Evaluation of the efficacy of immunomagnetic separation for the detection of Clavibacter michiganensis subsp. michiganensis in tomato seeds

Detection of Clavibacter michiganensis subsp. michiganensis in viable but nonculturable state from tomato seed using improved qPCR

Clavibacter michiganensis subsp. michiganensis (Cmm) is a seed-borne pathogen that causes bacterial canker disease of tomato. Cmm is typically detected in tomato seeds using quantitative real-time polymerase chain reaction (qPCR) combined with culture-based isolation. The viable but nonculturable (VBNC) state of Cmm may result in the underestimation or false negative detection of the pathogen. In the present study, propidium monoazide (PMA) and its improved structure PMAxx were used to pretreat Cmm prior to DNA extraction, followed by qPCR. Both PMA and PMAxx could bind to the chromosomal DNA of dead bacterial cells and therefore block DNA amplification by PCR. This effect, however, does not occur in living bacterial cells, as the chemicals cannot penetrate through the undamaged cell membrane. Both viable and dead Cmm cells were treated with PMA and PMAxx at various concentrations. With this treatment, the range of the cell population was determined for effective detection. PMAxx showed a better discrimination effect than PMA on the viable and dead cells of Cmm and was therefore used throughout the present study. VBNC cells of Cmm (108 CFU mL-1) was induced by 50 μM copper sulfate, which was detected at different sampling times up to a month by using both PMAxx-qPCR and flow cytometry assays. The optimal PMAxx concentration was 20 μM for detecting membrane-intact Cmm cells. High specificity and sensitivity were obtained at Cmm concentrations ranging from 103 to 107 CFU mL-1. The accurate and robust results of PMAxx-qPCR were confirmed by flow cytometry method to detect viable Cmm cells. Furthermore, the PMAxx-qPCR assay was successfully used in detecting VBNC Cmm cells in tomato seeds with as few as 10 seeds per set.

Bacterial Canker of Tomato: Current Knowledge of Detection, Management, Resistance, and Interactions

Clavibacter michiganensis subsp. michiganensis is the causal agent of bacterial canker of tomato. The disease was first described in 1910 in Michigan, USA. C. michiganensis subsp. michiganensis (from now on called clavibacter) was initially thought to be a phloem parasite, but was later found to be a xylem-invading bacterium. The host range comprises mainly solanaceous crops such as tomato, pepper, and eggplant. Strains show great variability in virulence and are usually described as being hypervirulent, hypovirulent, or nonvirulent. Clavibacter lacks a type III secretion system, and only a few virulence factors have been experimentally determined from the many putative virulence factors. As the molecular mode of infection by clavibacter is unknown, researchers have avoided intensive work on this organism. Genetic plant mechanisms conferring resistance to clavibacter are apparently complex, and breeders have yet to develop disease-resistant cultivars.

Applications of flow cytometry in plant pathology for genome size determination, detection and physiological status

Flow cytometers are probably the most multipurpose laboratory devices available. They can analyse a vast and very diverse range of cell parameters. This technique has left its mark on cancer, human immunodeficiency virus and immunology research, and is indispensable in routine clinical diagnostics. Flow cytometry (FCM) is also a well-known tool for the detection and physiological status assessment of microorganisms in drinking water, marine environments, food and fermentation processes. However, flow cytometers are seldom used in plant pathology, despite FCM's major advantages as both a detection method and a research tool. Potential uses of FCM include the characterization of genome sizes of fungal and oomycete populations, multiplexed pathogen detection and the monitoring of the viability, culturability and gene expression of plant pathogens, and many others. This review provides an overview of the history, advantages and disadvantages of FCM, and focuses on the current applications and future possibilities of FCM in plant pathology.

Detection of E. coli O157:H7 by immunomagnetic separation coupled with fluorescence immunoassay

Conventional culture-based methods for detection of E. coli O157:H7 in foods and water sources are time-consuming, and results can be ambiguous, requiring further confirmation by biochemical testing and PCR. A rapid immunoassay prior to cultivation to identify presumptive positive sample would save considerable time and resources. Immunomagnetic separation (IMS) techniques are routinely used for isolation of E. coli O157:H7 from enriched food and water samples, typically in conjunction with cultural detection followed by biochemical and serological confirmation. In this study, we developed a new method that combines IMS with fluorescence immunoassay, termed immunomagnetic fluorescence assay (IMFA), for the detection of E. coli O157:H7. E. coli O157:H7 cells were first captured by anti-O157 antibody-coated magnetic beads and then recognized by a fluorescent detector antibody, forming an immunosandwich complex. This complex was subsequently dissociated for measurement of fluorescence intensity with Signalyte™-II spectrofluorometer. Experiments were conducted to evaluate both linearity and sensitivity of the assay. Capture efficiencies were greater than 98%, as determined by cultural plating and quantitative real-time PCR, when cell concentrations were <10(5) cells/mL. Capture efficiency decreased at higher cell concentrations, due to the limitation of bead binding capacity. At lower cell concentrations (10-10(4) cells/mL), the fluorescence intensity of dissociated Cy5 solution was highly correlated with E. coli 157:H7 cell concentrations. The detection limit was 10 CFU per mL of water. The assay can be completed in less than 3 h since enrichment is not required, as compared to existing techniques that typically require a 24 h incubation for pre-enrichment, followed by confirmatory tests.