**2.1 Sample collection**

In order to confirm or discard the presence of *Salmonella* Saintpaul strain in Sinaloa fields, the U.S Food and Drug Administration (FDA), according to CDC statement, began the traceability of the strain in collaboration with Health and Agricultural Mexican authorities Comisión Federal para la Protección contra Riesgos Sanitarios (COFEPRIS, Federal Commission for Protection against Health Risks) and Servicio Nacional de Sanidad, Inocuidad y Calidad Agroalimentaria (SENASICA, National Health Service, Agri-food Safety and Quality). COFEPRIS and SENASICA are Mexican government institutions responsible to promote the adequate food production and to prevent the microbial risk ensuring food safety. Along with Health and Agricultural authorities of both countries, scientists from the Centro de Investigación en Alimentación y Desarrollo, A.C. (CIAD, Research Center for Food and Development), Culiacán station began collecting samples in Sinaloa fields to search for the *Salmonella enterica* serotype Saintpaul.

The sampling collection was conducted from June 23rd to June 27th of 2008. The sampling areas were divided in agricultural fields and packinghouse´s facilities. From the agricultural fields, canal water, reservoir water, water filtering equipment, soil, and tomato samples were collected; while from packinghouse´s facilities conveyor belts, tomato washing area, drying area and packing lines were sampled.

Sampling procedure was conducted according to the established by the American Public Health Association (APHA, 1998). Water and sediments samples were placed in sterile polypropylene flasks (Nalgene, Miami USA), while hermetic bags and sterile pre-wetted sampling sponges with 15 mL of phosphate-buffered solution (Whirl-Pak, Fort Atkinson, WI, USA) were used for soil and fruit.
