Evaluation of Escherichia coli O157:H7 growth media for use in test-and-hold procedures for ground beef processing

A New Few-Shot Learning Method of Bacterial Colony Counting Based on the Edge Computing Device

Bacterial colony counting is a time consuming but important task for many fields, such as food quality testing and pathogen detection, which own the high demand for accurate on-site testing. However, bacterial colonies are often overlapped, adherent with each other, and difficult to precisely process by traditional algorithms. The development of deep learning has brought new possibilities for bacterial colony counting, but deep learning networks usually require a large amount of training data and highly configured test equipment. The culture and annotation time of bacteria are costly, and professional deep learning workstations are too expensive and large to meet portable requirements. To solve these problems, we propose a lightweight improved YOLOv3 network based on the few-shot learning strategy, which is able to accomplish high detection accuracy with only five raw images and be deployed on a low-cost edge device. Compared with the traditional methods, our method improved the average accuracy from 64.3% to 97.4% and decreased the False Negative Rate from 32.1% to 1.5%. Our method could greatly improve the detection accuracy, realize the portability for on-site testing, and significantly save the cost of data collection and annotation over 80%, which brings more potential for bacterial colony counting.

Wet versus Dry Inoculation Methods Have a Significant Effect of Listeria monocytogenes Growth on Many Types of Whole Intact Fresh Produce

ABSTRACT

Listeria monocytogenes causes relatively few outbreaks linked to whole fresh produce but triggers recalls each year in the United States. There are limited data on the influence of wet versus dry inoculation methods on pathogen growth on whole produce. A cocktail of five L. monocytogenes strains that included clinical, food, and environmental isolates associated with foodborne outbreaks and recalls was used. Cultures were combined to target a final wet inoculum concentration of 4 to 5 log CFU/mL. The dry inoculum was prepared by mixing wet inoculum with 100 g of sterile sand and drying for 24 h. Produce investigated belonged to major commodity families: Ericaceae (blackberry, raspberry, and blueberry), Rutaceae (lemon and mandarin orange), Rosaceae (sweet cherry), Solanaceae (tomato), Brassaceae (cauliflower and broccoli), and Apiaceae (carrot). Whole intact, inoculated fruit and vegetable commodities were incubated at 2, 12, 22, and 35 ± 2°C. Commodities were sampled for up to 28 days, and the experiment was replicated six times. The average maximum growth increase was obtained by measuring the maximum absolute increase for each replicate within a specific commodity, temperature, and inoculation method. Data for each commodity, replicate, and temperature were used to create primary growth or survival models describing the lag phase and growth or shoulder and decline as a function of time. Use of a liquid inoculum (versus dry inoculum) resulted in a markedly increased L. monocytogenes growth rate and growth magnitude on whole produce surfaces. Temperature highly influenced this difference: a greater effect seen with more commodities at higher temperatures (22 and 35°C) versus lower temperatures (2 and 12°C). These findings need to be explored for other commodities and pathogens. The degree to which wet or dry inoculation techniques more realistically mimic contamination conditions throughout the supply chain (e.g., production, harvest, postharvest, transportation, or retail) should be investigated.

HIGHLIGHTS

Validation of Additional Approaches and Applications for Using the Continuous and Manual Sampling Devices for Raw Beef Trim

ABSTRACT

In this work, the goal was to determine the efficacy of MicroTally-based sampling in scenarios commonly encountered in the commercial beef processing industry, but outside of the parameters evaluated during the initial proof-of-concept work. The data were derived from 1,650 matched samples collected from 540 individual combo bins at six commercial beef processing plants, comparing MicroTally-based sampling (continuous and manual sampling devices [CSD and MSD]) to N60 Excision and/or N60 Plus methods. Mounting a 61-cm CSD cartridge to a 30-cm-wide conveyor provided sampling that is equivalent to N60 Excision and N60 Plus methods. Mounting a CSD to a chute instead of a conveyor was equivalent to the N60 Plus sampling method. The CSD was shown to be effective for sampling when used in conjunction with a "swinging arm trim diverter" and receiving product in batch mode as opposed to continuous flow. MSD sampling of oval combo bins with trim surface area (≈0.93 m2 [≈1,439 in2]) less than 1 m2 (1,600 in2) was shown to be equivalent to the N60 Plus sample collection method. Peracetic acid applied at the end of the trim conveyor did not negatively impact pathogen index target detection of the CSD even if the samples were shipped overnight before analysis. Pathogen index targets were demonstrated to be useful tools for validating methods designed to measure pathogen prevalence. The data presented herein support equivalency criteria of within 0.5 log CFU per sample for indicator organism counts. These data collectively support various alternative applications of MicroTally-based trim sampling and the application and interpretation of alternative methods for pathogen detection.

HIGHLIGHTS

Inhibitory Effects of Escherichia coli on the Formation and Development of Staphylococcus epidermidis Biofilm

In the present study, we examined whether a commensal gut bacterium Escherichia coli might prevent the formation and development of the biofilm of Staphylococcus epidermidis, a nosocomial extraintestinal pathogen but not a gut microorganism. When co-cultured with S. epidermidis, E. coli strain ATCC 35218, a non-pathogenic strain, was found to be dominant in the biofilm formed on the surface of wells of a microtiter plate. In addition, E. coli significantly incorporated and grew in a niche preoccupied by S. epidermidis biofilm. Two other E. coli strains (strain K-12 and B) also showed to interfere the biofilm formation by S. epidermidis. In contrast, S. epidermidis could not grow in a niche preoccupied by E. coli biofilm. These results suggest that, through inhibition of the formation and development of the biofilm, E. coli may eliminate S. epidermidis from the gastrointestinal tract.

Changes in STEC and bacterial communities during enrichment of manufacturing beef in selective and non-selective media

Detection and isolation of Shiga toxin-producing Escherichia coli (STEC) from manufacturing beef is challenging and it may be affected by microbial changes during enrichment. This study was designed to understand population changes during enrichment of beef from an integrated (Samples A and B) and a fragmented (Samples C and D) abattoir. The samples were enriched in buffered peptone water (BPW), Assurance GDS MPX top 7 STEC mEHEC®, BAX® E. coli O157:H7 MP and PDX-STEC media then were processed for 16 S rRNA sequencing. Escherichia dominated Sample B enrichment broths regardless of the media used (71.6-97.9%) but only in mEHEC broth (79.6%) of Sample A. Escherichia was dominant in Sample C in mEHEC (95.2%) and PDX-STEC (99.2%) broths but less in BPW (58.5%) and MP (64.9%) broths. In Sample D, Clostridium dominated in mEHEC (65.5%), MP (80.2%) and PDX-STEC (90.6%) broths. O157 STEC was isolated from Sample C only. The study suggested that MP may not be as effective as mEHEC and PDX-STEC and that Clostridium could interfere with enrichment of Escherichia. Understanding the ecological changes during enrichment provides meaningful insight to optimising the enrichment protocol for STEC and subsequently enhance the efficiency of STEC detection.

Quantifying the Influence of Relative Humidity, Temperature, and Diluent on the Survival and Growth of Enterobacter aerogenes

Survival of bacteria on surfaces plays an important role in the cross-contamination of food. Temperature, relative humidity (RH), surface type, and inoculum diluent can affect bacterial survival. This study was conducted to examine how temperature, RH, and diluent affect the survival of Enterobacter aerogenes on stainless steel, polyvinyl chloride, and ceramic tile. Although surface type had little effect on survival, temperature had a clear effect. E. aerogenes survival was highest at 7°C and 15 and 50% RH on all surfaces. Some diluents allowed growth under high RH conditions. Cell populations in distilled water inoculated onto each surface decreased initially compared with populations in 1% phosphate-buffered saline (PBS) and 0.1% peptone broth. At 15 and 50% RH, cell populations in 1% PBS declined more sharply after 120 h than did those 0.1% peptone, but populations in both diluents had similar declines up to 3 weeks. Cell populations in 0.1% peptone had the greatest growth and reached the highest population density (∼8 log CFU/mL). Cell populations in PBS and distilled water increased by ∼2 log CFU/mL. When cells in 0.1% peptone were inoculated onto stainless steel at 100% RH, populations increased to ∼7 log CFU per coupon, whereas cells in 1% PBS increased to ∼5 log CFU per coupon followed by a decline over 3 weeks. DMFit and GInaFiT software modeled inactivation on surfaces at all conditions other than 100% RH at 21°C. These findings have important implications for experiments in which microorganisms are inoculated onto foods or food contact surfaces because the growth observed may be affected more by the inoculum diluent at high or uncontrolled RH than by the type of inoculated surface.

Dynabeads protein G antibody conjugates combined with modified brain heart infusion broth for the enrichment and separation of Bacillus cereus in artificially contaminated vegetables

A modified brain heart infusion (MBHI) broth and a protocol of immunomagnetic separation (IMS) using antibody-coated Dynabeads^® protein G were developed for the enrichment and separation of Bacillus cereus in artificially contaminated vegetable samples. The MBHI consisted of BHI and 0.34 g/L magnesium sulfate, 12.08 g/L sodium pyruvate, 1.82 g/L yeast extract, and polymyxin B. The amount of immunomagnetic beads (IMBs) and immunoreaction time were optimized. The capture efficiency was 58.32% with 0.4 mg IMBs when the immunoreaction time was 20 min. Capture of B. cereus by IMBs did not interfere with competing flora. Pre-enrichment IMS was validated with four B. cereus strains in artificially contaminated baby sprouts, bean sprouts, lettuce, and spinach at two levels (∼0.1 and ∼1 CFU/g). We were able to detect and isolate B. cereus in 40/40 samples of vegetables contaminated at 0.1 CFU/g with IMS after 6 h of enrichment in MBHI.

Prospects for point-of-care pathogen diagnostics using surface-enhanced Raman scattering (SERS)

This review highlights recent advances in the application of surface-enhanced Raman scattering (SERS) in pathogen detection and discusses many of the challenges in moving this technology to the point-of-care (POC) arena.

Light scattering sensor for direct identification of colonies of Escherichia coli serogroups O26, O45, O103, O111, O121, O145 and O157

Background

Shiga-toxin producing Escherichia coli (STEC) have emerged as important foodborne pathogens, among which seven serogroups (O26, O45, O103, O111, O121, O145, O157) are most frequently implicated in human infection. The aim was to determine if a light scattering sensor can be used to rapidly identify the colonies of STEC serogroups on selective agar plates.

Methodology/Principal Findings

Initially, a total of 37 STEC strains representing seven serovars were grown on four different selective agar media, including sorbitol MacConkey (SMAC), Rainbow Agar O157, BBL CHROMagarO157, and R&F E. coli O157:H7, as well as nonselective Brain Heart Infusion agar. The colonies were scanned by an automated light scattering sensor, known as BARDOT (BActerial Rapid Detection using Optical scattering Technology), to acquire scatter patterns of STEC serogroups, and the scatter patterns were analyzed using an image classifier. Among all of the selective media tested, both SMAC and Rainbow provided the best differentiation results allowing multi-class classification of all serovars with an average accuracy of more than 90% after 10–12 h of growth, even though the colony appearance was indistinguishable at that early stage of growth. SMAC was chosen for exhaustive scatter image library development, and 36 additional strains of O157:H7 and 11 non-O157 serovars were examined, with each serogroup producing unique differential scatter patterns. Colony scatter images were also tested with samples derived from pure and mixed cultures, as well as experimentally inoculated food samples. BARDOT accurately detected O157 and O26 serovars from a mixed culture and also from inoculated lettuce and ground beef (10-h broth enrichment +12-h on-plate incubation) in the presence of natural background microbiota in less than 24 h.

Conclusions

BARDOT could potentially be used as a screening tool during isolation of the most important STEC serovars on selective agar plates from food samples in less than 24 h.

Multiplex PCR detection of Shiga toxin-producing Escherichia coli strains belonging to serogroups O157, O103, O91, O113, O145, O111, and O26 experimentally inoculated in beef carcass swabs, beef trim, and ground beef

Numerous foodborne outbreaks are attributed to Shiga toxin-producing Escherichia coli (STEC) and have been recognized for causing gastrointestinal disease in humans. Beef products have been considered the principal source of STEC. A multiplex PCR assay enabling simultaneous detection of STEC O103, O91, O113, O145, O111, O157, and O26 was developed and evaluated in artificially contaminated beef carcass swabs, beef trim, and ground beef after overnight enrichment. Individual serogroups were experimentally inoculated at low (1 to 10 CFU/ml) and high (11 to 100 CFU/ml) levels, and with a cocktail of strains belonging to two, four, and six serogroups. There was no significant difference in detecting single STEC strains under the different conditions. Only when strains were combined were there significant differences in detection of all cocktail isolates in some of the beef products. To address this issue, four serogroups were experimentally inoculated together at three different estimated levels (10, 10(2), and 10(3) CFU/ml) in all three beef products. Results yielded no significant difference in detecting STEC at the three inoculation levels (10, 10(2), and 10(3) CFU/ml) in trim and carcass swabs, but there was a significant difference in detecting STEC at the lowest levels (10 and 10(2) CFU/ml) in the 80:20 nonirradiated ground beef, and in the detection of STEC in irradiated ground beef. The findings from this study could provide industry and government agencies with a tool to evaluate the prevalence and incidence of STEC in beef products and their processing environments.

Comparative evaluation of a phage protein ligand assay with real-time PCR and a reference method for the detection of Escherichia coli O157:H7 in raw ground beef and trimmings

Enterohemorrhagic Escherichia coli O157:H7 is an important pathogen associated with infections caused by consumption of undercooked raw meat. Sensitive and rapid detection methods for E. coli O157:H7 are essential for the meat industry to ensure a safe meat supply. This study was conducted to compare the sensitivity of the VIDAS ultra performance E. coli test (ECPT UP) with a noncommercial real-time (RT) PCR method and the U.S. Department of Agriculture, Food Safety and Inspection Service (USDA-FSIS) reference method for detecting E. coli O157:H7 in raw ground beef. Optimal enrichment times and the efficacy of testing different types of raw meat, either as individual samples (25 g) or as composites (375 g), were examined. For 25-g samples of each type of raw ground beef tested, 6 h of enrichment was sufficient for both the VIDAS ECPT UP and RT-PCR methods, but for 375-g samples, 24 h of enrichment was required. Both the VIDAS ECPT UP and RT-PCR methods produced results similar to those obtained with the USDA-FSIS reference method after 18 to 24 h of enrichment. The primer specificity of the RT-PCR assay and the highly specific phage ligand used in the VIDAS ECPT UP for target recognition enabled the detection of low levels of E. coli O157:H7 in 25 g of various types of raw ground beef. The tests also allowed the detection of E. coli O157:H7 in composite raw ground beef and trimmings in samples of up to 375 g.

Inoculation of beef with low concentrations of Escherichia coli O157:H7 and examination of factors that interfere with its detection by culture isolation and rapid methods

Currently used industry testing programs require the ability to detect Escherichia coli O157:H7 in samples of beef trim or ground beef at levels as low as 1 CFU/375 g. We present a reliable protocol for generating a control inoculum for verification testing at this low concentration and evaluate its use. Results show that half of all samples received no cells when 1 CFU was the target concentration and that targets greater than 3 CFU were much more reliable. Detection by culture isolation and two commercial assays, Qualicon BAX-MP and BioControl GDS, detected 94% ± 11%, 92% ± 10%, and 92% ± 7% of samples inoculated with 5.4 CFU (range 1 to 9 CFU), respectively. We also examined the effect of background aerobic plate count (APC) bacteria and fat content effects on the detection of E. coli O157:H7. At APC concentrations below 6 log CFU/g, the rapid methods detected all beef trim samples inoculated with 26 CFU of E. coli O157:H7 per 65 g. At an APC of 6.7 log CFU/g, culture, BAX-MP, and GDS detected 100, 75, and 13%, respectively, of inoculated samples. Neither commercial method detected E. coli O157:H7 in the samples when APC was 7.7 log CFU/g, whereas culture was able to detect 63% of E. coli O157:H7 in the samples when APC was at this concentration. Increased fat content correlated with decreasing recovery of immunomagnetic separation beads, but this was not observed to interfere with detection of E. coli O157:H7.

Same-day detection of Escherichia coli O157:H7 from spinach by using electrochemiluminescent and cytometric bead array biosensors

Contamination of fresh produce with Escherichia coli O157:H7 and other pathogens commonly causes food-borne illness and disease outbreaks. Thus, screening for pathogens is warranted, but improved testing procedures are needed to allow reproducible same-day detection of low initial contamination levels on perishable foods, and methods for detecting numerous pathogens in a single test are desired. Experimental procedures were developed to enable rapid screening of spinach for E. coli O157:H7 by using multiplex-capable immunological assays that are analyzed using biosensors. Detection was achieved using an automated electrochemiluminescent (ECL) assay system and a fluorescence-based cytometric bead array. Using the ECL system, less than 0.1 CFU of E. coli O157:H7 per gram of spinach was detected after 5 h of enrichment, corresponding to 6.5 h of total assay time. Using the cytometric bead array, less than 0.1 CFU/g was detected after 7 h of enrichment, with a total time to detection of less than 10 h. These results illustrate that both biosensor assays are useful for rapid detection of E. coli O157:H7 on produce in time frames that are comparable to or better than those of other testing formats. Both methods may be useful for multiplexed pathogen detection in the food industry and other testing situations.

Comparative evaluation of practical functionality of rapid test format kits for detection of Escherichia coli O157:H7 on lettuce and leafy greens

Multistate outbreaks of Escherichia coli O157:H7 infection in 2005 and 2006 associated with fresh and especially minimally processed produce greatly escalated the application of rapid pathogen detection systems to safety management in this food category. Pathogen testing was rapidly integrated into preharvest qualification for field lots, incoming raw produce, or final product. The raw produce and final product were incorporated into test-and-hold programs, typically within a 10-h time frame. To enhance consumer safety and provide guidance for the industry, an assessment of selected kits in comparison to a culture-based method was undertaken. Four primary kits were compared: the Neogen Reveal, SDI RapidChek, BioControl GDS O157, and Qualicon BAX O157 MP. Nine different leafy greens were freshly harvested and inoculated with a five-isolate mixture of E. coli O157:H7 at 10 CFU/25 g of sample, and cultures were enriched following the specified protocol. The PCR method was most consistent for identifying the presence of the inoculated pathogen in the shortest period of time. For the red-pigmented leafy vegetables red butter lettuce, curly endive, red lettuce, and lollo rosa, 13, 38, 88, and 100% false-negative results, respectively, were obtained with the immunoassays, but PCR detection was minimally affected. Immunoassays were negatively affected by delays in achieving critical threshold populations during the allowed enrichment period. Leafy green type, temperature abuse, and preharvest environment were unlikely to affect the results of PCR-based kits. Findings strongly suggest that product testing systems using 8-h detection cutoffs may give false-negative results. These issues become very important in high-throughput testing and retest protocols for presumptive pathogen-positive lots of produce.

Sample preparation: the forgotten beginning

Advances in molecular technologies and automated instrumentation have provided many opportunities for improved detection and identification of microorganisms; however, the upstream sample preparation steps needed to apply these advances to foods have not been adequately researched or developed. Thus, the extent to which these advances have improved food microbiology has been limited. The purpose of this review is to present the current state of sample preparation, to identify knowledge gaps and opportunities for improvement, and to recognize the need to support greater research and development efforts on preparative methods in food microbiology. The discussion focuses on the need to push technological developments toward methods that do not rely on enrichment culture. Among the four functional components of microbiological analysis (i.e., sampling, separation, concentration, detection), the separation and concentration components need to be researched more extensively to achieve rapid, direct, and quantitative methods. The usefulness of borrowing concepts of separation and concentration from other disciplines and the need to regard the microorganism as a physicochemical analyte that may be directly extracted from the food matrix are discussed. The development of next-generation systems that holistically integrate sample preparation with rapid, automated detection will require interdisciplinary collaboration and substantially increased funding.

Validation of low-volume enrichment protocols for detection of Escherichia coli O157 in raw ground beef components, using commercial kits

Testing of beef destined for use in ground beef products for the presence of Escherichia coli O157:H7 has become an important cornerstone of control and verification activities within many meat supply chains. Validation of the ability of methods to detect low levels of E. coli O157:H7 is critical to confidence in test systems. Many rapid methods have been validated against standard cultural methods for 25-g samples. In this study, a number of previously validated enrichment broths and commercially available test kits were validated for the detection of low numbers of E. coli O157:H7 in 375-g samples of raw ground beef component matrices using 1 liter of enrichment broth (large-sample:low-volume enrichment protocol). Standard AOAC International methods for 25-g samples in 225 ml of enrichment broth, using the same media, incubation conditions, and test kits, were used as reference methods. No significant differences were detected in the ability of any of the tests to detect low levels of E. coli O157:H7 in samples of raw ground beef components when enriched according to standard or large-sample:low-volume enrichment protocols. The use of large-sample:low-volume enrichment protocols provides cost savings for media and logistical benefits when handling and incubating large numbers of samples.

Effects of using reduced volumes of nonselective enrichment medium in methods for the detection of Escherichia coli O157:H7 from raw beef

Recent work from our laboratory revealed that tryptic soy broth (TSB) was a superior enrichment medium for use in test-and-hold Escherichia coli O157:H7 methods at levels down to a ratio of three volumes of medium to one volume of sample. Lower ratios were examined for their effect on results obtained from culture isolation, the BAX E. coli O157:H7 MP assay, and the Assurance GDS E. coli O157:H7 assay. Ground beef and boneless beef trim were inoculated with a high level (170 CFU/65 g of ground beef and 43 CFU/65 g of trim) and a low level (17 CFU/65 g of ground beef and 4 CFU/65 g of trim) of E. coli O157:H7 and enriched in 3, 1, 0.5, and 0 volumes of TSB. The volume of TSB used did not affect E. coli O157:H7 detection by culture isolation, Assurance GDS detection in ground beef or trim, or the BAX MP assay detection in ground beef. However, BAX MP assay detection of E. coli O157:H7 in beef trim was 50, 42, and 33% positive when enrichment volumes of 0.5x, 1x, and 3x, respectively, were used. Optimum results with all methods were obtained using 1 volume of TSB. We concluded that detection test results can be considered valid as long as enrichment medium is used, even when it is less than the specified 3 or 10 volumes.

Comparison of Escherichia coli O157:H7 enrichment in spiked produce samples

Two strains of Escherichia coli O157:H7 were spiked into six varieties of produce at approximately 0.5 CFU g(-1). Samples were enriched by using the U.S. Food and Drug Administration (FDA) Bacteriological Analytical Manual (BAM) method and by using an experimental method incorporating acid shock. Target colonies were detectable on selective agars after 30 of 48 analyses with BAM enrichment and 48 of 48 analyses with acid enrichment. Real-time PCR screening of 24-h enrichment broths revealed the presence of the diagnostic stx1 or stx2 genes after 27 of 48 analyses with BAM enrichment and 42 of 48 analyses with acid enrichment. The efficiency of the enrichment varied with strain and type of produce spiked but overall was better with the experimental enrichment method. Modifications of both the acid enrichment and BAM enrichment methods also were tested. The acid method with a modified incubation temperature consistently yielded high rates of recovery (> 10(8) CFU ml(-1)), with no instances in which target cells could not be detected. Modification of the BAM procedure did not reproducibly improve enrichment efficiency.

Preparation-free method for detecting Escherichia coli O157:H7 in the presence of spinach, spring lettuce mix, and ground beef particulates

We show the detection of 100 cells per ml of Escherichia coli O157:H7 in the presence of spinach, spring lettuce mix, and ground beef washes and particulate matter with piezoelectric-excited millimeter-sized cantilever (PEMC) sensors. The PEMC sensors (sensing area, 2 mm2) were immobilized with polyclonal antibody specific to E. coli O157:H7 (EC) and were exposed to 10 aqueous washes of locally purchased spinach, spring lettuce mix, and ground beef for testing if EC was present. Absence of resonance frequency shift indicated that EC was not present in the 30 samples tested. Following the last sample in each food matrix, 1,000 cells per ml of EC were spiked into the sample container, and resonance frequency change was monitored. The total resonance frequency change was 880 +/- 5, 1,875 +/- 8, and 1,417 +/- 4 Hz for spinach, spring lettuce mix, and ground beef, respectively. A mixture of the three food matrices spiked with 100 cells per ml of EC gave a sensor response of 260 +/- 15 Hz. The resonance frequency changes are approximately 40% lower than our previously reported study on ground beef. It is suggested that the reduction in sensitivity is due to differences in pathogen adherence to food matrices, which affects target binding to the sensor surface. We conclude that detection selectivity is conserved in the three food matrices examined and that the magnitude of sensor response is a function of the food matrix.

Potential of surface-enhanced Raman spectroscopy for the rapid identification of Escherichia coli and Listeria monocytogenes cultures on silver colloidal nanoparticles

Surface-enhanced Raman (SERS) spectra of various batches of bacteria adsorbed on silver colloidal nanoparticles were collected to explore the potential of the SERS technique for rapid and routine identification of E. coli and L. monocytogenes cultures. Relative standard deviation (RSD) of SERS spectra from silver colloidal suspensions and ratios of SERS peaks from small molecules (K(3)PO(4)) were used to evaluate the reproducibility, stability, and binding effectiveness of citrate-reduced silver colloids over batch and storage processes. The results suggested consistent reproducibility of silver colloids over batch process and also stability and consistent binding effectiveness over an eight-week storage period. A variety of mixtures of E. coli/L. monocytogenes cultures with different colloidal batches revealed that, despite large variations in relative intensities and positions of SERS active bands, characteristic and unique bands at 712 and 390 cm(-1) were consistently observed and were the strongest in E. coli and L. monocytogenes cultures, respectively. Two specific bands were used to develop simple algorithms in the evaluation of binding effectiveness of silver colloids over storage and further to identify E. coli and L. monocytogenes cultures with a 100% success. A single spectrum acquisition took 5 approximately 6 min, and a minimum of 25 microL silver colloid was directly mixed with 25 microL volume of incubated bacterial culture. The short acquisition time and small volume of incubated bacterial culture make silver colloidal nanoparticle based SERS spectroscopy ideal for potential use in the routine and rapid screening of E. coli and L. monocytogenes cultures on large scales. This is the first report of the development of simple and universal algorithms for bacterial identification from the respective exclusive SERS peaks.

10-minute assay for detecting Escherichia coli O157:H7 in ground beef samples using piezoelectric-excited millimeter-size cantilever sensors

We detected Escherichia coli O157:H7 (EC) at approximately 10 cells per ml in spiked ground beef samples in 10 min using piezoelectric-excited millimeter-size cantilever (PEMC) sensors. The composite PEMC sensors have a sensing area of 2 mm2 and are prepared by immobilizing a polyclonal antibody specific to EC on the sensing surface. Ground beef (2.5 g) was spiked with EC at 10 to 10,000 cells per ml in phosphate-buffered saline (PBS). One milliliter of supernatant was removed from the blended samples and used to perform the detection experiments. The total resonant frequency change obtained for the inoculated samples was 138 +/- 9, 735 +/- 23, 2,603 +/- 51, and 7,184 +/- 606 Hz, corresponding to EC concentrations of 10, 100, 1,000, and 10,000 cells per ml, respectively. EC was detected in the sample solution within the first 10 min. The responses of the sensor to positive, negative, and buffer controls were 36 +/- 6, 27 +/- 2, and 2 +/- 7 Hz, respectively. Verification of EC attachment was confirmed by low-pH buffer release (PBS-HCl, pH 2.2), microscopy, and second antibody EC binding postdetection. The results indicate that PEMC sensors can reliably detect EC at less than 10 cells per ml in 10 min without sample preparation and with label-free reagents.