Colonization of plants by human pathogenic bacteria in the course of organic vegetable production

Abstract

In recent years, increasing numbers of outbreaks caused by the consumption of vegetables contaminated with human pathogenic bacteria were reported. The application of organic fertilizers during vegetable production is one of the possible reasons for contamination with those pathogens. In this study laboratory experiments in axenic and soil systems following common practices in organic farming were conducted to identify the minimal dose needed for bacterial colonization of plants and to identify possible factors like bacterial species or serovariation, plant species or organic fertilizer types used, influencing the success of plant colonization by human pathogenic bacteria. Spinach and corn salad were chosen as model plants and were inoculated with different concentrations of Salmonella enterica sv. Weltevreden, Listeria monocytogenes sv. 4b and EGD-E sv. 1/2a either directly (axenic system) or via agricultural soil amended with spiked organic fertilizers (soil system). In addition to PCR- and culture-based detection methods, fluorescence in situ hybridization (FISH) was applied in order to localize bacteria on or in plant tissues. Our results demonstrate that shoots were colonized by the pathogenic bacteria at inoculation doses as low as 4 × 10 CFU/ml in the axenic system or 4 × 105 CFU/g in the soil system. In addition, plant species dependent effects were observed. Spinach was colonized more often and at lower inoculation doses compared to corn salad. Differential colonization sites on roots, depending on the plant species could be detected using FISH-CLSM analysis. Furthermore, the transfer of pathogenic bacteria to plants via organic fertilizers was observed more often and at lower initial inoculation doses when fertilization was performed with inoculated slurry compared to inoculated manure. Finally, it could be shown that by introducing a simple washing step, the bacterial contamination was reduced in most cases or even was removed completely in some cases.

Distribution of phosphatase activity and various bacterial phyla in the rhizosphere of Hordeum vulgare L. depending on P availability

“…The labeling was conducted in a 2 ml tube according to Hofmann et al (2014) with modifications (probes and hybridization conditions are listed in Table 1). To avoid loss of rhizosphere soil adhering to the root, the samples were treated very carefully and all steps were subsequently performed in the same tube without touching the sample with the tip of the pipette.…”

Human pathogens in plant biofilms: Formation, physiology, and detection

“…Both animals and the environment in which agricultural plants are grown serve as reservoirs for these pathogens (Based on Barak and Schroeder, 2012). needed to improve food safety. For example, in the work of Hoffman et al (2014), studies were conducted to investigate the minimal number of Salmonella enterica sv. Weltevreden, Listeria monocytogenes EGD-E sv.…”

“…Contamination of plants by human pathogens can occur at the farm level and a greater understanding of key contamination routes is needed to improve food safety. For example, in the work of Hoffman et al (), studies were conducted to investigate the minimal number of Salmonella enterica sv. Weltevreden, Listeria monocytogenes EGD‐E sv.…”

Involvement of the Rcs regulon in the persistence of SalmonellaTyphimurium in tomatoes

“…However, it is also clear that the ability of opportunistic human pathogens (such as non-typhoidal strains of Salmonella enterica) to persist outside of their animal hosts, and to colonize and multiply within plants, is an important factor leading to the outbreaks of human salmonellosis from the consumption of fruits and vegetables Martinez-Vaz et al, 2014). Salmonella can persist in manure-amended soils for over 7 months, and the pathogen can be detected on aboveground parts of leafy greens and root vegetables grown in these amended soils for 2-3 months (Islam et al, 2004;Hofmann et al, 2014). These field reports are consistent with laboratory studies that demonstrated that plant-associated Salmonella and pathogenic E. coli ingested by vertebrate and invertebrate herbivores were shed by the animals in their feces and were capable of re-colonizing plants once excreted into the environment (Semenov et al, 2010).…”