May 19, 2015
Why do an enrichment?
- Food Safety
By Lauren Bambusch
No matter what protocol you’re using, when it comes to food safety, enrichments are king. Every approved testing method for foodborne pathogens requires some iteration of an enrichment. So what is that? Why is it required? Why do different pathogens have different enrichment times? And why do some protocols have more or longer enrichments than others?
What is an Enrichment?
In foodborne pathogen testing, enriching is the process of mixing your food matrix to be tested—the ground beef, the lettuce, the flour—with a growth media specifically designed to encourage bacterial growth. This mixture is often called an enrichment.
Why are Enrichments Required?
This becomes a mathematical thought problem. Let’s say there’s one Listeria cell on an apple. Because bacterial cells are so small, it’s impossible to search the entire surface of the apple for one microscopic cell. You could swab the entire surface of the apple, but that does not ensure that the transfer of the microscopic cell would happen, either from apple to swab or swab to testing method. And what if the Listeria cell is inside the apple? Statistically, the chances of detecting the pathogen are too low for the test to be accurate.
That’s where the enrichment comes in. This allows that one Listeria cell to replicate to a level that the cells are easily detectable in a growth medium.
Why do Different Pathogens have Different Enrichment Times?
Everyone’s different; that also goes for bacterial species. Some are round, some are more oval. Some have thick cells walls while some are thinner. Some even have little propellers called flagella that allow them to swim. And, understandably, some grow faster than others. Each enrichment time is tailored to how fast the organism can grow, given the enrichment media and temperature specified by the protocol. For example, Listeria tends to grow more slowly than E. coli or Salmonella and hence the recommended enrichment times for Listeria tend to be longer.
Why do some Protocols have More or Longer Enrichments than Others?
Depending on the type of testing protocol, you can have multiple types of enrichments for the same sample, and each protocol will have an enrichment procedure that is optimized for the testing method. For example, when testing with traditional plate methods, testing protocols often have a primary enrichment, which allows a wide selection of different bacteria to grow in a less harsh media. This ensures that sick or damaged cells have a change to recover and replicate. The primary enrichment is then followed by a secondary or sometimes called selective enrichment. This enrichment uses a harsher media that will inhibit the growth of organisms that are unlike the target pathogen. This allows the target organism and organisms like it to grow to a higher count within the enrichment, upping the chances of successfully finding the target organism farther down the line.
However, more advanced techniques like real-time PCR do not require the secondary or selective enrichment (and typically even allow a shortening of the primary enrichment) thus saving gobs of time. This is because the technology can seek out the DNA of specific organisms even in a background excess of other bacteria. In this case, the length of the enrichment needs to be just long enough ensure that sick cells recover and have time to replicate to detectable levels.
What questions do you have about enrichments? Email them to me at firstname.lastname@example.org and we will answer them!
Lauren Bambusch is a microbiologist by trade as well as a writer and baker by hobby. She lives in Baltimore with her husband, three cats, a super-sized mutt, and a school of fish, all of whom root for her Alma mater, Michigan State. Go Green!