Evidence for internalization of Escherichia coli into the aerial parts of maize via the root system.

Escherichia coli introduced into the hydroponic growing medium of maize plants was detected 48 h later in the shoot. Decapitation of root tips or severing of the plant root system at the root-shoot junction enhanced bacterial internalization. The density of the bacteria in shoots of plants with damaged roots or removed root systems was 27.8 and 23.9 times higher than that in plants with intact roots, respectively. The concentration of viable cells in the hydroponic solution decreased over time from 9.3 × 106 CFU/ml at the time of inoculation to 8.5 × 101 CFU/ml 4 days thereafter. The number of E. coli cells associated with the roots also decreased with time, but a significant decline appeared only at 4 days postinoculation. At the time of sampling for E. coli presence in the shoot, 102 CFU/ml was present in the nutrient solution and 8 × 103 CFU/g was associated with the roots. The present study is the first to demonstrate internalization of E. coli via the root in a monocotyledonous plant.