Chromium is a highly toxic metal for all living
organisms. Industrial use of chromium has resulted in
serious widespread pollution. Biological treatment (bioremediation)
has proven to be a cost-effective option for
cleanup of metal-contaminated sites. Several bacteria and
plant species are able to tolerate high levels of chromium
compounds that can be used for cleanup. An experiment
was designed to study the colonization behavior of two
indigenous Cr(VI)-reducing bacterial strain
Pseudomonas aeruginosa
Rb-1 and
Ochrobactrum intermedium
Rb-2 that
were grown in wheat system amended with and without
Cr(VI). Hydroponically grown wheat seedlings were coinoculated
with bacterial cultures to study the root colonization
potential by fluorescent and electron microscopy.
Bacterial inoculation caused significant increase in the
growth of seedlings under Cr(VI) stress. Fluorescent
microscopy showed good colonization potential of both
bacterial strains with roots of inoculated seedlings. Electron
micrographs revealed that Rb-1 tended to accumulate
in the form of clusters, while Rb-2 preferred to be attach in
groups of two or three cells to the root surface of inoculated
seedlings. Chromium stress led to the elongation of bacterial
rods along with uneven cell surface due to wrapping
of cells in mucilaginous material. Cr(VI) stress also
resulted in the damaging of plant root surface. Hence, few
cells of Rb-2 entered the damaged root cortex cells and
appeared as endophytes. Excessive production of fibrillar
material by both bacteria under chromium stress could
clearly be observed. Both strains displayed auxin production
and Cr(VI) reduction ability, showing promise for
bioremediation purposes.
