© Benaki Phytopathological Institute
Swarming motility in plant-associated bacteria
17
Such a mode of motility allows bacteria to
escape local stresses, translocate to a bet-
ter nutritional environment and efficiently
invade host tissue (Harshey, 2003). Thus, we
can infer that swarming motility must be an
important mean for overriding surface im-
pediments and claiming more space in the
bacteria’s natural habitat. However, despite
the benefits, this trait is energy expensive
and is dependent on surface wetness. The
loss of motility may be considered as anoth-
er adaptive strategy of bacteria to cope with
harsh environmental conditions
Swarming regulatory mechanisms and
strategies are diverse among the different
bacteria species and have recently been re-
viewed (Harshey, 2003; Kearns, 2010; Par-
tridge and Harshey, 2013; Harshey and Par-
tridge, 2015). In plant-associated bacteria
the ability to swarm can play an important
role in colonization of interior and exterior
surfaces of plants, in biofilm formation and
in virulence or protective functions (Xu
et al
.,
2012). In this review, we focus on highlight-
ing the recently emerging novel tactics of
plant-associated swarming bacteria to occu-
py, disperse and duel and/or cooperate on
plant surfaces.
Swarming bacteria dispersed over
fungi
While numerous studies have been focused
on identifying bacterial genes involved in
root colonization, limited attention was giv-
en to the involvement of fungi in facilitating
migration of bacteria (Hannula
et al.
, 2011).
Soils are heterogeneous particulate systems
exhibiting chemical heterogeneity. In the
majority of soils, the patchiness and thick-
ness of the liquid films restrict the dispersal
of individual cells or populations. Flagellum-
driven swimming requires bacterial cells to
be fully immersed in liquid while swarming
is restricted to a narrow range of wet condi-
tions (Partridge and Harshey, 2013; Partridge
and Harshey, 2015). Thus, flagellated bacte-
ria would be expected to swim or to swarm
under certain soil saturation levels. Recent
studies demonstrated that displacement
of
Bradyrhizobium japonicum
is achieved in
80% saturated soil (Covelli
et al.
, 2013) while
Pseudomonas fluorescens
strain X (Kremmy-
das
et al.
, 2013) displayed a fast movement
in 50% saturated soil (Fig. 1). Movement of
bacteria in bulk unsaturated soils or rhizo-
sphere, conditions that limit the dispersal of
microbes due to environments of low water
potential or discontinuous water films, may
not be achievable without additional aid.
Several lines of evidence suggested that my-
celia may also provide the appropriate con-
ditions for motile bacteria migration in un-
saturated soils. First, the abundance of fungi
which is ranging from 100 to 700 mgr per
g of soil, the extensive network of growing
mycelia which according to estimates sum
up to 20.000 km per m
3
of soil (Simon
et al.
,
2015). Second, their ability to colonize both
water-saturated and air-filled voids between
soil particles (
Wösten
,
2001). Third, flagellat-
ed bacterial strains could move along the
hyphal surface (Kohlmeier
et al.
, 2005).The
role of fungi in facilitating the dispersal of
bacteria was further substantiated in a re-
cent work, where it was shown that fungal
mycelia facilitate the spread of motile bacte-
Figure 1.
Motility of
Pseudomonas fluorescens
strain X in 100%
and 50% saturated soil.
The soil tablets included in solid (0.5% agar) Nutrient Agar
(NA) medium were inoculated with 3 μl of bacteria in the cen-
ter of each tablet and growth was recorded at the NA–soil in-
terfaces after incubation for 20 h.
