Our discussions will focus on the canonical features of each system and not the multitude of variations in each one ( Table 1). In this overview, we provide a brief introduction to a number of protein secretion systems, including those that are not discussed in depth in succeeding chapters, in order to highlight the structural and functional similarities and differences between these systems. This topic will be discussed in Section VII of this textbookįive secretion systems will be discussed in depth in subsequent chapters in this section: the Type III Secretion System (T3SS), T4SS, T5SS, T6SS, and T7SS. Due to the specificity of expression of some of these secretion systems in bacterial pathogens, antimicrobials are being developed against these systems to augment our current repertoire of antibiotics. There are several different classes of bacterial secretion systems, and their designs can differ based on whether their protein substrates cross a single phospholipid membrane, two membranes, or even three membranes, where two are bacterial and one is a host membrane. Other times, they are required to take advantage of an environmental niche, perhaps by secreting proteins that help bacteria to compete with nearby microorganisms. In certain cases, these dedicated secretions systems are used by bacterial pathogens to manipulate the host and establish a replicative niche. Some secretion systems are found in almost all bacteria and secrete a wide variety of substrates, while others have been identified in only a small number of bacterial species or are dedicated to secreting only one or a few proteins. Protein secretion systems are essential for the growth of bacteria and are used in an array of processes. Prokaryotes have developed numerous ways of transporting protein cargo between locations, which largely involve the assistance of dedicated protein secretion systems. One essential prokaryotic cell function is the transport of proteins from the cytoplasm into other compartments of the cell, the environment, and/or other bacteria or eukaryotic cells - a process known as protein secretion. Additionally, we address recent findings that indicate that the innate immune system of the host can detect and respond to the presence of protein secretion systems during mammalian infection. In this chapter, we review the canonical features of several common bacterial protein secretion systems, as well as their roles in promoting the virulence of bacterial pathogens. Some systems are conserved in all classes of bacteria and secrete a broad array of substrates, while others are only found in a small number of bacterial species and/or are specific to only one or a few proteins. In general, bacterial protein secretion apparatuses can be divided into different classes, based on their structures, functions, and specificity. Many pathogens use dedicated protein secretion systems to secrete virulence factors from the cytosol of the bacteria into host cells or the host environment. Secreted proteins can play many roles in promoting bacterial virulence, from enhancing attachment to eukaryotic cells, to scavenging resources in an environmental niche, to directly intoxicating target cells and disrupting their functions. One essential component of these strategies for many bacterial pathogens is the secretion of proteins across phospholipid membranes. Bacterial pathogens utilize a multitude of methods to invade mammalian hosts, damage tissue sites, and thwart the immune system from responding.

Type 3 secretion system