Bacterial colonization of same-age quailbush roots was examined using two sets of FISH primers, one specific for B. pumilus ES4 and one for the domain Bacteria. Originally, tailings had very limited populations of unidentified bacteria and most root surfaces were devoid of bacteria. This continued for the 60 days of the experiment ( Fig. 4A). Inoculation with B. pumilus ES4 significantly and consistently changed the visual rhizosphere profile of these roots. Specific detection of microcolonies of B. pumilus ES4 after day 15 was observed exclusively in the root tip and elongation zone area of the root ( Fig. 4B and C) and this was observed up to day 60 of the experiment ( Fig. 4D and E). With time, the population of bacteria of other species also increased and became dominant over the root surfaces ( Fig. 4D–F).