© Benaki Phytopathological Institute
Venieraki
et al.
18
ria in the soil, acting as highways for motile
bacteria (Nazir
et al.
, 2010). In a recent study,
this concept was further extended; the au-
thors using the bacterium
Paenibacillus vor-
tex
have shown that
P. vortex
swarms can
transport conidia of the
Aspergillus fumiga-
tus
over long distances. Inoculation of
A. fu-
migatus
conidia near to an artificial air re-
vealed that the fungi grown across the gap,
permitting a successful cross of
P. vortex
,
suggesting a role for swarming and/or fla-
gella in this mutually facilitating migration
process (Ingham
et al.
, 2011).
Dispersal along fungal hyphae appears
to be a widespread trait of swarming bacte-
ria (Bravo
et al.
, 2013; Furuno
et al.
, 2102; Pion
et al.
, 2013; Simon
et al.
, 2015).
In vitro
studies
have shown that migration along mycelium
surface facilitated the bacterial degradation
of organic soil pollutants (Banitz
et al.
, 2013)
and the migration of
Burkholderia terrae
BS001 along mycelium surface (Warmink
et
al.
, 2011). Shiga toxin-producing
Escherichia
coli
was found to spread over several food-
related fungi (Lee
et al.
, 2013). Enhanced col-
onization of rhizosphere by saprotrophic
fungi stimulated root surface colonization
by indigenous rhizosphere inhabiting bio-
control bacteria (de Boer
et al.
, 2014).
Bacterial swarms recruit cargo bacteria
or facilitate the migration of fellow
swarmers
Swarming offers a competitive advantage to
some bacteria in invading some plant habi-
tats (Barak
et al.
, 2009). However, co-swarm-
ing or transporting other bacterial species
may expand the abilities of the partners in
occupying and exploiting new territories.
This combination of properties is well illus-
trated by recent studies at laboratory condi-
tions. A non-swarming gentamycin resistant
Burkholderia cepacia
(cargo bacterium) al-
lowed the gentamycin-sensitive profi-
cient-swarming
Pseudomonas aeruginosa
to swarm and colonize a gentamycin-con-
taining area of the plate, dispersing both
bacteria (Venturi
et al
., 2010). Moreover, it
has been shown that an ampicillin-sensi-
tive
Paenibacillus vortex
was capable to
swarm and colonize an ampicillin plate us-
ing non-motile ampicillin resistant
Escher-
ichia
coli
as a cargo organism;
one species
provides an enzyme that detoxifies the an-
tibiotic (a sessile cargo bacterium carrying
a resistance gene), while the other (
P. vor-
tex
) moves itself and transports the cargo
bacterium
(
Finkelshtein
et al.
, 2015
).
Fast-
swarming
Myxococcus xanthus
strains coop-
erated with slower isolates, allowing the lat-
ter to keep pace with faster strains in mixed
groups (Kraemer and Velicer, 2014).
Whether these cooperative phenome-
na between swarming bacteria may occur in
natural habitats is not clear. However, in situ
experiments have provided evidence that
non-motile bacteria may behave as hitch-
hikers and thus are able to move along fun-
gal hyphae only with the aid of other motile
bacteria acting as ‘community migrators’ (Si-
mon
et al.
, 2015). Cooperation among non-
competitive swarming bacterial strains pro-
vide a mechanism for mixing, thus it would
be predicted that they may form mixed
strains biofilm (Reichenbach
et al.
, 2007). The
ability of three species (an antibiotic-produc-
ing
Pseudomonas aeruginosa
strain P1, a re-
sistant
Raoultella ornithinolytica
strain R1 and
a sensitive
Brevibacillus borstelensis
strain S1)
to establish biofilms (Narisawa
et al.
, 2008),
and the recent demonstration that kin but
not identical swarming
B. subtilis
strains were
able to co-exist in biofilms formed in
Arabi-
dopsis thaliana
roots (Stefanic
et al.
, 2015),
suggested that co-swarming but non-com-
petitive bacterial strains may lead to the es-
tablishment of an non-transitive competitive
network in natural habitat.
Interspecies communication affecting
swarming motility
In most environmental niches, multiple bac-
terial species coexist as dynamic commu-
nities. Bacteria have developed intercellu-
lar signaling to adapt and survive in natural
environments and to detect each other as
