Detecting dormant pathogens

Salmonella and Shiga toxigenic E. coli (STEC) are two pathogens that can go into a dormant state where they remain virulent but can’t be detected through traditional culturing methods.

This dormancy state is known as ‘viable but non-culturable’ or VBNC.

Canadian researchers are now looking for a quick, easy-to-use and economical test to detect VBNC on fresh and processed produce that can be performed in the field and doesn’t need cumbersome equipment.

“VBNC is a relatively new concept to the fresh produce industry,” explained Dr Xiaonan Lu, an associate professor of food science at the University of British Columbia.

Following the research, growers and processors may need to rethink the types and levels of antimicrobials they use to treat water to account for VBNC pathogens, Lu said.

The two-year project — Detection, validation and assessment of risks implied by the viable but non-culturable state of enteric bacterial pathogens in fresh produce — has three pillars.

First, the researchers plan to develop a new molecular detection method for VBNC that is user friendly. “The current detection methods for VBNC bacteria have some limitations,” Lu said. “During the past six months, after some optimisation and after a lot of hard work, we’ve already finished proof of concept that the new detection method is working.” The researchers still have a few parameters of the new assay to fine-tune, but Lu said they expect to have the work completed by this fall.

During the second part, the researchers plan to test the new detection method in lettuce fields at the Summerland Station. They also plan to spike lettuce, tomatoes and spinach with the VBNC pathogens to ensure the assay works with plants. Their earlier work was done in vitro in the laboratory.

“We want to know how robust the test is,” Lu said. Known technically as an isothermal PCR, it uses a warm water bath to keep the bacteria at 63°C as the population is amplified for better detection.

“This means we don’t need a special machine, such as a thermal cycler for the convectional PCR, so this method is very suitable for in-field studies. You don’t need to carry a very heavy thermal cycler to go to the field,” he said. The entire test can be completed in less than an hour, speeding decision-making for growers and processors. “The easier the technique and the quicker the technique, the better,” Lu said.

In addition, the researchers plan to survey lettuce fields to determine the survival of VBNC both in the field and during simulated processing. “We’re trying to do the simulated processing and prepare the fresh produce on different cutting boards and solid surfaces so we can see the (VBNC) survival,” he said.

The third pillar will be the development of a risk assessment of VBNC using data from the field trials and simulated processing.