Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

14.5 Physiological Characterization of Rhizobacteriaj265
presence of toxic metal concentrations may be more specifically related to climate,
geographic location and soil management or contamination [65]. In such environments,
most microorganisms have adapted through their physiological abilities to
resist the stress conditions. For example, Bacillus, as one of the oligotrophic groups
of soil bacteria, have shown relatively high growth rates in culture even at low
concentrations of energy-yielding substrates (e.g. carbon and nitrogen) and have
the ability to survive in nutrient-deficient soils. Arthrobacter are also capable of
surviving in carbon- and nitrogen-deficient soils by following energy-efficient metabolic
pathways [91]. Therefore, Arthrobacter are referred to as oligotrophs [92]. The
oligotrophs can reproduce in the presence of minimum concentrations of organic
matter, making them suitable candidates for plant root colonization in arid saline
soils [93].
Nutrients, in general, are limited in soil and bacteria, in response, reproduce
slowly and may even remain dormant for long periods. Some of these microorganisms
display considerable resistance to starvation [91]. A strong relationship between
cellular adaptation of Arthrobacter globiformis to the conditions of chemostat growth
and survival of starved suspensions for up to 56 days was reported [94]. A greater
starvation resistance of Arthrobacter than Pseudomonas and Bacillus in a carbon-free
medium was also reported in a number of studies [91,94,95].
Endogenous metabolism is another important pathway for microbial survival in
nature, and Arthrobacter may possibly use this mechanism to reduce metabolic rates
to enhance their survival under starvation [96]. Arthrobacters were also considered to
be the prominent members of the autochthonous microflora of soil that are able to
maintain population levels over long periods in nutrient-limiting environments [97].
By exhibiting a wide range of metabolic activities, they are able to utilize various lowmolecular-weight
organic compounds and some more complex compounds as
carbon and energy sources. In culture, they display a wide nutritional diversity and
can even utilize aromatic compounds to survive under nutrient deficiencies. In our
previous studies, some of the salt-tolerant bacterial strains such as A. globiformis
ArG1 and A. tumescens ArT16 isolated from nutrient-poor saline and arid soils were
able to grow in nitrogen-free medium and were considered as oligonitrophiles with
the ability to fix organic nitrogen (Figure 14.2).
A wide variety of free-living heterotrophic bacteria are known to utilize ammonium
nitrogen as the preferred nitrogen source. Under conditions of limited ammonium
nitrogen, many such bacteria are able to increase their affinity for ammonium
nitrogen through depression of the assimilatory system in which the task of ammonium-nitrogen
assimilation is performed by glutamine synthetase [98]. Among the
bacteria capable of nitrogen assimilation, glutamine synthetase is generally the catalyst
for assimilating ammonium nitrogen into organic compounds resulting from fixation
of atmospheric nitrogen [99]. The troposphere serves as a vast reservoir of nitrogen
(78%) that can be utilized when the preferred form of nitrogen in soil becomes limited,
so that oligonitrophiles, once adapted, can easily incorporate nitrogen through biological
nitrogen fixation. Adding carbon-enriched materials in excess could impose
nitrogen scarcity without removing the nitrogen source. Lactate, malate and succinate
are all labile carbon sources for microorganisms to enhance the nitrogenase activity.