Development of multiplex real-time PCR with Internal amplification control for simultaneous detection of Salmonella and Cronobacter in powdered infant formula

Contamination of powdered infant formula (PIF) by the bacteria Cronobacter spp. and Salmonella enterica was deemed a matter of great concern by the World Health Organization and the Food and Agriculture Organization of the United Nations in 2004. Therefore, we developed a rapid and sensitive multiplex real-time PCR assay for the simultaneous detection of Cronobacter and Salmonella in PIF. In addition, an internal amplification control (IAC) was also included for exclusion of false negative results in this study. The quantitative detection range for pure cultures in this optimized multiplex real-time PCR assay was 103 to 108 CFU/ml for both Salmonella and Cronobacter. When our established multiplex real-time PCR system was applied to artificially contaminated PIF, the detection limit was 103 CFU/ml for Salmonella and Cronobacter without enrichment. The commercial PIF was then inoculated with Salmonella and Cronobacter at 10, 1 and 0.1 CFU per gram of formula and the single enrichment broth samples were analyzed by multiplex real-time PCR after enrichment for 9, 12, and 24 h. At 12 h post-enrichment, we could detect Salmonella and Cronobacter at initial inoculation levels of approximately 0.1 CFU/g in PIF. Additionally, stable fluorescent IAC signals could be assessed between 29 and 34 cycles of PCR amplification. Results from this study showed that the multiplex real-time PCR assay is an effective method for the rapid and simultaneous detection and quantification of Cronobacter and Salmonella in PIF.