Characterization of a cyclophosphamide-induced murine model of immunosuppression to study Acinetobacter baumannii pathogenesis

Enterobacter hormaechei replaces virulence with carbapenem resistance via porin loss

Pathogenic Enterobacter species are of increasing clinical concern due to the multidrug-resistant nature of these bacteria, including resistance to carbapenem antibiotics. Our understanding of Enterobacter virulence is limited, hindering the development of new prophylactics and therapeutics targeting infections caused by Enterobacter species. In this study, we assessed the virulence of contemporary clinical Enterobacter hormaechei isolates in a mouse model of intraperitoneal infection and used comparative genomics to identify genes promoting virulence. Through mutagenesis and complementation studies, we found two porin-encoding genes, ompC and ompD, to be required for E. hormaechei virulence. These porins imported clinically relevant carbapenems into the bacteria, and thus loss of OmpC and OmpD desensitized E. hormaechei to the antibiotics. Our genomic analyses suggest porin-related genes are frequently mutated in E. hormaechei, perhaps due to the selective pressure of antibiotic therapy during infection. Despite the importance of OmpC and OmpD during infection of immunocompetent hosts, we found the two porins to be dispensable for virulence in a neutropenic mouse model. Moreover, porin loss provided a fitness advantage during carbapenem treatment in an ex vivo human whole blood model of bacteremia. Our data provide experimental evidence of pathogenic Enterobacter species gaining antibiotic resistance via loss of porins and argue antibiotic therapy during infection of immunocompromised patients is a conducive environment for the selection of porin mutations enhancing the multidrug-resistant profile of these pathogens.

A chronic murine model of pulmonary Acinetobacter baumannii infection enabling the investigation of late virulence factors, long-term antibiotic treatments, and polymicrobial infections

Acinetobacter baumannii can cause prolonged infections that disproportionately affect immunocompromised populations. Our understanding of A. baumannii respiratory pathogenesis relies on an acute murine infection model with limited clinical relevance that employs an unnaturally high number of bacteria and requires the assessment of bacterial load at 24-36 hours post-infection. Here, we demonstrate that low intranasal inoculums in immunocompromised mice with a tlr4 mutation leads to reduced inflammation, allowing for persistent infections lasting at least 3 weeks. Using this "chronic infection model," we determined the adhesin InvL is an imperative virulence factor required during later stages of infection, despite being dispensable in the early phase. We also demonstrate that the chronic model enables the distinction between antibiotics that, although initially reduce bacterial burden, either lead to complete clearance or result in the formation of bacterial persisters. To illustrate how our model can be applied to study polymicrobial infections, we inoculated mice with an active A. baumannii infection with Staphylococcus aureus or Klebsiella pneumoniae. We found that S. aureus exacerbates the infection, while K. pneumoniae enhances A. baumannii clearance. In all, the chronic model overcomes some limitations of the acute pulmonary model, expanding our capabilities to study of A. baumannii pathogenesis and lays the groundwork for the development of similar models for other important opportunistic pathogens.

Bacteriophage-resistant carbapenem-resistant Klebsiella pneumoniae shows reduced antibiotic resistance and virulence

Phage therapy has shown great promise in the treatment of bacterial infections. However, the effectiveness of phage therapy is compromised by the inevitable emergence of phage-resistant strains. In this study, a phage-resistant carbapenem-resistant Klebsiella pneumoniae strain SWKP1711R, derived from parental carbapenem-resistant K. pneumoniae strain SWKP1711 was identified. The mechanism of bacteriophage resistance in SWKP1711R was investigated and the molecular determinants causing altered growth characteristics, antibiotic resistance, and virulence of SWKP1711R were tested. Compared to SWKP1711, SWKP1711R showed slower growth, smaller colonies, filamentous cells visible under the microscope, reduced production of capsular polysaccharide (CPS) and lipopolysaccharide, and reduced resistance to various antibiotics accompanied by reduced virulence. Adsorption experiments showed that phage vB_kpnM_17-11 lost the ability to adsorb onto SWKP1711R, and the adsorption receptor was identified to be bacterial surface polysaccharides. Genetic variation analysis revealed that, compared to the parental strain, SWKP1711R had only one thymine deletion at position 78 of the open reading frame of the lpcA gene, resulting in a frameshift mutation that caused alteration of the bacterial surface polysaccharide and inhibition of phage adsorption, ultimately leading to phage resistance. Transcriptome analysis and quantitative reverse transcriptase PCR revealed that genes encoding lipopolysaccharide synthesis, ompK35, blaTEM-1, and type II and Hha-TomB toxin-antitoxin systems, were all downregulated in SWKP1711R. Taken together, the evidence presented here indicates that the phenotypic alterations and phage resistance displayed by the mutant may be related to the frameshift mutation of lpcA and altered gene expression. While evolution of phage resistance remains an issue, our study suggests that the reduced antibiotic resistance and virulence of phage-resistant strain derivatives might be beneficial in alleviating the burden caused by multidrug-resistant bacteria.

A novel Saclayvirus Acinetobacter baumannii phage genomic analysis and effectiveness in preventing pneumonia

Acinetobacter baumannii, which is resistant to multiple drugs, is an opportunistic pathogen responsible for severe nosocomial infections. With no antibiotics available, phages have obtained clinical attention. However, since immunocompromised patients are often susceptible to infection, the appropriate timing of administration is particularly important. During this research, we obtained a lytic phage vB_AbaM_P1 that specifically targets A. baumannii. We then assessed its potential as a prophylactic treatment for lung infections caused by clinical strains. The virus experiences a period of inactivity lasting 30 min and produces approximately 788 particles during an outbreak. Transmission electron microscopy shows that vB_AbaM_P1 was similar to the Saclayvirus. Based on the analysis of high-throughput sequencing and bioinformatics, vB_AbaM_P1 consists of 107537 bases with a G + C content of 37.68%. It contains a total of 177 open reading frames and 14 tRNAs. No antibiotic genes were detected. In vivo experiments, using a cyclophosphamide-induced neutrophil deficiency model, tested the protective effect of phage on neutrophil-deficient rats by prophylactic application of phage. The use of phages resulted in a decrease in rat mortality caused by A. baumannii and a reduction in the bacterial burden in the lungs. Histologic examination of lung tissue revealed a decrease in the presence of immune cells. The presence of phage vB_AbaM_P1 had a notable impact on preventing A. baumannii infection, as evidenced by the decrease in oxidative stress in lung tissue and cytokine levels in serum. Our research offers more robust evidence for the early utilization of bacteriophages to mitigate A. baumannii infection. KEY POINTS: •A novel Saclayvirus phage infecting A. baumannii was isolated from sewage. •The whole genome was determined, analyzed, and compared to other phages. •Assaying the effect of phage in preventing infection in neutrophil-deficient models.

Antimicrobial peptide thanatin fused endolysin PA90 (Tha-PA90) for the control of Acinetobacter baumannii infection in mouse model

BACKGROUND

This study addresses the urgent need for infection control agents driven by the rise of drug-resistant pathogens such as Acinetobacter baumannii. Our primary aim was to develop and assess a novel endolysin, Tha-PA90, designed to combat these challenges.

METHODS

Tha-PA90 incorporates an antimicrobial peptide (AMP) called thanatin at its N-terminus, enhancing bacterial outer membrane permeability and reducing host immune responses. PA90 was selected as the endolysin component. The antibacterial activity of the purified Tha-PA90 was evaluated using an in vitro colony-forming unit (CFU) reduction assay and a membrane permeability test. A549 cells were utilized to measure the penetration into the cytosol and the cytotoxicity of Tha-PA90. Finally, infection control was monitored in A. baumannii infected mice following the intraperitoneal administration of Tha-PA90.

RESULTS

Tha-PA90 demonstrated remarkable in vitro efficacy, completely eradicating A. baumannii strains, even drug-resistant variants, at a low concentration of 0.5 μM. Notably, it outperformed thanatin, achieving only a < 3-log reduction at 4 μM. Tha-PA90 exhibited 2-3 times higher membrane permeability than a PA90 and thanatin mixture or PA90 alone. Tha-PA90 was found within A549 cells' cytosol with no discernible cytotoxic effects. Furthermore, Tha-PA90 administration extended the lifespan of A. baumannii-infected mice, reducing bacterial loads in major organs by up to 3 logs. Additionally, it decreased proinflammatory cytokine levels (TNF-α and IL-6), reducing the risk of sepsis from rapid bacterial lysis. Our findings indicate that Tha-PA90 is a promising solution for combating drug-resistant A. baumannii. Its enhanced efficacy, low cytotoxicity, and reduction of proinflammatory responses render it a potential candidate for infection control.

CONCLUSIONS

This study underscores the significance of engineered endolysins in addressing the pressing challenge of drug-resistant pathogens and offers insights into improved infection management strategies.

A Combination of Blue Light at 460 nm and H2O2 for the Safe and Effective Eradication of Staphylococcus aureus in an Infected Mouse Skin Abrasion Model

Antibiotic-free approaches are more important than ever to address the rapidly growing problem of the antibiotic resistance crisis. The photolysis of the bacterial virulence factor staphyloxanthin using blue light at 460 nm (BL460 nm) has been found to effectively attenuate Staphylococcus aureus to chemical and physical agents. However, phototherapy using BL640 nm still needs to be investigated in detail for its safety in eradicating Staphylococcus aureus in vitro and in vivo. In this study, we employed a 460 nm continuous-wavelength LED source and a low concentration of hydrogen peroxide to treat S. aureus under a culturing condition and a wound abrasion mouse model. The results demonstrated the safety of the combined therapy when it did not modify the bacterial virulence factors or the susceptibility to widely used antibiotics. In addition, the results of the mouse model also showed that the combined therapy was safe to apply to mouse skin since it did not cause adverse skin irritation. More importantly, the therapy can aid in healing S. aureus-infected wounds with an efficacy comparable to that of the topical antibiotic Fucidin. The aforementioned findings indicate that the concurrent application of BL460 nm and hydrogen peroxide can be used safely as an alternative or adjunct to antibiotics in treating S. aureus-infected wounds.

Of mice and men: Laboratory murine models for recapitulating the immunosuppression of human sepsis

Prolonged immunosuppression is increasingly recognized as the major cause of late phase and long-term mortality in sepsis. Numerous murine models with different paradigms, such as lipopolysaccharide injection, bacterial inoculation, and barrier disruption, have been used to explore the pathogenesis of immunosuppression in sepsis or to test the efficacy of potential therapeutic agents. Nonetheless, the reproducibility and translational value of such models are often questioned, owing to a highly heterogeneric, complex, and dynamic nature of immunopathology in human sepsis, which cannot be consistently and stably recapitulated in mice. Despite of the inherent discrepancies that exist between mice and humans, we can increase the feasibility of murine models by minimizing inconsistency and increasing their clinical relevance. In this mini review, we summarize the current knowledge of murine models that are most commonly used to investigate sepsis-induced immunopathology, highlighting their strengths and limitations in mimicking the dysregulated immune response encountered in human sepsis. We also propose potential directions for refining murine sepsis models, such as reducing experimental inconsistencies, increasing the clinical relevance, and enhancing immunological similarities between mice and humans; such modifications may optimize the value of murine models in meeting research and translational demands when applied in studies of sepsis-induced immunosuppression.

Isolation and Characterization of vB_kpnM_17-11, a Novel Phage Efficient Against Carbapenem-Resistant Klebsiella pneumoniae

Phages and phage-encoded proteins exhibit promising prospects in the treatment of Carbapenem-Resistant Klebsiella pneumoniae (CRKP) infections. In this study, a novel Klebsiella pneumoniae phage vB_kpnM_17-11 was isolated and identified by using a CRKP host. vB_kpnM_17-11 has an icosahedral head and a retractable tail. The latent and exponential phases were 30 and 60 minutes, respectively; the burst size was 31.7 PFU/cell and the optimal MOI was 0.001. vB_kpnM_17-11 remained stable in a wide range of pH (4-8) and temperature (4-40°C). The genome of vB_kpnM_17-11 is 165,894 bp, double-stranded DNA (dsDNA), containing 275 Open Reading Frames (ORFs). It belongs to the family of Myoviridae, order Caudovirales, and has a close evolutionary relationship with Klebsiella phage PKO111. Sequence analysis showed that the 4530 bp orf022 of vB_kpnM_17-11 encodes a putative depolymerase. In vitro testing demonstrated that vB_kpnM_17-11 can decrease the number of K. pneumoniae by 10⁵-fold. In a mouse model of infection, phage administration improved survival and reduced the number of K. pneumoniae in the abdominal cavity by 10⁴-fold. In conclusion, vB_kpnM_17-11 showed excellent in vitro and in vivo performance against K. pneumoniae infection and constitutes a promising candidate for the development of phage therapy against CRKP.

Immunogenicity of loop 3 of Omp34 from A. Baumannii in loopless C-lobe of TbpB of N. meningitidis

Acinetobacter baumannii is a common causative agent of nosocomial infections, with a mortality rate of 43% in infected patients. Due to the emergence of multidrug-resistant (MDR) strains, vaccine development has become necessary. Since the 34 kDa outer membrane protein Omp34 has been identified as a potential vaccine target, we implemented a hybrid antigen approach to target its extracellular loops. Using bioinformatic and structural analyses, we selected Loop 3 from Omp34 and displayed it on the loopless C-lobe (LCL) of TbpB of Neisseria meningitidis. The hybrid antigen and the LCL were produced and used to immunize mice for passive and active immunization and challenge experiments in which the reactivity of the sera was assessed by ELISAs, the bacterial load in the tissues measured and the survival of immunized mice compared. LCL was ineffective in immunization against A. baumannii thus the resulting immunity was due to the presence of Omp34 loop 3. It resulted in increased survival and a reduced bacterial load in the tissues compared to the control groups. The findings indicate that the immunogenicity of Omp34 loops can induce protection against A. baumannii infection, and it could probably be used as a vaccine candidate to control the pathogenesis of A. baumannii.

In vitro and in vivo Activity of Combinations of Polymyxin B with Other Antimicrobials Against Carbapenem-Resistant Acinetobacter baumannii

Purpose

To study the in vitro and in vivo antibacterial activities of polymyxin B (PB) and other five antimicrobial agents, including imipenem (IMP), meropenem (MEM), tigecycline (TGC), sulbactam (SUL), and rifampicin (RIF), alone or in combination against carbapenem-resistant Acinetobacter baumannii (CRAB).

Methods

Microbroth dilution method was used to determine the minimum inhibitory concentration (MIC) of ten strains of CRAB against six antibacterial drugs, and the checkerboard method was used to determine the fractional inhibitory concentration index (FICI). A mouse pneumonia model was established by intranasal instillation of Ab5075 to evaluate the antibacterial activity in vivo.

Results

The resistance rate of ten CRAB strains to IMP, MEM, and SUL was 100%, that to PB and TGC was 0%, and that to RIF was 20%. When PB was used in combination with the other five antibiotics in vitro, it mainly showed synergistic and additive effects on CRAB. The synergistic effect of PB and RIF was maximal, followed by MEM and IMP but was weak with SUL and TGC. In vivo, compared to the model group (untreated with antibiotics), treatment group (six antibiotics alone and PB combined with the other five antibiotics) reduced the bacterial load in the lung tissue and the serum inflammatory factors (IL-1β, IL-6, and TNF-α). The bacterial load and the inflammatory factors of the combined group decreased significantly than that of the single group (P<0.05). The IL-6 and TNF-α values of the PB combined with the RIF group were significantly lower than the two drugs used individually.

Conclusion

The combination of PB and IMP, MEM, and RIF exerted robust in vitro synergistic effects on CRAB isolates. The combination of PB and the other five antimicrobial agents had a better effect in the mouse pneumonia model than single agent, while the combination of PB and RIF had the best effect.

Elizabethkingia anophelis, an emerging pathogen, inhibits RAW 264.7 macrophage function

Elizabethkingia anophelis is a pathogen that can cause a life-threatening infection in immunocompromised patients. The first case of E. anophelis infection was reported in 2013; subsequently, an increase in its incidence has been reported globally. Additionally, a mortality rate of more than 30% was observed in the US outbreak of 2015. To date, the pathogenic mechanisms underlying E. anophelis infection, such as toxin production, remain unclear. Since tissue macrophages act as the first line of defense against pathogens, in the present study the interactions between E. anophelis and a macrophage-like cell line RAW 264.7 were examined. Although E. anophelis showed no cytotoxicity toward RAW 264.7 macrophages, the infection inhibited LPS-induced morphological changes and activation of differentiation markers for the polarization of RAW 264.7 macrophages toward an M1-like phenotype. However, when the cell contact was restricted using Transwell inserts or bacterial culture supernatants were used instead of live bacteria, no such inhibition was observed. Moreover, it was shown that E. anophelis evaded phagocytosis. Overall, the results suggest that E. anophelis infection inhibits the differentiation of RAW 264.7 macrophages to a pro-inflammatory phenotype in a contact-dependent manner.

A conserved region of Acinetobacter trimeric autotransporter adhesion, Ata, provokes suppression of Acinetobacter baumannii virulence

The Acinetobacter trimeric autotransporter adhesin (Ata) is an important virulence factor. The conserved region from the genomic sequence of a 6777bp/2258 amino acid of Acinetobacter baumannii ATCC®19606™ ata was explored. A 263aa of the C-terminal of Ata (rcAta₂₆₃) was expressed. The effect of rcAta₂₆₃ on A. baumannii virulence was studied in a murine model. IgG and IgA were elicited and the mice groups challenged with A. baumannii showed significant survival rates from 66 to 100%. The bacterial loads were determined in the spleens, livers, and lungs of both control and test groups. The adhesion rate of A. baumannii to A549 cells in the presence of serum, cytotoxicity, mutagenicity, and biofilm disruption potential of rcAta₂₆₃ were determined. Intraperitoneally challenged groups showed a significantly reduced bacterial load in the organs of the immunized mice. Intranasal challenge reduced 4 logs of bacterial CFU/g in the test group. The immunized mice sera reduced adherence of A. baumannii to A549 cells to 80%. No cytotoxic or mutagenic effect was detected. Biofilm disruption was significantly increased in the presence of immunized mice sera. Immunization with the conserved region of Ata significantly combats the virulence of A. baumannii which could be considered as a therapeutic strategy to control A. baumannii infections.

Assessment of the safety and anti-inflammatory effects of three Bacillus strains in the respiratory tract

Chronic respiratory diseases are part of accumulating health problems partly due to worldwide increase in air pollution. By their antimicrobial and immunomodulatory properties, some probiotics constitute promising alternatives for the prevention and treatment of chronic respiratory diseases. We have isolated Bacillus strains from Korean fermented foods and selected three potentially probiotic strains (two Bacillus subtilis and one Bacillus amyloliquefaciens) based on safety, antimicrobial efficacy, activity against airborne pathogens and their immunomodulatory properties in vivo. Safety evaluation included in silico analysis for confirming absence of virulence genes. Safety for the respiratory tract was confirmed by an in vivo pathogenicity test using a murine model. Antimicrobial activity was displayed against several airborne pathogens. Potential antimicrobial metabolites such as 2,3-butanediol and propylene glycol were identified as possible antagonistic agents. Immunomodulatory properties in vitro were confirmed by upregulation of IL-10 expression in a macrophage cell line. Intranasal instillation and inhalation in an ovalbumin (OVA)-induced lung inflammation murine model reduced T helper type 2 (Th2) cytokines at transcriptional and protein levels in the lungs. The safety and potentially beneficial role of these Bacillus strains could be demonstrated for the respiratory tract of a murine model.

Epigallocatechin-3-gallate reduces liver and immune system damage in Acinetobacter baumannii-loaded mice with restraint stress

AIM

Due to the significant increase in the antimicrobial resistance of Acinetobacter baumannii (A. baumannii), new drugs to block the progression of infection are strongly needed. Epigallocatechin-3-gallate (EGCG), a major component of green tea, has exhibited potential activity against A. baumannii in vitro. The aim of this study was to determine if EGCG could be used for pretreating stress-related effects, liver damage, and immune dysfunction caused by A. baumannii infection in vivo.

METHODS

Levels of stress hormones, oxidative stress, liver damage, and immune components were analyzed in a murine infection model in which the mice were pretreated with EGCG for one week then intranasally inoculated with A. baumannii. The mice were restrained for 12 h to promote infection because A. baumannii is an opportunistic pathogen. The pretreatment efficacy of EGCG against A. baumannii in mice was assessed for 24 h after the bacterial infection.

RESULTS

Restraint stress strengthened the damage from the A. baumannii infection. Pretreatment with EGCG in the murine pneumonia model markedly reduced stress hormones, oxidative metabolites, and proinflammatory cytokine production. EGCG also increased the immune function by increasing the levels of sIgA, T cells and neutrophils after infection. Moreover, pretreatment with EGCG significantly decreased the liver damage by inhibiting the levels of transaminases, oxidative stress metabolites, and cytokines, while maintaining the normal activity of CYP450 enzymes in the liver.

CONCLUSION

EGCG was efficacious as a preventative treatment for the damage seen in an experimental model of A. baumannii infection.

Specific egg yolk immunoglobulin as a promising non-antibiotic biotherapeutic product against Acinetobacter baumannii pneumonia infection

Acinetobacter baumannii is a serious health threat with a high mortality rate. We have already reported prophylactic effects of IgYs raised against OmpA and Omp34 as well as against inactivated whole-cell (IWC) of A. baumannii in a murine pneumonia model. However, the infection was exacerbated in the mice group that received IgYs raised against the combination of OmpA and Omp34. The current study was conducted to propose reasons for the observed antibody-dependent enhancement (ADE) in addition to the therapeutic effect of specific IgYs in the murine pneumonia model. This phenomenon was hypothetically attributed to topologically inaccessible similar epitopes of OmpA and Omp34 sharing similarity with peptides of mice proteins. In silico analyses revealed that some inaccessible peptides of OmpA shared similarity with peptides of Omp34 and Mus musculus. Specific anti-OmpA and anti-Omp34 IgYs cross-reacted with Omp34 and OmpA respectively. Specific IgYs showed different protectivity against A. baumannii AbI101 in the murine pneumonia model. IgYs triggered against OmpA or IWC of A. baumannii were the most protective antibodies. IgY triggered against Omp34 is ranked next after those against OmpA. The lowest protection was observed in mice received IgYs raised against the combination of rOmpA and rOmp34. In conclusion, specific IgYs against OmpA, Omp34, and IWC of A. baumannii could serve as novel biotherapeutics against A. baumannii pneumonia.

Contamination of wounds with fecal bacteria in immuno-suppressed mice

Immunocompromised patients are predisposed to chronically infected wounds. Especially ulcers in the dorsal region often experience secondary polymicrobial infections. However, current wound infection models mostly use single-strain bacteria. To mimic clinically occurring infections caused by fecal contamination in immunocompromised/immobile patients, which differ significantly from single-strain infections, the present study aimed at the establishment of a new mouse model using infection by fecal bacteria. Dorsal circular excision wounds in immunosuppressed mice were infected with fecal slurry solution in several dilutions up to 1:8,000. Impact of immunosuppressor, bacterial load and timing on development of wound infections was investigated. Wounds were analyzed by scoring, 3D imaging and swab analyses. Autofluorescence imaging was not successful. Dose-finding of cyclophosphamide-induced immunosuppression was necessary for establishment of bacterial wound infections. Infection with fecal slurry diluted 1:166 to 1:400 induced significantly delayed wound healing (p < 0.05) without systemic reactions. Swab analyses post-infection matched the initial polymicrobial suspension. The customized wound score confirmed significant differences between the groups (p < 0.05). Here we report the establishment of a simple, new mouse model for clinically occurring wound infections by fecal bacteria and the evaluation of appropriate wound analysis methods. In the future, this model will provide a suitable tool for the investigation of complex microbiological interactions and evaluation of new therapeutic approaches.

Mouse Models and Tools for the in vivo Study of Neutrophils

Neutrophils are the most abundant leukocytes in human blood and critical actors of the immune system. Many neutrophil functions and facets of their activity in vivo were revealed by studying genetically modified mice or by tracking fluorescent neutrophils in animals using imaging approaches. Assessing the roles of neutrophils can be challenging, especially when exact molecular pathways are questioned or disease states are interrogated that alter normal neutrophil homeostasis. This review discusses the main in vivo models for the study of neutrophils, their advantages and limitations. The side-by-side comparison underlines the necessity to carefully choose the right model(s) to answer a given scientific question, and exhibit caveats that need to be taken into account when designing experimental procedures. Collectively, this review suggests that at least two models should be employed to legitimately conclude on neutrophil functions.

In vivo efficacy of combination of colistin with fosfomycin or minocycline in a mouse model of multidrug-resistant Acinetobacter baumannii pneumonia

Unfortunately, the options for treating multidrug-resistant (MDR) Acinetobacter baumannii (A. baumannii) infections are extremely limited. Recently, fosfomycin and minocycline were newly introduced as a treatment option for MDR A. baumannii infection. Therefore, we investigated the efficacy of the combination of colistin with fosfomycin and minocycline, respectively, as therapeutic options in MDR A. baumannii pneumonia. We examined a carbapenem-resistant A. baumannii isolated from clinical specimens at Severance Hospital, Seoul, Korea. The effect of colistin with fosfomycin, and colistin with minocycline on the bacterial counts in lung tissue was investigated in a mouse model of pneumonia caused by MDR A. baumannii. In vivo, colistin with fosfomycin or minocycline significantly (p < 0.05) reduced the bacterial load in the lungs compared with the controls at 24 and 48 h. In the combination groups, the bacterial loads differed significantly (p < 0.05) from that with the more active antimicrobial alone. Moreover, the combination regimens of colistin with fosfomycin and colistin with minocycline showed bactericidal and synergistic effects compared with the more active antimicrobial alone at 24 and 48 h. This study demonstrated the synergistic effects of combination regimens of colistin with fosfomycin and minocycline, respectively, as therapeutic options in pneumonia caused by MDR A. baumannii.

Two Novel Bacteriophages Improve Survival in Galleria mellonella Infection and Mouse Acute Pneumonia Models Infected with Extensively Drug-Resistant Pseudomonas aeruginosa

In this study, two novel P. aeruginosa phages, Bϕ-R656 and Bϕ-R1836, were evaluated in vitro, in silico, and in vivo for therapeutic efficacy and safety as an alternative antibacterial agent to control XDR-PA strains collected from pneumonia patients. Both phages exhibited potent bacteriolytic activity and greatly improved survival in G. mellonella larva infection and a mouse acute pneumonia model. Based on these results, we strongly predict that these two new phages could be used as fast-acting and safe alternative biological weapons against XDR-PA infections.

Extensively drug-resistant Pseudomonas aeruginosa (XDR-PA) is a life-threatening pathogen that causes serious global problems. Here, we investigated two novel P. aeruginosa bacteriophages (phages), Bϕ-R656 and Bϕ-R1836, in vitro, in silico, and in vivo to evaluate the potential of phage therapy to control XDR-PA clinical strains. Bϕ-R656 and Bϕ-R1836 belong to the Siphoviridae family and exhibited broad host ranges which could lyse 18 (64%) and 14 (50%) of the 28 XDR-PA strains. In addition, the two phages showed strong bacteriolytic activity against XDR-PA host strains from pneumonia patients. The whole genomes of Bϕ-R656 and Bϕ-R1836 have linear double-stranded DNA of 60,919 and 37,714 bp, respectively. The complete sequence of Bϕ-R656 had very low similarity to the previously discovered P. aeruginosa phages in GenBank, but phage Bϕ-R1836 exhibited 98% and 91% nucleotide similarity to Pseudomonas phages YMC12/01/R24 and PA1/KOR/2010, respectively. In the two in vivo infection models, treatment with Bϕ-R656 and Bϕ-R1836 enhanced the survival of Galleria mellonella larvae (50% and 60%, respectively) at 72 h postinfection and pneumonia-model mice (66% and 83%, respectively) at 12 days postinfection compared with untreated controls. Treatment with Bϕ-R656 or Bϕ-R1836 also significantly decreased the bacterial load in the lungs of the mouse pneumonia model (>6 log₁₀ CFU and >4 log₁₀ CFU, respectively) on day 5.

IMPORTANCE In this study, two novel P. aeruginosa phages, Bϕ-R656 and Bϕ-R1836, were evaluated in vitro, in silico, and in vivo for therapeutic efficacy and safety as an alternative antibacterial agent to control XDR-PA strains collected from pneumonia patients. Both phages exhibited potent bacteriolytic activity and greatly improved survival in G. mellonella larva infection and a mouse acute pneumonia model. Based on these results, we strongly predict that these two new phages could be used as fast-acting and safe alternative biological weapons against XDR-PA infections.

Efficacy of bacteriophage treatment against carbapenem-resistant Acinetobacter baumannii in Galleria mellonella larvae and a mouse model of acute pneumonia

Background

Acinetobacter baumannii is an opportunistic pathogen that causes serious nosocomial infection in intensive care units. In particular, carbapenem-resistant A. baumannii (CRAB) strains have been increasing in the past decade, and they have caused major medical problems worldwide. In this study, a novel A. baumannii lytic phage, the YMC 13/03/R2096 ABA BP (phage Βϕ-R2096), which specifically causes the lysis of CRAB strains, was characterized in detail in vitro and in silico, and the in vivo effectiveness of phage therapy was evaluated using Galleria mellonella and a mouse model of acute pneumonia.

Results

The A. baumannii phage Βϕ-R2096 was isolated from sewage water using CRAB clinical strains selected from patients at a university hospital in South Korea. The complete genome of the phage Βϕ-R2096, which belongs to the Myoviridae family, was analyzed. Phage Βϕ-R2096 inhibited bacterial growth in a dose-dependent manner and exhibited high bacteriolytic activity at MOI = 10. In the evaluation of its therapeutic potential against CRAB clinical isolates using two in vivo models, phage Βϕ-R2096 increased the survival rates of both G. mellonella larvae (from 0 to 50% at 24 h) and mice (from 30% with MOI = 0.1 to 100% with MOI = 10 for 12 days) in post-infection of CRAB. In particular, phage Βϕ-R2096 strongly ameliorated histologic damage to infected lungs, with bacterial clearance in the lungs observed on day 3 postinfection in the mouse acute pneumonia model. Moreover, in vivo studies revealed no mortality or serious side effects in phage-treated groups.

Conclusion

The results of this study strongly suggest that phage Βϕ-R2096, a novel A. baumannii lytic phage, could be an alternative antibacterial agent to control CRAB infections. This study is the first report to compare in vivo evaluations (G. mellonella larvae and a mouse acute pneumonia model) of the therapeutic efficacy of a phage against CRAB infections.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1443-5) contains supplementary material, which is available to authorized users.

Microbiome-Microbial Metabolome-Cancer Cell Interactions in Breast Cancer-Familiar, but Unexplored

Breast cancer is a leading cause of death among women worldwide. Dysbiosis, an aberrant composition of the microbiome, characterizes breast cancer. In this review we discuss the changes to the metabolism of breast cancer cells, as well as the composition of the breast and gut microbiome in breast cancer. The role of the breast microbiome in breast cancer is unresolved, nevertheless it seems that the gut microbiome does have a role in the pathology of the disease. The gut microbiome secretes bioactive metabolites (reactivated estrogens, short chain fatty acids, amino acid metabolites, or secondary bile acids) that modulate breast cancer. We highlight the bacterial species or taxonomical units that generate these metabolites, we show their mode of action, and discuss how the metabolites affect mitochondrial metabolism and other molecular events in breast cancer. These metabolites resemble human hormones, as they are produced in a "gland" (in this case, the microbiome) and they are subsequently transferred to distant sites of action through the circulation. These metabolites appear to be important constituents of the tumor microenvironment. Finally, we discuss how bacterial dysbiosis interferes with breast cancer treatment through interfering with chemotherapeutic drug metabolism and availability.

A Porcine Wound Model of Acinetobacter baumannii Infection

Objective: To better understand Acinetobacter baumannii pathogenesis and to advance drug discovery against this pathogen, we developed a porcine, full-thickness, excisional, monospecies infection wound model.

Approach: The research was facilitated with AB5075, a previously characterized, extensively drug-resistant A. baumannii isolate. The model requires cyclophosphamide-induced neutropenia to establish a skin and soft tissue infection (SSTI) that persists beyond 7 days. Multiple, 12-mm-diameter full-thickness wounds were created in the skin overlying the cervical and thoracic dorsum. Wound beds were inoculated with 5.0 × 10⁴ colony-forming units (CFU) and covered with dressing.

Results:A. baumannii was observed in the wound bed and on the dressing in what appeared to be biofilm. When bacterial burdens were measured, proliferation to at least 10⁶ CFU/g (log₁₀6) wound tissue was observed. Infection was further characterized by scanning electron microscopy (SEM) and peptide nucleic acid fluorescence in situ hybridization (PNA-FISH) staining. To validate as a treatment model, polymyxin B was applied topically to a subset of infected wounds every 2 days. Then, the treated and untreated wounds were compared using multiple quantitative and qualitative techniques to include gross pathology, CFU burden, histopathology, PNA-FISH, and SEM.

Innovation: This is the first study to use A. baumannii in a porcine model as the sole infectious agent.

Conclusion: The porcine model allows for an additional preclinical assessment of antibacterial candidates that show promise against A. baumannii in rodent models, further evaluating safety and efficacy, and serve as a large animal in preclinical assessment for the treatment of SSTI.

Effect of Cyclophosphamide Treatment on Central and Effector Memory T Cells in Mice

Immunological memory is a key feature of adaptive immunity. It provides the organism with long-lived and robust protection against infection. The important question is whether cyclophosphamide (CP), as immunosuppressive agent used in cancer therapy and in some autoimmune diseases, may act on the memory T-cell population. We investigated the effect of CP on the percentage of central memory T cells (T_CM) and effector memory T cells (T_EM) in the mouse model of CP-induced immunosuppression (8-10-week-old male C57BL/6 mice CP treated for 7 days at the daily dose of 50 μg/g body weight [bw], manifested the best immunosuppression status, as compared to lower doses of CP: 10 or 20 μg/g bw). The CP induced a significant decrease in the percentage of CD8⁺ (T_CM), compared to nonimmunosuppressed mice. This effect was not observed in the case of CD4⁺ T_CM population. The percentage of gated T_EM with CD4 and CD8 phenotype was significantly decreased in CP-treated mice, as compared to the control ones. Taken together, the above data indicate that CP-induced immunosuppression in mice leads to a reduction in the abundance of central memory cells possessing preferentially CD8⁺ phenotype as well as to a reduction in the percentage of effector memory cells (splenocytes both CD4⁺ and CD8⁺), compared to the cells from nonimmunosuppressed mice. These findings in mice described in this article may contribute to the understanding of the complexity of the immunological responses in humans and extend research on the impact of the CP model of immunosuppression in mice and memory T-cell populations.

Expression pattern of soluble triggering receptor expressed on myeloid cells-1 in mice with Acinetobacter baumannii colonization and infection in the lung

Background

Acinetobacter baumannii (A. baumannii) is one of the most troublesome opportunistic pathogens associated with hospital-acquired pneumonia (HAP). It is important to be able to discriminate A. baumannii colonization from infection in its early stages so that effective antibiotics can be promptly applied. Recent studies have reported that the secretion of soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) is markedly upregulated in pneumonia and sepsis, but the expression pattern of sTREM-1 in A. baumannii colonization and infection in the lung has not been explored.

Methods

C57BL/6J male mice were intraperitoneally injected with 1% streptozotocin for 5 consecutive days to establish diabetic models. Subsequently, aerosol inhalation of A. baumannii suspension was performed in these mice to induce pulmonary colonization or infection with saline as vehicle control. Mice were sacrificed and lung tissue was harvested on days 0, 1, 3, 5 and 7 after exposure. Pharyngeal swab culture, lung homogenate culture, and H&E staining of lung tissue were performed to assess the severity of infectious inflammation. sTREM-1 expressions in serum and lung supernatants, serum procalcitonin (PCT) and C-reactive protein (CRP) concentrations were measured by ELISA.

Results

A. baumannii colonization and infection models were verified by pharyngeal swab culture, lung homogenate culture, and H&E staining. While sTREM-1 concentrations in mice with A. baumannii colonization remained unchanged in serum and lung supernatants, sTREM-1 expression levels in infected animals were significantly upregulated. In addition, serum sTREM-1 concentration was positively correlated with serum levels of PCT and CRP.

Conclusions

Dynamic secretion of sTREM-1 is associated with the development of A. baumannii infection in the lung. Therefore, sTREM-1 expression level may be a promising biomarker for discriminating A. baumannii infection from colonization.

Phage Therapy as a Promising New Treatment for Lung Infection Caused by Carbapenem-Resistant Acinetobacter baumannii in Mice

Carbapenem-resistant Acinetobacter baumannii (CRAB) which is noted as a major pathogen associated with healthcare-associated infections has steadily developed beyond antibiotic control. Lytic bacteriophages with the characteristics of infecting and lysing specific bacteria have been used as a potential alternative to traditional antibiotics to solve multidrug-resistant bacterial infections. Here, we isolated A. baumannii-specific lytic phages and evaluated their potential therapeutic effect against lung infection caused by CRAB clinical strains. The combined lysis spectrum of four lytic phages' ranges was 87.5% (42 of 48) against CRAB clinical isolates. Genome sequence and analysis indicated that phage SH-Ab15519 is a novel phage which does not contain the virulence or antibiotic resistance genes. In vivo study indicated that phage SH-Ab15519 administered intranasally can effectively rescue mice from lethal A. baumannii lung infection without deleterious side effects. Our work explores the potential use of phages as an alternative therapeutic agent against the lung infection caused by CRAB strains.

The development and use of a drug-induced immunosuppressed rat-model to screen Phela for mechanism of immune stimulation

ETHNOPHARMACOLOGY RELEVANCY

Phela, is code name for a medicinal product made from four South African traditional medicinal plants (Clerodendrum glabrum E. Mey, Polianthes tuberosa (Linn.), Rotheca myricoides (Hochst.) Steane & Mabb. and Senna occidentalis (L.) Link). All these plants have established traditional use in a wide spectrum of diseases. Phela is under development for use as an immune booster in immunocompromised patients, which includes patients with the human immunodeficiency virus (HIV). Already several studies, both pre-clinical and clinical, have shown that Phela is a safe and effective immune booster. Despite some studies on the action of Phela, the mechanism of action by Phela is still not known. Understanding the mechanism of action will enable safer and effective use of the drug for the right indications. Unfortunately, there is no well characterized test-system for screening products for immune stimulant activity. Therefore, the objective of this study was to use Phela as the test article, to develop and validate a rat-model (test system) by which to screen medicines for immune stimulant activity.

MATERIAL AND METHODS

First, the batch of Phela used was authenticated by high performance liquid chromatography (HPLC) techniques; analytical methods for the immunosuppressant drugs, cyclosporine A (CsA), cyclophosphamide (CP) and dexamethasone (Dex) were developed and validated; and a slide-A-Lyzer dialysis was used to test for potential interactions in rat plasma of Phela with CsA, CP and Dex. Thereafter, using Sprague Dawley (SD) rats and in separate experiments, the effective dose of Phela in the study animals was determined in a dose ranging study with levamisole, a known immune stimulant as the positive control; the appropriate doses for immunosuppression by CsA, CP and Dex were determined; the time to reach 'established immunosuppression' with each drug was determined (it was also the time for intervention with Phela); and eventually, the effect of Phela on the immune system was tested separately for each drug induced immunosuppression. The immune system was monitored by observing for changes in plasma profiles of IL-2, IL-10, IgG, IgM, CD4 and CD8 cell counts at appropriate intervals, while in addition to function tests, the kidneys, liver, spleen, thymus, were weighed and examined for any pathology.

RESULTS

The chromatographic fingerprint certified this batch of Phela as similar to the authentic Phela. There was no significant interaction between Phela and CsA, CP and Dex. The effective dose of Phela was determined to be 15.4mg/kg/day. Phela led to a moderate increase in the immune parameters in the normal rats. Co-administration of Phela 15mg/kg/day orally for 21 days with CsA led to stoppage and reversal of the immunosppressive effects of CsA that were exhibited as increased IL-2, IL-10, CD4 and CD8 counts, implying that Phela stimulates the cell mediate immunity (CMI). For CP, Phela led to stoppage and reversal, though moderate, of CP-induced suppression of IL-10, IgM and IgG only, implying that Phela stimulates the humoral immunity (HI) too. Phela had no effect on Dex induced immunosuppression. Stimulation of the CMI means that Phela clinical testing programme should focus on diseases or disorders that compromise the CMI, e.g., HIV and TB. The stimulation of the HI immunity means that Phela may stimulate existing memory cells to produce antibodies.

CONCLUSION

The present study has revealed Phela's mechanism of action as mainly by stimulation of the CMI, implying that the use of Phela as immune booster in HIV patients is appropriate; and that using Phela as the test product, a rat model for screening medicinal products for immune stimulation has been successfully developed and validated, with a hope that it will lead to the testing of other related medicinal products.

Effect of intravenous immunoglobulin on the function of Treg cells derived from immunosuppressed mice with Pseudomonas aeruginosa pneumonia

AIM

The present study aimed to investigate the effect of intravenous immunoglobulin (IVIG) on regulatory T (Treg) cells derived from immunosuppressed mice with Pseudomonas aeruginosa (PA) pneumonia.

METHODS

A total of 108 BALB/c mice were randomly divided into the following groups: control group (Control), immunosuppressed group (IS), PA pneumonia group (PA), PA pneumonia in immunosuppressed group (IS + PA), PA pneumonia with IVIG treatment in immunocompetent group (PA + IVIG) and PA pneumonia with IVIG treatment in immunosuppressed group (IS + PA + IVIG). Each group comprised 18 mice. The combined PA pneumonia in immunosuppressed model and the treatment models were established. The mice in each group were sacrificed at 4, 8, and 24 h time points. The general condition and pathological changes in the lung tissues of the mice were monitored. Reverse transcription-polymerase chain reaction was used to detect the forkhead box P3 (FOXP3) mRNA relative expression level in the lung tissues. The enzyme-linked immunosorbent assay was used to detect the serum concentration of active transforming growth factor beta (TGF-β).

RESULTS

No inflammatory response were exhibited in the lung tissues of the mice in Control group and IS group, while varying degrees of acute lung injury were revealed in the mice in PA group, IS + PA group, PA + IVIG group and IS + PA + IVIG group. Lung tissue injury was most apparent at the 8 h time point, and it indicated the greatest effect in IS + PA group. Whereas tissue damages were alleviated in PA + IVIG group and IS + PA + IVIG group compared with IS + PA group. In addition, tissue damage lessened in PA + IVIG group compared with PA group and IS + PA + IVIG group. FOXP3 mRNA expression levels in the lung tissues and the serum concentration of TGF-β were lower in IS group, PA group, IS + PA group and IS + PA + IVIG group at the 4, 8 and 24 h time points, respectively compared with Control group. FOXP3 mRNA expression levels decreased in PA + IVIG group at the 4h time point and TGF-β serum concentrations decreased at the 4 and 8h time points compared with Control group, and subsequently increased.

CONCLUSIONS

In the immunosuppred model with PA pneumonia, the immune system was greatly compromised. IVIG partially restored the immunosuppressed functions of Treg cells, suppressed the overactivated immune system and ameliorated the development of the disease.

Expression of Toll-like receptor 4 in lungs of immune-suppressed rat with Acinetobacter baumannii infection

Toll-like receptor 4 (TLR4) is involved in the regulation of host responses to Acinetobacter baumannii (A. baumannii). The aim of the present study was to examine the function of TLR4 in lung inflammation in immune-suppressed rats with A. baumannii infection. A total of 72 Sprague-Dawley male rats were randomly divided into the control, A. baumannii infection and immune-suppressed infection groups. The immune-suppressed infection group was treated with 100 mg/kg hydrocortisone by subcutaneous injection every other day for 2 weeks prior to A. baumannii infection. Lung tissue was obtained on the 3rd and 7th day after tracheal inoculation with A. baumannii. The expression of TLR4 in bronchial and alveolar epithelial cells, and alveolar macrophage was examined using immunohistochemistry. The levels of interleukin (IL)-6, tumor necrosis factor (TNF)-α in bronchoalveolar lavage fluid were detected using ELISA. The results showed that in the control group, the expression of TLR4 was upregulated in the bronchial and alveolar epithelial, and alveolar macrophages, and the levels of IL-6 and TNF-α were increased in the early phase of A. baumannii infection. On the 7th day, no significant difference in the levels of IL-6 and TNF-α was observed between the A. baumannii infection and control groups. Conversely, the expression of TLR4 was downregulated in the immune-suppressed group, and the levels of IL-6 and TNF-α were reduced on the 3rd day after infection. In the subsequent observation period, the expression of TLR4 was upregulated and the levels of IL-6 and TNF-α were increased. In conclusion, the results show a critical role of TLR4 in mediating effective immune response in the lung of rat with A. baumannii infection.

In Vivo Application of Bacteriophage as a Potential Therapeutic Agent To Control OXA-66-Like Carbapenemase-Producing Acinetobacter baumannii Strains Belonging to Sequence Type 357

The increasing prevalence of carbapenem-resistant Acinetobacter baumannii (CRAB) strains in intensive care units has caused major problems in public health worldwide. Our aim was to determine whether this phage could be used as an alternative therapeutic agent against multidrug-resistant bacterial strains, specifically CRAB clinical isolates, using a mouse model. Ten bacteriophages that caused lysis in CRAB strains, including blaOXA-66-like genes, were isolated. YMC13/01/C62 ABA BP (phage Bϕ-C62), which showed the strongest lysis activity, was chosen for further study by transmission electron microscopy (TEM), host range test, one-step growth and phage adsorption rate, thermal and pH stability, bacteriolytic activity test, genome sequencing and bioinformatics analysis, and therapeutic effect of phage using a mouse intranasal infection model. The phage Bϕ-C62 displayed high stability at various temperatures and pH values and strong cell lysis activity in vitro The phage Bϕ-C62 genome has a double-stranded linear DNA with a length of 44,844 bp, and known virulence genes were not identified in silico. In vivo study showed that all mice treated with phage Bϕ-C62 survived after intranasal bacterial challenge. Bacterial clearance in the lung was observed within 3 days after bacterial challenge, and histologic damage also improved significantly; moreover, no side effects were observed.

IMPORTANCE

In our study, the novel A. baumannii phage Bϕ-C62 was characterized and evaluated in vitro, in silico, and in vivo These results, including strong lytic activities and the improvement of survival rates, showed the therapeutic potential of the phage Bϕ-C62 as an antimicrobial agent. This study reports the potential of a novel phage as a therapeutic candidate or nontoxic disinfectant against CRAB clinical isolates in vitro and in vivo.

Intranasal treatment with bacteriophage rescues mice from Acinetobacter baumannii-mediated pneumonia

AIM

With the emergence of drug-resistant bacteria, finding alternative agents to treat antibiotic-resistant bacterial infections is imperative.

MATERIALS & METHODS

A mouse pneumonia model was developed by combining cyclophosphamide pretreatment and Acinetobacter baumannii challenge, and a lytic bacteriophage was evaluated for its therapeutic efficacy in this model by examining the survival rate, bacterial load in the lung and lung pathology.

RESULTS

Intranasal instillation with bacteriophage rescued 100% of mice following lethal challenge with A. baumannii. Phage treatment reduced bacterial load in the lung. Microcomputed tomography indicated a reduction in lung inflammation in mice given phage.

CONCLUSION

This research demonstrates that intranasal application of bacteriophage is viable, and could provide complete protection from pneumonia caused by A. baumannii.

Neutropenia exacerbates infection by Acinetobacter baumannii clinical isolates in a murine wound model

The Gram negative coccobacillus Acinetobacter baumannii has become an increasingly prevalent cause of hospital-acquired infections in recent years. The majority of clinical A. baumannii isolates display high-level resistance to antimicrobials, which severely compromises our capacity to care for patients with A. baumannii disease. Neutrophils are of major importance in the host defense against microbial infections. However, the contribution of these cells of innate immunity in host resistance to cutaneous A. baumannii infection has not been directly investigated. Hence, we hypothesized that depletion of neutrophils increases severity of bacterial disease in an experimental A. baumannii murine wound model. In this study, the Ly-6G-specific monoclonal antibody (mAb), 1A8, was used to generate neutropenic mice and the pathogenesis of several A. baumannii clinical isolates on wounded cutaneous tissue was investigated. We demonstrated that neutrophil depletion enhances bacterial burden using colony forming unit determinations. Also, mAb 1A8 reduces global measurements of wound healing in A. baumannii-infected animals. Interestingly, histological analysis of cutaneous tissue excised from A. baumannii-infected animals treated with mAb 1A8 displays enhanced collagen deposition. Furthermore, neutropenia and A. baumannii infection alter pro-inflammatory cytokine release leading to severe microbial disease. Our findings provide a better understanding of the impact of these innate immune cells in controlling A. baumannii skin infections.

AB5075, a Highly Virulent Isolate of Acinetobacter baumannii, as a Model Strain for the Evaluation of Pathogenesis and Antimicrobial Treatments

Acinetobacter baumannii is recognized as an emerging bacterial pathogen because of traits such as prolonged survival in a desiccated state, effective nosocomial transmission, and an inherent ability to acquire antibiotic resistance genes. A pressing need in the field of A. baumannii research is a suitable model strain that is representative of current clinical isolates, is highly virulent in established animal models, and can be genetically manipulated. To identify a suitable strain, a genetically diverse set of recent U.S. military clinical isolates was assessed. Pulsed-field gel electrophoresis and multiplex PCR determined the genetic diversity of 33 A. baumannii isolates. Subsequently, five representative isolates were tested in murine pulmonary and Galleria mellonella models of infection. Infections with one strain, AB5075, were considerably more severe in both animal models than those with other isolates, as there was a significant decrease in survival rates. AB5075 also caused osteomyelitis in a rat open fracture model, while another isolate did not. Additionally, a Tn5 transposon library was successfully generated in AB5075, and the insertion of exogenous genes into the AB5075 chromosome via Tn7 was completed, suggesting that this isolate may be genetically amenable for research purposes. Finally, proof-of-concept experiments with the antibiotic rifampin showed that this strain can be used in animal models to assess therapies under numerous parameters, including survival rates and lung bacterial burden. We propose that AB5075 can serve as a model strain for A. baumannii pathogenesis due to its relatively recent isolation, multidrug resistance, reproducible virulence in animal models, and genetic tractability.

The incidence of A. baumannii infections has increased over the last decade, and unfortunately, so has antibiotic resistance in this bacterial species. A. baumannii is now responsible for more than 10% of all hospital-acquired infections in the United States and has a >50% mortality rate in patients with sepsis and pneumonia. Most research on the pathogenicity of A. baumannii focused on isolates that are not truly representative of current multidrug-resistant strains isolated from patients. After screening of a panel of isolates in different in vitro and in vivo assays, the strain AB5075 was selected as more suitable for research because of its antibiotic resistance profile and increased virulence in animal models. Moreover, AB5075 is susceptible to tetracycline and hygromycin, which makes it amenable to genetic manipulation. Taken together, these traits make AB5075 a good candidate for use in studying virulence and pathogenicity of this species and testing novel antimicrobials.