Simple and rapid method for the detection of Filobasidiella neoformans in a probiotic dairy product by using loop-mediated isothermal amplification

Yeast contamination is a serious problem in the food industry and a major cause of food spoilage. Several yeasts, such as Filobasidiella neoformans, which cause cryptococcosis in humans, are also opportunistic pathogens, so a simple and rapid method for monitoring yeast contamination in food is essential. Here, we developed a simple and rapid method that utilizes loop-mediated isothermal amplification (LAMP) for the detection of F. neoformans. A set of five specific LAMP primers was designed that targeted the 5.8S–26S rDNA internal transcribed spacer 2 region of F. neoformans, and the primer setʹs specificity was confirmed. In a pure culture of F. neoformans, the LAMP assay had a lower sensitivity threshold of 102 cells/mL at a runtime of 60 min. In a probiotic dairy product artificially contaminated with F. neoformans, the LAMP assay also had a lower sensitivity threshold of 102 cells/mL, which was comparable to the sensitivity of a quantitative PCR (qPCR) assay. We also developed a simple two-step method for the extraction of DNA from a probiotic dairy product that can be performed within 15 min. This method involves initial protease treatment of the test sample at 45 °C for 3 min followed by boiling at 100 °C for 5 min under alkaline conditions. In a probiotic dairy product artificially contaminated with F. neoformans, analysis by means of our novel DNA extraction method followed by LAMP with our specific primer set had a lower sensitivity threshold of 103 cells/mL at a runtime of 60 min. In contrast, use of our novel method of DNA extraction followed by qPCR assay had a lower sensitivity threshold of only 105 cells/mL at a runtime of 3 to 4 h. Therefore, unlike the PCR assay, our LAMP assay can be used to quickly evaluate yeast contamination and is sensitive even for crude samples containing bacteria or background impurities. Our study provides a powerful tool for the primary screening of large numbers of food samples for yeast contamination.