2. Material and methods
2.1. Bacterial isolates
A total of 336 Listeria isolates from ready-to-eat (RTE) meat
products and food-processing environments, including 206 L.
monocytogenes, and 130 L. innocua isolates, were characterized by
antimicrobial susceptibility tests. The strains were isolated between
May 2009 and April 2012 according to the ISO 11290-1 (ISO
11290-1: 1996/Amd 1, 2004a) and ISO-11290-2 (ISO 11290-2: 1998/
Amd 1, 2004b) standards as described in detail elsewhere (Gómez
et al., 2012). They were pre-identified (b-hemolysis and fermentation
of rhamnose, xylose and mannitol) and finally identified
with the gallery API-Listeria.
Table 1 shows the origin of the Listeria isolates used in this study.
Of the 336 strains investigated, 103 (72 L. monocytogenes and 31
L. innocua) were isolated from environmental samples taken from
32 Spanish meat industries located across NE and SW Spain. The
strains were isolated from food-contact surfaces made of stainless
steel, high molecular weight polyethylene (HMWP) and polyvinyl
chloride (PVC) conveyor belts. Another set of 147 strains (85
L. monocytogenes and 62 L. innocua) were isolated from the RTE
meat products produced within the sampled industries. The
remaining 86 strains of Listeria (49 L. monocytogenes and 37
L. innocua) were isolated from commercially available RTE meat
products purchased in butcher’s shops, supermarkets and