New developments in chromogenic and fluorogenic culture media

An Overview of the Public Health Challenges in Diagnosing and Controlling Human Foodborne Pathogens

Pathogens found in food are believed to be the leading cause of foodborne illnesses; and they are considered a serious problem with global ramifications. During the last few decades, a lot of attention has been paid to determining the microorganisms that cause foodborne illnesses and developing new methods to identify them. Foodborne pathogen identification technologies have evolved rapidly over the last few decades, with the newer technologies focusing on immunoassays, genome-wide approaches, biosensors, and mass spectrometry as the primary methods of identification. Bacteriophages (phages), probiotics and prebiotics were known to have the ability to combat bacterial diseases since the turn of the 20th century. A primary focus of phage use was the development of medical therapies; however, its use quickly expanded to other applications in biotechnology and industry. A similar argument can be made with regards to the food safety industry, as diseases directly endanger the health of customers. Recently, a lot of attention has been paid to bacteriophages, probiotics and prebiotics most likely due to the exhaustion of traditional antibiotics. Reviewing a variety of current quick identification techniques is the purpose of this study. Using these techniques, we are able to quickly identify foodborne pathogenic bacteria, which forms the basis for future research advances. A review of recent studies on the use of phages, probiotics and prebiotics as a means of combating significant foodborne diseases is also presented. Furthermore, we discussed the advantages of using phages as well as the challenges they face, especially given their prevalent application in food safety.

Current Scenario of Pathogen Detection Techniques in Agro-Food Sector

Over the past-decade, agricultural products (such as vegetables and fruits) have been reported as the major vehicles for foodborne diseases, which are limiting food resources. The spread of infectious diseases due to foodborne pathogens poses a global threat to human health and the economy. The accurate and timely detection of infectious disease and of causative pathogens is crucial in the prevention and treatment of disease. Negligence in the detection of pathogenic substances can be catastrophic and lead to a pandemic. Despite the revolution in health diagnostics, much attention has been paid to the agro-food sector regarding the detection of food contaminants (such as pathogens). The conventional analytical techniques for pathogen detection are reliable and still in operation. However, laborious procedures and time-consuming detection via these approaches emphasize the need for simple, easy-to-use, and affordable detection techniques. The rapid detection of pathogens from food is essential to avoid the morbidity and mortality originating from the suboptimal nature of empiric pathogen treatment. This review critically discusses both the conventional and emerging bio-molecular approaches for pathogen detection in agro-food.

Chemo-Enzymatic Production of 4-Nitrophenyl-2-acetamido-2-deoxy-α-D-galactopyranoside Using Immobilized β-N-Acetylhexosaminidase

α-Nitrophenyl derivatives of glycosides are convenient substrates used to detect and characterize α-N-acetylgalactosaminidase. A new procedure combining chemical and biocatalytic steps was developed to prepare 4-nitrophenyl-2-acetamido-2-deoxy-α-D-galactopyranoside (4NP-α-GalNAc). The α-anomer was prepared through chemical synthesis of an anomeric mixture followed by selective removal of the β-anomer using specific enzymatic hydrolysis. Fungal β-N-acetylhexosaminidase (Hex) from Penicillium oxalicum CCF 1959 served this purpose owing to its high chemo-and regioselectivity towards the β-anomeric N-acetylgalactosamine (GalNAc) derivative. The kinetic measurements of the hydrolytic reaction showed that the enzyme was not inhibited by the substrate or reaction products. The immobilization of Hex in lens-shaped polyvinyl alcohol hydrogel capsules provided a biocatalyst with very good storage and operational stability. The immobilized Hex retained 97% of the initial activity after ten repeated uses and 90% of the initial activity after 18 months of storage at 4 °C. Immobilization inactivated 65% of the enzyme activity. However, the effectiveness factor and kinetic and mass transfer phenomena approached unity indicating negligible mass transfer limitations.

Assessment of Reference Method Selective Broth and Plating Media with 19 Listeria Species Highlights the Importance of Including Diverse Species in Listeria Method Evaluations

ABSTRACT

Reference methods developed for detection of Listeria monocytogenes are commonly used for detection of Listeria at the genus level. Improved method performance data are needed because this genus has expanded from 6 to 26 species and now includes several Listeria sensu lato species, which can have phenotypes distinct from those of Listeria sensu stricto. We evaluated growth of 19 Listeria species, including 12 recently described Listeria sensu lato species, using the media specified by (i) the U.S. Food and Drug Administration (FDA) Bacteriological Analytical Manual, (ii) the U.S. Department of Agriculture (USDA) Microbiology Laboratory Guidebook, and (iii) the International Organization for Standardization (ISO). The FDA broth enrichment procedure allowed all species to grow to detectable levels (≥4 log CFU/mL), yielded the highest mean growth (7.58 log CFU/mL), and was the only procedure with which no Listeria sensu lato species yielded significantly higher growth than did a comparison Listeria sensu stricto species. With the USDA and ISO broth enrichment procedures, several Listeria sensu lato species yielded significantly higher growth than did either Listeria seeligeri or Listeria ivanovii, suggesting that these two Listeria sensu stricto species could be outgrown by Listeria sensu lato species. On selective and differential agar media, L. seeligeri, L. ivanovii, and Listeria grayi produced colonies with atypical morphology and/or growth of these species was inhibited (which may lead to incorrect classification of a sample as negative), whereas several newly described Listeria sensu lato species grew to high levels and produced colonies with typical morphology. Overall, our study results indicate that the ability to detect various Listeria species can be impacted by the specific broth and selective and differential agar used. Our data can help guide selection of appropriate media and detection methods for environmental Listeria monitoring programs and methods that are most likely to detect the targeted Listeria groups (e.g., Listeria sensu stricto, which appear to be the most appropriate index organisms for the pathogen L. monocytogenes).

HIGHLIGHTS

A Galactosidase-Activatable Fluorescent Probe for Detection of Bacteria Based on BODIPY

Pathogenic E. coli infection is one of the most widespread foodborne diseases, so the development of sensitive, reliable and easy operating detection tests is a key issue for food safety. Identifying bacteria with a fluorescent medium is more sensitive and faster than using chromogenic media. This study designed and synthesized a β-galactosidase-activatable fluorescent probe BOD-Gal for the sensitive detection of E. coli. It employed a biocompatible and photostable 4,4-difluoro-3a,4a-diaza-s-indancene (BODIPY) as the fluorophore to form a β-O-glycosidic bond with galactose, allowing the BOD-Gal to show significant on-off fluorescent signals for in vitro and in vivo bacterial detection. This work shows the potential for the use of a BODIPY based enzyme substrate for pathogen detection.

Advancement in Salmonella Detection Methods: From Conventional to Electrochemical-Based Sensing Detection

Large-scale food-borne outbreaks caused by Salmonella are rarely seen nowadays, thanks to the advanced nature of the medical system. However, small, localised outbreaks in certain regions still exist and could possess a huge threat to the public health if eradication measure is not initiated. This review discusses the progress of Salmonella detection approaches covering their basic principles, characteristics, applications, and performances. Conventional Salmonella detection is usually performed using a culture-based method, which is time-consuming, labour intensive, and unsuitable for on-site testing and high-throughput analysis. To date, there are many detection methods with a unique detection system available for Salmonella detection utilising immunological-based techniques, molecular-based techniques, mass spectrometry, spectroscopy, optical phenotyping, and biosensor methods. The electrochemical biosensor has growing interest in Salmonella detection mainly due to its excellent sensitivity, rapidity, and portability. The use of a highly specific bioreceptor, such as aptamers, and the application of nanomaterials are contributing factors to these excellent characteristics. Furthermore, insight on the types of biorecognition elements, the principles of electrochemical transduction elements, and the miniaturisation potential of electrochemical biosensors are discussed.

Citrobacter braakii Yield False-Positive Identification as Salmonella, a Note of Caution

BACKGROUND

Globally, Salmonella enterica is one of the leading causes of foodborne illness in humans. Food of animal origin is obligatorily tested for the presence of this pathogen. Unfortunately, in meat and meat products, this is often hampered by the presence of background microbiota, which may present as false-positive Salmonella.

METHODS

For the identification of Salmonella spp. from meat samples of beef, pork, and poultry, the authorized detection method is PN-EN ISO 6579-1:2017-04 with the White-Kauffmann-Le Minor scheme, two biochemical tests: API 20E and VITEK II, and a real-time PCR-based technique.

RESULTS

Out of 42 presumptive strains of Salmonella, 83.3% Salmonella enterica spp. enterica, 14.3% Citrobacter braakii, and 12.4% Proteus mirabilis were detected from 180 meat samples.

CONCLUSIONS

Presumptive strains of Salmonella should be identified based on genotypic properties such as DNA-based methods. The aim of this study was the isolation and identification of Salmonella spp. from miscellaneous meat sorts: beef, pork, and poultry.

Detection of Microbial Nitroreductase Activity by Monitoring Exogenous Volatile Organic Compound Production Using HS-SPME-GC-MS

Development of a rapid approach for universal microbial detection is required in the healthcare, food and environmental sectors to aid with medical intervention, food safety and environmental protection. This research investigates the use of enzymatic hydrolysis of a substrate by a microorganism to generate a volatile organic compound (VOC). One such enzyme activity that can be used in this context is nitroreductase as such activity is prevalent across a range of microorganisms. A study was developed to evaluate a panel of 51 microorganisms of clinical interest for their nitroreductase activity. Two enzyme substrates, nitrobenzene and 1-fluoro-2-nitrobenzene, were evaluated for this purpose with evolution, after incubation, of the VOCs aniline and 2-fluoroaniline, respectively. Detection of the VOCs was done using headspace-solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) with obtained limits of quantitation (LOQ) of 0.17 and 0.03 µg/mL for aniline and 2-fluoroaniline, respectively. The results indicated that both enzyme substrates were reduced by the same 84.3% of microorganisms producing the corresponding volatile anilines which were detected using HS-SPME-GC-MS. It was found that nitroreductase activity could be detected after 6–8 h of incubation for the selected pathogenic bacteria investigated. This approach shows promise as a rapid universal microbial detection system.

Validation Using Diverse, Difficult-to-Detect Salmonella Strains and a Dark Chocolate Matrix Highlights the Critical Role of Strain Selection for Evaluation of Simplified, Rapid PCR-Based Methods Offering Next-Day Time to Results

ABSTRACT

Modifications to pathogen detection kits to accomplish simplified protocols with reduced time to results may impact method performance, particularly when combining shortened enrichment times and simplified enrichment procedures. We used Salmonella detection in dark chocolate as a model to test the impact of different enrichment times (minimum and maximum validated times) and procedures on detection of low levels of difficult-to-detect Salmonella strains, for three PCR kits that were AOAC International Performance Tested Method certified for detection of Salmonella spp. in dark chocolate. Initial inclusivity studies with pure cultures showed that all three kits detected 70 of 70 Salmonella spp. strains at 1 log above the theoretical limit of detection, with some strains yielding later cycle threshold values or having variable detection among technical replicates, indicating reduced assay performance for these strains. Based on these data, we selected a S. enterica subsp. enterica serovar Poona strain as well as three non-subsp. enterica strains to test the ability of the three kits to detect Salmonella in dark chocolate inoculated at low levels (0.06 to 1.18 most probable number per 25 g). With primary enrichment in skim milk at 35°C, detection frequency for all assays did not significantly differ from the reference method for both the minimum and maximum validated enrichment times. However, a pilot study that used primary enrichment in buffered peptone water at 42°C yielded significantly fewer positive samples (13 of 80) than were obtained with the U.S. Food and Drug Administration Bacteriological Analytical Manual method using enrichment in skim milk at 35°C (40 of 80 positive samples); strains representing subsp. houtenae and salamae were detected in significantly fewer chocolate samples than enrichment with skim milk. Our data indicate that continued efforts to simplify rapid pathogen detection kits may reduce kit performance in a way that can only be detected with stringent evaluation protocols that are designed to identify kit failure modes.

HIGHLIGHTS

From hazard analysis to risk control using rapid methods in microbiology: A practical approach for the food industry

The prevention of foodborne diseases is one of the main objectives of health authorities. To this effect, analytical techniques to detect and/or quantify the microbiological contamination of foods prior to their release onto the market are required. Management and control of foodborne pathogens have generally been based on conventional detection methodologies, which are not only time-consuming and labor-intensive but also involve high consumable materials costs. However, this management perspective has changed over time given that the food industry requires efficient analytical methods that obtain rapid results. This review covers the historical context of traditional methods and their passage in time through to the latest developments in rapid methods and their implementation in the food sector. Improvements and limitations in the detection of the most relevant pathogens are discussed from a perspective applicable to the current situation in the food industry. Considering efforts that are being done and recent developments, rapid and accurate methods already used in the food industry will be also affordable and portable and offer connectivity in near future, which improves decision-making and safety throughout the food chain.

In vitro Antibiotic Susceptibility, Virulence Genes Distribution and Biofilm Production of Staphylococcus aureus Isolates from Bovine Mastitis in the Liaoning Province of China

Purpose

The aim of this study was to identify the subtype, characterize the antimicrobial resistance, determine the virulence gene distribution, and analyze the biofilm production of Staphylococcus aureus isolates from bovine mastitis milk samples in the Liaoning Province of China.

Materials and Methods

In total, 56 Staph. aureus isolates were collected and identified in this study; the isolates were divided into different spa types based on the sequence of the polymorphic X region of the spa gene. Additionally, antimicrobial susceptibility was investigated using the broth microdilution method, and 18 virulence genes were detected using PCR. Biofilm formation was measured by spectrophotometry with crystal violet staining and observed using confocal laser scanning microscopy.

Results

There were 12.12% (56/462) milk samples that were positive for Staph. aureus. These isolates were nonsusceptible to sulfamethoxazole (100%), penicillin (76.9%), daptomycin (76.79%), clindamycin (69.64%), and oxacillin (60.71%); however, the majority of the isolates (80.4%) were susceptible to amoxicillin/clavulanate. The predominant virulence genes encoded the cytotoxins, hla (94.64%) and hlb (89.29%), and the adhesion factors clfA (89.29%), clfB (89.29%), and fnbB (80.36%). Comparatively, virulence genes related to other adhesion factors such as cna (8.93%) and enterotoxins, such as seg (26.79%), sea (16.07%), seb (7.14%), and sec (7.14%) were detected at relatively lower rates. The following eight spa types were identified: t267 (35.84%), t730 (22.64%), t518 (15.09%), t1190 (11.32%), t1456 (9.43%), t224 (1.88%), t9129 (1.88%), and t177 (1.88%). The highest biofilm production was observed for t267. Staph. aureus exhibited various patterns of biofilm formation, with the biofilm often being associated with a tower-shaped structure or a thicker biofilm.

Conclusion

Our results indicated that Staph. aureus isolates from dairy cows with mastitis in the Liaoning Province of China were non-susceptible to sulfamethoxazole, penicillin, daptomycin, oxacillin, and clindamycin. Additionally, the most prevalent subtype was t267, which displayed resistance to multiple antimicrobial agents and harbored several virulence genes, including clfA, clfB, fnbB, hla, and hlb.

Quantification and Multidrug Resistance Profiles of Vancomycin-Resistant Enterococci Isolated from Two Wastewater Treatment Plants in the Same Municipality

Wastewater treatment plants (WWTPs) are points of control for the environmental dissemination of antimicrobial resistant bacteria. Vancomycin-resistant enterococci (VRE) were used as indicators of antimicrobial resistance (AMR) in two WWTPs (biologically aerated filter (BAF) and conventional activated sludge (CAS)) in the same municipality. The removal and abundance of enterococci and VRE as well as the species and antimicrobial resistance profiles of VRE were assessed. Enterococci and VRE from the primary and final effluents were enumerated. Results were assessed from an ecological context. VRE was not selected for by either WWTP but the BAF system outperformed the CAS system for the removal of enterococci/VRE. Enterococcus faecalis (n = 151), E. faecium (n = 94) and E. casseliflavus/E. gallinarum (n = 59) were the dominant VRE species isolated. A decrease in levofloxacin resistance in enterococci was observed in the BAF WWTP. An increase in nitrofurantoin resistant (p < 0.001) and a decrease in quinupristin/dalfopristin (p = 0.003) and streptomycin (p = 0.022) resistant enterococci were observed in the CAS WWTP, corresponding to a shift of VRE from E. faecalis to E. faecium. Wastewater treatment processes can be managed to limit the dissemination of antimicrobial resistance determinants into the surrounding environment.

Fecal indicator bacteria levels at beaches in the Florida Keys after Hurricane Irma

Hurricanes cause infrastructure failures which can lead to contamination of impacted areas. The objective of the current study was to evaluate whether Hurricane Irma contributed towards sewage contamination of coastal beaches. Through this study we evaluated indicators of fecal pollution (fecal indicator bacteria [FIB], enterococci and fecal coliform) and physico-chemical parameters (salinity, pH, turbidity, and temperature) in coastal waters of the Florida Keys shortly after the hurricane. To augment available county sampling data, two sets of sampling efforts were conducted; one focused on collecting samples spatially throughout the Keys to assess whether areas closer to hurricane landfall were more highly impacted. The second was to collect temporally intensive samples at one location during falling tide to evaluate the hypothesis of groundwater contamination. Samples were analyzed for FIB using a new method called timed appearance of culture signal (TACS), which was subsequently calibrated using traditional membrane filter and chromogenic substrate methods. Results showed that coastal beach waters were characterized by elevated but sporadic levels of fecal indicator bacteria up to two months after the hurricane. Spikes were not correlated with physico-chemical characteristics of the water. Our temporally intensive sampling effort did not support the hypothesis that groundwater was a source of elevated FIB. Competing factors could have played a role in the sporadic nature of the FIB levels after the hurricane. We suggest that beach erosion may have flushed out sediments at beaches closer to the hurricane landfall location thereby improving water quality during dry conditions. We also suggest that during wet conditions a source of FIB could include runoff from debris staging areas. Preemptive beach closures immediately after the hurricane were justified due to the sporadic nature of FIB contamination.

Effect of Time and Mixing in Thermal Pretreatment on Faecal Indicator Bacteria Inactivation

Faecal indicator bacteria of faecal coliform, Salmonella spp., and faecal Streptococcus are present at high levels in faecal sludge and affect human health. Mesophilic anaerobic digestion cannot reduce faecal indicator bacteria to meet the standards for biosolids; therefore, the thermal pretreatment of faecal sludge is essential. The main objectives of this research were to evaluate the effect of thermal (70 °C) pretreatment time (20, 40, 60, 80, 100, and 120 min) and mixing velocity gradient (no mixing, 133, 191, and 238 s⁻¹) on faecal indicator bacteria inactivation and determine the kinetics of the inactivation. The results showed that mixing has a more positive effect on pretreatment; thermal pretreatment with mixing was able to completely inactivate faecal indicator bacteria within 80 min, and inactivation followed first-order kinetics. In addition, under optimal mixing at a velocity gradient 191 s⁻¹, the thermal pretreatment with mixing had a positive effect on the sludge solubilisation. Soluble chemical oxygen demand (SCOD, 71,430 mg L⁻¹) and soluble protein (7.96 g L⁻¹) were higher than the values obtained with thermal pretreatment without mixing, which were a SCOD value of 63,600 mg L⁻¹ and soluble protein of 6.78 g L⁻¹.

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.

Prevalence, antimicrobial resistance profile and comparison of methods for the isolation of salmonella in chicken liver from Argentina

This study was conducted to estimate the apparent prevalence of Salmonella spp. in chicken livers obtained from markets in Entre Ríos, Argentina, using two culture methods (preenrichment and direct selective agar plating). We also determined the antimicrobial resistance of Salmonella isolated strains and evaluated the performance of the two culture methods and selective-differential plating media used for Salmonella isolation. Of 666 chicken livers studied, 32 organs (4.8%) related to 4 poultry slaughterhouse companies were positive for Salmonella sp. using one or two culture methods. Fifty Salmonella strains were isolated from the positive liver samples and were typed into 3 serovars: S. ser. Schwarzengrund (78%), S. ser. Enteritidis (18%), and S. ser. Typhimurium 4(%). More than one Salmonella serovar was found in livers belonging to two chicken slaughterhouse companies. All strains were susceptible to all antibiotics tested, with the exception of erythromycin (100% resistant) and streptomycin (22% intermediate sensitivity). Overall, 32 (4.80%) and 3 (0.45%) of the chicken liver samples were positive for Salmonella sp. in preenrichment method and direct selective agar plating method, respectively; these percentages were significantly different (P=0.0001; kappa=0.16). There was also a statistical difference in relative accuracy, sensitivity and negative predictive value between the preenrichment method and the direct selective agar plating method; the first had greater values for these parameters than the direct selective agar plating method. These parameters were statistically different between MacConkey agar (MCA) and modified lysine iron (MLIA) in the two culture methods; the second had greater values than MCA for both culture methods. This study shows that even though serovars that are important for public health were isolated, the prevalence of Salmonella sp. is low in chicken livers from Entre Rios, Argentina. The isolated strains do not have multi-resistance patterns. Furthermore, the preenrichment method and MLIA are superior to the direct selective agar plating method and MCA for Salmonella sp. isolation from chicken liver samples, respectively.

Synthesis of precipitating chromogenic/fluorogenic β-glucosidase/β-galactosidase substrates by a new method and their application in the visual detection of foodborne pathogenic bacteria

We developed a new efficient method for the synthesis of important indoxyl glycoside substrates for β-glucosidase and β-galactosidase by using 1-acetylindol-3-ones as intermediates. This method was used to synthesise novel precipitating fluorogenic substrates for β-glucosidase based on 2-(benzothiazol-2'-yl)-phenols. We also assessed the application of these substrates in the detection of foodborne pathogenic bacteria.

Evaluation of Two Standard and Two Chromogenic Selective Media for Optimal Growth and Enumeration of Isolates of 16 Unique Bacillus Species

The genus Bacillus is a group of gram-positive endospore-forming bacteria that can cause food poisoning and diarrheal illness in humans. A wide range of food products have been linked to foodborne outbreaks associated with these opportunistic pathogens. The U.S. Food and Drug Administration recommends (in their Bacteriological Analytical Manual) the use of Bacara or mannitol egg yolk polymyxin (MYP) agar plates and the most-probable-number (MPN) method for enumeration and confirmation of Bacillus cereus and related species isolated from foods, sporadic cases, outbreaks, and routine environmental surveillance samples. We performed a comparative analysis of two chromogenic media (Bacara and Brilliance) and two traditional media (MYP and polymyxin egg yolk mannitol bromothymol blue agar [PEMBA]) for the isolation and enumeration of 16 Bacillus species under modified growth conditions that included pH, temperature, and dilution factor. A total of 50 environmental, food, and American Type Culture Collection reference isolates from 16 distinct Bacillus species were evaluated. A food adulteration experiment also was carried out by artificially adulterating two baby food matrices with two isolates each of B. cereus and Bacillus thuringiensis . Our results clearly indicated that chromogenic plating media (Bacara and Brilliance) are better than conventional standard media (MYP and PEMBA) for the detection and enumeration of B. cereus in foods and other official regulatory samples. The comparison of the two chromogenic media also indicated that Brilliance medium to be more efficient and selective for the isolation of Bacillus.

Evaluation of Selective-Enrichment and Chromogenic Media for Salmonella Detection in Raw Shell Egg Contents with a Low Microbial Load

The current study was conducted to evaluate the ability to recover Salmonella from shell egg contents by culture methods. A total of 4,000 eggs were obtained from a grading and packing center located in the Gyeonggi Province of South Korea, and 200 samples were created by pooling 20 broken eggs. The pooled samples were held at room temperature for 4 d before a 25-mL aliquot of each pool was added to 225 mL of modified trypticase soy broth (mTSB) and incubated at 35°C for 24 ± 2 h. A loopful of the culture was streaked onto chromogenic Druggan-Forsythe-Iversen (DFI) agar and incubated at 36 ± 1°C for 18-24 h. In addition, 1 mL and/or 0.1 mL of the mTSB cultures were added to 10 mL of Muller-Kauffmann tetrathionate with novobiocin (MKTTn) or Rappaport-Vassiliadis (RV) broth, and they were incubated for 24 ± 2 h at 35 ± 2°C or 42 ± 0.2°C, respectively. A loopful from these cultures was streaked onto Brilliant Green (BG), xylose lysine deoxycholate (XLD), and bismuth sulfite (BS) agar plates, respectively. Directly streaking onto DFI agar revealed the presence of Salmonella in 14 out of the 200 pooled samples (7%); whereas the combination of RV medium and BG, XLD, and BS agar detected the pathogen in only 9 (4.5%), 7 (3.5%), and 3 (1.5%) of the pooled samples, respectively. When MKTTn broth was used, Salmonella was detected in 7 (3.5%), 2 (1%), and 0 (0%) of the samples when streaked onto BG, XLD, and BS agar, respectively. The results indicate that direct plating onto DFI agar without enrichment was the most suitable among the methods evaluated in this study for detecting Salmonella in raw shell egg contents with a low microbial load.

Naphthoquinone glycosides for bioelectroanalytical enumeration of the faecal indicator Escherichia coli

Microbial water quality monitoring for the presence of faecal indicator bacteria (FIB) is a mandatory activity in many countries and is key in public health protection. Despite technological advances and a need for methodological improvements, chromogenic and fluorogenic enzymatic techniques remain the mainstays of water quality monitoring for both public health agencies and regulated utilities. We demonstrated that bioelectroanalytical approaches to FIB enumeration are possible and can be achieved using commercially available enzyme-specific resorufin glycosides, although these are expensive, not widely available or designed for purpose. Following this, we designed two naphthoquinone glycosides which performed better, achieving Escherichia coli detection in the range 5.0 × 102 to 5.0 × 105 CFU ml-1 22-54% quicker than commercially available resorufin glycosides. The molecular design of the naphthoquinone glycosides requires fewer synthetic steps allowing them to be produced for as little as US$50 per kg. Tests with environmental samples demonstrated the low tendency for abiotic interference and that, despite specificity being maintained between β-glucuronidase and β-galactosidase, accurate enumeration of E. coli in environmental samples necessitates development of a selective medium. In comparison to a commercially available detection method, which has U.S. Environmental Protection Agency (EPA) approval, our approach performed better at high organism concentrations, detecting 500 organisms in 9 h compared with 13.5 h for the commercial method. Bioelectroanalytical detection is comparable to current approved methods and with further development could result in improved detection times. A recent trend for low-cost open-source hardware means that automated, potentiostatically controlled E. coli detection systems could be constructed for less than US$100 per channel.

A hydrogel based rapid test method for detection of Escherichia coli (E. coli) in contaminated water samples

We have formulated a new chemical composition for rapid detection ofEscherichia coli(E. coli) with currently available enzymatic substrates.

An Improved Helferich Method for the α/β-Stereoselective Synthesis of 4-Methylumbelliferyl Glycosides for the Detection of Microorganisms

An improved Helferich method is presented. It involves the glycosylation of 4-methyl-umbelliferone with glycosyl acetates in the presence of boron trifluoride etherate combined with triethylamine, pyridine, or 4-dimethylaminopyridine under mild conditions, followed by deprotection to give fluorogenic 4-methylumbelliferyl glycoside substrates. Due to the use of base, the glycosylation reaction proceeds more easily, is uncommonly α- or β-stereoselective, and affords the corresponding products in moderate to excellent yields (51%-94%) under appropriate conditions.

Rapid Detection of Escherichia coli via Enzymatically Triggered Reactions in Self-Reporting Chitosan Hydrogels

In this work, a self-reporting hydrogel for the rapid in situ detection of bacterial enzymes is reported. To implement the reporting function for the bacterium Escherichia coli into a film-based sensing format, chitosan hydrogel films on solid backing supports were equipped with a reporting function for the enzyme β-glucuronidase (β-GUS), which is secreted by >98% of all known E. coli strains. Covalent coupling of the fluorogenic substrate 4-methylumbelliferyl-β-D-glucuronide or the complementary chromogenic substrate 4-nitrophenyl-β-D-glucuronide via amide bond formation afforded an attachment that is stable for >24 h under physiological conditions. By contrast, in the presence of β-GUS, the reporter dyes were very rapidly cleaved and produced a signal for the presence of the enzyme, which was detectable by bare eye under appropriate illumination. Detailed investigations of the enzymatic reaction for both types of substrates in neat enzyme solution as well as in bacterial supernatant revealed the apparent reaction kinetics and allowed us to determine the concentration of β-GUS in the supernatant. Under optimized conditions, the 4-methylumbelliferyl-β-D-glucuronide-functionalized hydrogel reported the presence of β-GUS within 15 min with a limit of detection of <1 nM. Finally, the function of the generally applicable hydrogel-film-based sensing approach, which is compatible with polymer-film-based applications, including wound dressings and packaging materials, and is also amenable to address noncultivatable pathogenic bacteria by using appropriate fluorogenic or chromogenic substrates, was demonstrated by direct application with bacterial medium.

Efficacy of coral-hydroxyapatite and biphasic calcium phosphate for early bacterial detection

Nano- or microhydroxyapatites with microbiological properties are being used to detect pathogens in clinical samples and industrial environments. In this study, the calcium phosphates coral-hydroxyapatite and biphasic calcium phosphate were characterized physicochemically using x-ray diffraction, thermogravimetric, and differential thermal analysis. The morphology, texture, and chemical composition of the ceramics were also investigated using scanning electron microscopy with energy dispersive spectroscopy. The biocompatibility of the ceramics was evaluated using Escherichia coli and Enterococcus faecalis. Microorganisms were detected by incorporating the enzyme markers 4-metilumbelliferil-β-d-glucoside and 4-metilumbelliferil-β-d-glucuronide in the ceramic powders and evaluating fluorescence. The characterization of the ceramics revealed typical characteristics, such as crystallinity, thermal stability, and chemical composition, consistent with other calcium phosphates. The calcium phosphates coral-hydroxyapatite and biphasic calcium phosphate ceramics differed from one another in morphology, structural topography, particle size distribution, and the capacity to absorb water. These properties can influence the rates of microbiological responses and bacterial detection. Although both materials are suitable for use as structural supports in microbial diagnostic systems, BCP was more efficient and detected E. coli and E. faecalis more rapidly than CHA.

Hollow fiber concentrator for water quality monitoring: role of surfactant based elution fluids

Simple and efficient concentrators are indispensable to the development of portable biosensor based detection platforms for the determination of microbial contamination in water.

Continuous fluorometric method for measuring β-glucuronidase activity: comparative analysis of three fluorogenic substrates

A new continuous fluorometric method for measuring GUS activity shows a superior analytical performance to the established discontinuous method.

Novel chromogenic aminopeptidase substrates for the detection and identification of clinically important microorganisms

A series of amino acid derivatives 8-10, 42 and 43 have been prepared as chromogenic enzyme substrates in order to detect aminopeptidase activity in clinically important Gram-negative and Gram-positive bacteria. Enzymatic hydrolysis liberates the amino acid moiety and either a 4-aminophenol or a 4-dialkylaminoaniline derivative which undergoes oxidative coupling with 1-naphthol or a substituted 1-naphthol giving an indophenol dye. Substrates and 1-naphthols were incorporated into an agar-based culture medium and this allowed growth of intensely coloured bacterial colonies based on hydrolysis by specific enzymes. Red/pink coloured colonies were produced by the substrates 8-10 and blue coloured colonies were formed by the substrates 42 and 43. The L-alanyl aminopeptidase substrates 8 targeted L-alanyl aminopeptidase activity and gave coloured colonies with a range of Gram-negative bacteria. Substrates 9 targeted β-alanyl aminopeptidase activity and generated coloured colonies with selected Gram-negative species including Pseudomonas aeruginosa. Three substrates for L-pyroglutamyl acid aminopeptidase (10a, 10c and 43) were hydrolysed by enterococci and Streptococcus pyogenes to generate coloured colonies. Two yeasts were also included in the study, but they did not produce coloured colonies with any of the substrates examined.

Development of a calcium phosphate nanocomposite for fast fluorogenic detection of bacteria

Current procedures for the detection and identification of bacterial infections are laborious, time-consuming, and require a high workload and well-equipped laboratories. Therefore the work presented herein developed a simple, fast, and low cost method for bacterial detection based on hydroxyapatite nanoparticles with a nutritive mixture and the fluorogenic substrate. Calcium phosphate ceramic nanoparticles were characterized and integrated with a nutritive mixture for the early detection of bacteria by visual as well as fluorescence spectroscopy techniques. The composite was obtained by combining calcium phosphate nanoparticles (Ca:P ratio, 1.33:1) with a nutritive mixture of protein hydrolysates and carbon sources, which promote fast bacterial multiplication, and the fluorogenic substrate 4-methylumbellipheryl-β-D-glucuronide (MUG). The composite had an average particle size of 173.2 nm and did not show antibacterial activity against Gram-negative or Gram-positive bacteria. After an Escherichia coli suspension was in contact with the composite for 60-90 min, fluorescence detected under UV light or by fluorescence spectrophotometer indicated the presence of bacteria. Intense fluorescence was observed after incubation for a maximum of 90 min. Thus, this calcium phosphate nanocomposite system may be useful as a model for the development of other nanoparticle composites for detection of early bacterial adhesion.

Synthesis and application of resorufin β-D-glucuronide, a low-cost chromogenic substrate for detecting Escherichia coli in drinking water

The development of low-cost tests for Escherichia coli is hampered by the expense and limited choice of enzyme substrates. Most chromogenic substrates are required in costly amounts, while fluorogenic substrates require an additional apparatus (e.g., an ultraviolet lamp) to be detected. Herein, we propose an alternative chromogenic substrate, resorufin β-d-glucuronide (REG), which is exceptionally sensitive and may be employed in very small amounts. We show that REG can be produced similarly to other simple glucuronides and should therefore be no more expensive. The compound is used by both healthy and injured E. coli, resulting in a pronounced color change from orange to a bright pink. Because the released dye (resorufin) has a high extinction coefficient, substantially lower amounts are needed than for commercially available substrates. The potential of this substrate is demonstrated by a presence/absence test requiring just 0.1 mg of REG/100 mL of water sample, one hundredth of the quantity needed for common chromogenic substrates, with an estimated bulk cost of ≤0.1 U.S. cents/test. REG shows promise as a chromogenic substrate for E. coli detection and should be considered in the development of new water tests, especially for low-income settings.

CHROMagar Yersinia, a new chromogenic agar for screening of potentially pathogenic Yersinia enterocolitica isolates in stools

CHROMagar Yersinia (CAY) is a new chromogenic medium for the presumptive detection of virulent Yersinia enterocolitica in stools. Based on a comparative analysis of 1,494 consecutive stools from hospitalized patients, CAY was found to be just as sensitive as the reference medium (cefsulodin-irgasan-novobiocin agar) but was significantly more specific and had a very low false-positive rate. CAY reduces the workload (and thus costs) for stool analysis and can therefore be recommended for routine laboratory use.

Development of an improved selective and differential medium for isolation of Salmonella spp

We describe an improved selective, differential, and cost-effective medium, XA medium, which contains d-arabinose, to facilitate the selective isolation of Salmonella spp. The sensitivity and the specificity of XA medium were compared to those of xylose lysine desoxycholate agar (XLD) using stock cultures and naturally contaminated food samples. XA medium and XLD were evaluated with a total of 82 Salmonella and 69 non-Salmonella stock cultures. Of 82 strains of Salmonella spp. tested, 76 produced a characteristic black colony on XA medium and XLD. The remaining 6 strains belonged to Salmonella enterica serovars Berta (n = 1), Paratyphi A (n = 1), Gallinarum (n = 2), and Pullorum (n = 2). The sensitivities of XA medium and XLD were identical (92.7%). Citrobacter freundii (n = 21) and Proteus mirabilis (n = 21) stock cultures produced black colonies on XLD, whereas only 4 strains of P. mirabilis appeared as black colonies on XA medium. In the second phase of the study, a total of 180 food samples were cultured onto XA medium and XLD after selective enrichment. The sensitivities of XA medium and XLD were equal (100%), and a total of 6 Salmonella strains were isolated from the 180 food samples. The specificity of XA medium (92.0%) was superior to that of XLD (73.0%), with a total of 14 and 47 false-positive results found on XA medium and XLD, respectively. On the basis of its good specificity, XA medium is useful for the isolation of Salmonella spp. from food samples.

Usefulness of Chromogenic CromoCen® AGN agar medium for the identification of the genus Aeromonas: Assessment of faecal samples

Selective screening media for the detection and identification of Aeromonas strains are needed to guide primary isolation procedures in the clinical laboratory. This study compared the selective CromoCen® AGN chromogenic agar medium for the detection and identification of Aeromonas strains that were isolated from various samples against the conventional selective agar media that are commonly used for the isolation of this organism in food, environmental and clinical samples. The Miles and Misra and ecometric methods were used to evaluate the microbiological performance of CromoCen® AGN chromogenic agar medium, which was shown to be satisfactory. A total of 14 reference Aeromonas strains, 44 wild strains and 106 clinical stool specimens were examined using both non-chromogenic selective agars that are commonly used for Aeromonas isolation and CromoCen® AGN agar. The latter exhibited 94.73% sensitivity and 100% specificity for the various samples. On CromoCen® AGN agar medium, Aeromonas formed colonies with light green, greenish and salmon pigments with or without a surrounding wide transparent zone (halo) of 2-3mm in diameter around the entire border. This medium is recommended for the isolation and potential identification of the Aeromonas genus.

A summary catalogue of microbial drinking water tests for low and medium resource settings

Microbial drinking-water quality testing plays an essential role in measures to protect public health. However, such testing remains a significant challenge where resources are limited. With a wide variety of tests available, researchers and practitioners have expressed difficulties in selecting the most appropriate test(s) for a particular budget, application and setting. To assist the selection process we identified the characteristics associated with low and medium resource settings and we specified the basic information that is needed for different forms of water quality monitoring. We then searched for available faecal indicator bacteria tests and collated this information. In total 44 tests have been identified, 18 of which yield a presence/absence result and 26 of which provide enumeration of bacterial concentration. The suitability of each test is assessed for use in the three settings. The cost per test was found to vary from $0.60 to $5.00 for a presence/absence test and from $0.50 to $7.50 for a quantitative format, though it is likely to be only a small component of the overall costs of testing. This article presents the first comprehensive catalogue of the characteristics of available and emerging low-cost tests for faecal indicator bacteria. It will be of value to organizations responsible for monitoring national water quality, water service providers, researchers and policy makers in selecting water quality tests appropriate for a given setting and application.

False-positive identification ofEscherichia coliin treated municipal wastewater and wastewater-irrigated soils

The increasing use of treated wastewater for irrigation heightens the importance of accurate monitoring of water quality. Chromogenic media, because they are easy to use and provide rapid results, are often used for detection of Escherichia coli in environmental samples, but unique levels of organic and inorganic compounds alter the chemistry of treated wastewater, potentially hindering the accurate performance of chromogenic media. We used MI agar and molecular confirmatory methods to assess false-positive identification of E. coli in treated wastewater samples collected from municipal utilities, an irrigation holding pond, irrigated soils, and in samples collected from storm flows destined for groundwater recharge. False-positive rates in storm flows (4.0%) agreed closely with USEPA technical literature but were higher in samples from the pond, soils, and treatment facilities (33.3%, 38.0%, and 48.8%, respectively). Sequencing of false-positive isolates confirmed that most were, like E. coli, of the family Enterobacteriaceae, and many of the false-positive isolates were reported to produce the β-d-glucuronidase enzyme targeted by MI agar. False-positive identification rates were inversely related to air temperature, suggesting that seasonal variations in water quality influence E. coli identification. Knowledge of factors contributing to failure of chromogenic media will lead to manufacturer enhancements in media quality and performance and will ultimately increase the accuracy of future water quality monitoring programs.