A type VI secretion system is involved in Pseudomonas fluorescens bacterial competition

Area of Science:

• Microbiology
• Bacterial Pathogenesis
• Molecular Biology

Background:

• Type VI secretion systems (T6SSs) are widespread in Gram-negative bacteria, mediating interactions with other organisms by injecting toxins.
• T6SSs are found in human pathogens, and bacteria use immunity proteins to prevent self-intoxication.
• Environmental Pseudomonas fluorescens strain MFE01 exhibits unusual overexpression of T6SS components Hcp and VgrG.

Purpose of the Study:

• To investigate the role of the energy-consuming overexpression of the T6SS in P. fluorescens MFE01.
• To determine the function of the T6SS in the interactions of P. fluorescens MFE01 with competitor bacteria and eukaryotic cells.
• To explore the potential connection between T6SS activity and plant growth-promoting rhizobacterium (PGPR) properties.

Main Methods:

• Genomic analysis to identify T6SS gene clusters (e.g., hcp1, hcp2).
• Phenotypic studies assessing virulence against eukaryotic cell models (amebas, plant, animal cells).
• Antibacterial activity assays against various bacterial competitors.
• Gene mutagenesis (hcp2) and heterologous expression of immunity proteins to confirm T6SS-mediated killing.

Main Results:

• P. fluorescens MFE01 displays avirulence towards eukaryotic cells but exhibits potent antibacterial activity.
• Mutagenesis of the hcp2 gene significantly reduced or abolished antibacterial activity, depending on the prey.
• Heterologous expression of immunity proteins protected E. coli from MFE01-mediated killing, confirming T6SS effector targeting.
• MFE01 demonstrated protective effects against potato tuber soft rot caused by Pectobacterium atrosepticum.

Conclusions:

• The T6SS in P. fluorescens MFE01 is primarily involved in antibacterial competition, likely targeting peptidoglycan.
• The T6SS contributes to the protective role of P. fluorescens MFE01 against plant pathogens.
• The study suggests a link between T6SS function and the PGPR capabilities of P. fluorescens.