**2.2** *Salmonella* **isolation method**

*Salmonella* isolation from the collected samples was performed according to the APHA (1998), which consists in the Most Probable Number (MPN) technique by the use of 3x

The commercial relationship between Sinaloa state producers and the United States, is given in great majority for the exportation of Mexican fresh produce, which are extensively and carefully produced under strict guidelines of GAP and GMP, to prevent the misuse of pesticides and the presence of pathogens (SENASICA, 2011). As evidence of that, among the different foodborne outbreaks occurred in the United States of America, none of them had

From April to August of 2008, the US CDC Health Department confirmed the occurrence of a multistate *Salmonella* serotype Saintpaul outbreak affecting 43 US states, Columbia district and Canada (Figure 4). In August of 2008, a total of 1,442 cases and at least 286 hospitalizations and two deaths were reported. The US Health authorities argued high association (85%) of tomato and later jalapeño peppers as the pathogen transmission vehicles and pointed out Sinaloa tomato production as a possible source of the bacterial strain (CDC, 2008). The outbreak and the CDC call alerted both countries, which started to

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

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,

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,

*Salmonella* isolation from the collected samples was performed according to the APHA (1998), which consists in the Most Probable Number (MPN) technique by the use of 3x

been associated to fresh produce grown in fields of Sinaloa.

work together to source track the origin of the causative agent.

Sinaloa fields to search for the *Salmonella enterica* serotype Saintpaul.

drying area and packing lines were sampled.

WI, USA) were used for soil and fruit.

**2.2** *Salmonella* **isolation method** 

**2. Searching the causative agent in Sinaloa fields** 

**1.2** *Salmonella* **Saintpaul outbreak** 

**2.1 Sample collection** 

Fig. 4. Distribution and number of infected cases with *Salmonella* Saintpaul, in the United States (CDC, 2008).

Tripticase Soy Broth (3x TSB) (Bioxon, México), modified semi-solid Rappaport Vassiliadis (Difco, USA) and XLD agar (BIoxon, México) as pre enrichment, enrichment and selective isolation, respectively. The homogenized sample was diluted and distributed in 3 sets of 3 tubes each. Once inoculated the tubes were incubated at 37°C during 24 h. After incubation, aliquots were transferred to modified semi-solid Rappaport Vassiliadis (Difco, USA) and incubated at 42°C during 24 h. This process was done by triplicate. Finally a loop of the inoculated semi-solid medium was transferred to the XLD agar (Bioxon, México), which was incubated at 37°C during 24 h to identify presumptive colonies presenting round morphology, black central pigment and a well-defined

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transparent border. Presumptive colonies were prepared to DNA extraction for confirmation assay by Polymerase Chain Reaction (PCR).
