Date Published: January 23, 2017
Publisher: Public Library of Science
Author(s): Mathias Gallique, Victorien Decoin, Corinne Barbey, Thibaut Rosay, Marc G. J. Feuilloley, Nicole Orange, Annabelle Merieau, Eric Cascales.
Type VI secretion systems (T6SSs) are widespread in Gram-negative bacteria, including Pseudomonas. These macromolecular machineries inject toxins directly into prokaryotic or eukaryotic prey cells. Hcp proteins are structural components of the extracellular part of this machinery. We recently reported that MFE01, an avirulent strain of Pseudomonas fluorescens, possesses at least two hcp genes, hcp1 and hcp2, encoding proteins playing important roles in interbacterial interactions. Indeed, P. fluorescens MFE01 can immobilise and kill diverse bacteria of various origins through the action of the Hcp1 or Hcp2 proteins of the T6SS. We show here that another Hcp protein, Hcp3, is involved in killing prey cells during co-culture on solid medium. Even after the mutation of hcp1, hcp2, or hcp3, MFE01 impaired biofilm formation by MFP05, a P. fluorescens strain isolated from human skin. These mutations did not reduce P. fluorescens MFE01 biofilm formation, but the three Hcp proteins were required for the completion of biofilm maturation. Moreover, a mutant with a disruption of one of the unique core component genes, MFE01ΔtssC, was unable to produce its own biofilm or inhibit MFP05 biofilm formation. Finally, MFE01 did not produce detectable N-acyl-homoserine lactones for quorum sensing, a phenomenon reported for many other P. fluorescens strains. Our results suggest a role for the T6SS in communication between bacterial cells, in this strain, under biofilm conditions.
The type VI secretion system (T6SS) is a widespread macromolecular machinery in Gram-negative bacteria . This multiprotein complex delivers effectors into eukaryotic and/or bacterial cells [1–5]. This secretion system has a structure similar to that of the contractile tails of bacteriophages [6,7]. Hcp (hemolysin-coregulated protein) and VgrG (valine-glycine repeat protein G) are structural proteins of this machinery with structural similarities to the gp19 and gp5-gp27 proteins of bacteriophage T4, respectively [8,9]. The structural homologue of the phage tube is built from rings of Hcp hexamers with a tip complex composed of VgrG trimers, Paar protein, and effectors [10–15]. This tube is ejected by the contraction of a tubular sheath consisting of the conserved T6SS-associated cytoplasmic proteins, TssB and TssC [16–19]. Hcp proteins appear to be directly involved in effector recognition [13,20–23], acting in synergy with VgrG proteins. Moreover, many VgrG proteins possessing a variety of effector domains at their C-termini, are effectors as well as structural components .
We describe here several characteristics of the T6SS of the environmental P. fluorescens strain MFE01. We show that Hcp1, Hcp2, and Hcp3 may be involved in T6SS-mediated competition with other Pseudomonas strains on solid media, depending on the prey cell. MFE01 also impaired the formation of biofilms by P. fluorescens MFP05, a strain isolated from skin. Our findings highlight the potential of MFE01 as a biocontrol agent for preventing biofilm formation by another Pseudomonas strain isolated from skin. The protective barrier provided by human skin can be breached by burn wounds, which render the patient susceptible to bacterial infection. P. aeruginosa is frequently isolated from burn patients and causes serious infection [72,73], accounting for more than 50% of all deaths of these patients from infection . Infections with multidrug-resistant strains of P. aeruginosa may also be untreatable with antibiotics, highlighting the need to develop new therapies . For this reason, the ability of Lactobacillus to inhibit the growth of harmful bacteria in burn wounds has been investigated .