Rapid detection of respiring Escherichia coli O157:H7 in apple juice, milk, and ground beef by flow cytometry

Flow cytometry for the rapid detection of bacteria in cell culture production medium

The rapid and sensitive detection of microbial contaminants is critical for timely contamination investigations and monitoring of process control for in vitro protein production systems. A flow cytometric method was developed to monitor two types of cell culture media used in large scale protein production. The process used a DNA dye, thiazole orange, which binds to nucleic acids of viable and nonviable organisms. To ensure a representative sample was tested and to enhance the detection limit, a concentration step before staining and analysis included a double centrifugation. In addition, a washing step was included to eliminate background fluorescence caused by material in the media formulations. The staining and analysis of concentrated and washed samples takes approximately 0.5 h and provides objective results. The feasibility of the method was demonstrated by spiking sterile media with six bacterial species that represent the most commonly encountered bacterial contaminants. In addition to the bacterial spiking study, 164 lots of large scale production medium were tested by the flow cytometric method in parallel with conventional culture using Trypticase Soy Broth (TSB). In the bacterial spiking study, the concentration method increased the microbial titer by 2 logs. The detection limit of organisms within the medium was determined to be 1 CFU/ml. There was 100% correlation between the flow cytometric method and the standard aerobic plate count method using Trypticase Soy Agar. In the parallel study on 164 lots of large scale production media, the flow cytometric method was compared against culture in TSB. The sensitivity of the method was determined to be 100%, the specificity was 99.4%, the positive predictive value was 83%, and the negative predictive value was 100%. To reduce reporting of false positives, an initial positive result should be confirmed by an additional sample. The analytic sensitivity, specificity, and objectivity of this flow cytometric method indicate this method is valuable in the monitoring of production media for microbial contamination.

Disposable amperometric immunosensing strips fabricated by Au nanoparticles-modified screen-printed carbon electrodes for the detection of foodborne pathogen Escherichia coli O157:H7

A disposable amperometric immunosensing strip was fabricated for rapid detection of Escherichia coli O157:H7. The method uses an indirect sandwich enzyme-linked immunoassay with double antibodies. Screen-printed carbon electrodes (SPCEs) were framed by commercial silver and carbon inks. For electrochemical characterization the carbon electrodes were coupled with the first E. coli O157:H7-specific antibody, E. coli O157:H7 intact cells and the second E. coli O157:H7-specific antibody conjugated with horseradish peroxidase (HRP). Hydrogen peroxide and ferrocenedicarboxylic acid (FeDC) were used as the substrate for HRP and mediator, respectively, at a potential +300 mV vs. counter/reference electrode. The response current (RC) of the immunosensing strips could be amplified significantly by 13-nm diameter Au nanoparticles (AuNPs) attached to the working electrode. The results show that the combined effects of AuNPs and FeDC enhanced RC by 13.1-fold. The SPCE immunosensing strips were used to detect E. coli O157:H7 specifically. Concentrations of E. coli O157:H7 from 10(2) to 10(7)CFU/ml could be detected. The detection limit was approximately 6CFU/strip in PBS buffer and 50CFU/strip in milk. The SPCE modified with AuNPs and FeDC has the potential for further applications and provides the basis for incorporating the method into an integrated system for rapid pathogen detection.

Using oligonucleotide-functionalized Au nanoparticles to rapidly detect foodborne pathogens on a piezoelectric biosensor

A circulating-flow piezoelectric biosensor, based on an Au nanoparticle amplification and verification method, was used for real-time detection of a foodborne pathogen, Escherichia coli O157:H7. A synthesized thiolated probe (Probe 1; 30-mer) specific to E. coli O157:H7 eaeA gene was immobilized onto the piezoelectric biosensor surface. Hybridization was induced by exposing the immobilized probe to the E. coli O157:H7 eaeA gene fragment (104-bp) amplified by PCR, resulting in a mass change and a consequent frequency shift of the piezoelectric biosensor. A second thiolated probe (Probe 2), complementary to the target sequence, was conjugated to the Au nanoparticles and used as a "mass enhancer" and "sequence verifier" to amplify the frequency change of the piezoelectric biosensor. The PCR products amplified from concentrations of 1.2 x 10(2) CFU/ml of E. coli O157:H7 were detectable by the piezoelectric biosensor. A linear correlation was found when the E. coli O157:H7 detected from 10(2) to 10(6) CFU/ml. The piezoelectric biosensor was able to detect targets from real food samples.

Rapid quantification of bacterial cells in potable water using a simplified microfluidic device

A simplified microfluidic device for quantification of bacteria in potable water was fabricated and examined. Comparisons of counts of Escherichia coli by the microfluidic system and by epifluorescence microscopy closely correlated (r2=0.99). Bacteria in natural mineral water and in purified household tap water were accurately enumerated by using this system within 15 min after fluorescent staining.

Simple detection of small amounts of Pseudomonas cells in milk by using a microfluidic device

AIMS

Flow cytometry offers rapid and reliable analyses of bacteria in milk. However, a flow cytometer is relatively expensive and operation is rather complicated for an unskilled operator. We applied flow cytometry using a microfluidic device (on-chip flow cytometry) in detection of small amounts of milk-spoiling bacteria.

METHODS AND RESULTS

Pseudomonas cells in milk were in situ hybridized with Cy5-labelled probe specific for Pseudomonas spp. under optimized condition. Numbers of Pseudomonas cells in the stationary phase and in the starved state determined by on-chip flow cytometry were compared with those determined by conventional plate counting, and on-chip flow cytometry detected targeted cells in milk that were undetectable as colony forming units(CFU) on Standards Methods Agar.

CONCLUSIONS

The contamination in milk with fewer than 10 CFU ml(-1) of targeted cells in starved state was detectable with simple procedure (0.5 h milk-clearing, 1 h fixation, 2 h hybridization and 0.5 h on-chip flow cytometry following 12 h enrichment of cells).

SIGNIFICANCE AND IMPACT OF THE STUDY

On-chip flow cytometry following fluorescence in situ hybridization could be applicable to simple detection of milk-spoiling bacteria.

Application of a microfluidic device for counting of bacteria

AIMS

To develop a miniaturized analytical system for counting of bacteria.

METHODS AND RESULTS

Escherichia coli cells were used throughout the experiments. The system consists of a microfluidic chamber, a fluorescence microscope with a charge-coupled device (CCD) camera and syringe pumps. The chamber was made of a silicone rubber (30 x 30 mm and 4 mm high). The E. coli cells were flowed from a micro-nozzle fabricated in the chamber and detected with the CCD camera. The individual cells were indicated as signal peaks on a computer. The cell counts showed a good correlation compared with that of a conventional plate counting method, and results of the simultaneous detection of live and dead cells were also presented.

CONCLUSIONS, SIGNIFICANCE AND IMPACT OF THE STUDY

The system having a small disposable nozzle has the advantages for low cost and safe medical or environmental analysis, when compared with a conventional flow cytometer. This is the first step of the development of a one-chip microbe analyzer.

The use of Fourier transform infrared spectroscopy to differentiate Escherichia coli O157:H7 from other bacteria inoculated into apple juice

Fourier transform infrared spectroscopy (FT-IR) can discriminate Escherichia coli O157:H7 ATCC 35150 from other bacteria: E. coli ATCC 25522, Bacillus cereus ATCC 10876, and Listeria innocua ATCC 51742 inoculated in to apple juice. Spectra of bacterial suspensions (ca. 10(9) cfu/ml in 0.9% NaCl) on Anodisc (aluminum oxide) filters were tested. Unique FT-IR vibrational combination bands from mid-IR active components of bacterial cells are present in the "fingerprint region" at wavenumbers between 1500 and 800 cm(-1). Principal component analysis (PCA) revealed clear segregations between different bacterial strains. Also, soft independent modeling of class analogy (SIMCA) correctly classified E. coli O157:H7 ATCC 35150 from E. coli ATCC 25522 at an 82% confidence level; whereas a 77% confidence level was obtained when using SIMCA to classify E. coli O157:H7 from three other bacterial strains.

Multicolour digital image analysis system for identification of bacteria and concurrent assessment of their respiratory activity

AIMS

To develop a rapid and simple multicolour digital image analysis system for simultaneous identification of bacteria and assessment of their metabolic activity.

METHODS AND RESULTS

We developed an image analyser capable of distinguishing triple-stained bacterial cells. Bacteria were stained with a nucleic acid stain, a fluorescent antibody and a fluorescent metabolic indicator for enumeration, species identification and assessment of metabolic activity. This multicolour image analyser was used to simultaneously identify Escherichia coli O157:H7 in milk samples and assess their respiratory activity. The images of the triple-stained bacteria were captured using a combination of blue light and u.v. excitation and an epifluorescence microscope and were processed by our image analyser. We found a good correlation between the counts of actively respiring (r = 0.93) and total (r = 0.94) E. coli O157:H7 measured by digital image analysis and visual observation.

CONCLUSION

The multicolour digital image analysis system described here was able to quantify active pathogenic micro-organisms within 2 h.

SIGNIFICANCE AND IMPACT OF THE STUDY

This multicolour image analysis allows the rapid and simultaneous quantification of bacteria, identification of species and assessment of metabolic activity.

Enumeration of respiring Pseudomonas spp. in milk within 6 hours by fluorescence in situ hybridization following formazan reduction

Respiring Pseudomonas spp. in milk were quantified within 6 h by fluorescence in situ hybridization (FISH) with vital staining. FISH with an oligonucleotide probe based on 16S rRNA sequences was used for the specific detection of Pseudomonas spp. at the single cell level. 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) was used to estimate bacterial respiratory activity. The numbers of respiring Pseudomonas cells as determined by FISH with CTC staining (CTC-FISH) were almost the same or higher than the numbers of CFU as determined by the conventional culture method.

Rapid monitoring of microbial contamination on herbal medicines by fluorescent staining method

AIMS

To apply fluorescent staining method for fast assessment of microbial quality of herbal medicines.

METHODS AND RESULTS

The number of total bacteria and esterase-active bacteria on powdered traditional Chinese medicines were enumerated by fluorescent staining method using 6-carboxyfluorescein diacetate (6CFDA) and 4',6-diamidino-2-phenylindole (DAPI), and they were compared with colony-forming units (CFU). The CFU was approximately 10(3) per gram in ginseng radix, and no bacterial colonies were detected from others. However, the total bacterial number (TDC) was more than 10(7) per gram, and number of bacteria possessing esterase activity ranged from 1 to 3% of TDC.

CONCLUSIONS, SIGNIFICANCE AND IMPACT OF THE STUDY

Many bacteria in each Chinese medicine had enzyme activity and most of them could not be detected by conventional plate counting technique. Enumeration of bacterial cells on traditional Chinese medicines by fluorescent staining method requires less than 1 h. The double staining method with 6CFDA and DAPI could be applicable to rapid microbial monitoring of crude drugs.

Tetrazolium dyes as tools in cell biology: new insights into their cellular reduction

Tetrazolium salts have become some of the most widely used tools in cell biology for measuring the metabolic activity of cells ranging from mammalian to microbial origin. With mammalian cells, fractionation studies indicate that the reduced pyridine nucleotide cofactor, NADH, is responsible for most MTT reduction and this is supported by studies with whole cells. MTT reduction is associated not only with mitochondria, but also with the cytoplasm and with non-mitochondrial membranes including the endosome/lysosome compartment and the plasma membrane. The net positive charge on tetrazolium salts like MTT and NBT appears to be the predominant factor involved in their cellular uptake via the plasma membrane potential. However, second generation tetrazolium dyes that form water-soluble formazans and require an intermediate electron acceptor for reduction (XTT, WST-1 and to some extent, MTS), are characterised by a net negative charge and are therefore largely cell-impermeable. Considerable evidence indicates that their reduction occurs at the cell surface, or at the level of the plasma membrane via trans-plasma membrane electron transport. The implications of these new findings are discussed in terms of the use of tetrazolium dyes as indicators of cell metabolism and their applications in cell biology.