Rhizobacteria impact colonization of Listeria monocytogenes on Arabidopsis thaliana roots.

In spite of its relevance as a food-borne pathogen, we have limited knowledge about L. monocytogenes in the environment. L. monocytogenes' outbreaks have been linked to fruits and vegetables; thus, a better understanding of the factors influencing its ability to colonize plants is important. We tested how environmental factors and other soil- and plant-associated bacteria influenced L. monocytogenes' ability to colonize plant roots using Arabidopsis thaliana seedlings in a hydroponic growth system. We determined that the successful root colonization of L. monocytogenes 10403S was modestly but significantly enhanced by the bacterium being pre-grown at higher temperatures, and this effect was independent of the biofilm- and virulence-regulator PrfA. We tested 14 rhizosphere-derived bacteria for their impact on L. monocytogenes 10403S, identifying one that enhanced and ten that inhibited the association of 10403S with plant roots. We also characterized the outcomes of these interactions under both co-inoculation and invasion conditions. We characterized the physical requirements of five of these rhizobacteria to impact L. monocytogenes 10403S' association with roots, visualizing one of these interactions by microscopy. Furthermore, we determined that two rhizobacteria (one an inhibitor, the other an enhancer of 10403S root association) were able to similarly impact ten different L. monocytogenes strains, indicating that the effects of these rhizobacteria on L. monocytogenes are not strain specific. Taken together, our results advance our understanding of the parameters that effect L. monocytogenes plant root colonization, knowledge that may enable us to deter its association with, and thus downstream contamination of, food crops. IMPORTANCEListeria monocytogenes is ubiquitous in the environment, being found in or on soil, water, plants, and wildlife. However, little is known about the requirements for L. monocytogenes' existence in these settings. Recent L. monocytogenes outbreaks have been associated with contaminated produce; thus, we used a plant-colonization model to investigate factors that alter L. monocytogenes' ability to colonize plant roots. We show that L. monocytogenes' colonization of roots was enhanced when grown at higher temperatures prior to inoculation, but did not require a known regulator of virulence and biofilm formation. Additionally, we identified several rhizobacteria that altered the ability of eleven different strains of L. monocytogenes to colonize plant roots. Understanding the factors that impact L. monocytogenes' physiology and growth will be crucial for finding mechanisms (whether chemical or microbial) that enable its removal from plant surfaces to reduce L. monocytogenes' contamination of produce and eliminate food-borne illness.