The U.S. poultry industry places a top priority on food safety. Yet, the dynamic nature of daily processing and the ubiquitous nature of pathogens require a constant vigilance that today cannot be completely ensured through real-time, accurate, and sensitive sensing. As a result, colony culturing (bacterial enrichment) or plate counts remain the preferred laboratory procedures for the detection of viable pathogens (bacterial, viral, parasitic, plus yeast and mold) and microbial toxins in food products.
These traditional culturing methods, while very effective and sensitive if the plating conditions are optimal, have a number of less than desirable drawbacks. The most often cited concern is that even with pathogens that exhibit rapid growth rates, 24 to 48 hours are typically needed to enrich, isolate, and then enumerate those pathogens. In addition, pathogens like Shiga toxin-producing Escherichia coli (STEC) can become injured but not inactivated during poultry processing, thus growing slower or not at all on culture media. Lastly, all detection methods must consider sampling methods as bacteria are not evenly distributed on the product, and growth may also be inhibited by the product surface. Thus, even with all of its challenges and intricacies, the enrichment step remains a critical stage for rapid STEC detection.
Here, a team of researchers at the Georgia Tech Research Institute (GTRI) are examining whether detection times might be reduced. They have developed a new technique that allows for faster bacterial enrichment, which they believe holds promise for improving pathogen prevention and control in large-volume poultry processing samples.
“We began working on new ways to concentrate bacteria. In reality, food producers are interested in detecting any possible STEC bacteria in finished products, so biosensing devices require enrichment or pre-concentration before sensing to compensate for extremely low concentrations, if any, in typical sample volumes relative to the overall processing volumes. The real challenge is quickly detecting the presence or absence of viable pathogenic bacteria in a representative sample,” explains John Pierson, GTRI principal research engineer and project director.
The team has examined several techniques for processing larger sample volumes that have low concentrations of STEC bacteria. One of the approaches currently being explored focuses on rapid initiation of exponential growth rates.
Bacteria initially undergo a lag period during which the apparent metabolic activity is insubstantial, explains Stephanie Richter, GTRI research associate. This stage is often attributed to bacterial acclimation to a new environment.
“If we can find a technical approach for minimizing the lag time before cell division becomes exponential, the period needed to achieve a detectable bacteria concentration will be greatly decreased,” says Richter.
For instance, the doubling time for Salmonella is about 20 minutes, so if every cell grew exponentially, the enrichment step could be complete in less than 2 hours. However, due to nutrient limitations and the secretion of inhibitory metabolites, enrichment takes significantly longer.
Initial Results and Next Steps
“We have some preliminary data that shows we can manipulate the enrichment broth in a novel way to not only reduce the lag time before exponential growth starts, but to subsequently increase the overall growth rate. Our approach is not related to any of our previous fluidics work,” says Pierson.
Specifically, preliminary results with Salmonella enterica serovar Typhimurium indicated that the technology performed significantly better than the control, yielding a 37.6% shorter lag time followed by a 109.6% greater growth rate.
“We believe our advanced enrichment technique moves us significantly closer to real-time detection, and it may facilitate the use of either plate counts or biosensors for rapid detection that is crucial for effective food safety standards and microbiological quality control,” says Pierson.
Moving forward, the team plans on demonstrating the technique with low concentrations of bacteria.
“We now have a more efficient approach for enriching low concentrations in triplicate across seven dilutions. The data continues to be promising,” says Richter.