Quantitative assessments of the host range and strain specificity of endophytic colonization by Klebsiella pneumoniae 342

Enteric bacteria, particularly Klebsiella, are common endophytes of plants. Endophytic colonization is important as these bacteria may be beneficial, either by providing fixed N or growth hormones to the host plant. In this work, we assessed the host range and strain specificity for endophytic colonization with Klebsiella pneumoniae 342 (Kp342) on five host plants. This strain was inoculated onto seedlings of Medicago sativa, Medicago truncatula, Arabidopsis thaliana, Triticum aestivum, and Oryza sativa. The type strain of K. pneumoniae, ATCC13883, was also inoculated on all of these hosts except M. truncatula. Both strains were labeled with GFP. Eight inoculum levels were used from 1 CFU to 10⁷ CFU per plant plus uninoculated controls. Six days after inoculation, the number of cells colonizing the rhizosphere and interior were determined. Inoculation with about one CFU of Kp342 was adequate to obtain high colonization levels on the rhizosphere and roots of all host plants. The type strain could colonize the interior of the host plant but the highest colonization levels were generally 100-fold lower than those obtained from Kp342 and those levels required at least 1000 cells in the inoculum. Thus, Kp342 was a more efficient colonizer of the plant apoplast. In addition, the monocots inoculated in this work were colonized endophytically in much higher numbers than were the dicots. Cells of Kp342 congregate at lateral root junctions suggesting the cells enter the plant through cracks created by lateral root extensions. The strain and host effects observed here suggest that endophytic colonization is an active process controlled by genetic determinants from both partners.