Background: Swarming motility and biofilm formation are opposite, but related surface-associated behaviors that
allow various pathogenic bacteria to colonize and invade their hosts. In Sinorhizobium meliloti, the alfalfa endosymbiont,
these bacterial processes and their relevance for host plant colonization are largely unexplored. Our previous
work demonstrated distinct swarming abilities in two S. meliloti strains (Rm1021 and GR4) and revealed that both
environmental cues (iron concentration) and bacterial genes (fadD, rhb, rirA) play crucial roles in the control of
surface motility in this rhizobial species. In the current study, we investigate whether these factors have an impact
on the ability of S. meliloti to establish biofilms and to colonize host roots.
Results: We found that strain GR4, which is less prone to translocate on solid surfaces than strain Rm1021, is more
efficient in developing biofilms on glass and plant root surfaces. High iron conditions, known to prevent surface motility
in a wild-type strain of S. meliloti, promote biofilm development in Rm1021 and GR4 strains by inducing the formation of
more structured and thicker biofilms than those formed under low iron levels. Moreover, three different S. meliloti
mutants (fadD, rhb, and rirA) that exhibit an altered surface translocation behavior compared with the wild-type
strain, establish reduced biofilms on both glass and alfalfa root surfaces. Iron-rich conditions neither rescue the
defect in biofilm formation shown by the rhb mutant, which is unable to produce the siderophore rhizobactin
1021 (Rhb1021), nor have any impact on biofilms formed by the iron-response regulator rirA mutant. On the
other hand, S. meliloti FadD loss-of-function mutants do not establish normal biofilms irrespective of iron levels.
Conclusions: Our studies show that siderophore Rhb1021 is not only required for surface translocation, but also
for biofilm formation on glass and root surfaces by strain Rm1021. In addition, we present evidence for the
existence of control mechanisms that inversely regulate swarming and biofilm formation in S. meliloti, and that
contribute to efficient plant root colonization. One of these mechanisms involves iron levels and the iron global
regulator RirA. The other mechanism involves the participation of the fatty acid metabolism-related enzyme FadD.
