Acinetobacter baumannii as an oro-dental pathogen: a red alert!!

OBJECTIVES: This review highlights the existence and association of Acinetobacter baumannii with the oro-dental diseases, transforming this systemic pathogen into an oral pathogen. The review also hypothesizes possible reasons for the categorization of this pathogen as code blue due to its stealthy entry into the oral cavity. METHODOLOGY: Study data were retrieved from various search engines reporting specifically on the association of A. baumannii in dental diseases and tray set-ups. Articles were also examined regarding obtained outcomes on A. baumannii biofilm formation, iron acquisitions, magnitude of antimicrobial resistance, and its role in the oral cancers. RESULTS: A. baumannii is associated with the oro-dental diseases and various virulence factors attribute for the establishment and progression of oro-mucosal infections. Its presence in the oral cavity is frequent in oral microbiomes, conditions of impaired host immunity, age related illnesses, and hospitalized individuals. Many sources also contribute for its prevalence in the dental health care environment and the presence of drug resistant traits is also observed. Its association with oral cancers and oral squamous cell carcinoma is also evident. CONCLUSIONS: The review calls for awareness on the emergence of A. baumannii in dental clinics and for the need for educational programs to monitor and control the sudden outbreaks of such virulent and resistant traits in the dental health care settings.

Risk factors for bacteremic pneumonia and mortality (28-day mortality) in patients with Acinetobacter baumannii bacteremia.

BACKGROUND: Patients infected with Acinetobacter baumannii (AB) bacteremia in hospital have high morbidity and mortality. We analyzed the clinical characteristics of pneumonia and nonpneumonia-related AB bloodstream infections (AB BSIs) and explored the possible independent risk factors for the incidence and prognosis of pneumonia-related AB BSIs. METHODS: A retrospective monocentric observational study was performed. All 117 episodes of hospital-acquired AB bacteremia sorted into groups of pneumonia-related AB BSIs (n = 45) and nonpneumonia-related AB BSIs (n = 72) were eligible. Univariate/multivariate logistic regression analysis was used to explore the independent risk factors. The primary outcome was the antibiotic susceptibility in vitro of pneumonia-related AB BSIs group. The secondary outcome was the independent risk factor for the pneumonia-related AB BSIs group. RESULTS: Among 117 patients with AB BSIs, the pneumonia-related group had a greater risk of multidrug resistant A. baumannii (MDRAB) infection (84.44%) and carbapenem-resistant A. baumannii (CRAB) infection (80%). Polymyxin, minocycline and amikacin had relatively high susceptibility rates (> 80%) in the nonpneumonia-related group. However, in the pneumonia-related group, only polymyxin had a drug susceptibility rate of over 80%. Univariate analysis showed that survival time (day), CRAB, MDRAB, length of hospital stay prior to culture, length of ICU stay prior to culture, immunocompromised status, antibiotics used prior to culture (n > = 3 types), endotracheal tube, fiberoptic bronchoscopy, PITT, SOFA and invasive interventions (n > = 3 types) were associated with pneumonia-related AB bacteremia. The multivariate logistic regression analysis revealed that recent surgery (within 1 mo) [P = 0.043; 0.306 (0.098-0.962)] and invasive interventions (n > = 3 types) [P = 0.021; 0.072 (0.008-0.671)] were independent risk factors related to pneumonia-related AB bacteremia. Multivariate logistic regression analysis revealed that length of ICU stay prior to culture [P = 0.009; 0.959 (0.930-0.990)] and recent surgery (within 1 mo) [P = 0.004; 0.260 (0.105-0.646)] were independent risk factors for mortality in patients with pneumonia-related AB bacteremia. The Kaplan‒Meier curve and the timing test showed that patients with pneumonia-related AB bacteremia had shorter survival time compared to those with nonpneumonia-related AB bacteremia. CONCLUSIONS: Our study found that A. baumannii had a high rate of antibiotic resistance in vitro in the pneumonia-related bacteremia group, and was only sensitive to polymyxin. Recent surgery was a significantly independent predictor in patients with pneumonia-related AB bacteremia.

Characterization of Acinetobacter baumannii-calcoaceticus complex isolates and microbiological outcome for patients treated with sulbactam-durlobactam in a phase 3 trial (ATTACK).

Acinetobacter baumannii-calcoaceticus complex (ABC) causes severe, difficult-to-treat infections that are frequently antibiotic resistant. Sulbactam-durlobactam (SUL-DUR) is a targeted ß-lactam/ß-lactamase inhibitor combination antibiotic designed to treat ABC infections, including those caused by multidrug-resistant strains. In a global, pathogen-specific, randomized, controlled phase 3 trial (ATTACK), the efficacy and safety of SUL-DUR were compared to colistin, both dosed with imipenem-cilastatin as background therapy, in patients with serious infections caused by carbapenem-resistant ABC. Results from ATTACK showed that SUL-DUR met the criteria for non-inferiority to colistin for the primary efficacy endpoint of 28-day all-cause mortality with improved clinical and microbiological outcomes compared to colistin. This report describes the characterization of the baseline ABC isolates from patients enrolled in ATTACK, including an analysis of the correlation of microbiological outcomes with SUL-DUR MIC values and the molecular drivers of SUL-DUR resistance.

Phage-mediated colistin resistance in Acinetobacter baumannii.

AIMS: Antimicrobial resistance is a global threat to human health, and Acinetobacter baumannii is a paradigmatic example of how rapidly bacteria become resistant to clinically relevant antimicrobials. The emergence of multidrug-resistant A. baumannii strains has forced the revival of colistin as a last-resort drug, suddenly leading to the emergence of colistin resistance. We investigated the genetic and molecular basis of colistin resistance in A. baumannii, and the mechanisms implicated in its regulation and dissemination. METHODS: Comparative genomic analysis was combined with genetic, biochemical, and phenotypic assays to characterize Φ19606, an A. baumannii temperate bacteriophage that carries a colistin resistance gene. RESULTS: Ф19606 was detected in 41% of 523 A. baumannii complete genomes and demonstrated to act as a mobile vehicle of the colistin resistance gene eptA1, encoding a functional lipid A phosphoethanolamine transferase. The eptA1 gene is coregulated with its chromosomal homolog pmrC via the PmrAB two-component system and confers colistin resistance when induced by low calcium and magnesium levels. Resistance selection assays showed that the eptA1-harbouring phage Ф19606 promotes the emergence of spontaneous colistin-resistant mutants. CONCLUSIONS: Φ19606 is an unprecedented example of a self-transmissible phage vector implicated in the dissemination of colistin resistance.

Metabolomics Method in Understanding and Sensitizing Carbapenem-Resistant Acinetobacter baumannii to Meropenem.

Carbapenem-resistant Acinetobacter baumannii (CRAB) strains are prevalent worldwide and represent a major threat to public health. However, treatment options for infections caused by CRAB are very limited as they are resistant to most of the commonly used antibiotics. Consequently, understanding the mechanisms underlying carbapenem resistance and restoring bacterial susceptibility to carbapenems hold immense importance. The present study used gas chromatography-mass spectrometry (GC-MS)-based metabolomics to investigate the metabolic mechanisms of antibiotic resistance in clinically isolated CRAB. Inactivation of the pyruvate cycle and purine metabolism is the most typical characteristic of CRAB. The CRAB exhibited a reduction in the activity of enzymes involved in the pyruvate cycle, proton motive force, and ATP levels. This decline in central carbon metabolism resulted in a decrease in the metabolic flux of the α-ketoglutarate-glutamate-glutamine pathway toward purine metabolism, ultimately leading to a decline in adenine nucleotide interconversion. Exogenous adenosine monophosphate (AMP) and adenosine triphosphate (ATP) enhance the killing efficacy of Meropenem against CRAB. The combination of ATP and Meropenem also has a synergistic effect on eliminating CRAB persisters and the biofilm, as well as protecting mice against peritonitis-sepsis. This study presents a novel therapeutic modality to treat infections caused by CRAB based on the metabolism reprogramming strategy.

Heteroresistance to cefiderocol in carbapenem-resistant Acinetobacter baumannii in the CREDIBLE-CR study was not linked to clinical outcomes: a post hoc analysis.

IMPORTANCE: The population analysis profiling (PAP) test is considered the "gold standard" method to detect heteroresistance. It exposes bacteria to increasing concentrations of antibiotics at high cell densities to detect any minority resistant subpopulations that might be missed by the low inoculums used for reference susceptibility tests. However, its clinical relevance has not been well established. In the CREDIBLE-CR study, a numerically increased all-cause mortality was observed in the cefiderocol arm relative to the best available therapy arm for patients with Acinetobacter spp. infections. Heteroresistance has independently been proposed by another research group as a potential explanation of the mortality difference. An analysis of the baseline carbapenem-resistant Acinetobacter calcoaceticus-baumannii complex isolates from patients treated with cefiderocol in the CREDIBLE-CR study showed the highest clinical cure rate and the lowest mortality for patients with PAP-heteroresistant isolates compared with PAP-susceptible or PAP-resistant isolates. These findings contradict the abovementioned hypothesis that heteroresistance contributed to the increased mortality.

The efficacy of mesenchymal stem cell treatment and colistin-fosfomycin combination on colistin-resistant Acinetobacter baumannii sepsis model.

INTRODUCTION: This study examines the role of mesenchymal stem cells (MSCs) in an experimental sepsis model developed with colistin-resistant Acinetobacter baumannii (CRAB). MATERIALS AND METHODS: BALB-c mice were divided into treatment groups (MSC, MSC + colistin (C)-fosfomycin (F), and C-F and control groups (positive and negative)). CRAB was administered to mice through intraperitoneal injection. Three hours later, C, F, and MSC were given intraperitoneally to the treatment groups. Colistin administration was repeated every 12 h, F administration was done every 4 h, and the second dose of MSC was administered after 48 h. Mice were sacrificed at 24 and 72 h. The bacterial load was determined as colony-forming units per gram (cfu/g). Histopathological examination was conducted on the left lung, liver, and both kidneys. IL-6 and C-reactive protein (CRP) levels in mouse sera were determined by enzyme-linked immunosorbent assay. RESULTS: Among the treatment groups, the C-F group had the lowest colony count in the lung (1.24 ± 1.66 cfu/g) and liver (1.03 ± 1.08 cfu/g). The highest bacterial clearance was observed at 72 h compared to 24 h in the MSC-treated groups (p = 0.008). The MSC + C-F group showed the lowest histopathological score in the liver and kidney (p = 0.009). In the negative control group, the IL-6 level at the 24th hour was the lowest (p < 0.001). Among the treatment groups, the CRP level was the lowest in the MSC + C-F group at 24 and 72 h. CONCLUSION: In a CRAB sepsis model, adding MSCs to a colistin-fosfomycin treatment may be beneficial in terms of reducing bacterial loads and preventing histopathological damage.

The Pharmacokinetics/Pharmacodynamic Relationship of Durlobactam in Combination With Sulbactam in In Vitro and In Vivo Infection Model Systems Versus Acinetobacter baumannii-calcoaceticus Complex.

Sulbactam-durlobactam is a ß-lactam/ß-lactamase inhibitor combination currently in development for the treatment of infections caused by Acinetobacter, including multidrug-resistant (MDR) isolates. Although sulbactam is a ß-lactamase inhibitor of a subset of Ambler class A enzymes, it also demonstrates intrinsic antibacterial activity against a limited number of bacterial species, including Acinetobacter, and has been used effectively in the treatment of susceptible Acinetobacter-associated infections. Increasing prevalence of ß-lactamase-mediated resistance, however, has eroded the effectiveness of sulbactam in the treatment of this pathogen. Durlobactam is a rationally designed ß-lactamase inhibitor within the diazabicyclooctane (DBO) class. The compound demonstrates a broad spectrum of inhibition of serine ß-lactamase activity with particularly potent activity against class D enzymes, an attribute which differentiates it from other DBO inhibitors. When combined with sulbactam, durlobactam effectively restores the susceptibility of resistant isolates through ß-lactamase inhibition. The present review describes the pharmacokinetic/pharmacodynamic (PK/PD) relationship associated with the activity of sulbactam and durlobactam established in nonclinical infection models with MDR Acinetobacter baumannii isolates. This information aids in the determination of PK/PD targets for efficacy, which can be used to forecast efficacious dose regimens of the combination in humans.

Durlobactam, a Broad-Spectrum Serine ß-lactamase Inhibitor, Restores Sulbactam Activity Against Acinetobacter Species.

Sulbactam-durlobactam is a pathogen-targeted ß-lactam/ß-lactamase inhibitor combination in late-stage development for the treatment of Acinetobacter infections, including those caused by multidrug-resistant strains. Durlobactam is a member of the diazabicyclooctane class of ß-lactamase inhibitors with broad-spectrum serine ß-lactamase activity. Sulbactam is a first-generation, narrow-spectrum ß-lactamase inhibitor that also has intrinsic antibacterial activity against Acinetobacter spp. due to its ability to inhibit penicillin-binding proteins 1 and 3. The clinical utility of sulbactam for the treatment of contemporary Acinetobacter infections has been eroded over the last decades due to its susceptibility to cleavage by numerous ß-lactamases present in this species. However, when combined with durlobactam, the activity of sulbactam is restored against this problematic pathogen. The following summary describes what is known about the molecular drivers of activity and resistance as well as results from surveillance and in vivo efficacy studies for this novel combination.

Growth of Acinetobacter baumannii impacted by iron chelation.

Acinetobacter baumannii (AB) has become multidrug-resistant (MDR) in recent years, and, currently, there are limited effective treatment options. Nutrient metals (e.g. iron) are essential to the metabolic functions of AB. This study examined the impact of iron chelation on the growth of AB in vitro and in vivo. Susceptible and MDR-AB bloodstream isolates (n = 9) were recovered from different patients between 2011 and 2018. Clonal diversity was ascertained by Fourier-transform infrared spectroscopy. In vitro bacterial densities were measured over 20 h to determine growth profiles. Variable amounts of a chelating agent [deferiprone (DFP)] were added to create a concentration gradient. Galleria mellonella larvae were inoculated with an isolate, with and without DFP. Quantitative culture was used to ascertain the bacterial burden of aggregate larvae immediately and 4 h post-infection. Increasing concentrations of DFP caused a transient and concentration-dependent hindrance to in vitro growth, compared to the no-treatment group. In vivo bacterial burden immediately post-infection in both groups was comparable. After 4 h, the burden was much higher in the control group comparatively (8.7 and 6.7 log CFU g-1). These results support that micro-nutrient limitation has the potential of being a novel approach for treating high-risk infections due to MDR-AB.

Immunization with recombinant DcaP-like protein and AbOmpA revealed protections against sepsis infection of multi-drug resistant Acinetobacter baumannii ST2Pas in a C57BL/6 mouse model.

BACKGROUNDS: The prevalence of infections associated with multi-drug resistant (MDR) Acinetobacter baumannii is increasing worldwide. Therefore, the introduction of effective vaccines against this bacterium seems necessary. METHODS: AbOmpA and DcaP-like protein were selected as promising and putative immunogenic candidates based on previous in silico studies. Three formulations including AbOmpA, DcaP-like protein, and AbOmpA + DcaP-like protein were injected into C57BL/6 mice three times with Alum adjuvant. The specific production of IgG antibodies (e.g. total IgG, IgG1 and IgG2c) and cytokines (e.g. IL-4, IL-6, and IL-17A), were evaluated. LD50% of MDR A. baumannii ST2Pas was measured using Probit's method. After the challenge with bacteria, a decrease in bacterial loads (DLs) in the lung and spleen of mice was measured. Then serum bactericidal assay was performed to determine the function of antibodies on day 42. In addition, histopathological examinations of the spleen and lung, the number of macrophage and neutrophil, as well as the rate of lymphocyte infiltration were assessed. RESULTS: The highest level of total IgG was reported in the group immunized with DcaP-like protein on day 42. The survival rate of mice was 80% in the AbOmpA immunized group and 100% for the rest of two groups. DLs in the spleen of mice immunized with AbOmpA, DcaP-like protein, and combination form were 3.5, 3, and 3.4 Log10 (CFU/g), respectively. While in the lung, the DLs were 7.5 Log10 (CFU/g) for the AbOmpA group and 5 for the rest of two groups. The levels of IL-6, IL-4, and IL-17A were significantly decreased in all immunized groups after the bacterial challenge (except for IL-17A in the group of AbOmpA). The bactericidal effect of antibodies against DcaP-like protein was more effective. No histopathological damage was observed in the combination immunized group. The DcaP-like protein was more effective in neutrophil and macrophage deployment and decreased lymphocyte infiltration. CONCLUSION: The results of immunization with AbOmpA + DcaP-like protein induced a protective reaction against the sepsis infection of MDR A. baumannii. It seems that in the future, these proteins can be considered as promising components in the development of the A. baumannii vaccine.

Acinetobacter baumannii-calcoaceticus Complex-associated Orbital Cellulitis: A Case Report and Literature Review.

Acinetobacter baumannii-calcoaceticus complex is emerging as one of the most common causes of hospital-acquired infection globally. We present a case of orbital cellulitis caused by the A. baumannii-calcoaceticus complex. A 73-year-old Chinese woman with a history of rheumatoid arthritis presented with subacute onset of right upper eyelid swelling for 1 week. Computer tomography revealed a post-septal soft tissue lesion located at the right superior orbit that was enhanced with contrast, compressing on the superior aspect of the globe. Anterior orbitotomy with incisional biopsy of the right superior orbital lesion was performed, and histopathological examination was consistent with nonspecific inflammatory mass. The microbiological culture of the specimen yielded A. calcoaceticus and A. baumannii complex, which was sensitive to ciprofloxacin, ampicillin, and gentamicin. The infection resolved after a 1-week course of intravenous augmentin. Ophthalmologists should be alert to the possibility of patients having A. baumannii and A. calcoaceticus in periorbital cellulitis.

Whole-cell vaccine candidates induce a protective response against virulent Acinetobacter baumannii.

Acinetobacter baumannii causes multi-system diseases in both nosocomial settings and a pre-disposed general population. The bacterium is not only desiccation-resistant but also notoriously resistant to multiple antibiotics and drugs of last resort including carbapenem, colistin, and sulbactam. The World Health Organization has categorized carbapenem-resistant A. baumannii at the top of its critical pathogen list in a bid to direct urgent countermeasure development. Several early-stage vaccines have shown a range of efficacies in healthy mice, but no vaccine candidates have advanced into clinical trials. Herein, we report our findings that both an ionizing γ-radiation-inactivated and a non-ionizing ultraviolet C-inactivated whole-cell vaccine candidate protects neutropenic mice from pulmonary challenge with virulent AB5075, a particularly pathogenic isolate. In addition, we demonstrate that a humoral response is sufficient for this protection via the passive immunization of neutropenic mice.

Prevalence of Virulence Genes in Acinetobacter baumannii Isolated from Clinical Samples in Mymensingh Medical College Hospital, Bangladesh.

Acinetobacter baumannii is an opportunistic bacterial pathogen that is the most important cause of hospital-acquired infections. The objective of this study was to evaluate the predominance and determination of virulence encoding genes in A. baumannii isolates. During this cross-sectional study period from February 2019 to March 2020 of 380 clinical samples including endotracheal aspirates (70), wound swab or pus (175), urine (70) and blood (65) analysed in inpatients admitted to the hospital in different unit like ICU, Surgery and Burn unit of Mymensingh Medical College Hospital. Out of 380 studied samples, 130(34.21%) strains were yielded growth. Among 130 isolates, Acinetobacter spp. was 49(37.69%). Totally, 39(79.59%) were Acinetobacter baumannii which was detected by molecular technique PCR. Further more, the determination of virulence genes csgA and fimH detected by PCR. Among two studied virulence genes, csgA (38.46%) was the most prevalent virulent genes associated with disease severity and co-morbidity of the patient in A. baumannii infections.

Immune Response to Conjugates of Fragments of the Type K9 Capsular Polysaccharide of Acinetobacter baumannii with Carrier Proteins.

The clonal bacterial species Acinetobacter baumannii is an emerging multidrug-resistant pathogen which causes high-lethality infections. Cells of A. baumannii are surrounded by the type-specific capsular polysaccharide (CPS), which provides resistance to the protective mechanisms of the host and is considered a target for immunization. The conjugates of three inert carrier proteins and A. baumannii type K9 CPS fragments, which contained various numbers of oligosaccharide repeats (K-units), were synthesized by periodate oxidation and squaric acid chemistry. The conjugates were applied to immunize mice, and chemical synthesis by squaric acid was shown to significantly improve the immunogenic properties of glycoconjugate. In BALB/c mice, IgG antibodies were predominant among type K9 CPS reactive antibodies, and their total content was several times higher than that of IgM. Immune sera were characterized by their opsonization ability during practically the entire lives of the experimental mice. The sera were cross-reactive, but the highest specificity was observed against the antigen (type K9 CPS) used for immunization. The immunization of BALB/c and ICR-1 mice with a glycoconjugate without adjuvants led to varying degrees of stimulation of IL-10, IL-17A, and TNF-α production, but not IL-4 production in the ICR-1 mice. This is in contrast to the BALB/c mice, in which γ-IFN production was also activated. The protective effectiveness of the glycoconjugates obtained by squaric acid chemistry was demonstrated by experiments that involved challenging immunized and nonimmunized animals with a lethal dose of A. baumannii K9. IMPORTANCE Immunization by glycoconjugates with A. baumannii type K9 CPS fragments induced a high level of antibodies (predominantly IgG) in sera, which reacted specifically with the CPS of A. baumannii type K9, as well as a long immunological memory. The sera of immunized animals efficiently opsonized A. baumannii type K9. Immunization resulted in the balanced production of pro/anti-inflammatory lymphokines and protective antibodies to ensure the survival of the mice infected with A. baumannii. The level of specific antibodies was sufficient to provide protective immunity against the challenge by A. baumannii, making this approach applicable in the development of vaccine preparations.

Evaluation of a direct phage DNA detection-based Taqman qPCR methodology for quantification of phage and its application in rapid ultrasensitive identification of Acinetobacter baumannii.

BACKGROUND: Rapid phage enumeration/quantitation and viable bacteria determination is critical for phage application and treatment of infectious patients caused by the pathogenic bacteria. METHODS: In the current study, a direct phage DNA detection-based Taqman qPCR methodology for quantification of phage P53 and rapid ultrasensitive identification of Acinetobacter baumannii (A. baumannii) was evaluated. RESULTS: The assay was capable of quantifying P53 phage DNA without DNA extraction and the detection limit of the assay was 550 PFU/mL. The agreement bias between the quantitative results of three different phage concentrations in this assay and double agar overlay plaque assay were under 3.38%. Through the built detection system, down to 1 log CFU/mL of viable A. baumannii can be detected within 4 h in A. baumannii spiked swab and bronchoalveolar lavage fluid samples. Compared with the Taqman qPCR that targets the conserved sequence of A. baumannii, the sensitivity of the assay built in this study could increase four orders of magnitude. CONCLUSIONS: The methodology offers a valid alternative for enumeration of freshly prepared phage solution and diagnosis of bacterial infection caused by A. baumannii or other bacterial infection in complicated samples through switching to phages against other bacteria. Furthermore, the assay could offer drug adjustment strategy timely owing to the detection of bacteria vitality.

The role of quorum sensing, biofilm formation, and iron acquisition as key virulence mechanisms in Acinetobacter baumannii and the corresponding anti-virulence strategies.

Acinetobacter baumannii is a nosocomial pathogen responsible for several serious infections, including pneumonia, sepsis, and meningitis. The propensity of this bacterium to rapidly acquire antibiotic resistance leads to the emergence and spread of multidrug-resistant A. baumannii strains. As a result, antibiotics are becoming less effective in treating infections caused by this pathogen. In recent years, increasing efforts have focused on developing therapeutic compounds that could reduce the ability of A. baumannii to establish infection by inhibiting the virulence factors and pathogenesis of this pathogen without interfering with the bacterial viability. These alternative therapeutic options may impose milder selective pressure, reducing the likelihood of anti-virulence resistance development. To develop novel anti-virulence therapies, an in-depth understanding of the bacterial virulence mechanisms is crucial to identifying potential drug targets. This review summarises the latest discoveries about the virulence of A. baumannii, focusing on quorum sensing, biofilm formation, and iron acquisition, along with the corresponding anti-virulence strategies. This article also elaborates on the practical challenges involved in developing anti-virulence drugs. Therapeutic agents that target bacterial virulence factors may play a crucial role in controlling infection in the human host. Combining anti-virulence agents with existing antibiotics could enhance the therapeutic potential of these antibiotics against A. baumannii. Although anti-virulence therapy has been envisioned as an attractive alternative to overcome antimicrobial resistance, additional research on the possibility of developing resistance against anti-virulence drugs is encouraged to evaluate the sustainability of these strategies. Moreover, future studies on the efficacy of anti-virulence therapy against a diverse panel of clinical isolates and in polymicrobial A. baumannii infections are required to provide more valuable information about its clinical application.

Lipocalin2 as a potential antibacterial drug against Acinetobacter baumannii infection.

Available antibiotics to treat Acinetobacter baumannii infection is limited due to increasing resistance and the emergence of multiple drug-resistant strains. Hence, discovering effective agents against A. baumannii to reduce the number of infection-related deaths is imperative. In search of novel and alternative antibiotics, the antibacterial function of lipocalin2 (Lcn2) was investigated to treat systemic infections of A. baumannii using a mouse neutropenia model. We observed a significant increase in serum Lcn2 levels upon bacterial injection into the mouse, and the administration of recombinant Lcn2 (rmLcn2) extended their survival. Such protective effects were also observed in rmLcn2-pretreated macrophages, where rmLcn2 reduced the survival of the pathogen inside the macrophages. The underlying molecular mechanism of Lcn2 protection was also investigated. We observed that pretreatment of the Raw-264.7 macrophages with rmLcn2 markedly altered the expression of tonB3, which encodes a component of the transporter for ferrisiderophores in A. baumannii. However, the expression of katG, the gene encoding catalase, remained unaffected. These indicate that Lcn2-mediated defense against the pathogen is related to nutritional immunity rather than reactive oxygen species (ROS) production. Furthermore, the addition of rmLcn2 in infected mice diminished bacterial burden in multiple organs and enhanced the expression of tonB3 in the liver, spleen, and lungs of the infected mice. Increased survival rate due to rmLcn2 treatment declined when the infection model was established using lcn2-defective (lcn2-/-) mice, which indicated the necessity of endogenous Lcn2. Therefore, the antibacterial function of Lcn2 can be exploited to develop an alternative therapeutic agent against A. baumannii.

Correlative proteomics identify the key roles of stress tolerance strategies in Acinetobacter baumannii in response to polymyxin and human macrophages.

The opportunistic pathogen Acinetobacter baumannii possesses stress tolerance strategies against host innate immunity and antibiotic killing. However, how the host-pathogen-antibiotic interaction affects the overall molecular regulation of bacterial pathogenesis and host response remains unexplored. Here, we simultaneously investigate proteomic changes in A. baumannii and macrophages following infection in the absence or presence of the polymyxins. We discover that macrophages and polymyxins exhibit complementary effects to disarm several stress tolerance and survival strategies in A. baumannii, including oxidative stress resistance, copper tolerance, bacterial iron acquisition and stringent response regulation systems. Using the spoT mutant strains, we demonstrate that bacterial cells with defects in stringent response exhibit enhanced susceptibility to polymyxin killing and reduced survival in infected mice, compared to the wild-type strain. Together, our findings highlight that better understanding of host-pathogen-antibiotic interplay is critical for optimization of antibiotic use in patients and the discovery of new antimicrobial strategy to tackle multidrug-resistant bacterial infections.

Antiseptic 9-Meric Peptide with Potency against Carbapenem-Resistant Acinetobacter baumannii Infection.

Carbapenem-resistant A. baumannii (CRAB) infection can cause acute host reactions that lead to high-fatality sepsis, making it important to develop new therapeutic options. Previously, we developed a short 9-meric peptide, Pro9-3D, with significant antibacterial and cytotoxic effects. In this study, we attempted to produce safer peptide antibiotics against CRAB by reversing the parent sequence to generate R-Pro9-3 and R-Pro9-3D. Among the tested peptides, R-Pro9-3D had the most rapid and effective antibacterial activity against Gram-negative bacteria, particularly clinical CRAB isolates. Analyses of antimicrobial mechanisms based on lipopolysaccharide (LPS)-neutralization, LPS binding, and membrane depolarization, as well as SEM ultrastructural investigations, revealed that R-Pro9-3D binds strongly to LPS and impairs the membrane integrity of CRAB by effectively permeabilizing its outer membrane. R-Pro9-3D was also less cytotoxic and had better proteolytic stability than Pro9-3D and killed biofilm forming CRAB. As an LPS-neutralizing peptide, R-Pro9-3D effectively reduced LPS-induced pro-inflammatory cytokine levels in RAW 264.7 cells. The antiseptic abilities of R-Pro9-3D were also investigated using a mouse model of CRAB-induced sepsis, which revealed that R-Pro9-3D reduced multiple organ damage and attenuated systemic infection by acting as an antibacterial and immunosuppressive agent. Thus, R-Pro9-3D displays potential as a novel antiseptic peptide for treating Gram-negative CRAB infections.