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Stories from the Field

Skip Gossack, Colorado Public Health Laboratorian, Examines Listeria Culture Plates, October 2011
A Colorado Cantaloupe Saga

​Cantaloupe is valued for its sweet, juicy flesh, an excellent source of vitamin C. In the state of Colorado, the melon is big business, with more than 2,000 acres under cantaloupe cultivation and an annual crop worth nearly $8 million.

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Hugh Maguire, PhD, a senior microbiologist with the Colorado Department of Public Health & Environment (CDPHE), said, “People in this area look forward to the end of summer when local cantaloupe becomes available.”

Late last summer, Maguire learned that cantaloupe can also be a vehicle for one of the nastier foodborne bacteria: Listeria monocytogenes. In fact, cantaloupe and L. monocytogenes are a particularly bad combination, as the bacteria thrive in the refrigerator, where the melon is likely to be stored, and the simple act of slicing the fruit is sufficient to spread the bugs from rind to flesh.

In a typical year, the DPHE laboratory receives two patient specimens per month that test positive for L. monocytogenes.  So when the laboratory confirmed nine cases of L. monocytogenes infection between August 28 and September 2, red flags went up.

Using two DNA ‘fingerprinting’ techniques—PFGE and MLVA—Colorado scientists quickly determined that the patients could be assigned to one of three unique clusters, each with a Listeria bug with a different and distinct genetic ‘fingerprint.’ This information was crucial, as each cluster represented a potential outbreak source.

State epidemiologists interviewed cluster patients to look for common food exposures.  On September 7, they contacted the laboratory and, said Maguire, “asked if we could purchase cantaloupe from retail sites and test it for the presence of L. monocytogenes. Would we be able to find it if it were there? And the answer was, ‘Yes, we’ll get on it.’”

​​Would we be able to find it if it were there? And the answer was, ‘Yes, we’ll get on it.'



Hugh Maguire, PhD

Senior Microbiologist, Colorado Department of Public Health & Environment (CDPHE)

At the same time, investigators had a fortuitous break: one of the patients had purchased two cantaloupes, one stored as cut chunks in the refrigerator and the other still uncut. The scientists prepared to test these immediately, while two staff members went grocery shopping. The pair went to three stores—one outlet of store A and two outlets of store B—returning with 15 Colorado-grown cantaloupes.

In the meantime, the patient count continued to rise. Speed was all important.

“We fast-tracked everything to the limits of the protocol,” said Maguire. “If the protocol said incubate for four to 16 hours, we incubated for four hours on the dot.”

The scientists performed PFGE analysis in parallel with MLVA, both running at max speed.

They bagged each cantaloupe with an enrichment broth and let it incubate. Then they plated the broth on agar growth media and waited for dark blue dots to appear, a tell-tale sign of Listeria.

Working over the weekend, food microbiologist Skip Gossack recovered and confirmed those Listeria colonies, while molecular biologist Justin Nucci, MPH, initiated MLVA.

The results? All five melons from store A tested negative for L. monocytogenes. All ten from store B tested positive. This information alone, said Maguire, exonerated some cantaloupe farms on the list of suspected sources.

Even more importantly, all of the distinct Listeria fingerprints associated with patient isolates were represented in the isolates from store B cantaloupes. Maguire said, “This eliminated concerns that there might be an alternate source of Listeria. Everything we were seeing in the patients, we were seeing in the cantaloupe.”

The PFGE data, coming a day or so later, was “100% concordant” with MLVA data.

As the link to cantaloupe developed, regulators were already busy with traceback investigations. Ultimately, they identified one farm as the sole source. On September 14, the farm issued a voluntary cantaloupe recall.

However, since L. monocytogenes can incubate for up to 70 days before onset of illness, cases continued to mount.  By September 29, 84 cases from 19 states were reported to the CDC, among them, 15 deaths.

The good news is that the contamination did not occur three years earlier, before the CDPHE laboratory had the benefit of cooperative agreement funding from the US Food and Drug Administration and Food Emergency Response Network. Maguire said, “Without this support, the instrumentation, training and collaborative relationships [critical to breaking the investigation] would not have existed, because we wouldn’t have had the money to put them in place.”

Instead, with the infrastructure up and running, the laboratory moved quickly. Maguire said it met its major challenge:  “how to identify a dangerous product but not impact the entire Colorado cantaloupe industry.”

Colorado Department of Public Health & Environment, Laboratory Division Scientific Team Responsibl​e for Testing During the List
Colorado Department of Public Health & Environment, Laboratory Division Scientific Team Responsibl​e for Testing During the Listeria Outbreak, October 2011. (Left to right) Garth Summers (temporary​-Molecular lab), Kristin Mayo (lab technician​-Molecular lab), Skip Gossack (scientist​-Environme​ntal Microbiolo​gy lab), Margot Stuchin (APHL fellow-Mol​ecular lab) and Justin Nucci (scientist​-Molecular lab).