It is reasonable PLX4032 solubility dmso to suspect that modification of the PV microenvironment by additional secretion systems is also important in C. burnetii host cell parasitism. Gram-negative bacteria can employ several secretion systems to translocate proteins into the extracellular milieu [17]. However, bioinformatic analysis of the C. burnetii genome reveals canonical components of only a type I secretion system with the presence of a tolC homolog [18, 19]. Type I secretion is typically a one step process that transports proteins directly from the bacterial cytoplasm

into the surrounding environment [20]. However, a small number of proteins, such as heat-stable enterotoxins I and II of Escherichia coli[21, 22], and an ankyrin repeat protein of Rickettsia typhi[23], appear to access TolC via the periplasm after transport across the inner membrane by the Sec translocase. C. burnetii lacks typical constituents of a type II secretion system [24]. However, the organism encodes several genes involved in type IV pili (T4P) assembly, several of which are homologous to counterparts of type II secretion systems, indicating a common evolutionary

origin and possibly a similar function [25]. Accumulating data indicates core T4P proteins can constitute a secretion system [26–30]. In Francisella novicida, a collection of T4P proteins form a secretion system that Crizotinib secretes at least 7 proteins [27]. In Vibrio cholerae, T4P secrete a soluble colonization factor required for optimal intestinal colonization of infant mice [30]. Dichelobacter nodosus secrete proteases in a T4P-dependent manner [29, 31]. Like the well-studied type II secretion system of Legionella pneumophila, a close phylogenetic relative of C. burnetii[18],

substrates secreted by T4P are biased towards N-terminal signal sequence-containing enzymes [27, 32]. C. burnetii encodes several enzymes with predicted signal Pregnenolone sequences, such as an acid phosphatase (CBU0335) that inhibits neutrophil NADPH oxidase function and superoxide anion production [33, 34]. Along with PV detoxification, C. burnetii exoenzymes could presumably degrade macromolecules into simpler substrates that could then be transported by the organism’s numerous transporters [18]. Genome analysis indicates C. burnetii possesses a complete Sec translocase for translocation of signal sequence-containing proteins into the periplasm [18, 19]. Another secretion mechanism employed by Gram-negative bacteria is release of outer membrane vesicles (OMVs). OMVs capture periplasmic components before the vesicle pinches off from the cell envelope. This ‘packaging’ of proteins is thought to provide a protective environment for delivery of the contents. OMVs are implicated in a variety of functions including delivery of virulence factors, killing of competing bacteria, and suppression of host immune responses [35, 36]. The discovery of host cell-free growth of C.