Mycobacterium avium subsp. paratuberculosis (MAP) is the causative agent for Johne’s disease that affects cattle and leads to wasting of the infected animals. In a herd, most animals infected are asymptomatic but still propagate diseasethrough their milk and feces. This makes the most susceptible animals develop sickness and the within-herd transmission uncontrolled. It is therefore important to detect MAP infection early and even when it is causing only latent infection. The most used test for the detection of MAP in animal blood or milk samples is ELISA with MAP-specific antibodies. The problem with this essay is that the detection limit is high and this leads to false negatives, which limits the effective detection of infected animals. The most sensitive method available is liquid and solid culture. However, since MAP has a slow growth rate and no media is truly selective for these bacteria, there are a lot of instances where culture overgrowth by contaminating organisms impairs MAP viability and limits detection to 23% with many false negative results. Molecular based methods such as PCR are also used and are more rapid and sensitive but they do not assess the viability of MAP cells.
Therefore, it was extremely important to develop a new detection method that is both sensitive and rapid, and allows distinguishing between viable and dead cells of MAP. The use of a magnetic separation assay allowed researchers to overcome all these difficulties and to develop a new protocol with improved sensitivity and specificity when compared to the previously available ones. This method relies on the combination of peptide-mediated magnetic separation with a phage assay. The protocol consists in the isolation of MAP with paramagnetic beads coated with specific monoclonal antibodies and phage display-derived peptides. The obtained purified bacteria can then be detected by PCR or in culture. In the milk and feacal samples tested,the capture efficiency was higher than 85% even with low numbers of MAP (<10 cfu/ml). This work further shows the potential of magnetic separation methods for the improvement of diagnosis methods.
Novel Monoclonal Antibody and Peptide Binders for Mycobacterium avium subsp. paratuberculosis and Their Application for Magnetic Separation.O’Brien, L. M., Stewart, L. D., Strain, S. A. J. & Grant, I. R. PLoS One 11, e0147870 (2016).
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