Microbial Contamination of Organically and Conventionally Produced Fresh Vegetable Salads and Herbs from Retail Markets in Southwest Germany

Abstract

A total of 189 samples of fresh products (leafy salads, ready-to-eat mixed salads, and fresh herbs) bought in retail in Southwest Germany were investigated for their microbiological quality and the presence of pathogenic bacteria, including Salmonella spp., Listeria monocytogenes, and presumptive Bacillus cereus. Total aerobic mesophilic plate counts (TAC) ranged from 5.5 to 9.6 log colony-forming units (CFUs) per gram. Enterobacteria and pseudomonads were the predominant microorganisms and were detected in all samples with counts between 5.0 and 9.2 log CFU/g. Strains of Escherichia coli were detected in 9 salad (7.9%) and 25 herb samples (33.3%). Significant differences in bacterial counts were found between conventionally and organically-grown products: in herbs the counts of moulds were significantly higher in organically-grown products, while E. coli was only detected in conventionally-grown products. In conventionally-grown salad samples, yeast counts were significantly higher. Salmonella Enteritidis was only detected in two conventionally- and in one organically-produced salad samples (2.6%). No coagulase-positive staphylococci were detected in fresh salads as well as in herbs. High levels of B. cereus sensu lato (≥3 log CFU/g) were detected in 19 vegetable salads (16.7%) and even in 55 samples of fresh herbs (73.3%). Listeria monocytogenes could not be detected in fresh herbs; however, three L. monocytogenes strains were isolated from two conventionally-produced salad samples and belonged to PCR serogroup IIa. Although our results indicate a high microbial load in fresh salads and herbs in Southwest Germany in 2015, the incidences of human pathogenic bacteria, that is, L. monocytogenes, Salmonella spp., and coagulase-positive staphylococci strains, were low.

Keywords: fresh produce; human pathogenic bacteria; microbial quality and safety; ready-to-eat.