Role of bacterial secretion systems and effector proteins - insights into Aeromonas pathogenicity mechanisms

Gram-negative bacteria have developed several nanomachine channels known as type II, III, IV and VI secretion systems that enable export of effector proteins/toxins from the cytosol across the outer membrane to target host cells. Protein secretion systems are critical to bacterial virulence and interactions with other organisms. Aeromonas utilize various secretion machines e.g. two-step T2SS, a Sec-dependent system as well as one-step, Sec-independent T3SS and T6SS systems to transport effector proteins/toxins and virulence factors. Type III secretion system (T3SS) is considered the dominant virulence system in Aeromonas. The activity of bacterial T3SS effector proteins most often leads to disorders in signalling pathways and reorganization of the cell cytoskeleton. There are also scientific reports on the pathogenicity mechanism associated with host cell apopotosis/pyroptosis resulting from secretion of a cytotoxic enterotoxin, i.e. the Act protein, by the T2SS secretion system and an effector protein Hcp by the T6SS system. Type IV secretion system (T4SS) is the system which translocate protein substrates, protein-DNA complexes and DNA into eukaryotic or bacterial target cells. In this paper, the contribution of virulence determinants involved in the pathogenicity potential of Aeromonas is discussed. Considering that the variable expression of virulence factors has a decisive impact on the differences observed in the virulence of particular species of microorganisms, it is important to assess the correlation between bacterial pathogenicity and their virulence-associated genes.