Detection and Isolation of Low Levels of E. coli O157:H7 in Cilantro by Real-Time PCR, Immunomagnetic Separation, and Cultural Methods with and without an Acid Treatment

Validation of rapid detection methods for Salmonella enterica in green chile

The objective of this study is to validate the US Food and Drug Administration (FDA) rea-time polymerase chain reaction (qPCR) assay, the Neogen Amplified Nucleic Single Temperature Reaction (ANSR) assay, and the Vitek ImmunoDiagnostic Assay System (VIDAS) SLM procedure against the FDA cultural procedure for Salmonella detection in green chile pepper. Green chile was artificially contaminated with Salmonella according to the FDA guidelines (FDA. Guidelines for the Validation of Microbiological Methods for the FDA Foods Program, 3rd Edition. 2019. www.fda.gov/media/83812/download?attachment (17 March 2024, date last accessed)) at a fractional recovery level (where 50%-25% tests positive and at a level +1 log greater for each organism tested). Enriched samples were tested directly by the ANSR Salmonella test and by qPCR, and were subcultured into Rappaport-Vassiliadis and tetrathionate brilliant green broth for cultural detection and qPCR. For the VIDAS-SLM assay, the selective enrichments were further cultured in M broth before testing. Presumptive salmonellae were confirmed with biochemical tests, serology, and qPCR. All three rapid assays were compared favorably with the FDA-BAM (Bacteriological Analytical Manual) method. No significant differences at P < .05 were found between the procedures using McNemar's χ2 test. The three procedures were found to be rapid and reliable alternatives to cultural detection of Salmonella enterica in green chile.

The "Big Six": Hidden Emerging Foodborne Bacterial Pathogens

Non-O157 Shiga toxin-producing Escherichia coli (STEC) are emerging serogroups that often result in diseases ranging from diarrhea to severe hemorrhagic colitis in humans. The most common non-O157 STEC are O26, O45, O103, O111, O121, and O145. These serogroups are known by the name "big six" because they cause severe illness and death in humans and the United States Department of Agriculture declared these serogroups as food contaminants. The lack of fast and efficient diagnostic methods exacerbates the public impact of the disease caused by these serogroups. Numerous outbreaks have been reported globally and most of these outbreaks were caused by ingestion of contaminated food or water as well as direct contact with reservoirs. Livestock harbor a variety of non-O157 STEC serovars that can contaminate meat and dairy products, or water sources when used for irrigation. Hence, effective control and prevention approaches are required to safeguard the public from infections. This review addresses the disease characteristics, reservoirs, the source of infections, the transmission of the disease, and major outbreaks associated with the six serogroups ("big six") of non-O157 STEC encountered all over the globe.

Rapid and simultaneous purification of aflatoxin B1, zearalenone and deoxynivalenol using their monoclonal antibodies and magnetic nanoparticles

To develop a new simple and simultaneous purification method for mycotoxins in feeds and grains, magnetic nanoparticles (MNPs) conjugated with monoclonal antibodies (mAbs) against mycotoxins were used to separate aflatoxin B1 (AFB₁), zearalenone (ZEA) and deoxynivalenol (DON). For a single spike of each mycotoxin into the buffer solution (16% MeOH in PBS), mean recoveries were 93.1-95.0% for AFB₁ (5-20 ng/mL spiked), 87.2-96.0% for ZEA (125-500 ng/mL spiked) and 75.2-96.9% for DON (250-1,000 ng/mL spiked) by HPLC and ELISA. Recoveries of AFB₁ (20 ng/mL) and ZEA (500 ng/mL) simultaneously spiked into the buffer solution were 87.0 and 99.8%, respectively. Recovery rates of AFB₁/DON and DON/ZEA spiked simultaneously were 86.2%/76.6% and 92.0%/86.7%, respectively, at concentrations of 20 ng/mL AFB₁, 500 ng/mL ZEA, and 1,000 ng/mL DON. Recoveries using the novel mAb-MNP conjugated system in a buffer solution simultaneously spiked with AFB₁, ZEA and DON were 82.5, 94.6 and 73.4%, respectively. Recoveries of DON in animal feed were 107.7-132.5% at concentrations of 250-1,000 ng/g spiked in feed. The immunoaffinity chromatography (IAC) clean-up method was compared with the purification method using novel mAb-MNP. After fortification of animal feed with AFB₁ (5, 10 and 20 ng/g feed) and ZEA (125, 250 and 500 ng/g feed), AFB₁ and ZEA were purified using both the methods. In the case of the novel mAb-MNP conjugated system, mean recoveries for AFB₁ were 89.4, 73.1 and 88.3% at concentrations of 5, 10 and 20 ng/g feed, respectively. For ZEA, mean recoveries were 86.7, 85.9 and 79.1% at concentrations of 125, 250 and 500 ng/g, respectively. For IAC purification, recoveries were 42.9-45.1% for AFB₁ and 96.8-103.2% for ZEA. In conclusion, the present purification method using monoclonal antibodies conjugated to MNPs can be used for simple and simultaneous purification of mycotoxins from feed and maize.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-020-00083-w.

The progress of type II persisters of Escherichia coli O157:H7 to a non-culturable state during prolonged exposure to antibiotic stress with revival being aided through acid-shock treatment and provision of methyl pyruvate

Persisters are a form of dormancy in bacteria that provide temporary resistance to antibiotics. The following reports on the formation of Escherichia coli O157:H7 E318 type II persisters from a protracted (8 days) challenge with ampicillin. Escherichia coli O157:H7 followed a multiphasic die-off pattern with an initial rapid decline (Phase I) of susceptible cells that transitioned to a slower rate representing tolerant cells (Phase II). After 24 h post-antibiotic challenge, the E. coli O157:H7 levels remained relatively constant at 2 log CFU/mL (Phase III), but became non-culturable within 8-days (Phase IV). The revival of persisters in Phase III could be achieved by the removal of antibiotic stress, although those in Phase IV required an extended incubation period or application of acid-shock. The carbon utilization profile of persister cells was less diverse compared with non-persisters, with only methyl pyruvate being utilized from the range tested. Inclusion of methyl pyruvate in tryptic soy agar revived non-cultural persisters, presumably by stimulating metabolism. The results suggest that persisters could be subdivided into culturable or non-culturable cells, with the former representing a transition state to the latter. The study provided insights into how to revive cells from dormancy to aid enumeration and control.

Enhancement of Culture of Legionella longbeachae from Respiratory Samples by Use of Immunomagnetic Separation and Antimicrobial Decontamination

Legionella longbeachae is the commonest Legionella species identified in patients with community-acquired pneumonia in New Zealand. Isolation of the organism on culture is the gold standard for the diagnosis of Legionnaires disease, but it has poor sensitivity (40%) compared with quantitative PCR (qPCR). We have developed a selective decontamination process using glycine, vancomycin, polymyxin, and cycloheximide (GVPC) with immunomagnetic separation (IMS) for culturing L. longbeachae A polyclonal antibody specific for L. longbeachae was produced from New Zealand White rabbits and coupled to tosyl-activated magnetic beads. Stored L. longbeachae qPCR-positive respiratory samples were retrieved from -80°C storage for testing. One portion of test samples was mixed with GVPC and the antibody bead complex, separated, washed, and cultured on modified Wadowsky and Yee agar (MWY) agar. Another portion was exposed to HCl-KCl acidic buffer (pH 2.2) before incubation on MWY agar. qPCR used probes specific for the ITS (internal transcribed spacer) region of the L. longbeachae genome. Cultures were positive in 10/53 (19%) samples after acid wash and 26/53 (49%) after GVPC-IMS (P = 0.001). Growth of contaminants was rare. The mean qPCR threshold cycle values were lower in culture-positive samples after acid wash than in the culture-negative samples (mean, 29.9 versus 34.8; difference, 4.9; 95% confidence interval [CI], ±2.9; P = 0.001) but not after GVPC-IMS (mean, 33.0 versus 34.7; difference, 1.7; 95% CI, ±2.48; P = 0.16). The sensitivity of culture for L. longbeachae in respiratory specimens may be improved by using GVPC-IMS rather than acid wash for decontamination, but this should be confirmed in a prospective study of fresh specimens.

Short communication: Quantitative PCR coupled with sodium dodecyl sulfate and propidium monoazide for detection of culturable Escherichia coli in milk

Escherichia coli has been frequently reported as a major foodborne bacterium contaminating raw milk or pasteurized milk. Therefore, the aim of this study was to explore a quantitative real-time PCR (qPCR) technique combined with sodium dodecyl sulfate (SDS) and propidium monoazide (PMA) to detect culturable E. coli in milk. An internal amplification control was also added into this reaction system as an indicator of false-negative results. The inclusivity and exclusivity of the primers were tested using DNA from 7 E. coli and 14 other bacterial strains. The concentrations of SDS and PMA were determined according to plate counts and quantitative cycle values of qPCR, respectively. A standard curve was established using series diluted E. coli DNA. The reliability and specificity of this method were further determined by the detection of E. coli in spiked milk. The results showed that the optimal concentrations of SDS and PMA were 100 µg/mL and 40 μM, respectively. A standard curve with a good linear relationship (coefficient of determination = 0.997; amplification efficiency = 100.5%) was obtained. Compared with conventional PCR and PMA-qPCR, the SDS-PMA-qPCR assay was more specific and sensitive in culturable E. coli detection. Therefore, we evaluated and improved the SDS-PMA-qPCR method for detecting culturable E. coli in milk.

Use of bacteriophages as biological control agents in horticulture

Bacterial diseases in horticultural settings or infestation of fresh produce with human pathogenic bacteria can constitute a serious public health risk. To control horticultural bacterial diseases, chemical control strategies have traditionally been used, such as the application of bactericides and copper-based products, which resulted in development of resistance in bacteria against these agents. Moreover, the use of such chemical preventative measures on fresh produce can detrimentally affect human, animal and ecosystem health. Bacteriophages have been used to control pathogenic bacteria since the 1920s due to their specificity against host bacteria, as well as their ability to survive and infect their host without detrimental effects to the surrounding environments. As a result, their targeted host specific applications in horticultural settings can be of interest to growers as well as to the consumers. In this laboratory report, the efficacy of a bacteriophage cocktail when applied to fresh herbs inoculated with Escherichia coli was determined. Significant (P ≤ 0.001) reductions in E. coli colony forming units were observed in phage treated herb samples compared to counts in the control. These findings suggest that bacteriophage present as an alternative biocontrol for E. coli in horticulture.

Tellurite resistance profiles and performance of different chromogenic agars for detection of non-O157 Shiga toxin-producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) are globally important food-borne pathogens. The isolation of non-O157 STEC is a significant public health challenge due to the dramatic diversity of their phenotypes and genotypes. In the present study, 476 non-O157 STEC strains representing 95 different O-serogroups were used to evaluate tellurite resistance and the performance of 12 different chromogenic agars. Of 476 strains, only 108 (22.7%) strains showed the minimal inhibitory concentration (MIC) values for potassium tellurite being higher than 4μg/ml, and 96 (20.2%) strains harbored intact ter genes cluster. The presence of ter genes was significantly correlated with tellurite resistance. Six commercial chromogenic agars (TBX, MAC, SMAC, Rainbow® Agar O157, CHROMagar™ ECC, and Fluorocult O157) supported the growth of all strains. However, CT-SMAC, CHROMagar™ O157, and CHROMagar™ STEC agars exhibited 12.2%, 31.1%, and 38.0% of growth inhibition, respectively. Furthermore, 4.6%, 33.2%, and 45.0% of strains were inhibited on RBA-USDA, RBA-NT, and BCM O157 agar media. Variations in tellurite resistance and colony appearance might result in discrepant performance of non-O157 STEC recovery from different chromogenic agars. Using inclusive agars or less selective agar in combination with highly selective agar should be suggested to recover most non-O157 STEC strains, which would increase the probability of recovering STECs from complex background microflora.

An electrochemical biosensor for rapid detection of E. coli O157:H7 with highly efficient bi-functional glucose oxidase-polydopamine nanocomposites and Prussian blue modified screen-printed interdigitated electrodes

The presence of pathogenic bacteria in foods has always been a great threat to the wellbeing of people and the revenue of food manufacturers. Therefore, the demand for advanced detection methods that can sensitively and rapidly detect these pathogens has been of great importance. This study reports an electrochemical biosensor for rapid detection of E. coli O157:H7 with the integration of bifunctional glucose oxidase (GOx)-polydopamine (PDA) based polymeric nanocomposites (PMNCs) and Prussian blue (PB) modified screen-printed interdigitated microelectrodes (SP-IDMEs). The core-shell magnetic beads (MBs)-GOx@PDA PMNCs were first synthesized by the self-polymerization of dopamine (DA). Gold nanoparticles (AuNPs) were dispersed on the surface of PMNCs through biochemical synthesis to achieve further highly efficient adsorption of antibodies (ABs) and GOx. The final product ABs/GOxext/AuNPs/MBs-GOx@PDA PMNCs served as the carrier to separate target bacteria from food matrices as well as the amplifier for electrochemical measurement. The unbound PMNCs were separated by a filtration step and transferred into glucose solution to allow the enzymatic reaction to occur. The change of the current response was measured with an electrochemical detector using PB-modified SP-IDMEs. The constructed biosensor has been proven to be able to detect E. coli O157:H7 with the detection limit of 10(2) cfu ml(-1). The bifunctional PMNCs contain a high load of enzyme and can optimally utilize the binding sites on bacterial cells, which efficiently amplify the signals for measurement. The biosensor in this study exhibited good specificity, reproducibility, and stability and is expected to have a great impact on applications in the detection of foodborne pathogens.

Specific detection of live Escherichia coli O157:H7 using tetracysteine-tagged PP01 bacteriophage

Sensitive and rapid detection of Escherichia coli O157:H7, one of the most notorious bacterial pathogens, is urgently needed for public health protection. Yet, the existing methods are either lack of speed or limited in discriminating viable and dead cells. Using a recombinant bacteriophage, here we report the development of a rapid and sensitive method for live E. coli O157:H7 detection. First, the wild-type PP01 phage was engineered with a tetracysteine (TC)-tag fused with the small outer capsid (SOC) protein. Then, this PP01-TC phage was used to inoculate bacterial sample for 30min. Specific infection and rapid replication of PP01-TC phage in viable E. coli O157:H7 host cell yields a large number of progeny phages with capsids displaying TC tags that can be fluorescently labeled by a membrane permeable biarsenical dye (FlAsH). The bright green fluorescence of single E. coli O157:H7 cells can be readily detected by flow cytometry (FCM) and fluorescence microscopy. High specificity of the assay was verified with seven other bacterial strains. Practical application in E. coli O157:H7 detection in drinks was successfully demonstrated with artificially contaminated 100% apple juice. In less than three hours (including sample preconcentration) and with 40mL of sample volume, as low as 1cfu/mL E. coli O157:H7 can be detected in the presence of large excess of other nontarget bacteria via fluorescence microscopic measurement. The as-developed TC-PP01-FlAsH approach shows a great potential in the safeguard of liquid food products by providing rapid, sensitive, and specific detection of live E. coli O157:H7.

Detection, Characterization, and Typing of Shiga Toxin-Producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) are responsible for gastrointestinal diseases reported in numerous outbreaks around the world. Given the public health importance of STEC, effective detection, characterization and typing is critical to any medical laboratory system. While non-O157 serotypes account for the majority of STEC infections, frontline microbiology laboratories may only screen for STEC using O157-specific agar-based methods. As a result, non-O157 STEC infections are significantly under-reported. This review discusses recent advances on the detection, characterization and typing of STEC with emphasis on work performed at the Alberta Provincial Laboratory for Public Health (ProvLab). Candidates for the detection of all STEC serotypes include chromogenic agars, enzyme immunoassays (EIA) and quantitative real time polymerase chain reaction (qPCR). Culture methods allow further characterization of isolates, whereas qPCR provides the greatest sensitivity and specificity, followed by EIA. The virulence gene profiles using PCR arrays and stx gene subtypes can subsequently be determined. Different non-O157 serotypes exhibit markedly different virulence gene profiles and a greater prevalence of stx1 than stx2 subtypes compared to O157:H7 isolates. Finally, recent innovations in whole genome sequencing (WGS) have allowed it to emerge as a candidate for the characterization and typing of STEC in diagnostic surveillance isolates. Methods of whole genome analysis such as single nucleotide polymorphisms and k-mer analysis are concordant with epidemiological data and standard typing methods, such as pulsed-field gel electrophoresis and multiple-locus variable number tandem repeat analysis while offering additional strain differentiation. Together these findings highlight improved strategies for STEC detection using currently available systems and the development of novel approaches for future surveillance.

Assessment of commercial chromogenic solid media for the detection of non-O157 Shiga toxin-producing Escherichia coli (STEC)

Detection of Shiga toxin-producing Escherichia coli (STEC) has evolved significantly since the introduction of sorbitol-MacConkey agar. This study compares four chromogenic media (CHROMagar™ STEC, Rainbow® O157 agar, CHROMagar™ O157, and Colorex® O157) in their identification of non-O157 STEC. When 161 non-O157 STEC were directly inoculated onto each medium, detection rates on CHROMagar™ STEC, Rainbow® O157 agar, CHROMagar™ O157 and Colorex® O157 were 90%, 70%, 3.7% and 6.8%, respectively. Tellurite minimal inhibitory concentrations (MICs) correlated with growth on CHROMagar™ STEC as 20 of 22 isolates with poor or no growth had MICs ≤1μg/mL. Stool spiking experiments revealed that CHROMagar™ STEC had the highest recovery of the six most common non-O157 STEC, ranging from 30% (in mucoid stool) to 98% (in watery stool). When using clinical stool samples, CHROMagar™ STEC had a sensitivity, specificity, positive predictive value, and negative predictive value of 84.6%, 87%, 13.9%, and 99.6%, respectively.

Evaluation of detection methods for non-O157 Shiga toxin-producing Escherichia coli from food

Shiga toxin-producing Escherichia coli (STEC) remains a major foodborne pathogen of concern across the globe. Rapid detection and isolation of this pathogen is of great importance for public health reasons. In this study the detection and isolation of four non-O157 STEC strains (O26, O103, O111, O145) from different artificially contaminated matrices, namely ground (minced) beef, cattle carcass swab, lettuce mix and sprouted soy beans, were evaluated. Low amounts of STEC were used (0.25-1.40 cfu/g) to spike the samples. All samples were enriched in parallel in Buffered Peptone Water (BPW) and Brila broth. After enrichment, detection was performed using real-time PCR (qPCR), and isolation using two chromogenic agar media, CHROMagar™ STEC and ChromID™ EHEC. Inoculation on the agar media was performed either directly after enrichment or after the use of an acid treatment procedure. Furthermore, the use of this procedure was also tested on naturally contaminated food products, using 150 stx-positive samples. Although the qPCR Cycle Threshold (Ct) values were lower after enrichment in Brila broth, no significant differences in recovery were observed between both enrichment broths. Both agar media were equally suitable for the isolation of STEC, although a significantly higher recovery was obtained when using both agar media in parallel. For samples with a Ct value above 25, an acid treatment step prior to isolation ensured a significant improvement in the recovery of STEC due to the reduction in background microbiota. This acid treatment procedure proved especially useful for the isolation of STEC from sprouted soy bean samples.

Analytical bioconjugates, aptamers, enable specific quantitative detection of Listeria monocytogenes

As a major human pathogen in the Listeria genus, Listeria monocytogenes causes the bacterial disease listeriosis, which is a serious infection caused by eating food contaminated with the bacteria. We have developed an aptamer-based sandwich assay (ABSA) platform that demonstrates a promising potential for use in pathogen detection using aptamers as analytical bioconjugates. The whole-bacteria SELEX (WB-SELEX) strategy was adopted to generate aptamers with high affinity and specificity against live L. monocytogenes. Of the 35 aptamer candidates tested, LMCA2 and LMCA26 reacted to L. monocytogenes with high binding, and were consequently chosen as sensing probes. The ABSA platform can significantly enhance the sensitivity by employing a very specific aptamer pair for the sandwich complex. The ABSA platform exhibited a linear response over a wide concentration range of L. monocytogenes from 20 to 2×10(6) CFU per mL and was closely correlated with the following relationship: y=9533.3x+1542.3 (R(2)=0.99). Our proposed ABSA platform also provided excellent specificity for the tests to distinguish L. monocytogenes from other Listeria species and other bacterial genera (3 Listeria spp., 4 Salmonella spp., 2 Vibrio spp., 3 Escherichia coli and 3 Shigella spp.). Improvements in the sensitivity and specificity have not only facilitated the reliable detection of L. monocytogenes at extremely low concentrations, but also allowed for the development of a 96-well plate-based routine assay platform for multivalent diagnostics.