Characterization of the cultivable microbial community in a spinach-processing plant using MALDI-TOF MS.

Abstract A better and regular control of the production chain of fresh fruits and vegetables is necessary, because a contamination of the product by human- and phyto-pathogenic microorganisms may result in high losses during storage and poses a threat to human health. Therefore, detailed knowledge about the occurrence and the diversity of microorganisms within single processing steps is required to allow target-oriented produce safety control. Recently, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was successfully used to identify bacterial colonies. Bacteria can be identified with high accuracy by comparing them with generated spectra of a reference database. In this study, spinach and wash water samples were taken of the complete process line of a spinach-washing plant. Bacteria in the samples were grown on plate-count, Arcobacter selective, marine and blood agar. In total, 451 colonies were evaluated by MALDI-TOF MS, 16S rRNA gene sequence and phylogenetic analysis. 50% of the detected species belonged to the class of Gammaproteobacteria. Firmicutes were present with 22%. Mostly, the detected species showed 16S rRNA gene sequence dissimilarities larger than 1% to known reference species and, hence, could not be assigned to a distinct species. However, many isolated species belonged to genera which contain pathogenic or opportunistic pathogenic bacteria. In addition, the bacterial diversity on the spinach surface increased after the first washing step indicating a process-borne contamination of the spinach.