BACKGROUND
genus acinetobacter are important opportunistic pathogens in hospital-acquired infections  <cit> .
they cause various types of human infections, including pneumonia, wound infections, urinary tract infections, bacteremia, and meningitis.
of the currently known  <dig> acinetobacter species  <cit> , acinetobacter baumannii is the most prevalent in clinical specimens.
numerous outbreaks caused by a. baumannii have been reported, which are of great concern in clinical settings  <cit> .
despite convincing evidence linking a. baumannii with human infections, the study of its pathogenic mechanism is still in its elementary stage.
adherence of bacteria to epithelial cells is an essential step towards colonization and infection  <cit> .
bacterial adherence to host cells is mediated by fimbria or membrane components.
furthermore, many pathogenic bacteria are capable of invading non-phagocytic cells and evolve to survive within the host cells.
the cellular invasion of bacteria contributes to evasion of humoral immunity, persistence in the host, and penetration into deep tissues.
bacterial pathogens gain entry to non-phagocytic cells via two mechanisms; a zipper-like mechanism and a trigger mechanism, which were initially classified based on morphological differences  <cit> .
the zipper-like mechanism  requires the direct interaction of bacterial ligands to the host's cell surface receptors and involves local cytoskeletal rearrangement at the invasion site.
in contrast, the trigger mechanism is initiated by the injected bacterial effector proteins delivered by the type iii secretion system.
the effector proteins regulate cytoskeleton dynamics and induce dramatic cytoskeletal rearrangements such as membrane ruffles.
the previous study demonstrated that adherence of a. baumannii to human bronchial epithelial cells was mediated by fimbrial-like structures and entrapment of bacteria by cellular protrusions  <cit> .
however, the mechanisms underlying invasion of a. baumannii in epithelial cells have not been explored so far.
outer membrane proteins  of gram-negative bacteria are key players in bacterial pathogenesis.
ompa of escherichia coli, ospc of borrelia burgdorferi, and opa and opca of neisseria meningitidis facilitate adherence to and invasion of bacteria in host cells  <cit> .
ompa of a. baumannii  is the most abundant surface protein with a molecular mass of  <dig> kda and plays a role in permeability of small solutes.
abompa purified from a. baumannii atcc 19606t and recombinant abompa  bind to the surface of host cells and are localized in both the mitochondria and nuclei, which induce the death of host cells  <cit> .
based on previous findings that abompa bound to and entered host cells, we hypothesized that abompa is responsible for adherence to and invasion of a. baumannii in epithelial cells during the colonization and early stage of bacterial infection.
in the present study, we investigated the potential of a. baumannii to invade epithelial cells and studied the role of abompa in interactions of a. baumannii with epithelial cells.
RESULTS
a. baumannii invades epithelial cells dependently on both the bacterial strains and cell types
to determine whether a. baumannii invaded epithelial cells, a. baumannii atcc 19606t and four gentamicin-susceptible a. baumannii isolates from clinical specimens were selected and gentamicin protection assay was performed.
a multiplicity of infection  of  <dig> that was previously optimized in the adherence assay of a. baumannii was used in the cell invasion assay  <cit> .
to optimize infection time, human bronchial nci-h <dig> cells were infected for up to  <dig> h with a. baumannii atcc 19606t.
intracellular bacterial counts increased steadily with incubation times , but cell death and destruction of cell monolayer appeared  <dig> h after infection.
therefore, we chose  <dig> h as the maximum infection period in the following study to exclude any secondary effect of cell death.
nci-h <dig> cells were infected with four clinical a. baumannii isolates at an moi of  <dig> for  <dig> h. cell invasion of a. baumannii was different between bacterial strains: a. baumannii 05ka <dig> was the most invasive , whereas a. baumannii 05p <dig> was the least invasive  .
to determine whether the cell invasion of a. baumannii is dependent on epithelial cell types, human bronchial nci-h <dig> cells, human laryngeal hep- <dig> cells, and human cervical hela cells were infected with a. baumannii strains.
nci-h <dig> and hep- <dig> cells derived from the respiratory tract were more susceptible to cell invasion of a. baumannii than non-respiratory tract-derived hela cells .
confocal microscopic images provided direct evidence of intracellular localization of a. baumannii .
a. baumannii invades epithelial cells by a zipper-like mechanism
to investigate the involvement of microfilaments and microtubules in the cell invasion of a. baumannii, epithelial cells were pretreated with cytoskeleton inhibitors, cytochalasin d for microfilaments and vinblastine for microtubules, and a gentamicin protection assay was performed.
both cytochalasin d and vinblastine significantly inhibited the cell invasion of a. baumannii.
pretreatment of nci-h <dig> cells with cytochalasin d resulted in a decrease of intracellular bacteria of 93% and 90% in a. baumannii 05ka <dig> and type strain, respectively  .
cell invasion was also significantly decreased by vinblastine  .
these results suggest that a. baumannii invades epithelial cells through both microfilament- and microtubule-dependent uptake mechanisms.
the association of a. baumannii with epithelial cells was characterized in detail by electron microscopy.
scanning electron microscopy  exhibited the intimate interactions of a. baumannii with epithelial cells .
the cell membrane extended to and wrapped around bacteria, but the membrane ruffles were not appeared.
transmission electron microscopy  illuminated the sequential cell invasion of a. baumannii.
bacteria were loosely attached to the cell surface and small membrane process protruded from the cells .
after internalization of a. baumannii in epithelial cells, the cell membrane closed at the invasion site .
internalized bacteria were located in the membrane-bound vacuoles .
there was no sign of a membrane ruffle in sem and tem images.
cellular changes, such as membrane wrapping around bacteria and fitted vacuoles to bacterial size, suggest that a. baumannii is internalized by a zipper-like mechanism, but not a trigger mechanism.
abompa mediates the interaction of a. baumannii with epithelial cells
to determine the potential interaction of abompa with epithelial cells, the binding of rabompa to various types of epithelial cells was analyzed by using the flow cytometry.
it was found that rabompa specifically bound to the surface of epithelial cells tested .
the mean fluorescence intensities of nci-h <dig> and hep- <dig> cells were higher than those of hela cells, which was in accordance with the cell invasion of a. baumannii.
we determined whether rabompa inhibited the interactions of a. baumannii with epithelial cells.
the cytotoxicity of rabompa was first assessed, because ≥  <dig> μg/ml of abompa purified from a. baumannii atcc 19606t induced the cytotoxicity of hep- <dig> cells  <cit> .
however, ≤  <dig> μg/ml of rabompa did not induce any morphological and biochemical changes of epithelial cells for  <dig> h of incubation .
nci-h <dig> and hep- <dig> cells were pretreated with  <dig> μg/ml of rabompa for  <dig> h and infected with the highly invasive a. baumannii 05ka <dig> for  <dig> h. as a result, rabompa attenuated the binding of a. baumannii to epithelial cells .
adherence of a. baumannii 05ka <dig> to epithelial cells significantly decreased to 81% in hep- <dig> cells  and 87% in nci-h <dig> cells .
to determine whether rabompa also inhibited the cell invasion of a. baumannii, epithelial cells were pretreated with  <dig> μg/ml of rabompa for  <dig> h and infected with a. baumannii for  <dig> h. the cell invasion of a. baumannii 05ka <dig> significantly decreased to 69% in nci-h <dig> cells  and 67% in hep- <dig> cells  .
collectively, these results suggest that abompa mediates the adherence to and invasion of a. baumannii in epithelial cells.
cell invasion of isogenic abompa- mutant decreases as compared with wild-type bacteria
to compare the cell adherence of a. baumannii atcc 19606t and its isogenic abompa- mutant ks <dig>  nci-h <dig> and hep- <dig> cells were infected with bacteria for  <dig> h. wild-type bacteria exhibited dispersed adherence .
the number of adherent bacteria was 1– <dig> bacteria/infected cells and the percentage of infected cells was less than 3%.
the total number of adherent bacteria per  <dig> cells was  <dig>  ±  <dig>  in nci-h <dig> cells.
however, isogenic abompa- mutant formed microcolonies on the surface of epithelial cells, rather than dispersed adherence.
the number of adherent bacteria per  <dig> cells was not significantly different between wild-type bacteria and abompa- mutant.
to assess the cell invasion of wild-type bacteria and abompa- mutant, epithelial cells were infected with bacteria for  <dig> h. the cell invasion of abompa- mutant significantly decreased to 95% as compared with wild-type bacteria  .
a low cell invasion of abompa- mutant was not due to retarded bacterial growth, because there was no difference in bacterial growth between wild-type bacteria and abompa- mutant .
abompa plays a role in a. baumannii pathogenesis in vivo
to determine the role of abompa on a. baumannii pathogenesis in vivo, the murine pneumonia model was introduced.
the c57bl/ <dig> mice were intratracheally infected with  <dig> ×  <dig> cfu of wild-type bacteria and isogenic abompa- mutant, and euthanized two days after bacterial challenge.
total number of bacterial counts in the lungs were similar between mice infected with wild-type bacteria  and abompa- mutant , but lung histopathology such as the infiltration of polymorphonuclear leukocytes and destruction of alveolar structures was prominently observed in the mice infected with wild-type bacteria .
bacteremia occurred in mice infected with both bacterial strains, but there was a significant difference in the bacterial burden between the mice infected with wild-type bacteria  and isogenic abompa- mutant .
these results suggest that abompa is directly associated with the lung pathology and is responsible for the dissemination of a. baumannii into the bloodstream.
discussion
it was clearly demonstrated that a. baumannii has the potential to invade epithelial cells.
host cells play a significant role in determining the cell invasion of a. baumannii.
epithelial cells derived from the respiratory tract were more susceptible to a. baumannii invasion than epithelial cells derived from non-respiratory tract.
moreover, the binding capacity of rabompa in respiratory tract-derived epithelial cells was higher than that of non-respiratory tract-derived epithelial cells.
these findings may be partly responsible for the prevalence of a. baumannii in the respiratory tract, which is the most frequent colonization and infection site.
the efficiency of cell invasion of a. baumannii was lower than other invasive pathogens such as e. coli, campylobacter upsaliensis, helicobacter pylori, n. memingitidis, pseudomonas aeruginosa, and yersinia enterocolitica  <cit> .
a previous study showed that most clinical a. baumannii isolates infected only 20–30% of nci-h <dig> cells and the number of adhering bacteria was 1– <dig> bacteria per infected cell  <cit> .
this finding reflects the low cellular invasion of a. baumannii in the current study.
a relatively low adherence and invasion of a. baumannii in epithelial cells may contribute to a low virulence of this opportunistic pathogen, which particularly infects critically ill or severely wounded patients  <cit> .
a. baumannii invaded epithelial cells by a zipper-like mechanism, as demonstrated by the morphological characteristics.
a. baumannii was loosely attached to the surface of epithelial cells.
this observation is consistent with the previous study regarding adherence of a. baumannii to nci-h <dig> cells  <cit> .
however, the entrapment of bacteria by cellular protrusions, as noted in the previous study, was not observed.
after loose attachment of a. baumannii to epithelial cells, the cell membrane was extended to and wrapped around the bacteria, which are typical characteristics of the zipper-like mechanism.
these cellular responses against a. baumannii were mediated by actin rearrangement and membrane reorganization.
confocal images and gentamicin protection assay with cytochalasin d showed the involvement of actin filaments in the cell invasion of a. baumannii.
the microtubule inhibitor vinblastine also significantly inhibited the cell invasion of a. baumannii.
vinblastine may inhibit the transport of membrane-bound bacteria from the plasma membrane to the cytoplasm by preventing their movement in the microtubule-mediated contractile process.
once inside the host cells, internalized a. baumannii was located in the membrane-bound vacuoles.
this result, in conjunction with the results of the gentamicin protection assay, suggests that a. baumannii survives within membrane-bound vacuoles in the cytoplasm.
many intracellular bacteria are located in a membrane-bound vacuole compartment  <cit> , but some, such as neisseria spp., listeria monocytogenes, salmonella spp., and yersinia spp., escape the cytoplasmic vacuoles and are freely located in the cytoplasm  <cit> .
in regards to most pathogenic bacteria, a membrane-bound vacuole containing pathogens does not undergo sequential fusion with endosomes or lysosomes for its own benefit.
further studies are needed to characterize the fate of intracellular a. baumannii to understand the survival and dissemination of bacteria.
outer membrane proteins of gram-negative bacteria are known to be directly involved in the interaction of bacteria with host cells.
the present study focused on whether abompa is responsible for the interaction of a. baumannii with epithelial cells.
our data suggest that abompa is a major effector molecule of a. baumannii interacting with epithelial cells according to the following experimental results:  rabompa directly binds to the surface of epithelial cells;  pre-treatment of rabompa significantly attenuates the binding of a. baumannii to epithelial cells;  rabompa significantly inhibits the cell invasion of a. baumannii, and  the cell invasion of abompa- mutant decreased to 95% in comparison to wild-type bacteria.
in a murine pneumonia model, wild-type bacteria and isogenic abompa- mutant induced different lung pathologies.
moreover, wild-type a. baumannii had the capacity to disseminate into the bloodstream, whereas abompa- mutant rarely cross barriers to disseminate in this manner.
accordingly, abompa plays an important role in the a. baumannii pathogenesis regarding the induction of pneumonia and bacteremia.
a. baumannii can adhere to and invade epithelial cells during colonization and the early stage of infection.
in addition to its biological role as a porin, abompa plays a versatile role in the a. baumannii pathogenesis regarding its interaction with epithelial cells, the induction of apoptosis of host cells, and the dissemination of bacteria into the bloodstream.
our data provide a novel insight into a. baumannii pathogenesis and contribute to further our knowledge of a. baumannii infections.
CONCLUSIONS
a. baumannii invades epithelial cells depending on both bacterial strains and cell types.
cellular invasion is mediated by both microfilament- and microtubule-dependent uptake mechanisms.
abompa is a microbial component responsible for the adherence to and invasion of a. baumannii in epithelial cells.
the data obtained provide a novel insight into the a. baumannii pathogenesis in the early stage of bacterial infection.

 genus acinetobacter are important opportunistic pathogens in hospital-acquired infections. they cause various types of human infections, including pneumonia, wound infections, urinary tract infections, bacteremia, and meningitis. of the currently known acinetobacter species, acinetobacter baumannii is the most prevalent in clinical specimens. numerous outbreaks caused by a. baumannii have been reported, which are of great concern in clinical settings. despite convincing evidence linking a. baumannii with human infections, the study of its pathogenic mechanism is still in its elementary stage. adherence of bacteria to epithelial cells is an essential step towards colonization and infection. bacterial adherence to host cells is mediated by fimbria or membrane components. furthermore, many pathogenic bacteria are capable of invading non-phagocytic cells and evolve to survive within the host cells. the cellular invasion of bacteria contributes to evasion of humoral immunity, persistence in the host, and penetration into deep tissues. bacterial pathogens gain entry to non-phagocytic cells via two mechanisms; a zipper-like mechanism and a trigger mechanism, which were initially classified based on morphological differences. the zipper-like mechanism requires the direct interaction of bacterial ligands to the host's cell surface receptors and involves local cytoskeletal rearrangement at the invasion site. in contrast, the trigger mechanism is initiated by the injected bacterial effector proteins delivered by the type iii secretion system. the effector proteins regulate cytoskeleton dynamics and induce dramatic cytoskeletal rearrangements such as membrane ruffles. the previous study demonstrated that adherence of a. baumannii to human bronchial epithelial cells was mediated by fimbrial-like structures and entrapment of bacteria by cellular protrusions. however, the mechanisms underlying invasion of a. baumannii in epithelial cells have not been explored so far. outer membrane proteins of gram-negative bacteria are key players in bacterial pathogenesis. ompa of escherichia coli, ospc of borrelia burgdorferi, and opa and opca of neisseria meningitidis facilitate adherence to and invasion of bacteria in host cells. ompa of a. baumannii is the most abundant surface protein with a molecular mass of kda and plays a role in permeability of small solutes. abompa purified from a. baumannii atcc 19606t and recombinant abompa bind to the surface of host cells and are localized in both the mitochondria and nuclei, which induce the death of host cells. based on previous findings that abompa bound to and entered host cells, we hypothesized that abompa is responsible for adherence to and invasion of a. baumannii in epithelial cells during the colonization and early stage of bacterial infection. in the present study, we investigated the potential of a. baumannii to invade epithelial cells and studied the role of abompa in interactions of a. baumannii with epithelial cells.  a. baumannii invades epithelial cells dependently on both the bacterial strains and cell types to determine whether a. baumannii invaded epithelial cells, a. baumannii atcc 19606t and four gentamicin-susceptible a. baumannii isolates from clinical specimens were selected and gentamicin protection assay was performed. a multiplicity of infection of that was previously optimized in the adherence assay of a. baumannii was used in the cell invasion assay. to optimize infection time, human bronchial nci-h cells were infected for up to h with a. baumannii atcc 19606t. intracellular bacterial counts increased steadily with incubation times, but cell death and destruction of cell monolayer appeared h after infection. therefore, we chose h as the maximum infection period in the following study to exclude any secondary effect of cell death. nci-h cells were infected with four clinical a. baumannii isolates at an moi of for h. cell invasion of a. baumannii was different between bacterial strains: a. baumannii 05ka was the most invasive, whereas a. baumannii 05p was the least invasive. to determine whether the cell invasion of a. baumannii is dependent on epithelial cell types, human bronchial nci-h cells, human laryngeal hep- cells, and human cervical hela cells were infected with a. baumannii strains. nci-h and hep- cells derived from the respiratory tract were more susceptible to cell invasion of a. baumannii than non-respiratory tract-derived hela cells. confocal microscopic images provided direct evidence of intracellular localization of a. baumannii. a. baumannii invades epithelial cells by a zipper-like mechanism to investigate the involvement of microfilaments and microtubules in the cell invasion of a. baumannii, epithelial cells were pretreated with cytoskeleton inhibitors, cytochalasin d for microfilaments and vinblastine for microtubules, and a gentamicin protection assay was performed. both cytochalasin d and vinblastine significantly inhibited the cell invasion of a. baumannii. pretreatment of nci-h cells with cytochalasin d resulted in a decrease of intracellular bacteria of 93% and 90% in a. baumannii 05ka and type strain, respectively. cell invasion was also significantly decreased by vinblastine. these results suggest that a. baumannii invades epithelial cells through both microfilament- and microtubule-dependent uptake mechanisms. the association of a. baumannii with epithelial cells was characterized in detail by electron microscopy. scanning electron microscopy exhibited the intimate interactions of a. baumannii with epithelial cells. the cell membrane extended to and wrapped around bacteria, but the membrane ruffles were not appeared. transmission electron microscopy illuminated the sequential cell invasion of a. baumannii. bacteria were loosely attached to the cell surface and small membrane process protruded from the cells. after internalization of a. baumannii in epithelial cells, the cell membrane closed at the invasion site. internalized bacteria were located in the membrane-bound vacuoles. there was no sign of a membrane ruffle in sem and tem images. cellular changes, such as membrane wrapping around bacteria and fitted vacuoles to bacterial size, suggest that a. baumannii is internalized by a zipper-like mechanism, but not a trigger mechanism. abompa mediates the interaction of a. baumannii with epithelial cells to determine the potential interaction of abompa with epithelial cells, the binding of rabompa to various types of epithelial cells was analyzed by using the flow cytometry. it was found that rabompa specifically bound to the surface of epithelial cells tested. the mean fluorescence intensities of nci-h and hep- cells were higher than those of hela cells, which was in accordance with the cell invasion of a. baumannii. we determined whether rabompa inhibited the interactions of a. baumannii with epithelial cells. the cytotoxicity of rabompa was first assessed, because ≥ μg/ml of abompa purified from a. baumannii atcc 19606t induced the cytotoxicity of hep- cells. however, ≤ μg/ml of rabompa did not induce any morphological and biochemical changes of epithelial cells for h of incubation. nci-h and hep- cells were pretreated with μg/ml of rabompa for h and infected with the highly invasive a. baumannii 05ka for h. as a result, rabompa attenuated the binding of a. baumannii to epithelial cells. adherence of a. baumannii 05ka to epithelial cells significantly decreased to 81% in hep- cells and 87% in nci-h cells. to determine whether rabompa also inhibited the cell invasion of a. baumannii, epithelial cells were pretreated with μg/ml of rabompa for h and infected with a. baumannii for h. the cell invasion of a. baumannii 05ka significantly decreased to 69% in nci-h cells and 67% in hep- cells. collectively, these results suggest that abompa mediates the adherence to and invasion of a. baumannii in epithelial cells. cell invasion of isogenic abompa- mutant decreases as compared with wild-type bacteria to compare the cell adherence of a. baumannii atcc 19606t and its isogenic abompa- mutant ks nci-h and hep- cells were infected with bacteria for h. wild-type bacteria exhibited dispersed adherence. the number of adherent bacteria was 1– bacteria/infected cells and the percentage of infected cells was less than 3%. the total number of adherent bacteria per cells was ± in nci-h cells. however, isogenic abompa- mutant formed microcolonies on the surface of epithelial cells, rather than dispersed adherence. the number of adherent bacteria per cells was not significantly different between wild-type bacteria and abompa- mutant. to assess the cell invasion of wild-type bacteria and abompa- mutant, epithelial cells were infected with bacteria for h. the cell invasion of abompa- mutant significantly decreased to 95% as compared with wild-type bacteria. a low cell invasion of abompa- mutant was not due to retarded bacterial growth, because there was no difference in bacterial growth between wild-type bacteria and abompa- mutant. abompa plays a role in a. baumannii pathogenesis in vivo to determine the role of abompa on a. baumannii pathogenesis in vivo, the murine pneumonia model was introduced. the c57bl/ mice were intratracheally infected with × cfu of wild-type bacteria and isogenic abompa- mutant, and euthanized two days after bacterial challenge. total number of bacterial counts in the lungs were similar between mice infected with wild-type bacteria and abompa- mutant, but lung histopathology such as the infiltration of polymorphonuclear leukocytes and destruction of alveolar structures was prominently observed in the mice infected with wild-type bacteria. bacteremia occurred in mice infected with both bacterial strains, but there was a significant difference in the bacterial burden between the mice infected with wild-type bacteria and isogenic abompa- mutant. these results suggest that abompa is directly associated with the lung pathology and is responsible for the dissemination of a. baumannii into the bloodstream. discussion it was clearly demonstrated that a. baumannii has the potential to invade epithelial cells. host cells play a significant role in determining the cell invasion of a. baumannii. epithelial cells derived from the respiratory tract were more susceptible to a. baumannii invasion than epithelial cells derived from non-respiratory tract. moreover, the binding capacity of rabompa in respiratory tract-derived epithelial cells was higher than that of non-respiratory tract-derived epithelial cells. these findings may be partly responsible for the prevalence of a. baumannii in the respiratory tract, which is the most frequent colonization and infection site. the efficiency of cell invasion of a. baumannii was lower than other invasive pathogens such as e. coli, campylobacter upsaliensis, helicobacter pylori, n. memingitidis, pseudomonas aeruginosa, and yersinia enterocolitica. a previous study showed that most clinical a. baumannii isolates infected only 20–30% of nci-h cells and the number of adhering bacteria was 1– bacteria per infected cell. this finding reflects the low cellular invasion of a. baumannii in the current study. a relatively low adherence and invasion of a. baumannii in epithelial cells may contribute to a low virulence of this opportunistic pathogen, which particularly infects critically ill or severely wounded patients. a. baumannii invaded epithelial cells by a zipper-like mechanism, as demonstrated by the morphological characteristics. a. baumannii was loosely attached to the surface of epithelial cells. this observation is consistent with the previous study regarding adherence of a. baumannii to nci-h cells. however, the entrapment of bacteria by cellular protrusions, as noted in the previous study, was not observed. after loose attachment of a. baumannii to epithelial cells, the cell membrane was extended to and wrapped around the bacteria, which are typical characteristics of the zipper-like mechanism. these cellular responses against a. baumannii were mediated by actin rearrangement and membrane reorganization. confocal images and gentamicin protection assay with cytochalasin d showed the involvement of actin filaments in the cell invasion of a. baumannii. the microtubule inhibitor vinblastine also significantly inhibited the cell invasion of a. baumannii. vinblastine may inhibit the transport of membrane-bound bacteria from the plasma membrane to the cytoplasm by preventing their movement in the microtubule-mediated contractile process. once inside the host cells, internalized a. baumannii was located in the membrane-bound vacuoles. this result, in conjunction with the results of the gentamicin protection assay, suggests that a. baumannii survives within membrane-bound vacuoles in the cytoplasm. many intracellular bacteria are located in a membrane-bound vacuole compartment, but some, such as neisseria spp., listeria monocytogenes, salmonella spp., and yersinia spp., escape the cytoplasmic vacuoles and are freely located in the cytoplasm. in regards to most pathogenic bacteria, a membrane-bound vacuole containing pathogens does not undergo sequential fusion with endosomes or lysosomes for its own benefit. further studies are needed to characterize the fate of intracellular a. baumannii to understand the survival and dissemination of bacteria. outer membrane proteins of gram-negative bacteria are known to be directly involved in the interaction of bacteria with host cells. the present study focused on whether abompa is responsible for the interaction of a. baumannii with epithelial cells. our data suggest that abompa is a major effector molecule of a. baumannii interacting with epithelial cells according to the following experimental results: rabompa directly binds to the surface of epithelial cells; pre-treatment of rabompa significantly attenuates the binding of a. baumannii to epithelial cells; rabompa significantly inhibits the cell invasion of a. baumannii, and the cell invasion of abompa- mutant decreased to 95% in comparison to wild-type bacteria. in a murine pneumonia model, wild-type bacteria and isogenic abompa- mutant induced different lung pathologies. moreover, wild-type a. baumannii had the capacity to disseminate into the bloodstream, whereas abompa- mutant rarely cross barriers to disseminate in this manner. accordingly, abompa plays an important role in the a. baumannii pathogenesis regarding the induction of pneumonia and bacteremia. a. baumannii can adhere to and invade epithelial cells during colonization and the early stage of infection. in addition to its biological role as a porin, abompa plays a versatile role in the a. baumannii pathogenesis regarding its interaction with epithelial cells, the induction of apoptosis of host cells, and the dissemination of bacteria into the bloodstream. our data provide a novel insight into a. baumannii pathogenesis and contribute to further our knowledge of a. baumannii infections.  a. baumannii invades epithelial cells depending on both bacterial strains and cell types. cellular invasion is mediated by both microfilament- and microtubule-dependent uptake mechanisms. abompa is a microbial component responsible for the adherence to and invasion of a. baumannii in epithelial cells. the data obtained provide a novel insight into the a. baumannii pathogenesis in the early stage of bacterial infection. 
cell invasion of isogenic abompa- mutant significantly decreased as compared with wild-type bacteria. acinetobacter baumannii is a nosocomial pathogen of increasing importance, but the pathogenic mechanism of this microorganism has not been fully explored. in a murine pneumonia model, wild-type bacteria exhibited a severe lung pathology and induced a high bacterial burden in blood, whereas abompa- mutant was rarely detected in blood. these findings contribute to the understanding of a. baumannii pathogenesis in the early stage of bacterial infection. a. baumannii invaded epithelial cells by a zipper-like mechanism, which is associated with microfilament- and microtubule-dependent uptake mechanisms. this study investigated the potential of a. baumannii to invade epithelial cells and determined the role of a. baumannii outer membrane protein a in interactions with epithelial cells. pretreatment of recombinant abompa significantly inhibited the adherence to and invasion of a. baumannii in epithelial cells. a. baumannii adheres to and invades epithelial cells. abompa plays a major role in the interactions with epithelial cells. internalized bacteria were located in the membrane-bound vacuoles.
 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 in interactions with epithelial cells.  a. baumannii invaded epithelial cells by a zipper-like mechanism, which is associated with microfilament- and microtubule-dependent uptake mechanisms. internalized bacteria were located in the membrane-bound vacuoles. pretreatment of recombinant abompa significantly inhibited the adherence to and invasion of a. baumannii in epithelial cells. cell invasion of isogenic abompa- mutant significantly decreased as compared with wild-type bacteria. in a murine pneumonia model, wild-type bacteria exhibited a severe lung pathology and induced a high bacterial burden in blood, whereas abompa- mutant was rarely detected in blood.  a. baumannii adheres to and invades epithelial cells. abompa plays a major role in the interactions with epithelial cells. these findings contribute to the understanding of a. baumannii pathogenesis in the early stage of bacterial infection. 