Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/29/2003
Publication Date: 8/20/2003
Citation: COOLEY, M.B., MILLER, W.G., MANDRELL, R.E. COLONIZATION OF ARABIDOPSIS THALIANA WITH SALMONELLA ENTERICA AND ENTEROHEMORRHAGIC ESCHERICHIA COLI 157:H7 AND COMPETITIAN BY ENTEROBACTOR ASBURIAE. APPLIED AND ENVIRONMENTAL MICROBIOLOGY. 2003. v 68(8) p.4915-4926 Interpretive Summary: Several epidemics related to sprouts have been traced to bacterial contaminated seed. Human pathogens have been shown to grow on and invade plant tissue. To determine how the plant supports or controls this growth and invasion we have developed a model system using the plant thale cress. We have shown that two pathogens grow on thale cress and invade the root tissue and that this infection leads to infection of the seed once the plant matures. Furthermore, infected seed could not be cleaned, indicating that commercial attempts to sanitize seed may not be effective. Growth of the pathogens on thale cress is sensitive to the presense of competing microorganisms. One competing bacteria was found as a natural inhabitant of thale cress.
Technical 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 100% humidity S. enterica Newport and E. coli O157:H7 grew to 109 CFU g-1 on A. thaliana roots and to 2 X 107 CFU g-1 on shoots. Furthermore, root inoculation leads to contamination of the entire plant, indicating that the pathogens are capable of moving on or within the plant. Inoculation with GFP-labeled S. enterica and E. coli O157:H7 showed invasion of the roots at lateral root junctions. Movement was eliminated and invasion decreased using non-motile mutants of S. enterica. Survival of S. enterica 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 on un-autoclaved soil and amended soil, suggesting potential competition from indigenous epiphytes in 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 dependent on the method of inoculation. Seed contamination by pathogen was undetectable in the presence of the competitor, E. asburiae. Sampling of 74 pools of chaff indicated a strong correlation between the contamination of the chaff and seed (P=0.025). This suggested that contamination of the seed occurs directly from contaminated chaff or by invasion of the flower or silique. However, contaminated seed were not sanitized by extensive washing and chlorine treatment, indicating that some of the bacteria reside in a protected niche on the surface or within the seed.