Type 6 Secretion Systems

Bacterial Wars and Deadly Syringes

What exactly is a T6SS?

Syringe like structure designed to deliver effector proteins

Found in more than 25% of Gram negative bacteria

Delivers them in a cell-contact dependent manner

Used in a defensive manner against competitors

It is also able to inject effectors directly into eukaryotes during an infection

So where did it come from?

Structure and Phage Homology

Show structural and functional homology to the T4 Bacteriophage

Believed to actually be an inverted phage tail

Gene homology with Phage Genes Study by Leiman in 2009 found

homology between phage tail proteins and 3 T6SS components.

Study by Pell in 2009 found similariteis between the Hcp protein of T6SS and a major phage tail protein

Study by Pukatzki in 2007 showed that the VgrG are analogous to the Needle and spike complex of T4 Phage

Delivery of Effectors

The VgrG proteins are located at the distal end of the Hcp tube forming the spike

Following polymerisation of the outer sheath VipA/VipB proteins, the repeating Hcp proteins of the inner tube are ejected outward like a syringe

This is very similar to the mechanism employed by a phage tail

Following this extension effector proteins can be delivered to the target cell.

The exact mechanism however is still to be determined.

Friend or Foe?

Pseudomonas aeruginosa Tae effector proteins Tse1 and Tse3 Tse2 identified as a toxin but

mechanism remains unknown T6SS “duelling” between sister cells

Vibrio cholera effector proteins TseL, VasX and VgrG3

What are these used for? Against rivals of heterologous

bacteria Against sister cells Against eukaryotic cells during an

infection

Immunity to Self Harm

Immunity Determinants Some of the organisms these

have been identified in include Pseudomonas aeruginosa, Burkholderia thailandensis, Vibrio cholera and Serratia marcescens

Protects itself from self intoxication and sister cells by using immunity proteins Tsi1 and Tsi3

Genes encoding effector proteins are located close to ORFs encoding immunity proteins

Research and Future Implications

Pseudomonas T6SS has been shown to play a key role in chronic infections Biofilm formation Quorum Sensing Virulence

2006 Study by Mougous et al found that pulmonary secretions of cystic fibrosis patients contained Hcp1 Pseudomonas infects >80% of Cystic

Fibrosis sufferers A 2014 study by Ma et al reported on the

effctivness of Agrobacterium tumefaxciens T6SS system to kill Pseudomonas within a planta coinfection assay Possible way to turn non pathogenic

bacteria against pathogenic bacteria by using the T6SS machinery

A better understanding of T6SS could offer new antimicrobial treatments to sufferers of chronic diseases such as cystic fibrosis

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