**5. Phage based detection methods**

Bacteriophages are viruses infecting bacteria and by definition obligate intracellular parasites lacking their own metabolism, are extremely host-specific, and able only to infect specific species or even strains. Virulent phages with a broad host range within the *Salmonella* genus are ideally suited for detection purposes since they are unable to integrate

*Salmonella* Detection Methods for Food and Food Ingredients 385

processing and production. Further improvements to rapid methods for isolation and detection of *Salmonella* and other microbial pathogens will continue to focus on sample enrichment and preparation procedures to reduce test turn around times and increase the sensitivity of detection, and also on the application of novel technologies such as biosensors,

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Campylobacter lari, and Campylobacter upsaliensis by a multiplex PCR developed from the

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**7. References** 

into the host genome, with the successful infection always resulting in the death of their host (Hagens and Loessner 2007). Since the first report of the use of phage for detection by Ulitzur and Kuhn (1987), different strategies have been described for the detection of *Salmonella*. Generally, the majority of methods described involve measuring the activity of a reporter gene (generally, the luciferase *lux* genes from *Vibrio fisherii*), cloned into a vector carried by a phage, and expressed only after infection (Kuhn et al. 2002; Thouand et al. 2008). Luciferase genes have the enormous advantage in that background noise or photon emission is absent from food samples and the luminescence, when detected, reflects the presence of viable target bacteria. Other approaches include use of an ice nucleation reporter phage (Wolber and Green 1990); concentration by IMS followed by phage mediated release of adenylate kinase (AK) (Blasco et al. 1998; Wu et al. 2001); fluorescently labelled phage (Jiang et al 2009); and an IMS-bacteriophage plaque formation assay requiring the addition of a virucide to inactivate free phage particles (Fravrin et al. 2001). The usefulness of phagebased cell wall recognition proteins for magnetic capture has also been recently described utilizing cell-wall-binding domains (CBDs) highly specific for recognition and binding to target cells surfaces (Kretzer et al. 2007; Korndoerfer et al. 2006; Loessner et al. 2002). Paramagnetic beads coated with CBD molecules were shown to outperform commercially available antibody-based magnetic beads with respect to sensitivity and percent recovery (Kretzer et al. 2007). An extension to this approach has been the use of phage-tail-associated recognition proteins for the immobilization of gram-negative cells (Galikowska et al. 2011). For example, BioMerieux. has recently introduced *Salmonella* Up, an automated ELISA based VIDAS assay using a phage recombinant protein derived from specific bacteriophage tail fibers for the detection of *Salmonella* in food and food ingredients within 18-24 hours after enrichment in a non-selective broth.

Although at present commercial phage based detection systems are limited, the technology may circumvent the problem of viability presented by PCR, while promising to be more rapid than standard culture methods.
