Faecal microbial contamination of watercress (Nasturtium officinale) gathered by a Maori protocol in New Zealand streams

A study was carried out to determine the food safety status of watercress (Nasturtium officinale) harvested from rural streams under Maori protocol. Sixty‐five sample bunches (c. 500 g) were collected from four streams in the Waikato region of New Zealand each summer from 2005 to 2007. Two streams, Southern Enclosure and Te Waihou, were within reserves and the other two, Mangapiko and Piopio, flowed through pastoral farms. To assess faecal contamination status, Escherichia coli and thermotolerant Campylobacter were measured on watercress as collected and E. coli counts assessed against the New Zealand guideline for ready‐to‐eat foods (satisfactory: <3 E. coli per g). To determine whether washing would ensure the watercress met food safety standards, an additional 6 bunches were collected and washed in running tap water (household regime). A further 15 bunches were washed by a simulated commercial triple washing regime. At harvest, 16 of 17 watercress samples collected from the Southern Enclosure and 11 of 22 from Te Waihou met the satisfactory criterion for ready‐to‐eat food, but only 1 of 17 from Mangapiko and none of 9 from Piopio were satisfactory. No Campylobacter was recovered from any sample of watercress collected from the four sites. After washing in running tap water, E. coli numbers still exceeded the satisfactory criterion. Commercial triple washing was more effective in ensuring satisfactory watercress, but of the 15 samples subjected to this regime, only 6 met the satisfactory criterion. Escherichia coli remained firmly attached to watercress leaves after both washing regimes (presumably in biofilms). Overall, these findings suggest that it is not advisable to use watercress harvested from rural streams as a raw salad vegetable, particularly from those affected by pastoral farming.