2018 ASHS Annual Conference

An increasing number of foodborne illness outbreaks associated with cucumber has been observed in recent years. During the period from 2006 to 2016, a total of 15 cucumber-associated Salmonella outbreaks occurred in the United States. An outbreak in 2014, traced back to the Eastern Shore of Maryland, affected at least 275 people and an environmental source for the Salmonella contamination was suspected.

Contamination of produce can occur at multiple points throughout the food supply chain but a focus on pre-harvest contamination has increased as a result of knowledge that pathogens have the ability to become internalized within a variety of crops, including tomatoes, spinach and lettuce. Routes of fruit internalization remain largely unexplored, but enteric pathogens have been reported to undergo chemotaxis towards, and enter, leaf stomata. Stomata provide a micro-environment shielded from sanitizers and a source of photosynthetically-produced nutrients that facilitate survival.

Here we investigated the potential for the human enteropathogen Salmonella enterica to exhibit surface attachment on and internalization into cucumber fruit. Mini cucumbers purchased from a local store were exposed to light and inoculated in the mid-section with ~10⁸ CFU/mL of Salmonella serotype Newport or Poona. After incubation periods of either two or 24 hours, half of the cucumbers were dark-treated to close stomata while the other half were used to enumerate attached cells using direct plating on tryptic soy agar. Dark-treated fruit were then surface sterilized with 0.2% sodium hypochlorite to remove surface-associated cells. Inoculated sections were removed with a sterile scalpel, macerated with a pestle in buffered peptone water and enumerated similarly.

For each fruit analyzed, at least one of the two inoculated sections yielded recoverable internalized cells for both the 2- and 24-hour time points, with a statistically significant increase in internalized cells after 24 hours (p<0.05) for both serotypes. After a two-hour incubation and a water rinse to remove unattached cells, reductions of approximately 2 and 0.8 log CFU of Newport and Poona cells/fruit, respectively, were observed. Attachment values were significantly different between serotypes (p<0.05). Quantification of bacteria present after a two-hour incubation and surface sterilization yielded approximately 2 log CFU/fruit for both serotypes, indicating Salmonella was able to penetrate the cucumber fruit exocarp and escape application of sanitizing treatment. More work is needed to determine route of entry, however the high degree of internalization reveals a significant food safety risk, as internalized pathogens are protected during subsequent transport and storage.