A.V. Sturz, J. Nowak Applied Soil Ecology 15 2000 183–190 185
organizing forces that govern such communities need to be determined.
3. A strategy for creating stable microbial communities
When considering the anthropogenic introduction of new-colonists ‘beneficial microorganisms’ into
the root zone — through seed amendments or dur- ing seed-bed preparation — the potential for severe
negative interactions with autochthonous microbial populations should be borne in mind Atlas, 1986.
It is now appreciated that the microbiological pop- ulations of an ecosystem are able to interact with
one another through the production and reception of signalling molecules. Such signalling molecules
can subsequently influence gene expression, and thereby bacterial phenotype Salmond et al., 1995;
Albus et al., 1997; Surette and Bassler, 1998. ‘Quo- rum sensing’ describes one such signalling system,
whereby responses to bacterial population density are modulated through the accumulation of extracellular
signalling molecules, that can regulate an assorted range of metabolic processes Swift et al., 1996.
Similarly, the relationship between host and bacte- rial endophyte is not static. Communities of bacterial
endophytes may not only be host specific, but also plant tissue sensitive, reacting and adapting at certain
tissue sites and among certain tissue types within the host plant as it develops Sturz et al., 1999. The dy-
namic nature of bacterial phenotype expression, in this case antibiotic secretion, may be being governed by a
phenomenon analogous to ‘quorum sensing’ — which can also be influenced by environmental factors such
as oxygen concentration Sitnikov et al., 1995.
While positive interactions commensalism, mutu- alism, and synergism may enable some populations
to function as a community within a habitat Rayner, 1997, negative interactions may result in the exclu-
sion of microbial colonists from an established com- munity, or in a range of negative allelopathic events
Sturz and Christie, 1995, 1996.
In mature communities, positive interactions among autochthonous populations are usually better devel-
oped than in newly established communities. The successful establishment of beneficial organisms will
be influenced, to varying degrees, by the network of connections among species in a mature estab-
lished ecosystem. In essence, the establishment of the ‘new-colonist’ population can be prejudiced by
the dynamics of the ecosystem it is trying to invade, through a form of defensive mutualism Clay, 1988.
Thus, one component of an approach designed to favour the successful assimilation of selected organ-
isms into a rhizosphere, would be to introduce the ben- eficial microorganisms at the earliest possible stage
in the metapopulation continuum Levins, 1976; Hast- ings and Harrison, 1994. As endophytic bacteria have
been recovered from the ovules, seeds and tubers of a variety of plants Mundt and Hinkle, 1976; Holland
and Polacco, 1994, the creation of selected commu- nities of beneficial bacterial endophytes within these
germinal structures would form one of the earliest pio- neer colonization events possible. Initially, such com-
munities may be relatively stable and could compete with native soil bacteria once plant propagules had
been planted.
4. Engineering microbial communities
