The role of proteinaceous toxins secreted by Staphylococcus aureus in interbacterial competition

Abstract

Staphylococcus aureus is highly adapted to colonisation of the mammalian host. In humans the primary site of colonisation is the epithelium of the nasal cavity. A major barrier to colonisation is the resident microbiota, which have mechanisms to exclude S. aureus. As such, S. aureus has evolved mechanisms to compete with other bacteria, one of which is through secretion of proteinaceous toxins. S. aureus strains collectively produce a number of well characterised Class I, II and IV bacteriocins as well as several bacteriocin-like substances, about which less is known. These bacteriocins have potent antibacterial activity against several Gram positive organisms, with some also active against Gram negative species. S. aureus bacteriocins characterised to date are sporadically produced, and often encoded on plasmids. More recently the type VII secretion system (T7SS) of S. aureus has also been shown to play a role in interbacterial competition. The T7SS is encoded by all S. aureus isolates and so may represent a more widespread mechanism of competition used by this species. T7SS antagonism is mediated by the secretion of large protein toxins, three of which have been characterised to date: a nuclease toxin, EsaD; a membrane depolarising toxin, TspA; and a phospholipase toxin, TslA. Further study is required to decipher the role that these different types of secreted toxins play in interbacterial competition and colonisation of the host.