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Concept
Bacterial secretion system
Summary
Bacterial secretion systems are protein complexes present on the cell membranes of bacteria for secretion of substances. Specifically, they are the cellular devices used by pathogenic bacteria to secrete their virulence factors (mainly of proteins) to invade the host cells. They can be classified into different types based on their specific structure, composition and activity. Generally, proteins can be secreted through two different processes. One process is a one-step mechanism in which proteins from the cytoplasm of bacteria are transported and delivered directly through the cell membrane into the host cell. Another involves a two-step activity in which the proteins are first transported out of the inner cell membrane, then deposited in the periplasm, and finally through the outer cell membrane into the host cell.
These major differences can be distinguished between Gram-negative diderm bacteria and Gram-positive monoderm bacteria. But the classification is by no means clear and complete. There are at least eight types specific to Gram-negative bacteria, four to Gram-positive bacteria, while two are common to both. In addition, there is appreciable difference between diderm bacteria with lipopolysaccharide on the outer membrane (diderm-LPS) and those with mycolic acid (diderm-mycolate).
The export pathway is responsible for crossing the inner cell membrane in diderms, and the only cell membrane in monoderms.
Translocon
The general secretion (Sec) involves secretion of unfolded proteins that first remain inside the cells. In Gram-negative bacteria, the secreted protein is sent to either the inner membrane or the periplasm. But in Gram-positive bacteria, the protein can stay in the cell or is mostly transported out of the bacteria using other secretion systems. Among Gram-negative bacteria, Vibrio cholerae, Klebsiella pneumoniae, and Yersinia enterocolitica use the Sec system. Staphylococcus aureus and Listeria monocytogenes are Gram-positive bacteria that use the Sec system.
Official source
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