human norovirus GII.4 strain

Human noroviruses (NoV) are the leading cause of acute viral gastroenteritis worldwide. Significant antigenic diversity of NoV strains has limited the availability of broadly reactive ligands for design of detection assays. The purpose of this work was to produce and characterize single stranded (ss)DNA aptamers with binding specificity to human NoV using an easily produced NoV target-the P domain protein. Aptamer selection was done using SELEX (Systematic Evolution of Ligands by EXponential enrichment) directed against an Escherichia coli-expressed and purified epidemic NoV GII.4 strain P domain. Two of six unique aptamers (designated M1 and M6-2) were chosen for characterization. Inclusivity testing using an enzyme-linked aptamer sorbent assay (ELASA) against a panel of 14 virus-like particles (VLPs) showed these aptamers had broad reactivity and exhibited strong binding to GI.7, GII.2, two GII.4 strains, and GII.7 VLPs. Aptamer M6-2 exhibited at least low to moderate binding to all VLPs tested. Aptamers significantly (p<0.05) bound virus in partially purified GII.4 New Orleans outbreak stool specimens as demonstrated by ELASA and aptamer magnetic capture (AMC) followed by RT-qPCR. This is the first demonstration of human NoV P domain protein as a functional target for the selection of nucleic acid aptamers that specifically bind and broadly recognize diverse human NoV strains.

Dual labeled Ag@SiO₂ core-shell nanoparticle based optical immunosensor for sensitive detection of E. coli

An optical nanobiosensor is presented using a fluorescent dye and anti-E. coli McAb anchored Ag@Silica core shell nanoparticles, for rapid and sensitive Escherichia coli detection in environmental samples. The synthesized dual labeled core shell (DLCS) nanoparticle shows intense fluorescence at 620 nm in solution, having a narrow emission with full width at half maxima (FWHM) of 10 nm, as a prerequisite to develop a sensitive detection platform for various biosensing applications. The specific E. coli was captured using an anti-E. coli antibody functionalized quartz glass, followed by a treatment with DLCS, where the photoluminescence spectroscopy was used to detect the target pathogen. The fabrication of the quartz glass based optical-immunosensor was monitored, and the results show changes in the photoluminescent patterns, which substantiate that varied species were immobilized on the surface of the antibody modified quartz glass. Consequently, the optical immunosensor demonstrated specificity and improved sensitivity, as compared to the customary methods, and was able to detect as low as 5CFU/mL. The developed DLCS based optical immunosensor was evaluated with environmental water samples, which showed acceptable precision, reproducibility and stability, and could be readily applied to the routine monitoring of pathogenic microorganisms in the environmental samples, and most importantly, demonstrate the potential of a prototype development of a simple and inexpensive diagnostic technique.

Development of a robust method for isolation of shiga toxin-positive Escherichia coli (STEC) from fecal, plant, soil and water samples from a leafy greens production region in California

During a 2.5-year survey of 33 farms and ranches in a major leafy greens production region in California, 13,650 produce, soil, livestock, wildlife, and water samples were tested for Shiga toxin (stx)-producing Escherichia coli (STEC). Overall, 357 and 1,912 samples were positive for E. coli O157:H7 (2.6%) or non-O157 STEC (14.0%), respectively. Isolates differentiated by O-typing ELISA and multilocus variable number tandem repeat analysis (MLVA) resulted in 697 O157:H7 and 3,256 non-O157 STEC isolates saved for further analysis. Cattle (7.1%), feral swine (4.7%), sediment (4.4%), and water (3.3%) samples were positive for E. coli O157:H7; 7/32 birds, 2/145 coyotes, 3/88 samples from elk also were positive. Non-O157 STEC were at approximately 5-fold higher incidence compared to O157 STEC: cattle (37.9%), feral swine (21.4%), birds (2.4%), small mammals (3.5%), deer or elk (8.3%), water (14.0%), sediment (12.3%), produce (0.3%) and soil adjacent to produce (0.6%). stx1, stx2 and stx1/stx2 genes were detected in 63%, 74% and 35% of STEC isolates, respectively. Subtilase, intimin and hemolysin genes were present in 28%, 25% and 79% of non-O157 STEC, respectively; 23% were of the "Top 6″ O-types. The initial method was modified twice during the study revealing evidence of culture bias based on differences in virulence and O-antigen profiles. MLVA typing revealed a diverse collection of O157 and non-O157 STEC strains isolated from multiple locations and sources and O157 STEC strains matching outbreak strains. These results emphasize the importance of multiple approaches for isolation of non-O157 STEC, that livestock and wildlife are common sources of potentially virulent STEC, and evidence of STEC persistence and movement in a leafy greens production environment.

Detection of 5 CFU/g of Escherichia coli O157:H7 on lettuce using activated charcoal and real-time PCR without enrichment

A sample treatment method which separates Escherichia coli O157:H7 from lettuce and removes PCR inhibitors allowing 5 CFU/g of target cells to be detected using real-time PCR is described. Lettuce leaves inoculated with E. coli O157:H7 were rinsed with 0.025% sodium dodecyl sulfate (SDS). In this study, there were two major factors that strongly affected the recovery of E. coli O157:H7 during sample preparation, the amount of bentonite coated activated charcoal used to remove PCR inhibitors and the agitated contact time of the samples with the coated charcoal. When 3.0 g of activated carbon coated with bentonite were mixed with target cell suspensions (30 ml) derived from 50 g of lettuce, a high recovery of E. coli O157:H7 (93%) was obtained. Sample agitation with bentonite coated activated charcoal for 15 min resulted in 95% recovery of E. coli O157:H7. When a commercial DNA purification resin was used for detection of E. coli O157:H7 without the use of the bentonite treated charcoal, the real-time PCR (Rti-PCR) failed to detect 1 × 10(2) CFU/g. In contrast, with the use of use of bentonite coated activated charcoal and a commercial DNA purifying resin together, Rti-PCR was able to detect 5 CFU of E. coli O157:H7/g of lettuce which was equivalent to 2.8 CFU/Rti-PCR. Such a successful detection level was the result of the bentonite coated activated charcoal's ability to absorb the PCR inhibitors released from seeded lettuce during detachment. A standard curve was generated by plotting the Ct values against the log of CFU of target bacterial cells. A linear range of DNA amplification was exhibited from 5.0 × 10(0) to 1.0 × 10(4) CFU/g by using Rti-PCR.

Covalently linked immunomagnetic separation/adenosine triphosphate technique (Cov-IMS/ATP) enables rapid, in-field detection and quantification of Escherichia coli and Enterococcus spp. in freshwater and marine environments

AIMS

Developing a rapid method for detection of faecal pollution is among the critical goals set forth by the Environmental Protection Agency in its revision of water quality criteria. The purpose of this study is to devise and test covalently linked antibody-bead complexes for faecal indicator bacteria (FIB), specifically Escherichia coli or Enterococcus spp., in measuring water quality in freshwater and marine systems.

METHODS AND RESULTS

Covalently linked complexes were 58-89% more robust than antibody-bead complexes used in previous studies. Freshwater and marine water samples analysed using covalently linked immunomagnetic separation/adenosine triphosphate quantification technique (Cov-IMS/ATP) and culture-based methods yielded good correlations for E. coli (R = 0·87) and Enterococcus spp. (R = 0·94), with method detection limits below EPA recreational water quality health standards for single standard exceedances (E. coli- 38 cells per 100 ml; Enterococcus spp. - 25 cells per 100 ml). Cov-IMS/ATP correctly classified 87% of E. coli and 94% of Enterococcus spp. samples based on these water quality standards. Cov-IMS/ATP was also used as a field method to rapidly distinguish differential loading of E. coli between two stream channels to their confluence.

CONCLUSIONS

Cov-IMS/ATP is a robust, in-field detection method for determining water quality of both fresh and marine water systems as well as differential loading of FIB from two converging channels.

SIGNIFICANCE AND IMPACT OF THE STUDY

To our knowledge, this is the first work to present a viable rapid, in-field assay for measuring FIB concentrations in marine water environments. Cov-IMS/ATP is a potential alternative detection method, particularly in areas with limited laboratory support and resources, because of its increased economy and portability.

Development of monoclonal antibody-coated immunomagnetic beads for separation and detection of norovirus (genogroup II) in faecal extract samples

AIMS

The aim of this study is to develop an RT-PCR assay combined with immunomagnetic beads (IMS/RT-PCR) coating monoclonal antibody (Mab) for separation and detection of norovirus (genogroup II) in faecal samples. We furthermore compare its detection limits with IMS/RT-PCR using polyclonal antibody (Pab) and the TRIzol extraction method followed by RT-PCR (TRIzol-RT-PCR).

METHODS AND RESULTS

Mab-coated beads and Pab-coated beads were added to a series of tenfold dilutions of faecal extract containing norovirus in 1 ml PBS. After incubation and collection, the RNA was released by heating from virus separated by beads. The tenfold dilutions of faecal were also extracted with TRIzol reagent. The RNA was used as the template for RT-PCR detection (primers: JV12-JV13). IMS/RT-PCR using Mab showed an endpoint in the 10(-7) dilution and was 10(2) times more sensitive than IMS/RT-PCR using Pab and was at least 10(3) times more sensitive than TRIzol-RT-PCR method.

CONCLUSIONS

IMS/RT-PCR using Mab proved to be a more sensitive method of noroviruses (NVs) detection than IMS/RT-PCR using Pab and the TRIzol-RT-PCR method.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study to detect NVs with IMS/RT-PCR using Mab, and could serve as a model for future assays when broadly reactive NVs-specific Mabs are developed.

Immunomagnetic separation combined with real-time reverse transcriptase PCR assays for detection of norovirus in contaminated food

We developed an immunomagnetic separation (IMS) technique combined with real-time TaqMan reverse transcriptase PCR (RT-PCR), which allowed detection of norovirus at a level as low as 3 to 7 RT-PCR units from artificially contaminated strawberries. The inoculum recovery rate ranged from 14 to 30%. The data demonstrate that IMS combined with real-time RT-PCR will be useful as a rapid and sensitive method for detecting food-borne microbial contaminants.

Use of activated carbon coated with bentonite for increasing the sensitivity of pcr detection of Escherichia coli O157:H7 in Canadian oyster (Crassostrea gigas) tissue

A novel method for directly increasing the recovery of Escherichia coli O157:H7 and efficiently eliminating PCR inhibitors in oyster tissue without preenrichment was developed with the use of activated carbon coated with bentonite. The recovery of E. coli O157:H7 was significantly affected by the amount of bentonite used to coat the activated charcoal and the pH value of sample preparations. When 4.2 g of activated carbon were coated with 0.4 g of bentonite and seeded oyster samples were adjusted to a pH of 5.0, a high recovery of E. coli O157:H7 (91.6+/-4.4%) was obtained. Activated carbon, coated with bentonite, allowed the PCR detection of 1.5 x 10(2) CFU/g of oyster tissue which was equivalent to 30 genomic targets per PCR reaction. Without the use of activated carbon coated with bentonite, the minimum level of detection was 1.5 x 10(5) CFU/g of oyster tissue, which is equivalent to 3.0 x 10(4) genomic targets per PCR reaction. Three commercial DNA purification systems were used for comparison. The limit of detection with the Wizard DNA Clean-Up System and the Chelex(R)100 Resin was 1.5 x 10(3) CFU/g of oyster tissue which was equivalent to 3.0 x 10(2) CFU/PCR reaction. The QIAamp DNA Mini Kit resulted in a detection limit of 5 x 10(2) CFU/g of oyster tissue which was equivalent to 5 x 10(2) genomic targets per PCR reaction. The use of activated carbon coated with bentonite is an inexpensive method for removal of PCR inhibitors from tissue samples prior to the release of DNA from target cells resulting in relatively low numbers of target cells detected without enrichment.

Detection of Clavibacter michiganensis subsp. michiganensis in tomato seeds using immunomagnetic separation

The use of pathogen-free plant material is the main strategy for controlling bacterial canker of tomato caused by Clavibacter michiganensis subsp. michiganensis. However, detection and isolation of this pathogen from seeds before field or greenhouse cultivation is difficult when the bacterium is at low concentration and associated microbiota are present. Immunomagnetic separation (IMS), based on the use of immunomagnetic beads (IMBs) coated with specific antibodies, was used to capture C. michiganensis subsp. michiganensis cells, allowing removal of non-target bacteria from samples before plating on non-selective medium. Different concentrations of IMBs and of two antisera were tested, showing that IMS with 10(6)IMBs/ml coated with a polyclonal antiserum at 1/3200 dilution recovered more than 50% of target cells from initial inocula of 10(3) to 10(0)CFU/ml. Threshold detection was lower than 10CFU/ml even in seed extracts containing seed debris and high populations of non-target bacteria. The IMS permitted C. michiganensis subsp. michiganensis isolation from naturally infected seeds with higher sensitivity and faster than direct isolation on the semiselective medium currently used and could become a simple viable system for routinely testing tomato seed lots in phytosanitary diagnostic laboratories.

Detection of water-borne E. coli O157 using the integrating waveguide biosensor

Escherichia coli O157:H7, the most common serotype of enterohemorrhagic E. coli (EHEC), is responsible for numerous food-borne and water-borne infections worldwide. An integrating waveguide biosensor is described for the detection of water-borne E. coli O157, based on a fluorescent sandwich immunoassay performed inside a glass capillary waveguide. The genomic DNA of captured E. coli O157 cells was extracted and quantitative real-time PCR subsequently performed to assess biosensor-capture efficiency. In vitro microbial growth in capillary waveguide is also documented. The biosensor allows for quantitative detection of as few as 10 cells per capillary (0.075 ml volume) and can be used in conjunction with cell amplification, PCR and microarray technologies to positively identify a pathogen.

A review of conventional detection and enumeration methods for pathogenic bacteria in food

With continued development of novel molecular-based technologies for rapid, high-throughput detection of foodborne pathogenic bacteria, the future of conventional microbiological methods such as viable cell enumeration, selective isolation of bacteria on commercial media, and immunoassays seems tenuous. In fact, a number of unique approaches and variations on existing techniques are currently on the market or are being implemented that offer ease of use, reliability, and low cost compared with molecular tools. Approaches that enhance recovery of sublethally injured bacteria, differentiation among species using fluorogenics or chromogenics, dry plate culturing, differentiation among bacteria of interest using biochemical profiling, enumeration using impedence technology, techniques to confirm the presence of target pathogens using immunological methods, and bioluminescence applications for hygiene monitoring are summarized here and discussed in relation to their specific advantages or disadvantages when implemented in a food microbiology setting.Key words: food pathogen, detection, enumeration methods, food safety.

Rapid sample preparation method for PCR-based detection of Escherichia coli O157:H7 in ground beef

AIM

To develop an improved, rapid and sensitive sample preparation method for PCR-based detection of Escherichia coli O157:H7 in ground beef.

METHODS AND RESULTS

Fresh ground beef samples were experimentally inoculated with varying concentrations of E. coli O157:H7. PCR inhibitors were removed and bacterial cells were concentrated by filtration and centrifugation, and lysed using enzymatic digestion and successive freeze/thaw cycles. DNA was purified and concentrated via phenol/chloroform extraction and the Shiga toxin 1 gene (stx1) was amplified using PCR to evaluate the sample preparation method. Without prior enrichment of cells in broth media, the detection limit was 103 CFU g-1 beef. When a 6 h enrichment step was incorporated, the detection limit was 1 CFU g-1 beef. The total time required from beginning to end of the procedure was 12 h.

CONCLUSIONS

The sample preparation method developed here enabled substantially improved sensitivity in the PCR-based detection of E. coli O157:H7 in ground beef, as compared to previous reports.

SIGNIFICANCE AND IMPACT OF THE STUDY

Superb sensitivity, coupled with quick turn-around time, relative ease of use and cost-effectiveness, makes this a useful method for detecting E. coli O157:H7 in ground beef.

Deer meat as the source for a sporadic case of Escherichia coli O157:H7 infection, Connecticut

We report a case of Escherichia coli O157:H7, which was acquired by eating wild White-Tailed deer (Odocoileus virginianus). DNA fingerprint analysis verified venison as the source of infection. This pediatric case emphasizes the need for dissemination of information to hunters regarding the safe handling and processing of venison.

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.

Role of non-O157 VTEC

Non-O157 VTEC are typical Escherichia coli that differ only in their ability to produce verocytotoxins (VT). The transmission of VTEC is discussed in relation to the transmission of commensal E. coli. The emergence over the last few decades of a great variety of VTEC serotypes from healthy and diseased humans and animals is described. Particular attention is given to the distribution of the more important serogroups pathogenic for humans that have been described from around the world, particularly serogroups O26, O111, O128 and O103. The possible role of ruminants as reservoirs is discussed. The problems of laboratory diagnosis of non-O157 VTEC are considered and various laboratory methods are assessed. Evidence is presented that the particular E. coli serotypes now known to be VTEC were present in humans and animals many years ago, but have acquired the ability to produce VT and probably other virulence factors. Finally, predictions are made of the possible increase in problems associated with these emerging pathogens.

Hemolytic-Uremic Syndrome and Escherichia coli O121 at a Lake in Connecticut, 1999

OBJECTIVE

Non-O157 Shiga toxin-producing Escherichia coli (STEC) have emerged as an important public health problem. Outbreaks attributed to non-O157 STEC rarely are reported. In 1999, follow-up of routine surveillance reports of children with hemolytic- uremic syndrome (HUS) identified a small cluster of 3 cases of HUS, all of whom had spent overlapping time in a Connecticut lake community in the week before onset of symptoms. We conducted an investigation to determine the magnitude and source of the outbreak and to determine risk factors associated with the transmission of illness.

METHODS

We conducted a cohort study and an environmental investigation. The study population included all people who were at the lake in a defined geographic area during July 16-25, 1999. This time and area were chosen on the basis of interviews with the 3 HUS case-patients. A case was defined as diarrhea (>/=3 loose stools/d for >/=3 days) in a person who was at the lake during July 16-25, 1999. Stool samples were requested from any lake resident with diarrheal illness. Stools were cultured for Salmonella, Shigella, Campylobacter, and E coli O157. Broth cultures of stools were tested for Shiga toxin. Case-patients were asked to submit a serum specimen for antibody testing to lipopolysaccharides of selected STEC. Environmental samples from sediment, drinking water, lake water, and ice were obtained and cultured for E coli and tested for Shiga toxin. An environmental evaluation of the lake was conducted to identify any septic, water supply system, or other environmental condition that could be related to the outbreak.

RESULTS

Information was obtained for 436 people from 165 (78%) households. Eleven (2.5%) people had illnesses that met the case definition, including the 3 children with HUS. The attack rate was highest among those who were younger than 10 years and who swam in the lake on July 17 or 18 (12%; relative risk [RR]: 7.3). Illness was associated with swimming (RR = 8.3) and with swallowing water while swimming (RR = 7.0) on these days. No person who swam only after July 18 developed illness. Clinical characteristics of case-patients included fever (27%), bloody diarrhea (27%), and severe abdominal cramping (73%). Only the 3 children with HUS required hospitalization. No bacterial pathogen was isolated from the stool of any case-patient. Among lake residents outside the study area, E coli O121:H19 was obtained from a Shiga toxin-producing isolate from a toddler who swam in the lake. Serum was obtained from 7 of 11 case-patients. Six of 7 case-patients had E coli O121 antibody titers that ranged from 1:320 to >1:20 480. E coli indicative of fecal contamination was identified from sediment and water samples taken from a storm drain that emptied into the beach area and from a stream bed located between 2 houses, but no Shiga toxin-producing strain was identified.

CONCLUSIONS

Our findings are consistent with a transient local beach contamination in mid-July, probably with E coli O121:H19, which seems to be able to cause severe illness. Without HUS surveillance, this outbreak may have gone undetected by public health officials. This outbreak might have been detected sooner if Shiga toxin screening had been conducted routinely in HUS cases. Laboratory testing that relies solely on the inability of an isolate to ferment sorbitol will miss non-O157 STEC, such as E coli O121. Serologic testing can be used as an adjunct in the diagnosis of STEC infections. Lake-specific recommendations included education, frequent water sampling, and alternative means for toddlers to use lake facilities.

Solid-phase capture of proteins, spores, and bacteria

Current methods for the detection of pathogens in food and water samples generally require a preenrichment step that allows selective enrichment of the test organism. The objective of this research was to eliminate an enrichment step to allow detection of bacteria directly in food and water samples in 30 min. A high-flow-rate, fluidized bed to capture and concentrate large (bacteria and spores) and small (protein) molecules was developed. This format, ImmunoFlow, is volume independent and uses large beads (greater than 3 mm in diameter) when capturing bacteria to prevent sample clogging when testing food samples. Detection of bound targets was done using existing enzyme-linked immunosorbent assay (ELISA) protocols. Four antibodies (anti-Escherichia coli O157:H7, -Bacillus globigii, -bovine serum albumin [BSA], and -ovalbumin [OVA]) were covalently coupled to various glass and ceramic beads. Very small amounts of BSA (<1 ng) and OVA (0.2 to 4.0 microg) were detected. Various industrial and environmental samples were used to observe the effect of the sample composition on the capture of anti-B. globigii and anti-E. coli O157:H7 modified beads. The lower limit of detection for both E. coli O157:H7 and B. globigii was 1 spore/cell independent of the sample size. The activity of anti-B. globigii modified beads declined after 3 days. Anti-E. coli O157:H7 modified beads declined in their capture ability after 2 days in various storage buffers. Storage temperature (4 and 25 degrees C) did not influence the stability. The ImmunoFlow technology is capable of capturing bacteria and spores directly from samples, with subsequent detection in an ELISA format in 30 min.

Development of an immunomagnetic capture reverse transcription-PCR assay for the detection of Norwalk virus

Norwalk virus (NV) is the prototype human virus of the family Caliciviridae. A rapid immunomagnetic capture/reverse transcription-(IMC/RT-)PCR assay was developed for the detection of NV. Immunomagnetic capture (IMC) utilizes paramagnetic beads coupled to a virus-specific antibody and allows separation of virus from contaminating materials and virus concentration in a single step. The detection limit of the developed assay was approximately 250-750 genomic equivalents/ml of 10% stool suspension. The detection limit of the assay was not altered by the presence of excess hepatitis A virus (HAV), although non-specific binding of HAV to the paramagnetic beads was observed. A panel of 100 stools from experimental human infections was screened for NV using a previously described heat release method, an antigen ELISA, or IMC/RT-PCR. NV was detected in 65/100 of these samples by IMC/RT-PCR compared to only 38/99 by heat release and 31/95 by antigen detection ELISA. All samples that were negative by IMC were also negative by both heat release and antigen ELISA. The number of samples in which RT-PCR was inhibited was greatly reduced by the use of IMC/RT-PCR compared to the heat release method (1/100 and 16/95 samples inhibited, respectively). The ability of IMC to concentrate virus (> or =2000-fold greater than heat release) and effectively remove inhibitory substances gives this assay distinct advantages over both the heat release and antigen ELISAs.

Determination of the sensitivity of a rapid Escherichia coli O157:H7 assay for testing 375-gram composite samples

Both 25-g single-size ground beef samples and 375-g composite ground beef samples were tested by a method combining an immunomagnetic separation (IMS) technique with a sandwich enzyme-linked immunosorbent assay (ELISA) system (IMS-ELISA). The results demonstrated that IMS-ELISA could detect the target, Escherichia coli O157:H7, at the level of 10(-1) CFU/g of sample in either the 25- or 375-g sample size.

A comparison of methods for the detection of Escherichia coli O157:H7 from artificially-contaminated dairy products using PCR

Rapid nucleic acid-based methods to detect human pathogens in foods are dependent on the reliability of the DNA or RNA extraction method used. Skim milk, non-fat dry milk, Cheddar and Brie cheese, and reconstituted whey powder were seeded with serially diluted (10(0)-10(7) cfu 10 ml(-1)) Escherichia coli O157:H7 and subjected to DNA extraction (i) directly from the food product using a solvent-based procedure and (ii) using a guanidinium isothiocyanate (GITC) procedure after previous bacterial concentration. Both the efficiency of DNA extraction and the overall PCR detection limits were evaluated. In almost all instances, the total DNA yield using the solvent method was greater than that obtained for the concentration method. However, the purity of the DNA obtained after bacterial concentration was significantly better than that obtained after organic extraction alone. PCR detection limits after each DNA recovery method varied with the specific food, ranging from 10(1) to 10(4) cfu ml(-1) for all products except whey powder. DNA yields and subsequent PCR detection limits for reconstituted whey powder were extremely poor, and neither procedural changes nor the addition of PCR enhancement agents were able to improve recovery and/or detection. It is concluded that the efficiency of DNA extraction is an extremely important and frequently overlooked variable impacting the overall detection limits of PCR-based detection strategies.

Development of immunomagnetic separation technique for isolation of Pseudomonas syringae pv. phaseolicola

Immunomagnetic separation technique was developed for specific detection and selective isolation of Pseudomonas syringae pv. phaseolicola, the agent of halo-blight disease of beans. Whole-cell and exopolysaccharide fraction of the bacterium was used for polyclonal antibody production in rabbits. High specificity of the antisera was determined in agglutination reactions. The optimum immunocapture time for both antisera was determined as 1 h by using 1/nL CFU (i.e. 10(6) CFU per mL). No significant difference was observed in the binding capacity of cells to immunomagnetic particles with different antisera.