Assessing Acinetobacter baumannii Virulence and Persistence in a Murine Model of Lung Infection

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.

Diclofenac sensitizes multi-drug resistant Acinetobacter baumannii to colistin

Acinetobacter baumannii causes life-threatening infections that are becoming difficult to treat due to increasing rates of multi-drug resistance (MDR) among clinical isolates. This has led the World Health Organization and the CDC to categorize MDR A. baumannii as a top priority for the research and development of new antibiotics. Colistin is the last-resort antibiotic to treat carbapenem-resistant A. baumannii . Not surprisingly, reintroduction of colistin has resulted in the emergence of colistin-resistant strains. Diclofenac is a nonsteroidal anti-inflammatory drug used to treat pain and inflammation associated with arthritis. In this work, we show that diclofenac sensitizes colistin-resistant A. baumannii clinical strains to colistin, in vitro and in a murine model of pneumonia. Diclofenac also reduced the colistin MIC of Klebsiella pneumoniae and Pseudomonas aeruginosa isolates. Transcriptomic and proteomic analyses revealed an upregulation of oxidative stress-related genes and downregulation of type IV pili induced by the combination treatment. Notably, the concentrations of colistin and diclofenac effective in the murine model were substantially lower than those determined in vitro , implying a stronger synergistic effect in vivo compared to in vitro . A pilA mutant strain, lacking the primary component of the type IV pili, became sensitive to colistin in the absence of diclofenac. This suggest that the downregulation of type IV pili is key for the synergistic activity of these drugs in vivo and indicates that colistin and diclofenac exert an anti-virulence effect. Together, these results suggest that the diclofenac can be repurposed with colistin to treat MDR A. baumannii .

Aerosolized delivery of ESKAPE pathogens for murine pneumonia models

Murine pneumonia models for ESKAPE pathogens serve to evaluate novel antibacterials or to investigate immunological responses. The majority of published models uses intranasal or to a limited extent the intratracheal instillation to challenge animals. In this study, we propose the aerosol delivery of pathogens using a nebulizer. Aerosol delivery typically results in homogeneous distribution of the inoculum in the lungs because of lower particle size. This is of particular importance when compounds are assessed for their pharmacokinetic and pharmacodynamic (PK/PD) relationships as it allows to conduct several analysis with the same sample material. Moreover, aerosol delivery has the advantage that it mimics the 'natural route' of respiratory infection. In this short and concise study, we show that aerosol delivery of pathogens resulted in a sustained bacterial burden in the neutropenic lung infection model for five pathogens tested, whereas it gave a similar result in immunocompetent mice for three out of five pathogens. Moreover, a substantial bacterial burden in the lungs was already achieved 2 h post inhalation. Hence, this study constitutes a viable alternative for intranasal administration and a refinement of murine pneumonia models for PK/PD assessments of novel antibacterial compounds allowing to study multiple readouts with the same sample material.

The tRNA methyltransferase TrmB is critical for Acinetobacter baumannii stress responses and pulmonary infection

IMPORTANCE

As deficiencies in tRNA modifications have been linked to human diseases such as cancer and diabetes, much research has focused on the modifications' impacts on translational regulation in eukaryotes. However, the significance of tRNA modifications in bacterial physiology remains largely unexplored. In this paper, we demonstrate that the m7G tRNA methyltransferase TrmB is crucial for a top-priority pathogen, Acinetobacter baumannii, to respond to stressors encountered during infection, including oxidative stress, low pH, and iron deprivation. We show that loss of TrmB dramatically attenuates a murine pulmonary infection. Given the current efforts to use another tRNA methyltransferase, TrmD, as an antimicrobial therapeutic target, we propose that TrmB, and other tRNA methyltransferases, may also be viable options for drug development to combat multidrug-resistant A. baumannii.

Nasopharyngeal microbiome of COVID-19 patients revealed a distinct bacterial profile in deceased and recovered individuals

The bacterial co-infections in SARS-CoV-2 patients remained the least explored subject of clinical manifestations that may also determine the disease severity. Nasopharyngeal microbial community structure within SARS-CoV-2 infected patients could reveal interesting microbiome dynamics that may influence the disease outcomes. Here, in this research study, we analyzed distinct nasopharyngeal microbiome profile in the deceased (n = 48) and recovered (n = 29) COVID-19 patients and compared it with control SARS-CoV-2 negative individuals (control) (n = 33). The nasal microbiome composition of the three groups varies significantly (PERMANOVA, p-value <0.001), where deceased patients showed higher species richness compared to the recovered and control groups. Pathogenic genera, including Corynebacterium (LDA score 5.51), Staphylococcus, Serratia, Klebsiella and their corresponding species were determined as biomarkers (p-value <0.05, LDA cutoff 4.0) in the deceased COVID-19 patients. Ochrobactrum (LDA score 5.79), and Burkholderia (LDA 5.29), were found in the recovered group which harbors ordinal bacteria (p-value <0.05, LDA-4.0) as biomarkers. Similarly, Pseudomonas (LDA score 6.19), and several healthy nasal cavity commensals including Veillonella, and Porphyromonas, were biomarkers for the control individuals. Healthy commensal bacteria may trigger the immune response and alter the viral infection susceptibility and thus, may play important role and possible recovery that needs to be further explored. This research finding provide vital information and have significant implications for understanding the microbial diversity of COVID-19 patients. However, additional studies are needed to address the microbiome-based therapeutics and diagnostics interventions.

Assessing the Role of Cold-Shock Protein C: a Novel Regulator of Acinetobacter baumannii Biofilm Formation and Virulence

Acinetobacter baumannii is a formidable opportunistic pathogen that is notoriously difficult to eradicate from hospital settings. This resilience is often attributed to a proclivity for biofilm formation, which facilitates a higher tolerance toward external stress, desiccation, and antimicrobials. Despite this, little is known regarding the mechanisms orchestrating A. baumannii biofilm formation. Here, we performed RNA sequencing (RNA-seq) on biofilm and planktonic populations for the multidrug-resistant isolate AB5075 and identified 438 genes with altered expression. To assess the potential role of genes upregulated within biofilms, we tested the biofilm-forming capacity of their respective mutants from an A. baumannii transposon library. In so doing, we uncovered 24 genes whose disruption led to reduced biofilm formation. One such element, cold shock protein C (cspC), had a highly mucoid colony phenotype, enhanced tolerance to polysaccharide degradation, altered antibiotic tolerance, and diminished adherence to abiotic surfaces. RNA-seq of the cspC mutant revealed 201 genes with altered expression, including the downregulation of pili and fimbria genes and the upregulation of multidrug efflux pumps. Using transcriptional arrest assays, it appears that CspC mediates its effects, at least in part, through RNA chaperone activity, influencing the half-life of several important transcripts. Finally, we show that CspC is required for survival during challenge by the human immune system and is key for A. baumannii dissemination and/or colonization during systemic infection. Collectively, our work identifies a cadre of new biofilm-associated genes within A. baumannii and provides unique insight into the global regulatory network of this emerging human pathogen.

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

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

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

Polyphosphate Kinase Is Required for the Processes of Virulence and Persistence in Acinetobacter baumannii

Acinetobacter baumannii, one of the most successful bacteria causing severe nosocomial infection, was identified as a top-priority pathogen by the WHO. Thus, genetic manipulations to clarify the potential targets for fighting A. baumannii resistance and virulence are vital. Polyphosphate (polyP) kinase (PPK) is conserved in nearly all bacteria and is responsible for polyP formation, which is associated with bacterial pathogenicity and antibiotic resistance. In this study, ppk1-deficient (Δppk1::Apr), ppk1-complemented (Δppk1::Apr/PJL02-ppk1), and wild-type strains of A. baumannii ATCC 17978 were used to determine the influence of PPK1 on A. baumannii virulence and persistence mainly by polyP quantification, surface motility, biofilm formation, and bacterial persistence assays. Our work found that PPK1 is indispensable for polyP formation in vivo and that the motility of the PPK1-deficient strain was significantly impaired due to the lack of a pilus-like structure typically present compared with the complemented and wild-type strains. The deficiency of PPK1 also inhibited the biofilm formation of A. baumannii and decreased bacterial persistence under stimuli of high-concentration ampicillin (Amp) treatment, H₂O₂ stress, heat shock, and starvation stress. Furthermore, ppk1-deficient bacterium-infected mice showed a significantly reduced bacterial load and a decreased inflammatory response. However, complementation with PPK1 effectively rescued the impaired virulence and persistence of ppk1-deficient A. baumannii. In addition, metabonomic analysis revealed that PPK1 was associated with glycerophospholipid metabolism and fatty acid biosynthesis. Taken together, our results suggest that targeting PPK1 to control A. baumannii pathogenicity and persistence is a feasible strategy to fight this pathogen.

IMPORTANCEA. baumannii was identified as a top-priority pathogen by the WHO due to its antibiotic resistance. Meanwhile, the pathogenicity of A. baumannii mediated by several vital virulence factors also cannot be ignored. Here, the role of PPK1 in A. baumannii was also explored. We found that the motility ability and biofilm formation of a PPK1-deficient strain were significantly impaired. Furthermore, PPK1 was essential for its persistence maintenance to resist stimuli of high-concentration Amp treatment, H₂O₂ stress, heat shock, and starvation stress. Metabonomic analysis revealed that PPK1 was associated with glycerophospholipid metabolism and fatty acid biosynthesis. In addition, ppk1-deficient bacterium-infected mice showed significantly reduced bacterial loads and a decreased inflammatory responses in vivo. Together, our results suggest that PPK1 is vital for A. baumannii pathogenicity and persistence.

Light Regulates Acinetobacter baumannii Chromosomal and pAB3 Plasmid Genes at 37°C

The opportunistic pathogen A. baumannii has a remarkable capacity to persist in the hospital environment and cause devastating human infections. This capacity can be attributed partly to the sensing and regulatory systems that enable this pathogen to modify its physiology based on environmental cues. One of the signals that A. baumannii senses and responds to is light through the sensing and regulatory roles of the BlsA photoreceptor protein in cells cultured at temperatures below 30°C. This report presents evidence that a light stimulon is operational at 37°C, a condition at which the BlsA production and activity are drastically impaired. Global transcriptional analysis showed that the 37°C light stimulon includes the differential expression of chromosomal genes encoding a wide range of functions that are known to be involved in the adaptation to different metabolic conditions, as well as virulence and persistence in the host and the medical environment. Unexpectedly, the 37°C light stimulon also includes the differential expression of conjugation functions encoded by pAB3 plasmid genes. Our work further demonstrates that the TetR1 and H-NS regulators encoded by this conjugative plasmid control the expression of H2O2 resistance and surface motility, respectively. Furthermore, our data showed that pAB3 has an overall negative effect on the expression of these phenotypes and plays no significant virulence role. Although the nature of the bacterial factors and the mechanisms by which the regulation is attained at 37°C remain unknown, taken together, our work expands the current knowledge about light sensing and gene regulation in A. baumannii. IMPORTANCE As a facultative pathogen, Acinetobacter baumannii persists in various environments by sensing different environmental cues, including light. This report provides evidence of light-dependent regulation at 37°C of the expression of genes coding for a wide range of functions, including those involved in the conjugation of the pAB3 plasmid. Although this plasmid affects the expression of virulence traits when tested under laboratory conditions, it does not have a significant impact when tested using ex vivo and in vivo experimental models. These findings provide a better understanding of the interplay between light regulation and plasmid persistence in the pathobiology of A. baumannii.

Virulence Potential and Treatment Options of Multidrug-Resistant (MDR) Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic pathogen which is undoubtedly known for a high rate of morbidity and mortality in hospital-acquired infections. A. baumannii causes life-threatening infections, including; ventilator-associated pneumonia (VAP), meningitis, bacteremia, and wound and urinary tract infections (UTI). In 2017, the World Health Organization listed A. baumannii as a priority-1 pathogen. The prevalence of A. baumannii infections and outbreaks emphasizes the direct need for the use of effective therapeutic agents for treating such infections. Available antimicrobials, such as; carbapenems, tigecycline, and colistins have insufficient effectiveness due to the appearance of multidrug-resistant strains, accentuating the need for alternative and novel therapeutic remedies. To understand and overcome this menace, the knowledge of recent discoveries on the virulence factors of A. baumannii is needed. Herein, we summarized the role of various virulence factors, including; outer membrane proteins, efflux pumps, biofilm, penicillin-binding proteins, and siderophores/iron acquisition systems. We reviewed the recent scientific literature on different A. baumannii virulence factors and the effective antimicrobial agents for the treatment and management of bacterial infections.

Pharmacodynamic evaluation of suppression of in vitro resistance in Acinetobacter baumannii strains using polymyxin B-based combination therapy

The emergence of polymyxin resistance in Gram-negative bacteria infections has motivated the use of combination therapy. This study determined the mutant selection window (MSW) of polymyxin B alone and in combination with meropenem and fosfomycin against A. baumannii strains belonging to clonal lineages I and III. To evaluate the inhibition of in vitro drug resistance, we investigate the MSW-derived pharmacodynamic indices associated with resistance to polymyxin B administrated regimens as monotherapy and combination therapy, such as the percentage of each dosage interval that free plasma concentration was within the MSW (%T_MSW) and the percentage of each dosage interval that free plasma concentration exceeded the mutant prevention concentration (%T_>MPC). The MSW of polymyxin B varied between 1 and 16 µg/mL for polymyxin B-susceptible strains. The triple combination of polymyxin B with meropenem and fosfomycin inhibited the polymyxin B-resistant subpopulation in meropenem-resistant isolates and polymyxin B plus meropenem as a double combination sufficiently inhibited meropenem-intermediate, and susceptible strains. T_>MPC 90% was reached for polymyxin B in these combinations, while %T_MSW was 0 against all strains. T_MSW for meropenem and fosfomycin were also reduced. Effective antimicrobial combinations significantly reduced MSW. The MSW-derived pharmacodynamic indices can be used for the selection of effective combination regimen to combat the polymyxin B-resistant strain.

An overview of Acinetobacter baumannii pathogenesis: Motility, adherence and biofilm formation

The Gram-negative opportunistic pathogen Acinetobacter baumannii has gain notoriety in recent decades, primarily due to its propensity to cause nosocomial infections in critically ill patients. Its global spread, multi-drug resistance features and plethora of virulence factors make it a serious threat to public health worldwide. Though much effort has been expended in uncovering its successes, it continues to confound researchers due to its highly adaptive nature, mutating to meet the needs of a given environment. Its persistence in the clinical setting allows it to be in close proximity to a potential host, where contact can be made facilitating infection and colonization. In this article, we aim to provide a current overview of the bacterial virulence factors, specifically focusing on factors involved in the initial stages of infection, highlighting the role of adaptation facilitated by two-component systems and biofilm formation. Finally, the study of host-pathogen interactions using available animal models, their suitability, notable findings and some perspectives moving forward are also discussed.

Modulating Isoprenoid Biosynthesis Increases Lipooligosaccharides and Restores Acinetobacter baumannii Resistance to Host and Antibiotic Stress

Acinetobacter baumannii is a leading cause of ventilator-associated pneumonia and a critical threat due to multidrug resistance. The A. baumannii outer membrane is an asymmetric lipid bilayer composed of inner leaflet glycerophospholipids and outer leaflet lipooligosaccharides. Deleting mlaF of the maintenance of lipid asymmetry (Mla) system causes A. baumannii to become more susceptible to pulmonary surfactants and antibiotics and decreases bacterial survival in the lungs of mice. Spontaneous suppressor mutants isolated from infected mice contain an ISAba11 insertion upstream of the ispB initiation codon, an essential isoprenoid biosynthesis gene. The insertion restores antimicrobial resistance and virulence to ΔmlaF. The suppressor strain increases lipooligosaccharides, suggesting that the mechanism involves balancing the glycerophospholipids/lipooligosaccharides ratio on the bacterial surface. An identical insertion exists in an extensively drug-resistant A. baumannii isolate, demonstrating its clinical relevance. These data show that the stresses bacteria encounter during infection select for genomic rearrangements that increase resistance to antimicrobials.

Peptidoglycan editing provides immunity to Acinetobacter baumannii during bacterial warfare

Peptidoglycan (PG) is essential in most bacteria. Thus, it is often targeted by various assaults, including interbacterial attacks via the type VI secretion system (T6SS). Here, we report that the Gram-negative bacterium Acinetobacter baumannii strain ATCC 17978 produces, secretes, and incorporates the noncanonical d-amino acid d-lysine into its PG during stationary phase. We show that PG editing increases the competitiveness of A. baumannii during bacterial warfare by providing immunity against peptidoglycan-targeting T6SS effectors from various bacterial competitors. In contrast, we found that d-Lys production is detrimental to pathogenesis due, at least in part, to the activity of the human enzyme d-amino acid oxidase (DAO), which degrades d-Lys producing H2O2 toxic to bacteria. Phylogenetic analyses indicate that the last common ancestor of A. baumannii had the ability to produce d-Lys. However, this trait was independently lost multiple times, likely reflecting the evolution of A. baumannii as a human pathogen.

Synergistic therapeutic efficacy of ebselen and silver ions against multidrug-resistant Acinetobacter baumannii-induced urinary tract infections

Ebselen (EbSe), an organo-selenium compound with well-characterized toxicology and pharmacology, exhibited potent antibacterial activity against glutathione (GSH)-positive bacteria when combined with silver ions (Ag⁺).