Background: Acinetobacter baumannii is a nosocomial pathogen of increasing importance, but the pathogenic mechanism of this microorganism has not been fully explored. This study investigated the potential of A. baumannii to invade epithelial cells and determined the role of A. baumannii outer membrane protein A (AbOmpA) in interactions with epithelial cells.

SummaryAcinetobacter baumannii is an important opportunistic pathogen responsible for nosocomial infection. Despite considerable clinical and epidemiological data regarding the role of A. baumannii in nosocomial infection, the specific virulence factor or pathogenic mechanism of this organism has yet to be elucidated. This study investigated the molecular mechanism of apoptosis on the infection of human laryngeal epithelial HEp-2 cells with A. baumannii and examined the contribution of outer membrane protein 38 (Omp38) on the ability of A. baumannii to induce apoptosis of epithelial cells. A. baumannii induced apoptosis of HEp-2 cells through cell surface death receptors and mitochondrial disintegration. The Omp38-deficient mutant was not as able to induce apoptosis as the wild-type A. baumannii strain. Purified Omp38 entered the cells and was localized to the mitochondria, which led to a release of proapoptotic molecules such as cytochrome c and apoptosis-inducing factor (AIF). The activation of caspase-3, which is activated by caspase-9, degraded DNA approximately 180 bp in size, which resulted in the appearance of a characteristic DNA ladder. AIF degraded chromosomal DNA approximately 50 kb in size, which resulted in largescale DNA fragmentation. These results demonstrate that Omp38 may act as a potential virulence factor to induce apoptosis of epithelial cells in the early stage of A. baumannii infection.

Gram-negative bacteria produce outer membrane vesicles that play a role in the delivery of virulence factors to host cells. However, little is known about the membrane-derived vesicles (MVs) produced by Gram-positive bacteria. The present study examined the production of MVs from Staphylococcus aureus and investigated the delivery of MVs to host cells and subsequent cytotoxicity. Four S. aureus strains tested, two type strains and two clinical isolates, produced spherical nanovesicles during in vitro culture. MVs were also produced during in vivo infection of a clinical S. aureus isolate in a mouse pneumonia model. Proteomic analysis showed that 143 different proteins were identified in the S. aureus-derived MVs. S. aureus MVs were interacted with the plasma membrane of host cells via a cholesterol-rich membrane microdomain and then delivered their component protein A to host cells within 30 min. Intact S. aureus MVs induced apoptosis of HEp-2 cells in a dose-dependent manner, whereas lysed MVs neither delivered their component into the cytosol of host cells nor induced cytotoxicity. In conclusion, this study is the first report that S. aureus MVs are an important vehicle for delivery of bacterial effector molecules to host cells.

The use of antibiotics is strongly associated with antimicrobial resistance. E. coli isolates from different sources may select a specific gene cassette by antibiotic selective pressure, which results in differences in class 1 integrons. The horizontal transfer of class 1 integrons through conjugative plasmids seems to be responsible for wide dissemination of a particular type of class 1 integron.

Bacteremia is a common systemic disease caused by Acinetobacter baumannii, an important hospital-acquired pathogen among critically ill patients. The complement system is central to innate immune defense against invading bacteria in the blood. The present study investigated the susceptibility of clinical A. baumannii isolates to normal human sera (NHS), and determined the resistance mechanism of A. baumannii against complement-mediated lysis. The survival of A. baumannii isolates from bacteremic patients was significantly decreased in undiluted NHS, but they were resistant to 40% NHS. The alternative complement pathway was responsible for the direct killing of bacteria. The main regulator of the alternative complement pathway, factor H, bound to the surface of live A. baumannii treated with NHS. Factor H interacted with the outer membrane proteins with molecular sizes of 38 (AbOmpA), 32, and 24 kDa. The isogenic AbOmpA(-) mutant was highly susceptible to NHS in comparison with the wild-type A. baumannii strain, suggesting that AbOmpA was an important complement regulator-acquiring surface protein. These results indicate that A. baumannii evades complement attack through the acquisition of factor H to their surface.