Evaluation of a novel Listeria enrichment broth combined with a real‐time PCR diagnostics assay for the specific detection of Listeria monocytogenes in RTE pork products

Abstract

Foodborne outbreaks of listeriosis, human infection with Listeria monocytogenes, are relatively uncommon with approximately 0.1–1.1 cases /100 000 population globally, but high mortality rates of 20–30% are associated with infection (Siegman-Igra et al., 2002; Anonymous, 2012). Those at particular risk of infection include the immunocomprised, the elderly, neonates and pregnant women (Anonymous, 2011; Rocourt et al., 2003). Since the 1990s, over 40% of foodborne listeriosis outbreaks have been associated with ready to eat (RTE) meat products (Warriner & Namvar, 2009). The most significant outbreak of recent times took place in Canada in 2008, with a total of 57 listeriosis cases associated with RTE deli meats which resulted in 23 deaths (Anonymous, 2010; Gilmour et al., 2010). Other significant outbreaks were related with meat frankfurters [USA, 108 cases and 14 deaths in 1998–99] (Anonymous, 1998, 1999; Mead et al., 2006); pât e [Australia, 11 cases and 6 deaths in 1990] (Kittson, 1992; Watson & Ott, 1990); turkey deli meats [USA, 54 cases and 8 deaths in 2002] (Anonymous, 2002; Gottlieb et al., 2006); ‘Quargel’, a brand of acid ripened curd cheese [Austria, Germany and Czech Republic, 34 cases and 8 fatalities in 2009/2010] (Fretz et al., 2010a,b). Listeriosis outbreaks such as these highlight the importance of surveillance in RTE foods. In spite of high salt content and low water activity (Aw), cured and ripened RTE meat products such as those examined in this study can also harbour L. monocytogenes. The organism is capable of growing in 10% NaCl with a water activity (Aw) of 0.90 which is typical of production conditions (AFSSA, 2000; Ryser & Donnelly, 2001). Traditional methods for the specific detection of L. monocytogenes in food are both time consuming and laborious. Recently, a new culture-based method for the identification of L. monocytogenes from foods has been developed, namely the Listeria Precis method. This method uses a single enrichment in Oxoid Novel Enrichment Broth Listeria (ONE Broth-Listeria) followed by selective plating on chromogenic agar [ALOA (Agar Listeria selon Ottaviani & Agosti) One Day] and can provide presumptive results 48 h earlier than ISO 11290-1 (Anonymous, 2004a), a current standard method (Oxoid, 2010). This initial study was performed to determine the feasibility of reducing turnaround time of the Listeria Precis method (considered the reference method used in this study) by incorporating a rapid molecular test after culture enrichment in the ONE Broth-Listeria. A previously described qPCR assay for the specific detection of L. monocytogenes in food (O’Grady et al., 2008, 2009) was combined with the ONE Broth-Listeria culture enrichment step (24 h) of the Listeria Precis method (considered the alternative method used in this study). *Correspondent: Fax: +35391494598; e-mail: thomas.barry@nuigalway.ie Present address: Centre for Clinical Microbiology, University College London (Royal Free Hospital campus), London NW3 2PF