Multidrug-resistant Acinetobacter baumannii: mechanisms of virulence and resistance

Host-microbe interactions that shape the pathogenesis of Acinetobacter baumannii infection

Acinetobacter baumannii is an opportunistic pathogen that has emerged as a prevalent source of nosocomial infections, most frequently causing ventilator-associated pneumonia. The emergence of pan-drug resistant strains magnifies the problem by reducing viable treatment options and effectively increasing the mortality rate associated with Acinetobacter infections. In light of this rising threat, research on A. baumannii epidemiology, antibiotic resistance, and pathogenesis is accelerating. The recent development of both in vitro and in vivo models has enabled studies probing the host-Acinetobacter interface. Bacterial genetic screens and comparative genomic studies have led to the identification of several A. baumannii virulence factors. Additionally, investigations into host defence mechanisms using animal models or cell culture have provided insight into the innate immune response to infection. This review highlights some of the key attributes of A. baumannii virulence with an emphasis on bacterial interactions with the innate immune system.

Identification of a general O-linked protein glycosylation system in Acinetobacter baumannii and its role in virulence and biofilm formation

Acinetobacter baumannii is an emerging cause of nosocomial infections. The isolation of strains resistant to multiple antibiotics is increasing at alarming rates. Although A. baumannii is considered as one of the more threatening “superbugs” for our healthcare system, little is known about the factors contributing to its pathogenesis. In this work we show that A. baumannii ATCC 17978 possesses an O-glycosylation system responsible for the glycosylation of multiple proteins. 2D-DIGE and mass spectrometry methods identified seven A. baumannii glycoproteins, of yet unknown function. The glycan structure was determined using a combination of MS and NMR techniques and consists of a branched pentasaccharide containing N-acetylgalactosamine, glucose, galactose, N-acetylglucosamine, and a derivative of glucuronic acid. A glycosylation deficient strain was generated by homologous recombination. This strain did not show any growth defects, but exhibited a severely diminished capacity to generate biofilms. Disruption of the glycosylation machinery also resulted in reduced virulence in two infection models, the amoebae Dictyostelium discoideum and the larvae of the insect Galleria mellonella, and reduced in vivo fitness in a mouse model of peritoneal sepsis. Despite A. baumannii genome plasticity, the O-glycosylation machinery appears to be present in all clinical isolates tested as well as in all of the genomes sequenced. This suggests the existence of a strong evolutionary pressure to retain this system. These results together indicate that O-glycosylation in A. baumannii is required for full virulence and therefore represents a novel target for the development of new antibiotics.

Multidrug resistant (MDR) Acinetobacter baumannii strains are an increasing cause of nosocomial infections worldwide. Due to the remarkable ability of A. baumannii to gain resistance to antibiotics, this bacterium is now considered to be a “superbug”. A. baumannii strains resistant to all clinically relevant antibiotics known have also been isolated. Although MDR A. baumannii continues to disseminate globally, very little is known about its pathogenesis mechanisms. Our experiments revealed that A. baumannii ATCC 17978 has a functional O-linked protein glycosylation system, which seems to be present in all strains of A. baumannii sequenced to date and several clinical isolates. We identified seven glycoproteins and elucidated the structure of the glycan moiety. A glycosylation-deficient strain was generated. This strain produced severely reduced biofilms, and exhibited attenuated virulence in amoeba, insect, and murine models. These experiments suggest that glycosylation may play an important role in virulence and may lay the foundation for new drug discovery strategies that could stop the dissemination of this emerging human pathogen, which has become a major threat for healthcare systems.

Extremophilic Acinetobacter strains from high-altitude lakes in Argentinean Puna: remarkable UV-B resistance and efficient DNA damage repair

High-Altitude Andean Lakes (HAAL) of the South American Andes are almost unexplored ecosystems of shallow lakes. The HAAL are recognized by a remarkably high UV exposure, strong changes in temperature and salinity, and a high content of toxic elements, especially arsenic. Being exposed to remarkably extreme conditions, they have been classified as model systems for the study of life on other planets. Particularly, Acinetobacter strains isolated from the HAAL were studied for their survival competence under strong UV-B irradiation. Clinical isolates, Acinetobacter baumannii and Acinetobacter johnsonii, served as reference material. Whereas the reference strains rapidly lost viability under UV-B irradiation, most HAAL-derived strains readily survived this exposure and showed less change in cell number after the treatment. Controls for DNA repair activity, comparing dark repair (DR) or photo repair (PR), gave evidence for the involvement of photolyases in the DNA repair. Comparative measurements by HPLC-mass spectrometry detected the number of photoproducts: bipyrimidine dimers under both PR and DR treatments were more efficiently repaired in the HAAL strains (up to 85 % PR and 38 % DR) than in the controls (31 % PR and zero DR ability). Analysis of cosmid-cloned total genomic DNA from the most effective DNA-photorepair strain (Ver3) yielded a gene (HQ443199) encoding a protein with clear photolyase signatures belonging to class I CPD-photolyases. Despite the relatively low sequence similarity of 41 % between the enzymes from Ver3 and from E. coli (PDB 1DNPA), a model-building approach revealed a high structural homology to the CPD-photolyase of E. coli.

Comparison of the virulence potential of Acinetobacter strains from clinical and environmental sources

Several Acinetobacter strains have utility for biotechnology applications, yet some are opportunistic pathogens. We compared strains of seven Acinetobacter species (baumannii, Ab; calcoaceticus, Ac; guillouiae, Ag; haemolyticus, Ah; lwoffii, Al; junii, Aj; and venetianus, Av-RAG-1) for their potential virulence attributes, including proliferation in mammalian cell conditions, haemolytic/cytolytic activity, ability to elicit inflammatory signals, and antibiotic susceptibility. Only Ah grew at 10² and 10⁴ bacteria/well in mammalian cell culture medium at 37°C. However, co-culture with colonic epithelial cells (HT29) improved growth of all bacterial strains, except Av-RAG-1. Cytotoxicity of Ab and Ah toward HT29 was at least double that of other test bacteria. These effects included bacterial adherence, loss of metabolism, substrate detachment, and cytolysis. Only Ab and Ah exhibited resistance to killing by macrophage-like J774A.1 cells. Haemolytic activity of Ah and Av-RAG-1 was strong, but undetectable for other strains. When killed with an antibiotic, Ab, Ah, Aj and Av-RAG-1 induced 3 to 9-fold elevated HT29 interleukin (IL)-8 levels. However, none of the strains altered levels of J774A.1 pro-inflammatory cytokines (IL-1β, IL-6 and tumor necrosis factor-α). Antibiotic susceptibility profiling showed that Ab, Ag and Aj were viable at low concentrations of some antibiotics. All strains were positive for virulence factor genes ompA and epsA, and negative for mutations in gyrA and parC genes that convey fluoroquinolone resistance. The data demonstrate that Av-RAG-1, Ag and Al lack some potentially harmful characteristics compared to other Acinetobacter strains tested, but the biotechnology candidate Av-RAG-1 should be scrutinized further prior to widespread use.

Surface-associated motility, a common trait of clinical isolates of Acinetobacter baumannii, depends on 1,3-diaminopropane

While flagella-independent motility has long been described in representatives of the genus Acinetobacter, the mechanism of motility remains ambiguous. Acinetobacter baumannii, a nosocomial pathogen appearing increasingly multidrug-resistant, may profit from motility during infection or while persisting in the hospital environment. However, data on the frequency of motility skills among clinical A. baumannii isolates is scarce. We have screened a collection of 83 clinical A. baumannii isolates of different origin and found that, with the exception of one isolate, all were motile on wet surfaces albeit to varying degrees and exhibiting differing morphologies. Screening a collection of transposon mutants of strain ATCC 17978 for motility defects, we identified 2 akinetic mutants carrying transposon insertions in the dat and ddc gene, respectively. These neighbouring genes contribute to synthesis of 1,3-diaminopropane (DAP), a polyamine ubiquitously produced in Acinetobacter. Supplementing semi-solid media with DAP cured the motility defect of both mutants. HPLC analyses confirmed that DAP synthesis was abolished in ddc and dat mutants of different A. baumannii isolates and was re-established after genetic complementation. Both, the dat and ddc mutant of ATCC 17978 were attenuated in the Galleria mellonella caterpillar infection model. Taken together, surface-associated motility is a common trait of clinical A. baumannii isolates that requires DAP and may play a role in its virulence.

Acinetobacter baumannii: an emerging opportunistic pathogen

Acinetobacter baumannii is an opportunistic bacterial pathogen primarily associated with hospital-acquired infections. The recent increase in incidence, largely associated with infected combat troops returning from conflict zones, coupled with a dramatic increase in the incidence of multidrug-resistant (MDR) strains, has significantly raised the profile of this emerging opportunistic pathogen. Herein, we provide an overview of the pathogen, discuss some of the major factors that have led to its clinical prominence and outline some of the novel therapeutic strategies currently in development.

In vitro activity of telavancin in combination with colistin versus Gram-negative bacterial pathogens

The treatment of Gram-negative infections is increasingly compromised by the spread of resistance. With few agents currently in development, clinicians are now considering the use of unorthodox combination therapies for multidrug-resistant strains. Here we assessed the in vitro activity of the novel lipoglycopeptide telavancin (TLV) when combined with colistin (COL) versus 13 Gram-negative type strains and 66 clinical isolates. Marked synergy was observed in either checkerboard (fractional inhibitory concentration index [FICI], <0.5; susceptibility breakpoint index [SBPI], >2) or time-kill assays (>2-log reduction in viable counts compared with starting inocula at 24 h) versus the majority of COL-susceptible enterobacteria, Stenotrophomonas maltophilia, and Acinetobacter baumannii isolates, but only limited effects were seen against Pseudomonas aeruginosa or strains with COL resistance. Using an Etest/agar dilution method, the activity of TLV was potentiated by relatively low concentrations of COL (0.25 to 0.75 μg/ml), reducing the MIC of TLV from >32 μg/ml to ≤ 1 μg/ml for 35% of the clinical isolates. This provides further evidence that glycopeptide-polymyxin combinations may be a useful therapeutic option in the treatment of Gram-negative infections.

Acinetobacter in modern warfare

Increasing appreciation of the role of Acinetobacter baumannii in the aetiology of severe nosocomial infections, together with its ability to employ several mechanisms to rapidly develop resistance to multiple classes of antimicrobial agents, has led to growing interest in this organism over recent years. Recognition and subsequent investigation of the significance of pathogenic Acinetobacter infections in military personnel sustaining injuries during the conflicts in Afghanistan and Iraq has provided an important contribution to the epidemiology of infections with Acinetobacter spp. The following review examines this recent military experience.

Colistin resistance of Acinetobacter baumannii: clinical reports, mechanisms and antimicrobial strategies

Colistin is the last resort for treatment of multidrug-resistant Acinetobacter baumannii. Unfortunately, resistance to colistin has been reported all over the world. The highest resistance rate was reported in Asia, followed by Europe. The heteroresistance rate of A. baumannii to colistin is generally higher than the resistance rate. The mechanism of resistance might be loss of lipopolysaccharide or/and the PmrAB two-component system. Pharmacokinetic/pharmacodynamic studies revealed that colistin monotherapy is unable to prevent resistance, and combination therapy might be the best antimicrobial strategy against colistin-resistant A. baumannii. Colistin/rifampicin and colistin/carbapenem are the most studied combinations that showed promising results in vitro, in vivo and in the clinic. New peptides showing good activity against colistin-resistant A. baumannii are also being investigated.

Heteroresistance to cephalosporins and penicillins in Acinetobacter baumannii

Heteroresistance to antimicrobial agents may affect susceptibility test results and therapeutic success. In this study, we investigated heteroresistance to cephalosporins and penicillins in Acinetobacter baumannii, a major pathogen causing nosocomial infections. Two A. baumannii isolates exhibited heteroresistance to ampicillin-sulbactam, ticarcillin-clavulanic acid, cefepime, and cefpirome, showing a distinct colony morphology of circular rings within the inhibition halos. Pulsed-field gel electrophoresis (PFGE) and outer membrane protein (OMP) analysis demonstrated that subpopulations around the disks/Etest strips and the original strains all belonged to the same PFGE type and OMP profile. Population analysis profile (PAP) showed the presence of heteroresistant subpopulations with high cefepime resistance levels in two isolates (008 and 328). Interestingly, A. baumannii 008 contained two peaks: one was grown in the presence of up to 1 μg of cefepime/ml, the other apparently occurred when the concentration of cefepime was raised to 256 μg/ml. After serial passages without exposure to cefepime, the PAP curve maintained the same trend observed for the original strain of A. baumannii 008. However, the PAP curve showed a shift to relatively lower cefepime resistance (from 256 to 64 μg/ml) in A. baumannii 328 after 10 passages in antibiotic-free Mueller-Hinton agar plates. Convergence to a monotypic resistance phenotype did not occur. Growth rate analysis revealed that slower growth in resistant subpopulations may provide a strategy against antibiotic challenge. To our knowledge, this is the first report of heteroresistance to cephalosporins and penicillins in A. baumannii.

Endemic Acinetobacter baumannii in a New York hospital

Background

Acinetobacter baumannii is an increasingly multidrug-resistant (MDR) cause of hospital-acquired infections, often associated with limited therapeutic options. We investigated A. baumannii isolates at a New York hospital to characterize genetic relatedness.

Methods

Thirty A. baumannii isolates from geographically-dispersed nursing units within the hospital were studied. Isolate relatedness was assessed by repetitive sequence polymerase chain reaction (rep-PCR). The presence and characteristics of integrons were assessed by PCR. Metabolomic profiles of a subset of a prevalent strain isolates and sporadic isolates were characterized and compared.

Results

We detected a hospital-wide group of closely related carbapenem resistant MDR A. baumannii isolates. Compared with sporadic isolates, the prevalent strain isolates were more likely to be MDR (p = 0.001). Isolates from the prevalent strain carried a novel Class I integron sequence. Metabolomic profiles of selected prevalent strain isolates and sporadic isolates were similar.

Conclusion

The A. baumannii population at our hospital represents a prevalent strain of related MDR isolates that contain a novel integron cassette. Prevalent strain and sporadic isolates did not segregate by metabolomic profiles. Further study of environmental, host, and bacterial factors associated with the persistence of prevalent endemic A. baumannii strains is needed to develop effective prevention strategies.

Growth phase-dependent expression of RND efflux pump- and outer membrane porin-encoding genes in Acinetobacter baumannii ATCC 19606

OBJECTIVES

To analyse the expression of resistance-nodulation-division (RND) efflux pumps and outer membrane porins during various growth phases of Acinetobacter baumannii ATCC 19606.

METHODS

Expression of five different RND pump-encoding genes (adeB, adeG, adeJ, AciBau_2436 and AciBau_2746) and two outer membrane porin-encoding genes (carO and oprD) was analysed at four growth timepoints, representing early, mid and late log phases and the stationary phase using quantitative RT-PCR.

RESULTS

The adeB and adeJ RND pump genes were expressed at higher levels than all of the other RND pumps, though their expression was reduced at higher cell density. The adeG gene was found to be constitutively expressed, albeit at much lower levels than those seen for adeB or adeJ. The previously uncharacterized AciBau_2436 was found to be expressed in the early exponential phase. The expression level of oprD was found to be inversely proportional to cell density, while that of carO was found to increase in the mid-log phase but then decrease in the later stages of the growth phase.

CONCLUSIONS

This work shows cell density-dependent expression of adeB, adeJ, AciBau_2436, carO and oprD genes, suggesting a role of global regulatory mechanisms in the expression of these genes in A. baumannii ATCC 19606.

Acinetobacter baumannii is able to gain and maintain a plasmid harbouring In35 found in Enterobacteriaceae isolates from Argentina

The aim of this study was to determine the presence of bla (CTX-M-2) in our A. baumannii population and their putative role as an alternative mechanism of resistance to third-generation cephalosporins in this species. The bla (CTX-M-2) gene is widespread among the Enterobacteriaceae isolates from our country; however, it was not found in 76 isolates A. baumannii non-epidemiologically related clinical isolates resistant to third-generation cephalosporins isolated since 1982 in hospitals from Buenos Aires City. A plasmid isolated from Proteus mirabilis that possesses the complex class 1 integron In35::ISCR1::bla (CTX-M-2) was used to transform the natural competent A. baumannii clinical strain A118. PCR, plasmid extraction, DNA restriction, and susceptibility test confirmed that A118 could gain and maintain the plasmid possessing In35::ISCR1::bla (CTX-M-2), the genetic platform where the bla (CTX-M-2) gene is dispersing in Argentina.

Colistin-resistant, lipopolysaccharide-deficient Acinetobacter baumannii responds to lipopolysaccharide loss through increased expression of genes involved in the synthesis and transport of lipoproteins, phospholipids, and poly-β-1,6-N-acetylglucosamine

We recently demonstrated that colistin resistance in Acinetobacter baumannii can result from mutational inactivation of genes essential for lipid A biosynthesis (Moffatt JH, et al., Antimicrob. Agents Chemother. 54:4971-4977). Consequently, strains harboring these mutations are unable to produce the major Gram-negative bacterial surface component, lipopolysaccharide (LPS). To understand how A. baumannii compensates for the lack of LPS, we compared the transcriptional profile of the A. baumannii type strain ATCC 19606 to that of an isogenic, LPS-deficient, lpxA mutant strain. The analysis of the expression profiles indicated that the LPS-deficient strain showed increased expression of many genes involved in cell envelope and membrane biogenesis. In particular, upregulated genes included those involved in the Lol lipoprotein transport system and the Mla-retrograde phospholipid transport system. In addition, genes involved in the synthesis and transport of poly-β-1,6-N-acetylglucosamine (PNAG) also were upregulated, and a corresponding increase in PNAG production was observed. The LPS-deficient strain also exhibited the reduced expression of genes predicted to encode the fimbrial subunit FimA and a type VI secretion system (T6SS). The reduced expression of genes involved in T6SS correlated with the detection of the T6SS-effector protein AssC in culture supernatants of the A. baumannii wild-type strain but not in the LPS-deficient strain. Taken together, these data show that, in response to total LPS loss, A. baumannii alters the expression of critical transport and biosynthesis systems associated with modulating the composition and structure of the bacterial surface.

[Emerging Acinetobacter baumannii infections and factors favouring their occurrence]

During the last decade, Acinetobacter baumannii (AB) has been increasingly responsible for infections occurring in three particular contexts (in terms of patients and environment). Community AB pneumonia is severe infections, mainly described around the Indian Ocean, and which mainly concern patients with major co-morbidities. AB is also responsible for infections occurring among soldiers wounded in action during operations conducted in Iraq or Afghanistan. Lastly, this bacterium is responsible for infections occurring among casualties from natural disasters like earthquakes and tsunamis. Those infections are often due to multidrug-resistant strains, which can be implicated in nosocomial outbreaks when patients are hospitalized in a local casualty department or during their repatriation thereafter. The source of the contaminations which lead to AB infections following injuries (warfare or natural disasters) is still poorly known. Three hypotheses are usually considered: a contamination of wounds with environmental bacteria, a wound contamination from a previous cutaneous or oropharyngeal endogenous reservoir, or hospital acquisition. The implication of telluric or agricultural primary reservoirs in human AB infections is a common hypothesis which remains to be demonstrated by further specifically designed studies.

Protection against Acinetobacter baumannii infection via its functional deprivation of biofilm associated protein (Bap)

Acinetobacter baumannii, a major nosocomial pathogen, has remarkable capacity to acquire antimicrobial resistance attributable to its biofilm formation ability. Biofilm associated protein (Bap), a specific cell surface protein, is directly involved in biofilm formation by A. baumannii and plays a major role in bacterial infectious processes. In the present study we cloned, expressed and purified a 371 amino acid subunit of Bap. Mice were immunized using recombinant Bap subunit. They were then challenged with A. baumannii to evaluate the immunogenicity and protectivity of Bap subunit. Humoral immune response to Bap was determined by ELISA. Injection of Bap subunit resulted in high antibody titers. Decrease in bacterial cell counts of the immunized mice was evident 18 h after challenge. Reaction of antibodies against Bap with several strains suggests that not only immunodominant regions of Bap in A. baumannii strains are conserved but also have the same epitope presenting pattern in different strains. Immunodominant region of Bap possesses target sites for a protective humoral immune response to A. baumannii. This seems to be a conserved region erecting efficacy of Bap as an appropriate vaccine candidate.

Clinical characteristics and outcomes of bacteremia due to different genomic species of Acinetobacter baumannii complex in patients with solid tumors

PURPOSE

Acinetobacter baumannii, Acinetobacter genomic species 3 (AGS 3), and Acinetobacter genomic species sensu Tjernberg and Ursing (AGS 13TU) are phenotypically indistinguishable and are often reported together as the A. baumannii complex (ABC). Few studies have investigated the difference in outcome caused by these different species, and all involved heterogeneous groups of patients. This study aimed to delineate whether there are differences in the clinical characteristics and outcome among patients with solid tumors and bacteremia caused by A. baumannii or two other non-baumannii ABC species (AGS 3 plus AGS 13TU).

METHODS

Patients with solid tumors and ABC bacteremia over a period of 5 years in a medical center were identified. The patient data were retrospectively reviewed and analyzed.

RESULTS

We identified 103 patients with ABC bacteremia during the study period. Bacteremia was due to A. baumannii in 30 patients, AGS 3 in 24 patients, and AGS 13TU in 49 patients. Among the 103 patients with ABC bacteremia, recent stay in the intensive care unit (ICU) (p = 0.008) was independently associated with the acquisition of A. baumannii bacteremia. Multivariate analysis revealed that bacteremia caused by A. baumannii (hazard ratio [HR] 2.990, 95% confidence interval [CI], 1.021-8.752, p = 0.046) and Acute Physiology and Chronic Health Evaluation (APACHE) II score ≥21 (HR 4.623, 95% CI 1.348-15.859, p = 0.015) were independent factors associated with 14-day mortality.

CONCLUSIONS

Infection with A. baumannii and a high APACHE II score (≥21) might be associated with poor outcome in patients with solid tumors and ABC bacteremia.

Outcomes in patients infected with carbapenem-resistant Acinetobacter baumannii and treated with tigecycline alone or in combination therapy

PURPOSE

Acinetobacter baumannii is a non-fermenting aerobic gram-negative bacteria and one of the important nosocomial pathogens, especially in intensive care units (ICUs). In recent years, multidrug-resistant (MDR) isolates have been an emerging problem, with limited therapeutic options. Tigecycline is a novel antimicrobial, with its in vitro activity against most gram-positive and gram-negative pathogens.

METHODS

This is a retrospective study that was conducted in a tertiary care hospital with 550 beds in Ankara, Turkey, from January 2009 to July 2010. Thirty-three patients who had carbapenem-resistant Acinetobacter spp. infections and received tigecycline alone or in combination with other antibiotics for at least 3 days were included.

RESULTS

The median age of the patients was 62 (18-87) years. All of the patients were diagnosed and treated in the ICU. Clinical responses were observed in 23 patients (69.7%). Ten patients (30%) had clinical failure. There was no significant difference between ventilator-associated pneumonia (VAP) and bloodstream infection (BSI) in terms of clinical or microbiological outcome (p > 0.05). The microbiological response rate was 50%. Superinfection was detected in 13 patients (43.3%) and Pseudomonas aeruginosa was the most frequently isolated pathogen. The 30-day overall mortality rate and attributable mortality rates were 57.6 and 24.2%, respectively. The attributable mortality rate was higher in the group in which microbiological eradication was not provided.

CONCLUSIONS

Although it is approved by the Food and Drug Administration (FDA) for the treatment of complicated intra-abdominal infections, complicated skin and soft tissue infections, and community-acquired bacterial pneumonia, emerged resistance of Acinetobacter spp. and limited therapeutic options left physicians no choice but to use tigecycline for off-label indications.

Horizontal gene transfer and assortative recombination within the Acinetobacter baumannii clinical population provide genetic diversity at the single carO gene, encoding a major outer membrane protein channel

We described previously the presence in Acinetobacter baumannii of a novel outer membrane (OM) protein, CarO, which functions as an L-ornithine OM channel and whose loss was concomitant with increased carbapenem resistance among clonally related nosocomial isolates of this opportunistic pathogen. Here, we describe the existence of extensive genetic diversity at the carO gene within the A. baumannii clinical population. The systematic analysis of carO sequences from A. baumannii isolates obtained from public hospitals in Argentina revealed the existence of four highly polymorphic carO variants among them. Sequence polymorphism between the different A. baumannii CarO variants was concentrated in three well-defined protein regions that superimposed mostly to predicted surface-exposed loops. Polymorphism among A. baumannii CarO variants was manifested in differential electrophoretic mobilities, antigenic properties, abilities to form stable oligomeric structures, and l-ornithine influx abilities through the A. baumannii OM under in vivo conditions. Incongruence between the phylogenies of the clinical A. baumannii isolates analyzed and those of the carO variants they harbor suggests the existence of assortative (entire-gene) carO recombinational exchange within the A. baumannii population. Exchange of carO variants possessing differential characteristics mediated by horizontal gene transfer may constitute an A. baumannii population strategy to survive radically changing environmental conditions, such as the leap from inanimate sources to human hosts and vice versa, persistence in a compromised host, and/or survival in health care facilities.

Spread of imipenem-resistant Acinetobacter baumannii of European clone II in Western China

The aim of this study was to investigate the distribution of resistance genes and the clonal relationship amongst imipenem-resistant Acinetobacter baumannii isolates from ten hospitals in Western China as well as to compare the molecular epidemiological data with those of isolates from two hospitals in Hangzhou and Beijing. Genes encoding OXA carbapenemases, metallo-β-lactamases, AmpC cephalosporinase and carbapenem resistance-associated outer membrane protein (CarO) were screened using polymerase chain reaction (PCR) and sequencing. PCR mapping was performed to determine whether insertion sequence ISAba1 elements preceded OXA carbapenemases and AmpC cephalosporinase. International clonal lineages were identified by sequence type multiplex PCR. Multilocus sequence typing (MLST) was performed to determine the sequence types (STs), and then eBURST algorithm was applied to assign clonal complexes (CCs). In this study, dissemination of acquired ISAba1 preceding the bla(OXA-23-like) gene was the predominant enzymatic resistance mechanism amongst 272 imipenem-resistant isolates. Five isolates harboured the carO gene disrupted by insertion of ISAba1 and three isolates lacked carO. All of the 36 representative isolates belonged to European clone II. Ten STs, including three novel types, were identified. These STs were clustered into CC92 and two distinct singletons. These observations suggest that imipenem-resistant A. baumannii of European clone II, which carries acquired ISAba1 preceding the bla(OXA-23-like) gene and belongs to CC92, has spread within Western China.

In vitro activity of several antimicrobial peptides against colistin-susceptible and colistin-resistant Acinetobacter baumannii

At present, colistin is among the few antibiotics effective against Acinetobacter baumannii clinical isolates. However, in the last few years, colistin-resistant A. baumannii strains have been isolated. Therefore, antibiotics effective against these usually pan-resistant colistin-resistant A. baumannii strains are required. The main objective of this study was to analyse the activity of 15 peptides against colistin-susceptible and colistin-resistant A. baumannii. The MICs were determined by microdilution. Among these 15 antimicrobial peptides (AMPs), melittin, indolicidin and mastoparan showed good activity against both colistin-susceptible and colistin-resistant A. baumannii. Further studies of mastoparan with time-killing curves showed bactericidal activity at MIC ×8 for both colistin-susceptible and colistin-resistant A. baumannii. In conclusion, mastoparan may be a potential alternative for the treatment of colistin-resistant A. baumannii infections.

Innate immune responses to systemic Acinetobacter baumannii infection in mice: neutrophils, but not interleukin-17, mediate host resistance

Acinetobacter baumannii is a nosocomial pathogen with a high prevalence of multiple-drug-resistant strains, causing pneumonia and sepsis. The current studies further develop a systemic mouse model of this infection and characterize selected innate immune responses to the organism. Five clinical isolates, with various degrees of antibiotic resistance, were assessed for virulence in two mouse strains, and between male and female mice, using intraperitoneal infection. A nearly 1,000-fold difference in virulence was found between bacterial strains, but no significant differences between sexes or mouse strains were observed. It was found that microbes disseminated rapidly from the peritoneal cavity to the lung and spleen, where they replicated. A persistent septic state was observed. The infection progressed rapidly, with mortality between 36 and 48 h. Depletion of neutrophils with antibody to Ly-6G decreased mean time to death and increased mortality. Interleukin-17 (IL-17) promotes the response of neutrophils by inducing production of the chemokine keratinocyte-derived chemoattractant (KC/CXCL1), the mouse homolog of human IL-8. Acinetobacter infection resulted in biphasic increases in both IL-17 and KC/CXCL1. Depletion of neither IL-17 nor KC/CXCL1, using specific antibodies, resulted in a difference in bacterial burdens in organs of infected mice at 10 h postinfection. Comparison of bacterial burdens between IL-17a(-/-) and wild-type mice confirmed that the absence of this cytokine did not sensitize mice to Acinetobacter infection. These studies definitely demonstrate the importance of neutrophils in resistance to systemic Acinetobacter infection. However, neither IL-17 nor KC/CXCL1 alone is required for effective host defense to systemic infection with this organism.

In vivo efficacy of glycopeptide-colistin combination therapies in a Galleria mellonella model of Acinetobacter baumannii infection

The treatment of Acinetobacter baumannii infections poses a significant clinical challenge, with isolates resistant to all commonly used agents increasingly being reported. With few new agents in the pipeline, clinicians are increasingly turning to combinations of antimicrobials in the hope that they may act synergistically together. In this study we assessed the activities of two glycopeptide-colistin combinations both in vitro and using a Galleria mellonella caterpillar model of A. baumannii infection. In checkerboard assays both vancomycin and teicoplanin were highly active against susceptible and multidrug-resistant strains of A. baumannii when combined with colistin (fractional inhibitory concentration [FIC] of <0.25). Treatment of G. mellonella caterpillars infected with lethal doses of A. baumannii resulted in significantly enhanced survival rates when either vancomycin or teicoplanin was given with colistin compared to colistin treatment alone (P < 0.05). This effect was most marked when vancomycin was the glycopeptide administered, although this agent was also highly effective as monotherapy, possibly through an immunomodulatory action on the G. mellonella response to A. baumannii infection. This work suggests that glycopeptide-colistin combinations are highly active against A. baumannii both in vitro and in a simple animal model of infection. They should be considered further as potential treatments for difficult-to-treat A. baumannii infections.

In vitro activity of teicoplanin combined with colistin versus multidrug-resistant strains of Acinetobacter baumannii

OBJECTIVES

Antimicrobial treatment of multidrug-resistant Acinetobacter baumannii (MDRAB) remains an important therapeutic challenge. With isolates resistant to all conventional agents now reported, clinicians are increasingly forced to turn to unorthodox combination treatments in the hope that these may be efficacious. Although a potent interaction between vancomycin and colistin has been demonstrated, there are concerns regarding the inherent toxicity of combining these agents in clinical practice. As teicoplanin has less nephrotoxic potential than vancomycin, we assessed whether a colistin/teicoplanin combination would have similar antimicrobial activities in vitro.

METHODS

The antimicrobial activity of colistin alone and in combination with teicoplanin was assessed versus a collection of MDRAB belonging to a number of epidemic lineages present in the UK. Synergy studies were undertaken using microtitre plate chequerboard assays, an Etest agar dilution method and standard time-kill methodology.

RESULTS

The combination of teicoplanin and colistin was bactericidal versus all of the strains tested. In chequerboard assays, fractional inhibitory concentration indices of <0.5 were obtained, consistent with significant in vitro synergy. Using the Etest method the MIC of teicoplanin fell from >256 mg/L to ≤2 mg/L in the presence of subinhibitory concentrations of colistin.

CONCLUSIONS

Significant synergy was observed when colistin was combined with teicoplanin versus MDRAB in vitro. This may represent a useful therapeutic combination for the treatment of A. baumannii infections, especially when renal toxicity is a significant concern.

Acinetobacter baumannii secretes cytotoxic outer membrane protein A via outer membrane vesicles

Acinetobacter baumannii is an important nosocomial pathogen that causes a high morbidity and mortality rate in infected patients, but pathogenic mechanisms of this microorganism regarding the secretion and delivery of virulence factors to host cells have not been characterized. Gram-negative bacteria naturally secrete outer membrane vesicles (OMVs) that play a role in the delivery of virulence factors to host cells. A. baumannii has been shown to secrete OMVs when cultured in vitro, but the role of OMVs in A. baumannii pathogenesis is not well elucidated. In the present study, we evaluated the secretion and delivery of virulence factors of A. baumannii to host cells via the OMVs and assessed the cytotoxic activity of outer membrane protein A (AbOmpA) packaged in the OMVs. A. baumannii ATCC 19606^T secreted OMVs during in vivo infection as well as in vitro cultures. Potential virulence factors, including AbOmpA and tissue-degrading enzymes, were associated with A. baumannii OMVs. A. baumannii OMVs interacted with lipid rafts in the plasma membranes and then delivered virulence factors to host cells. The OMVs from A. baumannii ATCC 19606^T induced apoptosis of host cells, whereas this effect was not detected in the OMVs from the ΔompA mutant, thereby reflecting AbOmpA-dependent host cell death. The N-terminal region of AbOmpA_22-170 was responsible for host cell death. In conclusion, the OMV-mediated delivery of virulence factors to host cells may well contribute to pathogenesis during A. baumannii infection.

Microbial electrodialysis cell for simultaneous water desalination and hydrogen gas production

A new approach to water desalination is to use exoelectrogenic bacteria to generate electrical power from the biodegradation of organic matter, moving charged ions from a middle chamber between two membranes in a type of microbial fuel cell called a microbial desalination cell. Desalination efficiency using this approach is limited by the voltage produced by the bacteria. Here we examine an alternative strategy based on boosting the voltage produced by the bacteria to achieve hydrogen gas evolution from the cathode using a three-chambered system we refer to as a microbial electrodialysis cell (MEDC). We examined the use of the MEDC process using two different initial NaCl concentrations of 5 g/L and 20 g/L. Conductivity in the desalination chamber was reduced by up to 68 ± 3% in a single fed-batch cycle, with electrical energy efficiencies reaching 231 ± 59%, and maximum hydrogen production rates of 0.16 ± 0.05 m(3) H(2)/m(3) d obtained at an applied voltage of 0.55 V. The advantage of this system compared to a microbial fuel cell approach is that the potentials between the electrodes can be better controlled, and the hydrogen gas that is produced can be used to recover energy to make the desalination process self-sustaining with respect to electrical power requirements.

ISAba825, a functional insertion sequence modulating genomic plasticity and bla(OXA-58) expression in Acinetobacter baumannii

ISAba825, an insertion sequence found inactivating Acinetobacter baumannii carO, was tagged with a kanamycin (Kn) resistance cassette. ISAba825::Kn effectively transposed in A. baumannii, showing preference for short, AT-enriched target sequences, generating 6- to 9-bp target duplications. Additionally, we detected the presence of ISAba825 upstream of a plasmid-borne bla(OXA-58) gene, generating a hybrid promoter largely enhancing its expression and leading to carbapenem resistance. Overall, a role for ISAba825 in carbapenem resistance modulation in A. baumannii is proposed.

Potent and rapid bactericidal action of alyteserin-1c and its [E4K] analog against multidrug-resistant strains of Acinetobacter baumannii

The emergence of multidrug-resistant strains of Acinetobacter baumannii (MDRAB) constitutes a serious threat to public health and necessitates the discovery of new types of antimicrobial agents. Alyteserin-1c (GLKEIFKAGLGSLVKGIAAHVAS·NH(2)) is a cationic, α-helical peptide that was first isolated from skin secretions of the midwife toad Alytes obstetricans. Synthetic alyteserin-1c displayed potent activity against clinical isolates of MDRAB (minimum inhibitory concentration, MIC=5-10 μM; minimum bactericidal concentration, MBC=5-10 μM) while displaying low hemolytic activity against human erythrocytes (LD(50)=220 μM). Increasing the cationicity of alyteserin-1c by the substitution Glu(4)→Lys enhanced the potency against MDRAB (MIC=1.25-5 μM; MBC=1.25-5 μM) as well as decreasing hemolytic activity (HC(50)>400 μM). More than 99.9% of the bacteria were killed within 30 min by the [E4K] analog at a concentration of 1 × MBC. Increasing the cationicity of [E4K]alyteserin-1c further by the additional substitutions of Ala(8),Val(14) or Ala(18) by l-Lys did not enhance antimicrobial potency. Derivatives of [E4K]alyteserin-1c containing a palmitate group coupled either to α-amino group at the N-terminus or to ɛ-amino group on the Lys(18) residue of the [E4K,A18K] analog retained antimicrobial activity but showed dramatically increased hemolytic activities (>40- and >13-fold, respectively).