The Role of Biofilms as a Reservoir for Escherichia coli in Irrigation Systems

Introduction: Shiga toxin-producing Escherichia coli (STEC) are responsible for numerous produce-associated outbreaks. Arizona ranks third in the nation in the production of fresh market vegetables, all of which are grown by irrigated agriculture. Several outbreaks have been linked to contaminated irrigation waters; nevertheless, STEC are rarely found in water and thus it is often difficult to determine the source of contamination. 

Purpose: To determine the occurrence and levels of STEC in irrigation biofilms to determine their potential role as a reservoir for STEC. 

Methods: A total of 1,115 biofilm samples were collected from sprinkler heads/pipes/gaskets, pumps, and canals in Yuma, Arizona during the growing season (October-April). Samples were enriched in buffered peptone water for 24h at 37°C, followed by selection in EC broth for 24h at 44.5°C. Cultures producing gas were subcultured onto mEndo agar plates; darkly pigmented colonies were evaluated using API20E biochemical strips for species identification. Confirmed E. coli isolates were tested for the presence of Shiga toxin (stx1, stx2) and intimin (eae) genes using qPCR.

Results: E. coli was isolated from a total of 51 (4.6%) samples; however, none possessed any of the virulence genes. Generic E. coli (non-STEC) was found throughout most of the irrigation systems from the canals, to the pumps, to the sprinkler pipes (proximal and distal ends of the fields). Nevertheless, the organism was identified with greater frequency in irrigation canal biofilms (11.1% of samples) than in other areas of the irrigation systems (4.1% of samples).

Significance: There is likely a low risk of contamination of produce from biofilms containing STEC in Yuma, Arizona. It appears that such contamination either occurs in the field or via sporadic contamination of irrigation waters.