Effect of ethanol on differential protein production and expression of potential virulence functions in the opportunistic pathogen Acinetobacter baumannii

Presence of Methicillin-Resistant Staphylococci and Carbapenemase-Positive Acinetobacter Isolates on Surfaces in German Dog Daycare Facilities and Correlation with Cleaning Practices

Many bacteria, including methicillin-resistant staphylococci and opportunistic pathogens such as Acinetobacter (A.) species, in particular members of the A. calcoaceticus-A. baumannii (Acb) complex, are known to survive in environmental settings. This increases the risk of bacterial spreading and transmission to animals and humans, especially in institutions with a high animal population density. This study aimed to identify the presence of multidrug-resistant (MDR) staphylococci and Acinetobacter species in dog daycare facilities (DDFs). The surfaces of 16 DDFs were sampled and information about cleaning practices was obtained. After microbial identification, the detected isolates were investigated by multiplex PCR for antimicrobial resistance genes. In 200 location samples, 38 staphylococci and 109 Acinetobacter spp. isolates were identified. Methicillin-resistant genes were confirmed in 18 staphylococci isolates from four DDFs, including Staphylococcus (S.) equorum, S. saprophyticus, S. cohnii, S. lentus, and S. haemolyticus. Eight A. radioresistens isolates comprising the blaOXA-23 carbapenemase gene and seventeen isolates belonging to the Acb complex were also isolated. This is the first investigation for MDR pathogens in DDFs, a close human-animal interaction environment. Though the role of DDFs in the transmission and spreading of MDR bacteria is not known, the findings should contribute to the public awareness and underscore the necessity of adequate cleaning protocols.

P. aeruginosa interactions with other microbes in biofilms during co-infection

This review addresses the topic of biofilms, including their development and the interaction between different counterparts. There is evidence that various diseases, such as cystic fibrosis, otitis media, diabetic foot wound infections, and certain cancers, are promoted and aggravated by the presence of polymicrobial biofilms. Biofilms are composed by heterogeneous communities of microorganisms protected by a matrix of polysaccharides. The different types of interactions between microorganisms gives rise to an increased resistance to antimicrobials and to the host's defense mechanisms, with the consequent worsening of disease symptoms. Therefore, infections caused by polymicrobial biofilms affecting different human organs and systems will be discussed, as well as the role of the interactions between the gram-negative bacteria Pseudomonas aeruginosa, which is at the base of major polymicrobial infections, and other bacteria, fungi, and viruses in the establishment of human infections and diseases. Considering that polymicrobial biofilms are key to bacterial pathogenicity, it is fundamental to evaluate which microbes are involved in a certain disease to convey an appropriate and efficacious antimicrobial therapy.

Gut microbiota in alcohol-related liver disease: pathophysiology and gut-brain cross talk

Alcohol-related liver disease (ALD) from excessive alcohol intake has a unique gut microbiota profile. The disease progression-free survival in ALD patients has been associated with the degree of gut dysbiosis. The vicious cycles between gut dysbiosis and the disease progression in ALD including: an increase of acetaldehyde production and bile acid secretion, impaired gut barrier, enrichment of circulating microbiota, toxicities of microbiota metabolites, a cascade of pro-inflammatory chemokines or cytokines, and augmentation in the generation of reactive oxygen species. The aforementioned pathophysiology process plays an important role in different disease stages with a spectrum of alcohol hepatitis, ALD cirrhosis, neurological dysfunction, and hepatocellular carcinoma. This review aims to illustrate the pathophysiology of gut microbiota and clarify the gut-brain crosstalk in ALD, which may provide the opportunity of identifying target points for future therapeutic intervention in ALD.

Cross-protection and cross-feeding between Klebsiella pneumoniae and Acinetobacter baumannii promotes their co-existence

Acinetobacter baumannii and Klebsiella pneumoniae are opportunistic pathogens frequently co-isolated from polymicrobial infections. The infections where these pathogens co-exist can be more severe and recalcitrant to therapy than infections caused by either species alone, however there is a lack of knowledge on their potential synergistic interactions. In this study we characterise the genomes of A. baumannii and K. pneumoniae strains co-isolated from a single human lung infection. We examine various aspects of their interactions through transcriptomic, phenomic and phenotypic assays that form a basis for understanding their effects on antimicrobial resistance and virulence during co-infection. Using co-culturing and analyses of secreted metabolites, we discover the ability of K. pneumoniae to cross-feed A. baumannii by-products of sugar fermentation. Minimum inhibitory concentration testing of mono- and co-cultures reveals the ability for A. baumannii to cross-protect K. pneumoniae against the cephalosporin, cefotaxime. Our study demonstrates distinct syntrophic interactions occur between A. baumannii and K. pneumoniae, helping to elucidate the basis for their co-existence in polymicrobial infections.

Lipocalin-2 is an essential component of the innate immune response to Acinetobacter baumannii infection

Acinetobacter baumannii is an opportunistic pathogen and an emerging global health threat. Within healthcare settings, major presentations of A. baumannii include bloodstream infections and ventilator-associated pneumonia. The increased prevalence of ventilated patients during the COVID-19 pandemic has led to a rise in secondary bacterial pneumonia caused by multidrug resistant (MDR) A. baumannii. Additionally, due to its MDR status and the lack of antimicrobial drugs in the development pipeline, the World Health Organization has designated carbapenem-resistant A. baumannii to be its priority critical pathogen for the development of novel therapeutics. To better inform the design of new treatment options, a comprehensive understanding of how the host contains A. baumannii infection is required. Here, we investigate the innate immune response to A. baumannii by assessing the impact of infection on host gene expression using NanoString technology. The transcriptional profile observed in the A. baumannii infected host is characteristic of Gram-negative bacteremia and reveals expression patterns consistent with the induction of nutritional immunity, a process by which the host exploits the availability of essential nutrient metals to curtail bacterial proliferation. The gene encoding for lipocalin-2 (Lcn2), a siderophore sequestering protein, was the most highly upregulated during A. baumannii bacteremia, of the targets assessed, and corresponds to robust LCN2 expression in tissues. Lcn2^-/- mice exhibited distinct organ-specific gene expression changes including increased transcription of genes involved in metal sequestration, such as S100A8 and S100A9, suggesting a potential compensatory mechanism to perturbed metal homeostasis. In vitro, LCN2 inhibits the iron-dependent growth of A. baumannii and induces iron-regulated gene expression. To elucidate the role of LCN2 in infection, WT and Lcn2^-/- mice were infected with A. baumannii using both bacteremia and pneumonia models. LCN2 was not required to control bacterial growth during bacteremia but was protective against mortality. In contrast, during pneumonia Lcn2^-/- mice had increased bacterial burdens in all organs evaluated, suggesting that LCN2 plays an important role in inhibiting the survival and dissemination of A. baumannii. The control of A. baumannii infection by LCN2 is likely multifactorial, and our results suggest that impairment of iron acquisition by the pathogen is a contributing factor. Modulation of LCN2 expression or modifying the structure of LCN2 to expand upon its ability to sequester siderophores may thus represent feasible avenues for therapeutic development against this pathogen.

A lack of therapeutic options has prompted the World Health Organization to designate multidrug-resistant Acinetobacter baumannii as its priority critical pathogen for research into new treatment strategies. The mechanisms employed by A. baumannii to cause disease and the host tactics exercised to constrain infection are not fully understood. Here, we further characterize the innate immune response to A. baumannii infection. We identify nutritional immunity, a process where the availability of nutrient metals is exploited to restrain bacterial growth, as being induced during infection. The gene encoding for lipocalin-2 (Lcn2), a protein that can impede iron uptake by bacteria, is highly upregulated in infected mice, and corresponds to robust LCN2 detection in the tissues. We find that LCN2 is crucial to reducing mortality from A. baumannii bacteremia and inhibits dissemination of the pathogen during pneumonia. In wild-type and Lcn2-deficient mice, broader transcriptional profiling reveals expression patterns consistent with the known response to Gram-negative bacteremia. Although the role of LCN2 in infection is likely multifactorial, we find its antimicrobial effects are at least partly exerted by impairing iron acquisition by A. baumannii. Facets of nutritional immunity, such as LCN2, may be exploited as novel therapeutics in combating A. baumannii infection.

Maximizing the recovery of the native p28 bacterial peptide with improved activity and maintained solubility and stability in Escherichia coli BL21 (DE3)

p28 is a natural bacterial product, which recently has attracted much attention as an efficient cell penetrating peptide (CPP) and a promising anticancer agent. Considering the interesting biological qualities of p28, maximizing its expression appears to be a prominent priority. The optimization of such bioprocesses might be facilitated by utilizing statistical approaches such as Design of Experiment (DoE). In this study, we aimed to maximize the expression of "biologically active" p28 in Escherichia coli BL21 (DE3) host by harnessing statistical tools and experimental methods. Using Minitab, Plackett-Burman and Box-Behnken Response Surface Methodology (RSM) designs were generated to optimize the conditions for the expression of p28. Each condition was experimentally investigated by assessing the biological activity of the purified p28 in the MCF-7 breast cancer cell line. Seven independent variables were investigated, and three of them including ethanol concentration, OD600 of the culture at the time of induction, and the post-induction temperature were demonstrated to significantly affect the p28 expression in E. coli. The cytotoxicity, penetration efficiency, and total process time were measured as dependent variables. The optimized expression conditions were validated experimentally, and the final products were investigated in terms of expression yield, solubility, and stability in vitro. Following the optimization, an 8-fold increase of the concentration of p28 expression was observed. In this study, we suggest an optimized combination of effective factors to produce soluble p28 in the E. coli host, a protocol that results in the production of a significantly high amount of the biologically active peptide with retained solubility and stability.

Customized low-cost high-throughput amplifier for electro-fluidic detection of cell volume changes in point-of-care applications

Physical parameters of the pathogenic cells, like its volume, shape, and stiffness, are important biomarkers for diseases, chemical changes within the cell, and overall cell health. The response of pathogenic bacteria and viruses to different chemical disinfectants is studied widely. Some of the routinely employed techniques to measure these changes require elaborate and expensive equipment which limits any study to a non-mobile research lab facility. Recently, we showed a micropore-based electro-fluidic technique to have great promise in measuring subtle changes in cell volumes at high throughput and resolution. This method, however, requires commercial amplifiers, which makes this technique expensive and incompatible for in-field use. In this paper, we develop a home-built amplifier to make this technique in-field compatible and apply it to measure changes in bacterial volumes upon exposure to alcohol. First, we introduce our low-cost and portable transimpedance amplifier and characterize the maximum range, absolute error percentage, and RMS noise of the amplifier in the measured current signal, along with the amplifier's bandwidth, and compared these characteristics with the commercial amplifiers. Using our home-built amplifier, we demonstrate a high throughput detection of ~1300 cells/second and resolve cell diameter changes down to 1 μm. Finally, we demonstrate measurement of cell volume changes in E. coli bacteria when exposed to ethanol (5% v/v), which is otherwise difficult to measure via imaging techniques. Our low-cost amplifier (~100-fold lower than commercial alternatives) is battery-run, completely portable for point-of-care applications, and the electro-fluidic devices are currently being tested for in-field applications.

Convergence of Biofilm Formation and Antibiotic Resistance in Acinetobacter baumannii Infection

Acinetobacter baumannii (A. baumannii) is a leading cause of nosocomial infections as this pathogen has certain attributes that facilitate the subversion of natural defenses of the human body. A. baumannii acquires antibiotic resistance determinants easily and can thrive on both biotic and abiotic surfaces. Different resistance mechanisms or determinants, both transmissible and non-transmissible, have aided in this victory over antibiotics. In addition, the propensity to form biofilms (communities of organism attached to a surface) allows the organism to persist in hospitals on various medical surfaces (cardiac valves, artificial joints, catheters, endotracheal tubes, and ventilators) and also evade antibiotics simply by shielding the bacteria and increasing its ability to acquire foreign genetic material through lateral gene transfer. The biofilm formation rate in A. baumannii is higher than in other species. Recent research has shown how A. baumannii biofilm-forming capacity exerts its effect on resistance phenotypes, development of resistome, and dissemination of resistance genes within biofilms by conjugation or transformation, thereby making biofilm a hotspot for genetic exchange. Various genes control the formation of A. baumannii biofilms and a beneficial relationship between biofilm formation and "antimicrobial resistance" (AMR) exists in the organism. This review discusses these various attributes of the organism that act independently or synergistically to cause hospital infections. Evolution of AMR in A. baumannii, resistance mechanisms including both transmissible (hydrolyzing enzymes) and non-transmissible (efflux pumps and chromosomal mutations) are presented. Intrinsic factors [biofilm-associated protein, outer membrane protein A, chaperon-usher pilus, iron uptake mechanism, poly-β-(1, 6)-N-acetyl glucosamine, BfmS/BfmR two-component system, PER-1, quorum sensing] involved in biofilm production, extrinsic factors (surface property, growth temperature, growth medium) associated with the process, the impact of biofilms on high antimicrobial tolerance and regulation of the process, gene transfer within the biofilm, are elaborated. The infections associated with colonization of A. baumannii on medical devices are discussed. Each important device-related infection is dealt with and both adult and pediatric studies are separately mentioned. Furthermore, the strategies of preventing A. baumannii biofilms with antibiotic combinations, quorum sensing quenchers, natural products, efflux pump inhibitors, antimicrobial peptides, nanoparticles, and phage therapy are enumerated.

Human Serum Proteins and Susceptibility of Acinetobacter baumannii to Cefiderocol: Role of Iron Transport

Cefiderocol, a recently introduced antibiotic, has a chemical structure that includes a cephalosporin that targets cell wall synthesis and a chlorocatechol siderophore moiety that facilitates cell penetration by active iron transporters. Analysis of the effect that human serum, human serum albumin, and human pleural fluid had on growing Acinetobacter baumannii showed that genes related to iron uptake were down-regulated. At the same time, β-lactamase genes were expressed at higher levels. The minimum inhibitory concentrations of this antimicrobial in A. baumannii cells growing in the presence of human serum, human serum albumin, or human pleural fluid were higher than those measured when these fluids were absent from the culture medium. These results correlate with increased expression levels of β-lactamase genes and the down-regulation of iron uptake-related genes in cultures containing human serum, human serum albumin, or human pleural fluid. These modifications in gene expression could explain the less-than-ideal clinical response observed in patients with pulmonary or bloodstream A. baumannii infections. The exposure of the infecting cells to the host's fluids could cause reduced cefiderocol transport capabilities and increased resistance to β-lactams. The regulation of genes that could impact the A. baumannii susceptibility to cefiderocol, or other antibacterials, is an understudied phenomenon that merits further investigation.

Galleria mellonella as a Good Model to Study Acinetobacter baumannii Pathogenesis

The invertebrate model, Galleria mellonella, has been widely used to study host-pathogen interactions due to its cheapness, ease of handling, and similar mammalian innate immune system. G. mellonella larvae have been proven to be useful and a reliable model for analyzing pathogenesis mechanisms of multidrug resistant Acinetobacter baumannii, an opportunistic pathogen difficult to kill. This review describes the detailed experimental design of G. mellonella/A. baumannii models, and provides a comprehensive comparison of various virulence factors and therapy strategies using the G. mellonella host. These investigations highlight the importance of this host-pathogen model for in vivo pathogen virulence studies. On the long term, further development of the G. mellonella/A. baumannii model will offer promising insights for clinical treatments of A. baumannii infection.

Role of Iron-Containing Alcohol Dehydrogenases in Acinetobacter baumannii ATCC 19606 Stress Resistance and Virulence

Most bacteria possess alcohol dehydrogenase (ADH) genes (Adh genes) to mitigate alcohol toxicity, but these genes have functions beyond alcohol degradation. Previous research has shown that ADH can modulate quorum sensing in Acinetobacter baumannii, a rising opportunistic pathogen. However, the number and nature of Adh genes in A. baumannii have not yet been fully characterized. We identified seven alcohol dehydrogenases (NAD⁺-ADHs) from A. baumannii ATCC 19606, and examined the roles of three iron-containing ADHs, ADH3, ADH4, and ADH6. Marker-less mutation was used to generate Adh3, Adh4, and Adh6 single, double, and triple mutants. Disrupted Adh4 mutants failed to grow in ethanol-, 1-butanol-, or 1-propanol-containing mediums, and recombinant ADH4 exhibited strongest activity against ethanol. Stress resistance assays with inorganic and organic hydroperoxides showed that Adh3 and Adh6 were key to oxidative stress resistance. Virulence assays performed on the Galleria mellonella model organism revealed that Adh4 mutants had comparable virulence to wild-type, while Adh3 and Adh6 mutants had reduced virulence. The results suggest that ADH4 is primarily involved in alcohol metabolism, while ADH3 and ADH6 are key to stress resistance and virulence. Further investigation into the roles of other ADHs in A. baumannii is warranted.

Analysis of Antimicrobial and Antibiofilm Activity of Human Milk Lactoferrin Compared to Bovine Lactoferrin against Multidrug Resistant and Susceptible Acinetobacter baumannii Clinical Isolates

Acinetobacter baumannii is an opportunistic bacterial pathogen that causes severe infections in immunocompromised patients. The emergence of multi- and pan-drug resistant strains of A. baumannii from clinical sources has confounded treatment and enhanced morbidity and mortality associated with these infections. One way that A. baumannii circumnavigates environmental and antimicrobial challenge is by forming tertiary architectural structures of cells known as biofilms. Biofilm-inhibiting molecules could be deployed as a potential chemotherapeutic strategy to inhibit or disrupt A. baumannii biofilms and mitigate adverse outcomes due to infection. Lactoferrin is an innate immune glycoprotein produced in high concentrations in both human and bovine milk which has previously been shown to have antibacterial and antibiofilm activities. We sought to test lactoferrin against a bank of clinical isolates of A. baumannii to determine changes in bacterial growth or biofilm formation. Our results indicate that human lactoferrin has slightly more potent antibacterial activities than bovine lactoferrin against certain strains of A. baumannii and that these effects are associated with anatomical site of isolation. Additionally, we have shown that both bovine and human lactoferrin can inhibit A. baumannii biofilm formation and that these effects are associated with anatomical site of isolation and whether the strain forms robust or weak biofilms.

Antimicrobial photodynamic therapy against Acinetobacter baumannii

Acinetobacter baumannii (A. baumannii) has emerged as a pathogen of global importance able to cause opportunistic infections on the skin, urinary tract, lungs, and bloodstream, being frequently involved in hospital outbreaks. Such bacterium can resist a variety of environmental conditions and develop resistance to different classes of antibiotics. Antimicrobial photodynamic therapy (aPDT) has been considered a promising approach to overcome bacterial resistance once it does not cause selective environmental pressure on bacteria. In this review, studies on aPDT were accessed on PubMed, and their findings were summarized regarding its efficacy against A. baumannii. The data obtained from the literature show that exogenous photosensitizers belonging to different chemical classes are effective against multidrug-resistant A. baumannii strains. However, most of such data is from in vitro studies, and additional studies are necessary to evaluate if the exogenous photosensitizers may induce selective pressure on A. baumannii and the effectiveness of such photosensitizers in clinical practice.

Pentamidine enhances photosensitization of Acinetobacter baumannii using diode lasers with emission of light at wavelength of ʎ = 405 nm and ʎ = 635 nm

Antimicrobial photodynamic inactivation is currently one of the most promising trends in the modern bactericidal protocols. Under the conditions defined in our studies, we found that in vitro photosensitization of A. baumannii with 5-ALA as a precursor of protoporphyrin IX (photosensitizer) reduces the concentration of viable cells in planktonic cultures, and this process can be strongly enhanced by pentamidine. Diode lasers with the peak-power wavelength of ʎ = 405 nm (radiation intensity of 26 mW cm^-2) and ʎ = 635 nm (radiation intensity of 55 mW cm^-2) were used in this study. It was found that a blue laser light (energy fluence of 64 J cm^-2; no external photosensitizer) in the presence of pentamidine resulted in a reduction of CFU of 99.992 % compared to 99.97 % killing without pentamidine. When a red laser light was used in the experiments (energy fluence of 136 J cm^-2; no external photosensitizer), the mortality rate was 99.98 % in the presence of pentamidine compared to 99.93 % of those killed without the addition of this drug. The lethal effect with 5-ALA was achieved under blue light fluence of 16 J cm^-2 (in the presence of pentamidine) and 32 J cm^-2 (without pentamidine). In the case of laser light of 635 nm, the lethal effect with 5-ALA was attained with energy fluence of 51 J cm^-2 (with pentamidine) and 102 J cm^-2 (without pentamidine). The possible roles of pentamidine in enhancing photodynamic inactivation of A. baumannii have been discussed.

Outer membrane vesicles containing OmpA induce mitochondrial fragmentation to promote pathogenesis of Acinetobacter baumannii

Acinetobacter baumannii is a highly antibiotic resistant Gram-negative bacterium that causes life-threatening infections in humans with a very high mortality rate. A. baumannii is an extracellular pathogen with poorly understood virulence mechanisms. Here we report that A. baumannii employs the release of outer membrane vesicles (OMVs) containing the outer membrane protein A (OmpAAb) to promote bacterial pathogenesis and dissemination. OMVs containing OmpAAb are taken up by mammalian cells where they activate the host GTPase dynamin-related protein 1 (DRP1). OmpAAb mediated activation of DRP1 enhances its accumulation on mitochondria that causes mitochondrial fragmentation, elevation in reactive oxygen species (ROS) production and cell death. Loss of DRP1 rescues these phenotypes. Our data show that OmpAAb is sufficient to induce mitochondrial fragmentation and cytotoxicity since its expression in E. coli transfers its pathogenic properties to E. coli. A. baumannii infection in mice also induces mitochondrial damage in alveolar macrophages in an OmpAAb dependent manner. We finally show that OmpAAb is also required for systemic dissemination in the mouse lung infection model. In this study we uncover the mechanism of OmpAAb as a virulence factor in A. baumannii infections and further establish the host cell factor required for its pathogenic effects.

New Provisional Function of OmpA from Acinetobacter sp. Strain SA01 Based on Environmental Challenges

Acinetobacter OmpA is known as a multifaceted protein with multiple functions, including emulsifying properties. Bioemulsifiers are surface-active compounds that can disperse hydrophobic compounds in water and help increase the bioavailability of hydrophobic hydrocarbons to be used by degrading microorganisms.

An outer membrane protein A (OmpA) from Acinetobacter sp. strain SA01 was identified and characterized in-depth based on the structural and functional characteristics already known of its homologues. In silico structural studies showed that this protein can be a slow porin, binds to peptidoglycan, and exhibits emulsifying properties. Characterization of the recombinant SA01-OmpA, based on its emulsifying properties, represented its promising potentials in biotechnology. Also, the presence of SA01-OmpA in outer membrane vesicles (OMV) and biofilm showed that this protein, like its homologues in Acinetobacter baumannii, can be secreted into the extracellular environment through OMVs and play a role in the formation of biofilm. After ensuring the correct selection of the protein of interest, the role of oxidative stress induced by cell nutritional parameters (utilization of specific carbon sources) on the expression level of OmpA was carefully studied. For this purpose, the oxidative stress level of SA01 cell cultures in the presence of three nonrelevant carbon sources (sodium acetate, ethanol, and phenol) was examined under each condition. High expression of SA01-OmpA in ethanol- and phenol-fed cells with higher levels of oxidative stress than acetate suggested that oxidative stress could be a substantial factor in the regulation of SA01-OmpA expression. The significant association of SA01-OmpA expression with the levels of oxidative stress induced by cadmium and H₂O₂, with oxidative stress-inducing properties and lack of nutritional value, confirmed that the cells tend to harness their capacities with a possible increase in OmpA production. Collectively, this study suggests a homeostasis role for OmpA in Acinetobacter sp. SA01 under oxidative stress besides assuming many other roles hitherto attributed to this protein.

IMPORTANCEAcinetobacter OmpA is known as a multifaceted protein with multiple functions, including emulsifying properties. Bioemulsifiers are surface-active compounds that can disperse hydrophobic compounds in water and help increase the bioavailability of hydrophobic hydrocarbons to be used by degrading microorganisms. In this study, an OmpA from Acinetobacter sp. SA01 was identified and introduced as an emulsifier with a higher emulsifying capacity than Pseudomonas aeruginosa rhamnolipid. We also showed that the expression of this protein is not dependent on the nutritional requirements but is more influenced by the oxidative stress caused by stressors. This finding, along with the structural role of this protein as a slow porin or its role in OMV biogenesis and biofilm formation, suggests that this protein can play an important role in maintaining cellular homeostasis under oxidative stress conditions. Altogether, the present study provides a new perspective on the functional performance of Acinetobacter OmpA, which can be used both to optimize its production as an emulsifier and a target in the treatment of multidrug-resistant strains.

Antimicrobial Resistance in ESKAPE Pathogens

Antimicrobial-resistant ESKAPE ( Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens represent a global threat to human health. The acquisition of antimicrobial resistance genes by ESKAPE pathogens has reduced the treatment options for serious infections, increased the burden of disease, and increased death rates due to treatment failure and requires a coordinated global response for antimicrobial resistance surveillance. This looming health threat has restimulated interest in the development of new antimicrobial therapies, has demanded the need for better patient care, and has facilitated heightened governance over stewardship practices.

Antimicrobial Resistance in ESKAPE Pathogens

Antimicrobial-resistant ESKAPE ( Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens represent a global threat to human health. The acquisition of antimicrobial resistance genes by ESKAPE pathogens has reduced the treatment options for serious infections, increased the burden of disease, and increased death rates due to treatment failure and requires a coordinated global response for antimicrobial resistance surveillance. This looming health threat has restimulated interest in the development of new antimicrobial therapies, has demanded the need for better patient care, and has facilitated heightened governance over stewardship practices.

Carbapenemases: Transforming Acinetobacter baumannii into a Yet More Dangerous Menace

Acinetobacter baumannii is a common cause of serious nosocomial infections. Although community-acquired infections are observed, the vast majority occur in people with preexisting comorbidities. A. baumannii emerged as a problematic pathogen in the 1980s when an increase in virulence, difficulty in treatment due to drug resistance, and opportunities for infection turned it into one of the most important threats to human health. Some of the clinical manifestations of A. baumannii nosocomial infection are pneumonia; bloodstream infections; lower respiratory tract, urinary tract, and wound infections; burn infections; skin and soft tissue infections (including necrotizing fasciitis); meningitis; osteomyelitis; and endocarditis. A. baumannii has an extraordinary genetic plasticity that results in a high capacity to acquire antimicrobial resistance traits. In particular, acquisition of resistance to carbapenems, which are among the antimicrobials of last resort for treatment of multidrug infections, is increasing among A. baumannii strains compounding the problem of nosocomial infections caused by this pathogen. It is not uncommon to find multidrug-resistant (MDR, resistance to at least three classes of antimicrobials), extensively drug-resistant (XDR, MDR plus resistance to carbapenems), and pan-drug-resistant (PDR, XDR plus resistance to polymyxins) nosocomial isolates that are hard to treat with the currently available drugs. In this article we review the acquired resistance to carbapenems by A. baumannii. We describe the enzymes within the OXA, NDM, VIM, IMP, and KPC groups of carbapenemases and the coding genes found in A. baumannii clinical isolates.

Insights into Acinetobacter baumannii: A Review of Microbiological, Virulence, and Resistance Traits in a Threatening Nosocomial Pathogen

Being a multidrug-resistant and an invasive pathogen, Acinetobacter baumannii is one of the major causes of nosocomial infections in the current healthcare system. It has been recognized as an agent of pneumonia, septicemia, meningitis, urinary tract and wound infections, and is associated with high mortality. Pathogenesis in A. baumannii infections is an outcome of multiple virulence factors, including porins, capsules, and cell wall lipopolysaccharide, enzymes, biofilm production, motility, and iron-acquisition systems, among others. Such virulence factors help the organism to resist stressful environmental conditions and enable development of severe infections. Parallel to increased prevalence of infections caused by A. baumannii, challenging and diverse resistance mechanisms in this pathogen are well recognized, with major classes of antibiotics becoming minimally effective. Through a wide array of antibiotic-hydrolyzing enzymes, efflux pump changes, impermeability, and antibiotic target mutations, A. baumannii models a unique ability to maintain a multidrug-resistant phenotype, further complicating treatment. Understanding mechanisms behind diseases, virulence, and resistance acquisition are central to infectious disease knowledge about A. baumannii. The aims of this review are to highlight infections and disease-producing factors in A. baumannii and to touch base on mechanisms of resistance to various antibiotic classes.

New Shuttle Vectors for Real-Time Gene Expression Analysis in Multidrug-Resistant Acinetobacter Species: In Vitro and In Vivo Responses to Environmental Stressors

The Acinetobacter genus includes species of opportunistic pathogens and harmless saprophytes. The type species, Acinetobacter baumannii, is a nosocomial pathogen renowned for being multidrug resistant (MDR). Despite the clinical relevance of infections caused by MDR A. baumannii and a few other Acinetobacter spp., the regulation of their pathogenicity remains elusive due to the scarcity of adequate genetic tools, including vectors for gene expression analysis. Here, we report the generation and testing of a series of Escherichia coli-Acinetobacter promoter-probe vectors suitable for gene expression analysis in Acinetobacter spp. These vectors, named pLPV1Z, pLPV2Z, and pLPV3Z, carry both gentamicin and zeocin resistance markers and contain lux, lacZ, and green fluorescent protein (GFP) reporter systems downstream of an extended polylinker, respectively. The presence of a toxin-antitoxin gene system and the high copy number allow pLPV plasmids to be stably maintained even without antibiotic selection. The pLPV plasmids can easily be introduced by electroporation into MDR A. baumannii belonging to the major international lineages as well as into species of the Acinetobacter calcoaceticus-A. baumannii complex. The pLPV vectors have successfully been employed to study the regulation of stress-responsive A. baumannii promoters, including the DNA damage-inducible uvrABC promoter, the ethanol-inducible adhP and yahK promoters, and the iron-repressible promoter of the acinetobactin siderophore biosynthesis gene basA A lux-tagged A. baumannii ATCC 19606^T strain, carrying the iron-responsive pLPV1Z::PbasA promoter fusion, allowed in vivo and ex vivo monitoring of the bacterial burden in the Galleria mellonella infection model.IMPORTANCE The short-term adaptive response to environmental cues greatly contributes to the ecological success of bacteria, and profound alterations in bacterial gene expression occur in response to physical, chemical, and nutritional stresses. Bacteria belonging to the Acinetobacter genus are ubiquitous inhabitants of soil and water though some species, such as Acinetobacter baumannii, are pathogenic and cause serious concern due to antibiotic resistance. Understanding A. baumannii pathobiology requires adequate genetic tools for gene expression analysis, and to this end we developed user-friendly shuttle vectors to probe the transcriptional responses to different environmental stresses. Vectors were constructed to overcome the problem of antibiotic selection in multidrug-resistant strains and were equipped with suitable reporter systems to facilitate signal detection. By means of these vectors, the transcriptional response of A. baumannii to DNA damage, ethanol exposure, and iron starvation was investigated both in vitro and in vivo, providing insights into A. baumannii adaptation during stress and infection.

Ethanol Decreases Pseudomonas aeruginosa Flagellar Motility through the Regulation of Flagellar Stators

Pseudomonas aeruginosa frequently encounters microbes that produce ethanol. Low concentrations of ethanol reduced P. aeruginosa swim zone area by up to 45% in soft agar. The reduction of swimming by ethanol required the flagellar motor proteins MotAB and two PilZ domain proteins (FlgZ and PilZ). PilY1 and the type 4 pilus alignment complex (comprising PilMNOP) were previously implicated in MotAB regulation in surface-associated cells and were required for ethanol-dependent motility repression. As FlgZ requires the second messenger bis-(3'-5')-cyclic dimeric GMP (c-di-GMP) to represses motility, we screened mutants lacking genes involved in c-di-GMP metabolism and found that mutants lacking diguanylate cyclases SadC and GcbA were less responsive to ethanol. The double mutant was resistant to its effects. As published previously, ethanol also represses swarming motility, and the same genes required for ethanol effects on swimming motility were required for its regulation of swarming. Microscopic analysis of single cells in soft agar revealed that ethanol effects on swim zone area correlated with ethanol effects on the portion of cells that paused or stopped during the time interval analyzed. Ethanol increased c-di-GMP in planktonic wild-type cells but not in ΔmotAB or ΔsadC ΔgcbA mutants, suggesting c-di-GMP plays a role in the response to ethanol in planktonic cells. We propose that ethanol produced by other microbes induces a regulated decrease in P. aeruginosa motility, thereby promoting P. aeruginosa colocalization with ethanol-producing microbes. Furthermore, some of the same factors involved in the response to surface contact are involved in the response to ethanol.IMPORTANCE Ethanol is an important biologically active molecule produced by many bacteria and fungi. It has also been identified as a potential marker for disease state in cystic fibrosis. In line with previous data showing that ethanol promotes biofilm formation by Pseudomonas aeruginosa, here we report that ethanol reduces swimming motility using some of the same proteins involved in surface sensing. We propose that these data may provide insight into how microbes, via their metabolic byproducts, can influence P. aeruginosa colocalization in the context of infection and in other polymicrobial settings.

Ethanol Stimulates Trehalose Production through a SpoT-DksA-AlgU-Dependent Pathway in Pseudomonas aeruginosa

Pseudomonas aeruginosa frequently resides among ethanol-producing microbes, making its response to the microbially produced concentrations of ethanol relevant to understanding its biology. Our transcriptome analysis found that genes involved in trehalose metabolism were induced by low concentrations of ethanol, and biochemical assays showed that levels of intracellular trehalose increased significantly upon growth with ethanol. The increase in trehalose was dependent on the TreYZ pathway but not other trehalose-metabolic enzymes (TreS or TreA). The sigma factor AlgU (AlgT), a homolog of RpoE in other species, was required for increased expression of the treZ gene and trehalose levels, but induction was not controlled by the well-characterized proteolysis of its anti-sigma factor, MucA. Growth with ethanol led to increased SpoT-dependent (p)ppGpp accumulation, which stimulates AlgU-dependent transcription of treZ and other AlgU-regulated genes through DksA, a (p)ppGpp and RNA polymerase binding protein. Ethanol stimulation of trehalose also required acylhomoserine lactone (AHL)-mediated quorum sensing (QS), as induction was not observed in a ΔlasR ΔrhlR strain. A network analysis using a model, eADAGE, built from publicly available P. aeruginosa transcriptome data sets (J. Tan, G. Doing, K. A. Lewis, C. E. Price, et al., Cell Syst 5:63-71, 2017, https://doi.org/10.1016/j.cels.2017.06.003) provided strong support for our model in which treZ and coregulated genes are controlled by both AlgU- and AHL-mediated QS. Consistent with (p)ppGpp- and AHL-mediated quorum-sensing regulation, ethanol, even when added at the time of culture inoculation, stimulated treZ transcript levels and trehalose production in cells from post-exponential-phase cultures but not in cells from exponential-phase cultures. These data highlight the integration of growth and cell density cues in the P. aeruginosa transcriptional response to ethanol.IMPORTANCE Pseudomonas aeruginosa is often found with bacteria and fungi that produce fermentation products, including ethanol. At concentrations similar to those produced by environmental microbes, we found that ethanol stimulated expression of trehalose-biosynthetic genes and cellular levels of trehalose, a disaccharide that protects against environmental stresses. The induction of trehalose by ethanol required the alternative sigma factor AlgU through DksA- and SpoT-dependent (p)ppGpp. Trehalose accumulation also required AHL quorum sensing and occurred only in post-exponential-phase cultures. This work highlights how cells integrate cell density and growth cues in their responses to products made by other microbes and reveals a new role for (p)ppGpp in the regulation of AlgU activity.

Lon Protease Has Multifaceted Biological Functions in Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative opportunistic pathogen that is known to survive harsh environmental conditions and is a leading cause of hospital-acquired infections. Specifically, multicellular communities (known as biofilms) of A. baumannii can withstand desiccation and survive on hospital surfaces and equipment. Biofilms are bacteria embedded in a self-produced extracellular matrix composed of proteins, sugars, and/or DNA. Bacteria in a biofilm are protected from environmental stresses, including antibiotics, which provides the bacteria with selective advantage for survival. Although some gene products are known to play roles in this developmental process in A. baumannii, mechanisms and signaling remain mostly unknown. Here, we find that Lon protease in A. baumannii affects biofilm development and has other important physiological roles, including motility and the cell envelope. Lon proteases are found in all domains of life, participating in regulatory processes and maintaining cellular homeostasis. These data reveal the importance of Lon protease in influencing key A. baumannii processes to survive stress and to maintain viability.IMPORTANCEAcinetobacter baumannii is an opportunistic pathogen and is a leading cause of hospital-acquired infections. A. baumannii is difficult to eradicate and to manage, because this bacterium is known to robustly survive desiccation and to quickly gain antibiotic resistance. We sought to investigate biofilm formation in A. baumannii, since much remains unknown about biofilm formation in this bacterium. Biofilms, which are multicellular communities of bacteria, are surface attached and difficult to eliminate from hospital equipment and implanted devices. Our research identifies multifaceted physiological roles for the conserved bacterial protease Lon in A. baumannii These roles include biofilm formation, motility, and viability. This work broadly affects and expands understanding of the biology of A. baumannii, which will permit us to find effective ways to eliminate the bacterium.

Analysis of colony phase variation switch in Acinetobacter baumannii clinical isolates

Reversible switching between opaque and translucent colony formation is a novel feature of Acinetobacter baumannii that has been associated with variations in the cell morphology, surface motility, biofilm formation, antibiotic resistance and virulence. Here, we assessed a number of phenotypic alterations related to colony switching in A. baumannii clinical isolates belonging to different multi-locus sequence types. Our findings demonstrated that these phenotypic alterations were mostly strain-specific. In general, the translucent subpopulations of A. baumannii produced more dense biofilms, were more piliated, and released larger amounts of outer membrane vesicles (OMVs). In addition, the translucent subpopulations caused reduced fertility of Caenorhabditis elegans. When assessed for effects on the immune response in RAW 264.7 macrophages, the OMVs isolated from opaque subpopulations of A. baumannii appeared to be more immunogenic than the OMVs from the translucent form. However, also the OMVs from the translucent subpopulations had the potential to evoke an immune response. Therefore, we suggest that OMVs may be considered for development of new immunotherapeutic treatments against A. baumannii infections.

A Light-Regulated Type I Pilus Contributes to Acinetobacter baumannii Biofilm, Motility, and Virulence Functions

Transcriptional analyses of Acinetobacter baumannii ATCC 17978 showed that the expression of A1S_2091 was enhanced in cells cultured in darkness at 24°C through a process that depended on the BlsA photoreceptor. Disruption of A1S_2091, a component of the A1S_2088-A1S_2091 polycistronic operon predicted to code for a type I chaperone/usher pilus assembly system, abolished surface motility and pellicle formation but significantly enhanced biofilm formation on plastic by bacteria cultured in darkness. Based on these observations, the A1S_2088-A1S_2091 operon was named the photoregulated pilus ABCD (prpABCD) operon, with A1S_2091 coding for the PrpA pilin subunit. Unexpectedly, comparative analyses of ATCC 17978 and prpA isogenic mutant cells cultured at 37°C showed the expression of light-regulated biofilm biogenesis and motility functions under a temperature condition that drastically affects BlsA production and its light-sensing activity. These assays also suggest that ATCC 17978 cells produce alternative light-regulated adhesins and/or pilus systems that enhance bacterial adhesion and biofilm formation at both 24°C and 37°C on plastic as well as on the surface of polarized A549 alveolar epithelial cells, where the formation of bacterial filaments and cell chains was significantly enhanced. The inactivation of prpA also resulted in a significant reduction in virulence when tested by using the Galleria mellonella virulence model. All these observations provide strong evidence showing the capacity of A. baumannii to sense light and interact with biotic and abiotic surfaces using undetermined alternative sensing and regulatory systems as well as alternative adherence and motility cellular functions that allow this pathogen to persist in different ecological niches.

The role of multispecies social interactions in shaping Pseudomonas aeruginosa pathogenicity in the cystic fibrosis lung

Pseudomonas aeruginosa is a major pathogen in the lungs of cystic fibrosis (CF) patients. However, it is now recognised that a diverse microbial community exists in the airways comprising aerobic and anaerobic bacteria as well as fungi and viruses. This rich soup of microorganisms provides ample opportunity for interspecies interactions, particularly when considering secreted compounds. Here, we discuss how P. aeruginosa-secreted products can have community-wide effects, with the potential to ultimately shape microbial community dynamics within the lung. We focus on three well-studied traits associated with worsening clinical outcome in CF: phenazines, siderophores and biofilm formation, and discuss how secretions can shape interactions between P. aeruginosa and other commonly encountered members of the lung microbiome: Staphylococcus aureus, the Burkholderia cepacia complex, Candida albicans and Aspergillus fumigatus. These interactions may shape the evolutionary trajectory of P. aeruginosa while providing new opportunities for therapeutic exploitation of the CF lung microbiome.

The higBA Toxin-Antitoxin Module From the Opportunistic Pathogen Acinetobacter baumannii - Regulation, Activity, and Evolution

Acinetobacter baumannii is one of the major causes of hard to treat multidrug-resistant hospital infections. A. baumannii features contributing to its spread and persistence in clinical environment are only beginning to be explored. Bacterial toxin-antitoxin (TA) systems are genetic loci shown to be involved in plasmid maintenance and proposed to function as components of stress response networks. Here we present a thorough characterization of type II system of A. baumannii, which is the most ubiquitous TA module present in A. baumannii plasmids. higBA of A. baumannii is a reverse TA (the toxin gene is the first in the operon) and shows little homology to other TA systems of RelE superfamily. It is represented by two variants, which both are functional albeit exhibit strong difference in sequence conservation. The higBA2 operon is found on ubiquitous 11 Kb pAB120 plasmid, conferring carbapenem resistance to clinical A. baumannii isolates and represents a higBA variant that can be found with multiple sequence variations. We show here that higBA2 is capable to confer maintenance of unstable plasmid in Acinetobacter species. HigB2 toxin functions as a ribonuclease and its activity is neutralized by HigA2 antitoxin through formation of an unusually large heterooligomeric complex. Based on the in vivo expression analysis of gfp reporter gene we propose that HigA2 antitoxin and HigBA2 protein complex bind the higBA2 promoter region to downregulate its transcription. We also demonstrate that higBA2 is a stress responsive locus, whose transcription changes in conditions encountered by A. baumannii in clinical environment and within the host. We show elevated expression of higBA2 during stationary phase, under iron deficiency and downregulated expression after antibiotic (rifampicin) treatment.

Uncovering the mechanisms of Acinetobacter baumannii virulence

Acinetobacter baumannii is a nosocomial pathogen that causes ventilator-associated as well as bloodstream infections in critically ill patients, and the spread of multidrug-resistant Acinetobacter strains is cause for concern. Much of the success of A. baumannii can be directly attributed to its plastic genome, which rapidly mutates when faced with adversity and stress. However, fundamental virulence mechanisms beyond canonical drug resistance were recently uncovered that enable A. baumannii and, to a limited extent, other medically relevant Acinetobacter species to successfully thrive in the health-care environment. In this Review, we explore the molecular features that promote environmental persistence, including desiccation resistance, biofilm formation and motility, and we discuss the most recently identified virulence factors, such as secretion systems, surface glycoconjugates and micronutrient acquisition systems that collectively enable these pathogens to successfully infect their hosts.

Current and Emerging Topical Antibacterials and Antiseptics: Agents, Action, and Resistance Patterns

Bacterial skin infections represent some of the most common infectious diseases globally. Prevention and treatment of skin infections can involve application of a topical antimicrobial, which may be an antibiotic (such as mupirocin or fusidic acid) or an antiseptic (such as chlorhexidine or alcohol). However, there is limited evidence to support the widespread prophylactic or therapeutic use of topical agents. Challenges involved in the use of topical antimicrobials include increasing rates of bacterial resistance, local hypersensitivity reactions (particularly to older agents, such as bacitracin), and concerns about the indiscriminate use of antiseptics potentially coselecting for antibiotic resistance. We review the evidence for the major clinical uses of topical antibiotics and antiseptics. In addition, we review the mechanisms of action of common topical agents and define the clinical and molecular epidemiology of antimicrobial resistance in these agents. Moreover, we review the potential use of newer and emerging agents, such as retapamulin and ebselen, and discuss the role of antiseptic agents in preventing bacterial skin infections. A comprehensive understanding of the clinical efficacy and drivers of resistance to topical agents will inform the optimal use of these agents to preserve their activity in the future.

Effects of binge alcohol exposure on Burkholderia thailandensis-alveolar macrophage interaction

Alcohol consumption has diverse and well-documented effects on the human immune system and its ability to defend against infective agents. One example is melioidosis, a disease caused by infection with Burkholderia pseudomallei, which is of public health importance in Southeast Asia and Northern Australia, with an expanding global distribution. While B. pseudomallei infections can occur in healthy humans, binge alcohol use is progressively being recognized as a major risk factor. Although binge alcohol consumption has been considered as a risk factor for the development of melioidosis, no experimental studies have investigated the outcomes of alcohol exposure on Burkholderia spp. infection. Therefore, we proposed the use of non-pathogenic B. thailandensis E264 as a useful BSL-1 model system to study the effects of binge alcohol exposure on bacteria and alveolar macrophage interactions. The MH-S alveolar macrophage (AMs) cell line was used to characterize innate immune responses to infection in vitro. Our results showed that alcohol exposure significantly suppressed the uptake and killing of B. thailandensis by AMs. Alveolar macrophages incubated in alcohol (0.08%) for 3 h prior to infection showed significantly lower bacterial uptake at 2 and 8 h post infection. Activated AMs with IFN-γ and pre and post-incubation in alcohol when exposed to B. thailandensis released lower nitric oxide (NO) concentrations, compared to activated AMs with IFN-γ from non-alcoholic controls. As a result, B. thailandensis survival and replication increased ∼2.5-fold compared to controls. The presence of alcohol (1%) also increased bacterial survival within AMs. Alcohol significantly decreased bacterial motility compared to non-alcoholic controls. Increased biofilm formation was observed at 3 and 6 h when bacteria were pre-incubated in (0.08%) alcohol. These results provide insights into binge alcohol consumption, a culturally prevalent risk factor, as a predisposing factor for melioidosis.

Clinical and Pathophysiological Overview of Acinetobacter Infections: a Century of Challenges

Acinetobacter is a complex genus, and historically, there has been confusion about the existence of multiple species. The species commonly cause nosocomial infections, predominantly aspiration pneumonia and catheter-associated bacteremia, but can also cause soft tissue and urinary tract infections. Community-acquired infections by Acinetobacter spp. are increasingly reported. Transmission of Acinetobacter and subsequent disease is facilitated by the organism's environmental tenacity, resistance to desiccation, and evasion of host immunity. The virulence properties demonstrated by Acinetobacter spp. primarily stem from evasion of rapid clearance by the innate immune system, effectively enabling high bacterial density that triggers lipopolysaccharide (LPS)-Toll-like receptor 4 (TLR4)-mediated sepsis. Capsular polysaccharide is a critical virulence factor that enables immune evasion, while LPS triggers septic shock. However, the primary driver of clinical outcome is antibiotic resistance. Administration of initially effective therapy is key to improving survival, reducing 30-day mortality threefold. Regrettably, due to the high frequency of this organism having an extreme drug resistance (XDR) phenotype, early initiation of effective therapy is a major clinical challenge. Given its high rate of antibiotic resistance and abysmal outcomes (up to 70% mortality rate from infections caused by XDR strains in some case series), new preventative and therapeutic options for Acinetobacter spp. are desperately needed.

Modeling Acinetobacter baumannii wound infections: The critical role of iron

BACKGROUND

Acinetobacter baumannii has emerged as an increasingly important and successful opportunistic human pathogen due to its ability to withstand harsh environmental conditions, its characteristic virulence factors, and quick adaptability to stress.

METHODS

We developed a clinically relevant murine model of A. baumannii traumatic wound infection to determine the effect of local wound environment on A. baumannii virulence. Mice underwent rectus muscle crush injury combined with ischemia created by epigastric vessel ligation, followed by A. baumannii inoculation. Reiterative experiments were performed using (1) a mutant deficient in the production of the siderophore acinetobactin, or (2) iron supplementation of the wound milieu. Mice were euthanized 7 days later, and rectus muscle analyzed for signs of clinical infection, HIF1α accumulation, bacterial abundance, and colony morphotype. To determine the effect of wound milieu on bacterial virulence, Galleria mellonella infection model was used.

RESULTS

The combination of rectus muscle injury with ischemia and A. baumannii inoculation resulted in 100% incidence of clinical wound infection that was significantly higher compared with other groups (n = 15/group, p < 0.0001). The highest level of wound infection was accompanied by the highest level of A. baumannii colonization (p < 0.0001) and the highest degree of HIF1α accumulation (p < 0.05). A. baumannii strains isolated from injured/ischemic muscle with clinical infection displayed a rough morphotype and a higher degree of virulence as judged by G. mellonella killing assay as compared with smooth morphotype colonies isolated from injured muscle without clinical infection (100% vs. 60%, n = 30 Log-Rank test, p = 0.0422). Iron supplementation prevented wound infection (n = 30, p < 0.0001) and decreased HIF1α (p = 0.039643). Similar results of decrease in wound infection and HIF1α were obtained when A. baumannii wild type was replaced with its derivative mutant [INCREMENT]BasD deficient in acinetobactin production.

CONCLUSION

The ability of A. baumannii to cause infections in traumatized wound relies on its ability to scavenge iron and can be prevented by iron supplementation to the wound milieu.

Iron-Regulated Phospholipase C Activity Contributes to the Cytolytic Activity and Virulence of Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic Gram-negative pathogen that causes a wide range of infections including pneumonia, septicemia, necrotizing fasciitis and severe wound and urinary tract infections. Analysis of A. baumannii representative strains grown in Chelex 100-treated medium for hemolytic activity demonstrated that this pathogen is increasingly hemolytic to sheep, human and horse erythrocytes, which interestingly contain increasing amounts of phosphatidylcholine in their membranes. Bioinformatic, genetic and functional analyses of 19 A. baumannii isolates showed that the genomes of each strain contained two phosphatidylcholine-specific phospholipase C (PC-PLC) genes, which were named plc1 and plc2. Accordingly, all of these strains were significantly hemolytic to horse erythrocytes and their culture supernatants tested positive for PC-PLC activity. Further analyses showed that the transcriptional expression of plc1 and plc2 and the production of phospholipase and thus hemolytic activity increased when bacteria were cultured under iron-chelation as compared to iron-rich conditions. Testing of the A. baumannii ATCC 19606^Tplc1::aph-FRT and plc2::aph isogenic insertion derivatives showed that these mutants had a significantly reduced PC-PLC activity as compared to the parental strain, while testing of plc1::ermAM/plc2::aph demonstrated that this double PC-PLC isogenic mutant expressed significantly reduced cytolytic and hemolytic activity. Interestingly, only plc1 was shown to contribute significantly to A. baumannii virulence using the Galleria mellonella infection model. Taken together, our data demonstrate that both PLC1 and PLC2, which have diverged from a common ancestor, play a concerted role in hemolytic and cytolytic activities; although PLC1 seems to play a more critical role in the virulence of A. baumannii when tested in an invertebrate model. These activities would provide access to intracellular iron stores this pathogen could use during growth in the infected host.

Multiple roles for a novel RND-type efflux system in Acinetobacter baumannii AB5075

Colony opacity phase variation in Acinetobacter baumannii strain AB5075 is regulated by a reversible high‐frequency switch. Transposon mutagenesis was used to generate mutations that decreased the opaque to translucent switch and a gene encoding a predicted periplasmic membrane fusion component of a resistance–nodulation–cell division (RND)‐type efflux system was isolated. This gene was designated arpA and immediately downstream was a gene designated arpB that encodes a predicted membrane transporter of RND‐type systems. A nonpolar, in‐frame deletion in arpA resulted in a 70‐fold decrease in the opaque to translucent switch. An arpB::Tc mutant exhibited a 769‐fold decrease in the opaque to translucent switch. However, the translucent to opaque switch was largely unchanged in both the arpA and arpB mutants. The arpA and arpB mutants also exhibited increased surface motility in the opaque form and the arpB mutant exhibited increased susceptibility to aminoglycosides. The arpA and arpB mutants were both attenuated in a Galleria mellonella model of virulence. A divergently transcribed TetR‐type regulator ArpR was capable of repressing the arpAB operon when this TetR regulator was overexpressed. The arpR gene was also involved in regulating the opaque to translucent switch as an in‐frame arpR mutation decreased this switch by 1,916‐fold.

Characterization of surface antigen protein 1 (SurA1) from Acinetobacter baumannii and its role in virulence and fitness

Acinetobacter baumannii is a Gram-negative bacillus that causes nosocomial infections, such as bacteremia, pneumonia, and meningitis and urinary tract and wound infections. In the present study, the surface antigen protein 1 (SurA1) gene of A. baumannii strain CCGGD201101 was identified, cloned and expressed, and then its roles in fitness and virulence were investigated. Virulence was observed in the human lung cancer cell lines A549 and HEp-2 at one week after treatment with recombinant SurA1. One isogenic SurA1 knock-out strain, GR0015, which was derived from the A. baumannii strain CCGGD201101 isolated from diseased chicks in a previous study, highlighted the effect of SurA1 on fitness and growth. Its growth rate in LB broth and killing activity in human sera were significantly decreased compared with strain CCGGD201101. In the Galleria mellonella insect model, the isogenic SurA1 knock-out strain exhibited a lower survival rate and decreased dissemination. These results suggest that SurA1 plays an important role in the fitness and virulence of A. baumannii.

Community-acquired Acinetobacter baumannii: clinical characteristics, epidemiology and pathogenesis

Community-acquired Acinetobacter baumannii (CA-Ab) is a rare but serious cause of community-acquired pneumonia in tropical regions of the world. CA-Ab infections predominantly affect individuals with risk factors, which include excess alcohol consumption, diabetes mellitus, smoking and chronic lung disease. CA-Ab pneumonia presents as a surprisingly fulminant course and is characterized by a rapid onset of fever, severe respiratory symptoms and multi-organ dysfunction, with a mortality rate reported as high as 64%. It is unclear whether the distinct clinical syndrome caused by CA-Ab is because of host predisposing factors or unique bacterial characteristics, or a combination of both. Deepening our understanding of the drivers of overwhelming CA-Ab infection will provide important insights into preventative and therapeutic strategies.

Optimisation of the Caenorhabditis elegans model for studying the pathogenesis of opportunistic Acinetobacter baumannii

This study aimed to increase the sensitivity of Caenorhabditis elegans as an infection model for detection of minor differences in virulence or fitness between different Acinetobacter baumannii strains with known resistance and virulence mechanisms. Selected A. baumannii strains and mutants, comprising wild-type strains (ATCC 17978 and 19606), colistin-resistant strains (ATCC 19606 ΔlpxA and ATCC 19606 ΔlpxC), a clinical encapsulated isolate (AB307-0294), an imipenem-resistant strain (ATCC 17978 Δomp33-36) and an sRNA knock-out strain (ATCC 17978 Δ13573), were employed in developing killing and fertility assays in a C. elegans infection model. Because virulence levels of the strains were known, they could be used to assess assays in the nematode model for their ability to discriminate between degrees of virulence. The model was validated by microscopic analysis and in a murine sepsis infection model. The fertility assay, specifically utilising nematode growth medium, was able to detect virulence differences between the wild-type strains, ATCC 19606 ΔlpxA and isolate AB307-0294. Moreover, modification of an alternative culture medium by incremental changes in osmolarity facilitated detection of subtle virulence differences between isogenic mutants (ATCC 17978 Δomp33-36 and 17978 Δ13573). The success of the proposed fertility model depends on establishing a balance between optimal C. elegans reproduction and environmental stress leading to maximum pathogen-induced damage. This invertebrate model may reduce the need for mammalian in vivo studies of A. baumannii resistance and pathogenicity and may additionally be validated for the study of other low-virulence bacterial pathogens.

Transcriptional regulation of the iac locus from Acinetobacter baumannii by the phytohormone indole-3-acetic acid

The iac locus is involved in indole-3-acetic acid (IAA) catabolism in Acinetobacter baumannii. Nine structural genes of iac are transcribed in the same direction, whereas iacR, which encodes a MarR-type transcriptional regulator, is transcribed in the opposite direction. The IacA protein, which is encoded by the second structural gene of the iac locus, is expressed in an IAA-dependent manner. Here, we characterized gene expression from this locus in wild type A. baumannii and in an iacR mutant; this revealed that the iacH promoter is negatively regulated by IacR. The transcriptional site of iacH was determined by using 5' rapid amplification of cDNA ends; one IacR-binding site was identified between positions -35 and +28 of the iacH promoter. Sequence analysis and an electrophoretic mobility shift assay indicated that recombinant IacR binds specifically to a sequence with dyad symmetry in the iacR-iacH overlapping promoters in the absence of IAA. In addition, a two-plasmid expression system in Escherichia coli showed that IAA probably serves as a ligand that binds to IacR and releases it from the iacH promoter, thereby allowing RNA polymerase to transcribe iac. Thus, iac is expressed in order to promote IAA degradation, whereas free IacR is required for iac repression. We conclude that IacR serves as a key regulator of IAA degradation in A. baumannii in the rhizosphere. These results provide new insights into the possible role of A. baumannii in the environment.

Role of the carboxy terminus of SecA in iron acquisition, protein translocation, and virulence of the bacterial pathogen Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative opportunistic nosocomial pathogen that causes pneumonia and soft tissue and systemic infections. Screening of a transposon insertion library of A. baumannii ATCC 19606T resulted in the identification of the 2010 derivative, which, although capable of growing well in iron-rich media, failed to prosper under iron chelation. Genetic, molecular, and functional assays showed that 2010's iron utilization-deficient phenotype is due to an insertion within the 3' end of secA, which results in the production of a C-terminally truncated derivative of SecA. SecA plays a critical role in protein translocation through the SecYEG membrane channel. Accordingly, the secA mutation resulted in undetectable amounts of the ferric acinetobactin outer membrane receptor protein BauA while not affecting the production of other acinetobactin membrane protein transport components, such as BauB and BauE, or the secretion of acinetobactin by 2010 cells cultured in the presence of subinhibitory concentrations of the synthetic iron chelator 2,2'-dipyridyl. Outer membrane proteins involved in nutrient transport, adherence, and biofilm formation were also reduced in 2010. The SecA truncation also increased production of 30 different proteins, including proteins involved in adaptation/tolerance responses. Although some of these protein changes could negatively affect the pathobiology of the 2010 derivative, its virulence defect is mainly due to its inability to acquire iron via the acinetobactin-mediated system. These results together indicate that although the C terminus of the A. baumannii ATCC 19606T SecA is not essential for viability, it plays a critical role in the production and translocation of different proteins and virulence.

Candida albicans ethanol stimulates Pseudomonas aeruginosa WspR-controlled biofilm formation as part of a cyclic relationship involving phenazines

In chronic infections, pathogens are often in the presence of other microbial species. For example, Pseudomonas aeruginosa is a common and detrimental lung pathogen in individuals with cystic fibrosis (CF) and co-infections with Candida albicans are common. Here, we show that P. aeruginosa biofilm formation and phenazine production were strongly influenced by ethanol produced by the fungus C. albicans. Ethanol stimulated phenotypes that are indicative of increased levels of cyclic-di-GMP (c-di-GMP), and levels of c-di-GMP were 2-fold higher in the presence of ethanol. Through a genetic screen, we found that the diguanylate cyclase WspR was required for ethanol stimulation of c-di-GMP. Multiple lines of evidence indicate that ethanol stimulates WspR signaling through its cognate sensor WspA, and promotes WspR-dependent activation of Pel exopolysaccharide production, which contributes to biofilm maturation. We also found that ethanol stimulation of WspR promoted P. aeruginosa colonization of CF airway epithelial cells. P. aeruginosa production of phenazines occurs both in the CF lung and in culture, and phenazines enhance ethanol production by C. albicans. Using a C. albicans adh1/adh1 mutant with decreased ethanol production, we found that fungal ethanol strongly altered the spectrum of P. aeruginosa phenazines in favor of those that are most effective against fungi. Thus, a feedback cycle comprised of ethanol and phenazines drives this polymicrobial interaction, and these relationships may provide insight into why co-infection with both P. aeruginosa and C. albicans has been associated with worse outcomes in cystic fibrosis.

In many human infections, several species of microbes are often present. This is typically the case with the disease cystic fibrosis, characterized by thick mucus in the lungs that is colonized by bacteria and fungi. Here, we show evidence that interactions between the bacterium Pseudomonas aeruginosa and the fungus Candida albicans result in attributes of infection that are worse for the human host. We found that ethanol, such as that produced by C. albicans, causes increased levels of a signaling molecule in P. aeruginosa that promotes biofilm formation. Biofilm formation by P. aeruginosa is associated with infections that are more difficult to treat. Ethanol stimulated P. aeruginosa colonization of plastic surfaces and airway cells, and we identified components of this mechanism. Fungally-produced ethanol also changes the spectrum of phenazine toxins produced by P. aeruginosa, and phenazines are associated with worse lung function in people with cystic fibrosis. In light of the fact that phenazines interact with C. albicans to promote ethanol production, we propose a positive feedback loop between C. albicans and P. aeruginosa that contributes to worse disease. Our findings could have implications for the study and treatment of multi-species infections.

Impact of Acinetobacter baumannii superoxide dismutase on motility, virulence, oxidative stress resistance and susceptibility to antibiotics

Acinetobacter baumannii is a Gram-negative bacterium appearing as an opportunistic pathogen in hospital settings. Superoxide dismutase (SOD) contributes to virulence in several pathogenic bacteria by detoxifying reactive oxygen species released in the course of host defense reactions. However, the biological role of SODs in A. baumannii has not yet been elucidated. Here, we inactivated in A. baumannii ATCC 17978 gene A1S_2343, encoding a putative SOD of the Fe-Mn type by transposon insertion, resulting in mutant ATCC 17978 sod2343::Km. The mutation was also introduced in two naturally competent A. baumannii isolates by transformation with chromosomal DNA derived from mutant ATCC 17978 sod2343::Km. We demonstrate that inactivation of sod2343 leads to significant motility defects in all three A. baumannii strains. The mutant strains were more susceptible to oxidative stress compared to their parental strains. Susceptibility to colistin and tetracycline was increased in all mutant strains while susceptibility of the mutants to gentamicin, levofloxacin and imipenem was strain-dependent. In the Galleria mellonella infection model the mutant strains were significantly attenuated. In conclusion, sod2343 plays an important role in motility, resistance to oxidative stress, susceptibility to antibiotics and virulence in A. baumannii.

Alcohol enhances Acinetobacter baumannii-associated pneumonia and systemic dissemination by impairing neutrophil antimicrobial activity in a murine model of infection

Acinetobacter baumannii (Ab) is a common cause of community-acquired pneumonia (CAP) in chronic alcoholics in tropical and sub-tropical climates and associated with a >50% mortality rate. Using a murine model of alcohol (EtOH) administration, we demonstrated that EtOH enhances Ab-mediated pneumonia leading to systemic infection. Although EtOH did not affect neutrophil recruitment to the lungs of treated mice, it decreased phagocytosis and killing of bacteria by these leukocytes leading to increased microbial burden and severity of disease. Moreover, we determined that mice that received EtOH prior to Ab infection were immunologically impaired, which was reflected in increased pulmonary inflammation, sequential dissemination to the liver and kidneys, and decreased survival. Furthermore, immunosuppression by EtOH was associated with deregulation of cytokine production in the organs of infected mice. This study establishes that EtOH impairs immunity in vivo exacerbating Ab infection and disease progression. The ability of Ab to cause disease in alcoholics warrants the study of its virulence mechanisms and host interactions.

Stress responses in the opportunistic pathogen Acinetobacter baumannii

Acinetobacter baumannii causes a wide range of severe infections among compromised and injured patients worldwide. The relevance of these infections are, in part, due to the ability of this pathogen to sense and react to environmental and host stress signals, allowing it to persist and disseminate in medical settings and the human host. This review summarizes current knowledge on the roles that environmental and cellular stressors play in the ability of A. baumannii to resist nutrient deprivation, oxidative and nitrosative injury, and even the presence of the commonly used antiseptic ethanol, which could serve as a nutrient- and virulence-enhancing signal rather than just being a convenient disinfectant. Emerging experimental evidence supports the role of some of these responses in the pathogenesis of the infections A. baumannii causes in humans and its capacity to resist antibiotics and host response effectors.

Molecular epidemiological study of clinical Acinetobacter baumannii isolates: phenotype switching of antibiotic resistance

BACKGROUND

The presence of clinical Acinetobacter baumannii (A. baumannii) isolates with differing antibiotic resistance phenotypes in the same patient causes difficulties and confusion in treatment. This phenomenon may be caused by reasons such as cross-infection from neighboring patients that switches to different A. baumannii strain, natural mutation of A. baumannii, inducing of different antibiotic resistance genes expression or acquisition of genes conferring resistance from another source. To elucidate this question, clinical A. baumannii strains, isolated from the same individual patients, showed antibiotic resistance phenotypes switching during the same hospitalization period, were attentively collected for further analysis. Molecular approaches for phylogenetic analysis, including pulsed field gel electrophoresis, multilocus sequence typing, and short tandem repeat analysis, were employed for the chronological studies.

FINDINGS

Our results showed that antibiotic resistance phenotype switching could have occurred as a result through both cross-infection and natural mutation roots. Our results also suggest that rapid phenotype switching between paired isolates could occur during one single course of antibiotic treatment.

CONCLUSIONS

Though cross infection caused antibiotic resistance phenotype switching does occur, natural mutation of A. baumannii isolates is particularly cautious for antibiotic treatment.