Agriculture Reference
In-Depth Information
been described essentially as a mechanism that coordinates and reinforces community
behaviors in many bacterial species (An et al., 2006). These signaling molecules, which
include
N
-acyl homoserine lactone, peptides and a modified ribose, accumulate in the
surrounding environment until a concentration threshold of such compounds is attained,
which then instigates the expression of certain genes. A study involving spatial analysis of
bacterial “patches” or population concentrations in the soil matrix concluded that the
observed close proximity of cells to each other suggests concerted activity, that such
clustering would better explain the reported time course of the nitrification process and
further suggested the possibility of quorum sensing in such a case (Nunan et al., 2003). It has
been increasingly recognized that quorum sensing is a far more complex process than solely
for census taking (Dunn and Stabb, 2007). Other than its mention in the spatial analysis paper
cited above, this aspect of bacterial ecology has not to our knowledge been considered in
relation to soil aggregation. Many soil and rhizosphere bacteria have been shown to secrete
quorum-sensing molecules. Among these are strains belonging to
Agrobacterium
(Fuqua and
Winans, 1994; Zhang et al., 1993),
Burkholderia
(Lewenza et al., 1999),
Chromobacterium
(Chernin et al., 1998; McClean et al., 1997),
Pseudomonas
(Gray et al., 1994; Wood and
Pierson III, 1996; Wood et al., 1997),
Ralstonia
(Flavier et al., 1997),
Rhizobium
species (and
other related genera involved in nitrogen fixation symbioses with legumes (Pierson et al.,
1998, Cha et al., 1998),
Rhodobacte
r (Schaefer et al., 2002) and
Serratia
species (Christensen
et al., 2003). The behaviors controlled or affected by quorum sensing mechanisms include
biofilm formation (Davies et al., 1998; Huber et al., 2001), expression of pathogenicity genes
(Pierson et al., 1998; Andersson et al.,
2000) and as in the original study on this phenomenon,
bioluminescence (Fuqua et al., 1994). A newer alternative perspective postulates that
individual cells practice quorum sensing to detect the degree of diffusion in their
surroundings (Redfield, 2002) and not necessarily for any cooperative purposes. However,
regardless of either interpretation, the net effect is that members of the same species appear to
act as a single organism and threshold population levels must be reached (Bassler and Miller,
2006). This is interesting in light of the existence of structured biofilm networks described
above. Such networks may be an answer to the question of how signaling is transmitted or
signaling molecules propagated among communities of bacteria
in situ
(Battin et al., 2007)
.
The theory that quorum sensing is actually done to assess the degree of diffusion in the
surroundings of bacterial cells and the existence of structured biofilms would be compatible
in the context of soil aggregate formation by bacteria. Biofilm formation, however structured,
is affected both by quorum sensing and by the nutritional status of the bacteria (De Beer and
Stoodley, 2006). Bacterial biofilm formation, which is preceded by EPS production (a stage
of the process of soil aggregation involving the bacterial component), and shown to be under
the control of cell-cell communication or quorum sensing, thus would require threshold
population levels of the species involved.
Anaerobic bacteria have seldom to our knowledge been studied in relation to soil
aggregation. Anaerobic microbial processes in soil have been discussed and the anaerobic
conditions occurring in soil aggregates have been documented (Tiedje et al., 1984), but to our
knowledge few studies have subsequently investigated the relationship between anaerobic
bacteria and water stable aggregates (Andrade et al., 1998; Bethlenflavay et al., 1999).
Interestingly, these two studies found correlations between population levels, culturable
anaerobic bacteria, and water stable aggregates. Soil anaerobic conditions are supposed to be
periodic or transient in occurrence and this along with over all low metabolic activity under
