Colonization of Arabidopsis thaliana with Salmonella enterica and Enterohemorrhagic Escherichia coli O157:H7 and Competition by Enterobacter asburiae

Abstract

Enteric pathogens, such as Salmonella enterica and Escherichia coli O157:H7, have been shown to contaminate fresh produce. Under appropriate conditions, these bacteria will grow on and invade the plant tissue. We have developed Arabidopsis thaliana (thale cress) as a model system with the intention of studying plant responses to human pathogens. Under sterile conditions and at 100% humidity, S. enterica serovar Newport and E. coli O157:H7 grew to 10 9 CFU g ؊1 on A. thaliana roots and to 2 ؋ 10 7 CFU g ؊1 on shoots. Furthermore, root inoculation led to contamination of the entire plant, indicating that the pathogens are capable of moving on or within the plant in the absence of competition. Inoculation with green fluorescent protein-labeled S. enterica and E. coli O157:H7 showed invasion of the roots at lateral root junctions. Movement was eliminated and invasion decreased when nonmotile mutants of S. enterica were used. Survival of S. enterica serovar Newport and E. coli O157:H7 on soil-grown plants declined as the plants matured, but both pathogens were detectable for at least 21 days. Survival of the pathogen was reduced in unautoclaved soil and amended soil, suggesting competition from indigenous epiphytes from the soil. Enterobacter asburiae was isolated from soil-grown A. thaliana and shown to be effective at suppressing epiphytic growth of both pathogens under gnotobiotic conditions. Seed and chaff harvested from contaminated plants were occasionally contaminated. The rate of recovery of S. enterica and E. coli O157:H7 from seed varied from undetectable to 19% of the seed pools tested, depending on the method of inoculation. Seed contamination by these pathogens was undetectable in the presence of the competitor, Enterobacter asburiae. Sampling of 74 pools of chaff indicated a strong correlation between contamination of the chaff and seed (P ‫؍‬ 0.025). This suggested that contamination of the seed occurred directly from contaminated chaff or by invasion of the flower or silique. However, contaminated seeds were not sanitized by extensive washing and chlorine treatment, indicating that some of the bacteria reside in a protected niche on the seed surface or under the seed coat.

Differential interaction of Salmonella enterica serovars with lettuce cultivars and plant-microbe factors influencing the colonization efficiency

“…This research presents plant and microbial factors that influence the (endophytic) colonization efficiency of human pathogenic S. enterica serovars in association with lettuce. In this study S. enterica serovars colonized lettuce endophytically as was shown before (Cooley et al, 2003;Klerks et al, 2007). However, we showed for the first time a differential interaction between S. enterica serovars and plant cultivars, besides serovar-dependent host susceptibility with respect to the degree of colonization.…”

“…Since no bacteria are present that colonize the roots in an axenic system, the roots are easily accessible for the inoculated Salmonellae. The rhizosphere of roots grown in soil however are known to contain many different soil bacteria that colonize the roots already at the sprouting stage (Yang and Crowley, 2000), herewith protecting the roots with a shield of indigenous soil bacteria (Cooley et al, 2003;Berg et al, 2005). Moreover, the steep gradient of rootexuded compounds in soil is continually modulated by the indigenous soil microflora.…”

“…Moreover, the steep gradient of rootexuded compounds in soil is continually modulated by the indigenous soil microflora. The Salmonellae have to compete with these environmental bacteria to establish in the rhizosphere (Gagliardi and Karns, 2000;Cooley et al, 2003;Ibekwe et al, 2006), which leads to lower S. enterica cell densities close to the roots. This suggests that the colonization efficiency is strongly dependent on accessibility of the plant roots to the S. enterica serovars.…”

Wastewater reuse in irrigation: A microbiological perspective on implications in soil fertility and human and environmental health

“…Moreover, because plants are privileged hosts for these bacteria, if supplied in irrigation water, they can proliferate and survive for longer periods when plants are present than in their absence (Gagliardi and Karns, 2002;Ibekwe et al, 2004;Tyler and Triplett, 2008). Another risk is that bacteria transmitted by this pathway may be inherited through plant seeds, as has been described for some endophytic bacteria (Burnett et al, 2000;Cooley et al, 2003;Guo et al, 2001;Rosenblueth and Martínez-Romero, 2006;Tyler and Triplett, 2008;Wang et al, 2006). The risks associated with the dis-semination of human pathogens due to the use of treated wastewater for irrigation can be considered low (Bichai et al, 2012;Shuval and Fattal, 2003).…”

Characterization of Bacteria Isolated from the Saffron (Crocus sativus L.) Rhizosphere

“…E. coli (BA16) was isolated from the soil of the 2 years-old saffron plantations (Table 1). These bacteria have not yet been proposed as plant pathogens, and were characterized as providing suitable conditions for plant growth by root colonization (Cooley et al 2003).…”