Secretion systems, bacterial

Introduction

A remarkable evolution of secretion systems to transport substances across the cell envelope (=cytoplasmic membrane, cell wall and possibly outer membrane) has occured in bacteria. Bacterial secretion systems are "nanomachines" in the form of protein complexes located to the cell membranes. In pathogenic bacteria, their function is to secrete different substances, so-called effector molecules, which are predominantly proteins, which allow the bacterium to colonize or invade the host cell. The effector molecules can also manipulate the host cell so that it is less likely to survive the bacterial infection, because they constitute toxins, adhesins, or degradation enzymes, etc. The secretion systems are, thus, very important virulence factors and they can be divided into different types depending on their composition, structure, mechanism and evolutionary relationship. Gram-negative and gram-positive bacteria can have (at least) eight and (at least) four different secretion systems, respectively, and there are two fundamentally different mechanisms (one-step and two-step mechanism).

Mechanisms

Through the one-step mechanism, the effector molecules are transported directly across the bacterial cell membrane(s) into the host cell and this can occur in both gram-negative and gram-positive bacteria. Through the two-step mechanism, the effector molecules are first transported through the plasma membrane to the periplasmic space of the bacterium and then by means of other protein complexes through the outer membrane and into the host cell. This can only happen in gram-negative bacteria.

Secretion across the plasma membrane

Secretion across the plasma membrane occurs in both gram-positive and gram-negative bacteria and there are three main systems for this type of transport:

• Sec (general secretion or GSP)
• SRP (signal recognition particle)
• Tat (twin arginine translocation)

Secretion across the outer membrane

Of course, secretion across the outer membrane only needs to occur in gram-negative bacteria and through either the one-step mechanism or the two-step mechanism.

Different principle types of secretion

The secretion systems are usually divided into the following seven main types, but this division is not complete and sometimes not entirely clear. Then there are also subtypes of these main types:

• Type I secretion system (T1SS or TOSS)
  A simple one-step mechanism that requires only three different proteins. For instance Escherichia coli and Pseudomonas fluorescens possess this system.
• Type II secretion system (T2SS)
  A mechanism that only transports proteins over the outer membrane and generally transports the protein first by means of Sec across the cytoplasmic membrane. 
• Type III secretion system (T3SS or TTSS)
  This type is the most complicated of all secretion systems described so far. It is a one-step mechanism utilizing a cannula-like protrusion (appendix) which can make holes in the host's cell membrane so that the secreted protein can be injected. This secretion system is also called injectosome because of its similarity to a syringe. Up to 25 different proteins are required to build up the injectosome and it has the same evolutionary origin as bacterial flagellas. Certain species within the following gram-negative bacterial genera possess T3SS, for instance Pseudomonas, Salmonella, Shigella, Vibrio and Yersinia.
• Type IV secretion system (T4SS or TFSS)
  Can translocate proteins through both a one-step mechanism and a two-step mechanism.  Bordetella pertussis, Helicobacter pylori and Legionella pneumophila, for instance, possess T4SS. This system can also translocate DNA.
• Type V secretion system (T5SS)
  This secretion system can be divided into three main categories.
• Type VI secretion system (T6SS)
  This secretion system consists of a "phage-tail-nail-like injectosome reminiscent of T3SS. T6SS has structural and functional homology with the bacteriophage T4 and is required for the following bacteria to be virulent: Burkholderia mallei, Edwarsiella tarda, Francisella tularensis, Pseudomonas aeruginosa and Vibrio cholerae.
• Type VII secretion system (T7SS)
  This type of secretion system consists of a family of similar systems, which are specialized for bacteria that have specific lipids in their cell wall (eg, mycobacteria). Corynebacterium diphtheria and Nocardia sp. have been found to possess T7SS.

References

You can find nice pictures of the different bacterial secretion systems at the website SlideShare (www.slideshare.net) if you search for "bacterial secretion system".