In addition , the extracellular esterase of group AStreptococcusreduces phagocyte recruitment by hydrolyzing platelet-activating factor (PAF) (73), and lipases produced and secreted by the opportunistic fungal pathogenCandida albicanssupport colonization and penetration of host cells (74, 75). while their growth characteristics are indistinguishable from those of the wild-type strain in nutrient-rich broth. Genetic inactivation of the T2S system and its substrate, LipA, also has a negative impact onin vivofitness in a neutropenic murine model for bacteremia. Both thegspDandlipAmutants are outcompeted by the wild-type strain as judged by their reduced numbers in spleen and liver following intravenous coinoculation. Collectively, our findings suggest that the T2S system plays a hitherto-unrecognized role inin vivosurvival ofA. baumanniiby transporting a lipase that may contribute to fatty acid metabolism. IMPORTANCEInfections by multidrug-resistantAcinetobacter Calcipotriol baumanniiare a growing health concern worldwide, underscoring the need for a better understanding of the molecular mechanisms by which this pathogen causes disease. In this study, we demonstrated thatA. baumanniiexpresses a functional type II secretion (T2S) system that is responsible for secretion of LipA, an extracellular lipase required for utilization of exogenously added lipids. The T2S system and the secreted lipase supportin vivocolonization and thus contribute to the pathogenic potential ofA. baumannii. == INTRODUCTION == Acinetobacter baumannii, an increasingly common nosocomial Gram-negative pathogen, is responsible for a wide range of infections, including pneumonia, urinary tract infections, bacteremia, meningitis, and skin and wound infections (14). Immunocompromised and severely ill patients in the intensive care unit, individuals with extensive wounds or invasive devices, and those who are undergoing or have recently undergone antibiotic regimens are particularly susceptible toA. baumanniiinfections (57). Ventilator-associated pneumonia and bloodstream infections are the most severe, resulting in 25% to 35% mortality rates (8, 9). The pathogenic success ofA. baumanniiis likely multifactorial, but its ability to persist on dry surfaces, form biofilms, resist complement-mediated killing, and survive antibiotic treatment are of importance (1, 1013). The escalating frequency of multidrug-resistant (MDR) strains ofA. baumanniiis of particular concern. In the past 10 years, there has been an alarming 60% increase in the number of MDR clinical isolates reported (http://www.cddep.org). An important and clinically relevant aspect of bacterial infections is the ability of bacteria to grow as a biofilm (14). These matrix-encased, multilayer bacterial Calcipotriol communities are exceptionally resistant to antibiotic treatment and are prone to spreading antibiotic resistance through horizontal gene transfer (15, 16). Clinical isolates that form biofilms survive for long periods of time on dry surfaces and are able to colonize common hospital equipment, such as ventilator tubes (17). Several factors have been shown to be necessary for abiotic biofilm formation, including a pilus assembly system, the OmpA outer membrane protein, and capsular polysaccharide, which is also protective against complement-mediated killing (13, 1820). While research has focused on the mechanisms of antibiotic resistance and biofilm formation and the epidemiology ofA. baumannii, our understanding ofA. baumanniipathogenesis COG3 is lagging and little is known about the contribution of secreted proteins toA. baumanniisurvival and propagation during infection. Sequencing of severalA. baumanniigenomes has revealed thatA. baumanniicontains genes for a variety of transport systems, including the assembly and translocation system for type IV pilus and type VI and type IV secretion systems. The Calcipotriol type IV pilus supports the twitching motility ofA. baumannii(21) but may also contribute to adhesion, colonization, biofilm formation, and transformation like type IV pili in other Gram-negative pathogens, while the type VI secretion system is utilized for bacterial competition and the type IV secretion system may be required for virulence (22, 23). In addition , A. baumanniipossesses genes for a type II secretion (T2S) system (24, 25). Bacteria that use the T2S system are typically environmental bacteria; however , they also include pathogens such asVibrio cholerae, enterotoxigenicEscherichia coli, Pseudomonas aeruginosa, andLegionella pneumophila(2629). The T2S system mediates.