Bacterial effector protein

Bacterial effectors are proteins secreted by pathogenic bacteria into the cells of their host, usually using a type 3 secretion system (TTSS/T3SS), a type 4 secretion system (TFSS/T4SS) or a Type VI secretion system (T6SS). Some bacteria inject only a few effectors into their host’s cells while others may inject dozens or even hundreds. Effector proteins may have many different activities, but usually help the pathogen to invade host tissue, suppress its immune system, or otherwise help the pathogen to survive. Effector proteins are usually critical for virulence. For instance, in the causative agent of plague (Yersinia pestis), the loss of the T3SS is sufficient to render the bacteria completely avirulent, even when they are directly introduced into the bloodstream.Gram negative microbes are also suspected to deploy bacterial outer membrane vesicles to translocate effector proteins and virulence factors via a novel membrane vesicle trafficking secretory pathway, in order to modify their environment or attack/invade target cells, for example, at the host-pathogen interface.

Many pathogenic bacteria are known to have secreted effectors but for most species the exact number is unknown. Once a pathogen genome has been sequenced, effectors can be predicted based on protein sequence similarity, but such predictions are not always precise. More importantly, it is difficult to prove experimentally that a predicted effector is actually secreted into a host cell because the amount of each effector protein is tiny. For instance, Tobe et al. (2006) predicted more than 60 effectors for pathogenic E. coli but could only show for 39 that they are secreted into human Caco-2 cells. Finally, even within the same bacterial species, different strains often have different repertoires of effectors. For instance, the plant pathogen Pseudomonas syringae has 14 effectors in one strain, but more than 150 have been found in multiple different strains.

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