Quantification of low and high levels of Salmonella enterica serovar Typhimurium on leaves

Precise and rapid quantification of low levels of pathogens associated with fresh produce may be particularly challenging and yet evermore necessary to guarantee microbial safety of the produce and to carry out research on the subject of persistence of the pathogens. Here, microbiological and molecular based methods were examined for their ability to precisely quantify different amounts of Salmonella enterica serovar Typhimurium artificially inoculated on parsley leaves. Recovery of S. Typhimurium from parsley by mechanical detachment using stomacher, mortar and pestle, vortex, sonicator or homogenizer followed by plating resulted in underestimation with less than 1% recovery when leaves were inoculated with 3.5-6.5 log CFU/g. Lower levels were undetectable by most assayed methods, and only recovery with mortar and pestle or adding of enrichment step resulted in partial detection of 300 CFU/g. Implementation of PCR based methods with/without pre extraction of the DNA from the contaminated leaves resulted in more accurate values of the pathogen (about 20% of the initial inocula) and as low as 300 CFU/g were detected even without an enrichment step. These methods can be applied to study transfer of Salmonlla from contaminated water or soil to plants using low and more reasonable levels of contamination.