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Background   Pseudomonas fluorescens  strain MFE01 secretes in abundance two Hcp proteins (haemolysin co-regulated proteins) Hcp1 and Hcp2, characteristic of a functional type 6 secretion system. Phenotypic studies have shown that MFE01 has antibacterial activity against a wide range of competitor bacteria, including rhizobacteria and clinically relevant bacteria. Mutagenesis of the  hcp2  gene abolishes or reduces, depending on the target strain, MFE01 antibacterial activity. Hcp1, encoded by  hcp1 , may also be involved in bacterial competition. We therefore assessed the contribution of Hcp1 to competition of  P. fluorescens  MFE01 with other bacteria, by studying MFE01 mutants in various competitive conditions.    Results  Mutation of  hcp1  had pleiotropic effects on the MFE01 phenotype. It affected mucoidy of the strain and its motility and was associated with the loss of flagella, which were restored by introduction of plasmid expressing  hcp1 . The  hcp1  mutation had no effect on bacterial competition during incubation in solid medium. MFE01 was able to sequester another  P. fluorescens  strain, MFN1032, under swimming conditions. The  hcp2  mutant but not the  hcp1  mutant conserved this ability. In competition assays on swarming medium, MFE01 impaired MFN1032 swarming and displayed killing activity. The  hcp2  mutant, but not the  hcp1  mutant, was able to reduce MFN1032 swarming. The  hcp1  and  hcp2  mutations each abolished killing activity in these conditions.    Conclusion  Our findings implicate type 6 secretion of Hcp1 in mucoidy and motility of MFE01. Our study is the first to establish a link between a type 6 secretion system and flagellin and mucoidy. Hcp1 also appears to contribute to limiting the motility of prey cells to facilitate killing mediated by Hcp2. Inhibition of motility associated with an Hcp protein has never been described. With this work, we illustrate the importance and versatility of type 6 secretion systems in bacterial adaptation and fitness.

A Pseudomonas fluorescens type 6 secretion system is related to mucoidy, motility and bacterial competition