Persistence of Two Campylobacter jejuni Strains in Soil and on Spinach Plants

There are indications that the more frequent use of untreated organic residues for fertilization results in increased risk of contamination with human pathogens. Here, we evaluate the ability of two different strains of Campylobacter jejuni to persist in manure and soil as well as spread to spinach plants. It was revealed that different strategies for inoculation of C. jejuni contribute to the persistence of the bacterium in soil, roots, and shoots. Upon inoculation of the bacteria into manure prior to soil application, the amount of C. jejuni subsequently recovered in soil was higher than that from treatments involving the addition of C. jejuni cells to the soil after plant emergence. Irrespective of the bacterial inoculation dose and strategy employed, the C. jejuni content in soil remained relatively constant, whereas the majority of C. jejuni cells applied to spinach leaves could be recovered during the whole evaluation period of 21 days. 1. Introduction In contrast to infections by Salmonella spp. and pathogenic Escherichia coli that cause massive foodborne outbreaks, campylobacteriosis is mainly presented as sporadic illness [1]. In view of its sporadic nature combined with an unusual microaerophilic and thermophilic lifestyle, recovery of Campylobacter spp. outside its host is a major challenge, predominantly resulting in unidentified point sources of Campylobacter contamination [2]. One possible means of Campylobacter entry into the human food chain is through application of untreated animal manures and/or biosolids to agricultural crop land. Organic manure is an important source of plant nutrients and organic matter, particularly within organic farming where no mineral fertilizers are allowed. The risk for Campylobacter contamination of crops is highest in cases where the produce is likely to be eaten raw, including crops such as salads, spinach, fruit, and various vegetables. Consistently, Campylobacter spp. has been detected on produce sampled at the marketplace, such as spinach, lettuce, radish, green onion, potatoes, and parsley [3], carrots and cabbage [4], mixed salad vegetables [5], mushrooms [6], and spinach and fenugreek [7]. The extent of Campylobacter spp. survival in manure is affected by the type of animal, their diet stress, and age [8, 9], respectively, as well as manure management and method of application [10, 11]. The majority of studies performed to date indicate that Campylobacter species are not able to persist effectively in solid manure once excreted [12–17]. However, contrasting results have been obtained in the most