Modifications of the U.S. food and drug administration validated method for detection of Cyclospora cayetanensis oocysts in prepared dishes: Mexican-style salsas and guacamole

Abstract

Although multiple outbreak clusters of Cyclospora cayetanensis have been traced back to consumption of dishes in Mexican-style restaurants, the FDA Bacteriological Analytical Manual (BAM) does not currently provide methods to detect C. cayetanensis in dishes that contain multiple produce ingredients, such as salsas and guacamole. These complex food matrices also may contain high levels of fats, which can interfere with the detection. Several modifications to the BAM Chapter 19b method (washing produce, DNA extraction, and a TaqMan real-time PCR assay targeting the 18S rRNA gene of C. cayetanensis) were assessed with the goal to detect as few as 5 oocysts of C. cayetanensis in 25 g samples of commercial salsa/pico de gallo, guacamole, and salsa verde. Both freshly prepared and frozen versions of these foods were seeded with 5, 10 and 200 oocysts. For salsa samples, using a gentler washing step than recommended by BAM, we achieved detection of 5 oocysts in the samples (81.8%, n = 11). Increasing the amount of Alconox® in the wash solution to 1%, rather than the 0.1% used in BAM, and adjusting the DNA extraction protocol to process large wash pellets, enabled detection of 5 oocysts in guacamole. To reach the desired level of detection in salsa verde, two types of modifications were necessary: gentler washing and DNA extraction modifications. The use of these same method modifications on previously frozen food samples, provided levels of detection similar to those achieved with fresh dishes. Our modifications enabled robust and reproducible detection of C. cayetanensis in multi-ingredient Mexican dishes, detecting as few as 5 oocysts in 25 g samples. Validating and deploying effective methods to detect C. cayetanensis in high risk fresh produce and prepared dishes are critically important for prevalence studies and outbreak investigations of this parasite.