Secretion systems, bacterial
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).
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:
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: