The ability of Acinetobacter baumannii to adhere to and persist on surfaces as biofilms could be central to its pathogenicity. The production of pili and a biofilm-associated protein and the expression of antibiotic resistance are needed for robust biofilm formation on abiotic and biotic surfaces. This multistep process also depends on the expression of transcriptional regulatory functions, some of which could sense nutrients available to cells. This report extends previous observations by showing that although outer membrane protein A (OmpA) of A. baumannii 19606 plays a partial role in the development of robust biofilms on plastic, it is essential for bacterial attachment to Candida albicans filaments and A549 human alveolar epithelial cells. In contrast to abiotic surfaces, the interaction with biotic surfaces is independent of the CsuA/BABCDE-mediated pili. The interaction of A. baumannii 19606 with fungal and epithelial cells also results in their apoptotic death, a response that depends on the direct contact of bacteria with these two types of eukaryotic cells. Furthermore, the bacterial adhesion phenotype correlates with the ability of bacteria to invade A549 epithelial cells. Interestingly, the killing activity of cell-free culture supernatants proved to be protease and temperature sensitive, suggesting that its cytotoxic activity is due to secreted proteins, some of which are different from OmpA.

Acinetobacter baumannii is a Gram-negative opportunistic nosocomial pathogen. This microorganism survives in hospital environments despite unfavorable conditions such as desiccation, nutrient starvation and antimicrobial treatments. It is hypothesized that its ability to persist in these environments, as well as its virulence, is a result of its capacity to form biofilms. A. baumannii forms biofilms on abiotic surfaces such as polystyrene and glass as well as biotic surfaces such as epithelial cells and fungal filaments. Pili assembly and production of the Bap surface-adhesion protein play a role in biofilm initiation and maturation after initial attachment to abiotic surfaces. Furthermore, the adhesion and biofilm phenotypes of some clinical isolates seem to be related to the presence of broad-spectrum antibiotic resistance. The regulation of the formation and development of these biofilms is as diverse as the surfaces on which this bacterium persists and as the cellular components that participate in th...

Acinetobacter baumannii , which causes serious infections in immunocompromised patients, expresses high-affinity iron acquisition functions needed for growth under iron-limiting laboratory conditions. In this study, we determined that the initial interaction of the ATCC 19606 ^T type strain with A549 human alveolar epithelial cells is independent of the production of BasD and BauA, proteins needed for acinetobactin biosynthesis and transport, respectively. In contrast, these proteins are required for this strain to persist within epithelial cells and cause their apoptotic death. Infection assays using Galleria mellonella larvae showed that impairment of acinetobactin biosynthesis and transport functions significantly reduces the ability of ATCC 19606 ^T cells to persist and kill this host, a defect that was corrected by adding inorganic iron to the inocula. The results obtained with these ex vivo and in vivo approaches were validated using a mouse sepsis model, which showed that expression of the acinetobactin-mediated iron acquisition system is critical for ATCC 19606 ^T to establish an infection and kill this vertebrate host. These observations demonstrate that the virulence of the ATCC 19606 ^T strain depends on the expression of a fully active acinetobactin-mediated system. Interestingly, the three models also showed that impairment of BasD production results in an intermediate virulence phenotype compared to those of the parental strain and the BauA mutant. This observation suggests that acinetobactin intermediates or precursors play a virulence role, although their contribution to iron acquisition is less relevant than that of mature acinetobactin.

Light is a ubiquitous environmental signal that many organisms sense and respond to by modulating their physiological responses accordingly. While this is an expected response among phototrophic microorganisms, the ability of chemotrophic prokaryotes to sense and react to light has become a puzzling and novel issue in bacterial physiology, particularly among bacterial pathogens. In this work, we show that the opportunistic pathogenAcinetobacter baumanniisenses and responds to blue light. Motility and formation of biofilms and pellicles were observed only when bacterial cells were incubated in darkness. In contrast, the killing ofCandida albicansfilaments was enhanced when they were cocultured with bacteria under light. These bacterial responses depend on the expression of theA. baumanniiATCC 17978 A1S_2225 gene, which codes for an 18.6-kDa protein that contains an N-terminalblue-light-sensing-usingflavin (BLUF) domain and lacks a detectable output domain(s). Spectral analyses of the purified recombinant protein showed its ability to sense light by a red shift upon illumination. Therefore, the A1S_2225 gene, which is present in several members of theAcinetobactergenus, was namedblue-light-sensingA(blsA). Interestingly, temperature plays a role in the ability ofA. baumanniito sense and respond to light via the BlsA photoreceptor protein.

TheAcinetobacter baumanniitype strain, ATCC 19606, secretes acinetobactin, a catechol siderophore highly related to the iron chelator anguibactin produced by the fish pathogenVibrio anguillarum(Listonella anguillarum). This paper reports the initial characterization of the genes and gene products involved in the acinetobactin-mediated iron-acquisition process. Insertional mutagenesis resulted in the isolation of several derivatives whose ability to grow in medium containing the iron chelator 2,2′-dipyridyl was affected. One of the insertions disrupted a gene encoding a predicted outer-membrane protein, named BauA, highly similar to FatA, the receptor for ferric anguibactin. Immunological relatedness of BauA with FatA was confirmed by Western blot analysis. Another transposon insertion was mapped to a gene encoding a protein highly similar to FatD, the permease component of the anguibactin transport system. Further DNA sequencing and nucleotide sequence analysis revealed that theseA. baumannii19606 genes are part of a polycistronic locus that contains thebauDCEBAORFs. While the translation products ofbauD, -C, -Band -Aare highly related to theV. anguillarumFatDCBA iron-transport proteins, the product ofbauEis related to the ATPase component of Gram-positive ATP-binding cassette (ABC) transport systems. This entire locus is flanked by genes encoding predicted proteins related to AngU and AngN,V. anguillarumproteins required for the biosynthesis of anguibactin. These protein similarities, as well as the structural similarity of anguibactin and acinetobactin, suggested that these two siderophores could be utilized by both bacterial strains, a possibility that was confirmed by siderophore utilization bioassays. Taken together, these results demonstrate that these pathogens, which cause serious infections in unrelated hosts, express very similar siderophore-mediated iron-acquisition systems.