Insights into mucoid Acinetobacter baumannii: A review of microbiological characteristics, virulence, and pathogenic mechanisms in a threatening nosocomial pathogen

Genomic epidemiology of a novel Pandoraea pneumonica group caused severe bloodstream infection in Hainan, China, 2021-2024

Introduction

Rarely does Pandoraea occur in bloodstream infections (BSI), although it's typically found in cystic fibrosis. This study aims to decipher the genetic map and obtain insights of clinical symptoms into Pandoraea from BSI patients.

Methods

30 suspected BSI patients' diagnostic records and medical histories were recorded. Pandoraea spp. isolates were collected and subjected to antimicrobial susceptibility testing, Sanger sequencing and Whole-genome sequencing (WGS).

Results

Of the 30 clinical cases, five (16.67%) ultimately died, whereas 25 (83.33%) are alive. 30 purified Pandoraea isolates showed high degree of MIC values to Meropenem, Amoxicillin and Potassium Clavulanate, Gentamicin, and Ceftazidime. Then, all isolates were identified as P. pneumonica based on the 16S rRNA-based phylogenetic analysis. Among 28 genomes of them, the average genome size and average GC contents were 5,397,568 bp, and 62.43%, respectively. However, WP1 displayed high similarity (90.6%) to reference Pandoraea sp. LMG 31114. Genetic differences between the tested isolates and LMG 31114 suggested that the outbreak's causative pathogen could be a novel cluster of P. pneumonica. The genomes accumulated mutations at an estimated rate of 1.3 × 10-7 mutations/year/site. Moreover, 26 clinical isolates within the P. pneumonica cluster were formed in July 2014, revealing a tendency to develop regional endemic patterns.

Conclusion

BSI caused by this novel cluster of P. pneumonica is linked to significant morbidity and mortality. Such cluster remains a critical public health challenge due to their regional epidemiological patterns and antibiotic treatment risk. This study contributed to the basis on pathogen identification, disease diagnosis, and BSI treatment.

Depolymerase as a potent adjunct to polymyxin for targeting KL160 pandrug-resistant Acinetobacter baumannii in a murine bacteremia model

OBJECTIVES

Acinetobacter baumannii bacteremia caused by pandrug-resistant strains poses a major challenge in intensive care units, necessitating novel therapeutic approaches. Phage-derived depolymerases offer a promising adjunct to conventional antibiotics. However, studies on A. baumannii phage depolymerases have been limited to non-mammalian models. This study investigates the therapeutic efficacy, safety, and potential mechanisms of action of DPO-HL, both as a monotherapy and in combination with polymyxin B, in a murine model of A. baumannii bacteremia.

METHODS

DPO-HL was expressed and purified via Ni-NTA affinity chromatography. Its bactericidal activity was assessed through dynamic killing and biofilm disruption assays. Interaction with human plasma was examined to determine its impact on plasma's bactericidal activity. Synergy with polymyxin B was evaluated by MIC reduction. Safety was assessed via cytotoxicity, haemolysis, and acute toxicity tests. A mouse bacteremia model was established to evaluate therapeutic efficacy via intraperitoneal and intravenous administration.

RESULTS

DPO-HL, targeting KL160 capsular polysaccharide, exhibited stability in plasma and enhanced plasma's bactericidal effect. It showed strong synergy with polymyxin B, reducing its MIC by 16-fold, and efficiently eradicated mature biofilms. DPO-HL alone reduced bacterial load and endotoxin levels but rescued only 30% of bacteremia mice. Combination therapy (1.45 mg/kg DPO-HL + 0.5 mg/kg polymyxin B) significantly reduced endotoxin levels and achieved 100% survival, regardless of administration route.

CONCLUSIONS

This study identifies a KL160-targeting depolymerase and demonstrates its potent synergy with polymyxin B in treating A. baumannii bacteremia, supporting its potential for clinical application.

Current innovations in mRNA vaccines for targeting multidrug-resistant ESKAPE pathogens

The prevalence of multidrug-resistant (MDR) ESKAPE pathogens, including Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa, represents a critical global public health challenge. In response, mRNA vaccines offer an adaptable and scalable platform for immunotherapy against ESKAPE pathogens by encoding specific antigens that stimulate B-cell-driven antibody production and CD8+ T-cell-mediated cytotoxicity, effectively neutralizing these pathogens and combating resistance. This review examines recent advancements and ongoing challenges in the development of mRNA vaccines targeting MDR ESKAPE pathogens. We explore antigen selection, the nuances of mRNA vaccine technology, and the complex interactions between bacterial infections and antibiotic resistance. By assessing the potential efficacy of mRNA vaccines and addressing key barriers to their paraclinical implementation, this review highlights the promising function of mRNA-based immunization in combating MDR ESKAPE pathogens.

Metagenomic insights on promoting the removal of resistome in aerobic composting pig manure by lightly burned modified magnesite

The antibiotic resistance genes (ARGs) have become a serious issue facing public health. In this study, light-burned magnesite with a high specific surface area at 650 °C (MS650) was used for aerobic composting, evaluating its effect on the resistome during pig manure composting. Different concentrations of MS650 reduced the abundance of the resistome, including seven high-risk ARGs, class two metal and biocide resistance genes (MBRGs), and human pathogenic bacteria (HPBs). The addition of 2.5 % MS650 (L1) in the composting had the best reduction effect on ARGs, MBRGs and HPBs. ARG and microbial community assembly are deterministic processes. Proteobacteria and Actinobacteria was the main factor associated with the decrease in ARGs, followed by virulence factor genes (VFGs, 44.2 %). The reduction in MBRGs by MS650 mainly suppressed HGT by reducing the Isfinder abundance. To summarize, MS650 is an effective method to improve emission reduction of ARGs and MBRGs. This study provided a theoretical basis for improving the engineering application potential of MS650.

Clinical, phenotypic characterization and genomic analysis of the mucoid Acinetobacter baumannii from a teaching hospital

BACKGROUND

Acinetobacter baumannii (A. baumannii) has become a significant nosocomial pathogen globally over the past decade due to the increasing prevalence of antibiotic-resistant isolates. The formation of the mucoid phenotype is a crucial adaptive defense response to external pressure, but the clinical, phenotypic and genotypic characteristics and their relationship with sequence types (ST) and K locus (KL) types remain unclear.

METHODS

In this study, we screened a total of 736 A. baumannii isolates, from which we identified and characterized 13 mucoid isolates. The study explored the clinical characteristics of patients with mucoid isolates, investigated the mucoid phenotype, performed capsule observation, quantified capsule production, and assessed antimicrobial susceptibility. Subsequently, whole-genome sequencing (WGS) was used to analyze the sequence types (ST), loci for capsular polysaccharide (KL), antibiotic resistance genes, virulence genes, and core-genome single-nucleotide polymorphisms (SNPs). Additionally, the virulence of all mucoid strains was evaluated through serum resistance assay, biofilm-forming assay, and Galleria mellonella survival assay.

RESULTS

All mucoid A. baumannii isolates were found to be encapsulated and extremely drug-resistant. Among patients infected with these isolates, 92.3 % had pulmonary infections, and the 30-day mortality rate was 61.5 %. The analysis revealed that not all strains are highly virulent. Whole-genome sequencing (WGS) identified the sequence types as ST136, ST208, ST381, ST195, and ST281, and the capsular types as KL77, KL7, KL33, KL2, and KL3. The ST208 and KL7 isolates exhibited higher virulence and greater biofilm formation, with KL7 isolates also showing higher capsule production. Despite these differences, no significant variations in virulence genes were observed among the mucoid isolates, except for biofilm-associated and quorum-sensing genes. The highly virulent ST208/KL7 strains (AB276, AB313, and AB552) lacked biofilm-associated genes (csuA/BABCDE), indicating these genes do not directly cause differences in biofilm formation.

CONCLUSION

The mucoid A. baumannii isolates were extensively drug-resistant, and infections caused by these isolates could lead to higher mortality. However, not all strains had high virulence, with variations likely related to specific sequence types (ST) and K locus (KL) types.

Clinical and microbiological features of a cohort of patients with Acinetobacter baumannii bloodstream infections

PURPOSE

Acinetobacter baumannii is emerging as a pathogen that is a focus of global concern due to the frequent occurrence of the strains those are extensively resistant to antibiotics. This study was aimed to analyze the clinical and microbiological characteristics of a cohort of patients with A. baumannii bloodstream infections (BSIs) in western China.

METHODS

A retrospective study of the patients at West China Hospital of Sichuan University with A. baumannii BSIs between Jan, 2018 and May, 2023 was conducted. Antimicrobial susceptibility of A. baumannii isolates was tested by microdilution broth method. Whole-genome sequencing and genetic analysis were also performed for these isolates.

RESULTS

Among the 117 patients included, longer intensive care unit stay, higher mortality, and more frequent invasive procedures and use of more than 3 classes of antibiotics were observed among the carbapenem-resistant A. baumannii (CRAB)-infected group (n = 76), compared to the carbapenem-susceptible A. baumannii (CSAB)-infected group (n = 41, all P ≤ 0.001). Twenty-four sequence types (STs) were determined for the 117 isolates, and 98.7% (75/76) of CRAB were identified as ST2. Compared to non-ST2 isolates, ST2 isolates exhibited higher antibiotic resistance, and carried more resistance and virulence genes (P < 0.05). In addition, 80 (68.4%) isolates were CRISPR-positive, showed higher antibiotic susceptibility, and harbored less resistance and virulence genes, in comparison to CRISPR-negative ones (P < 0.05). Phylogenetic clustering based on coregenome SNPs indicated a sporadic occurrence of clonal transmission.

CONCLUSION

Our findings demonstrate a high frequency of ST2 among A. baumannii causing BSIs, and high antibiotic susceptibility of non-ST2 and CRISPR-positive isolates. It is necessary to strengthen the surveillance of this pathogen.

Removal of intracellular and extracellular antibiotic resistance genes and virulence factor genes using electricity-intensified constructed wetlands

Constructed wetland (CW) is considered a promising technology for the removal of emerging contaminants. However, its removal performance for antibiotic resistance genes (ARGs) is not efficient and influence of virulence factor genes (VFGs) have not been elucidated. Here, removal of intracellular and extracellular ARGs as well as VFGs by electricity-intensified CWs was comprehensively evaluated. The two electrolysis-intensified CWs can improve the removal of intracellular ARGs and MGEs to 0.96- and 0.85-logs, respectively. But cell-free extracellular ARGs (CF-eARGs) were significantly enriched with 1.8-logs in the electrolysis-intensified CW. Interestingly, adding Fe-C microelectrolysis to the electrolysis-intensified CW is conducive to the reduction of CF-eARGs. However, the detected number and relative abundances of intracellular and extracellular VFGs were increased in all of the three CWs. The biofilms attached onto the substrates and rhizosphere are also hotspots of both intracellular and particle-associated extracellular ARGs and VFGs. Structural equation models and correlation analysis indicated that ARGs and VFGs were significantly cooccurred, suggesting that VFGs may affect the dynamics of ARGs. The phenotypes of VFGs, such as biofilm, may act as protective matrix for ARGs, hindering the removal of resistance genes. Our results provide novel insights into the ecological remediation technologies to enhance the removal of ARGs.

Roles of DJ41_1407 and DJ41_1408 in Acinetobacter baumannii ATCC19606 Virulence and Antibiotic Response

Acinetobacter baumannii is a major cause of nosocomial infections, and its highly adaptive nature and broad range of antibiotic resistance enable it to persist in hospital environments. A. baumannii often employs two-component systems (TCSs) to regulate adaptive responses and virulence-related traits. This study describes a previously uncharacterized TCS in the A. baumannii ATCC19606 strain, consisting of a transcriptional sensor, DJ41_1407, and its regulator, DJ41_1408, located adjacent to GacA of the GacSA TCS. Markerless mutagenesis was performed to construct DJ41_1407 and DJ41_1408 single and double mutants. DJ41_1408 was found to upregulate 49 genes and downregulate 43 genes, most of which were associated with carbon metabolism and other metabolic pathways, such as benzoate degradation. MEME analysis revealed a putative binding box for DJ41_1408, 5'TGTAAATRATTAYCAWTWAT3'. Colony size, motility, biofilm-forming ability, virulence, and antibiotic resistance of DJ41_1407 and DJ41_1408 single and double mutant strains were assessed against wild type. DJ41_1407 was found to enhance motility, while DJ41_1408 was found to upregulate biofilm-forming ability, and may also modulate antibiotic response. Both DJ41_1407 and DJ41_1408 suppressed virulence, based on results from a G. mellonella infection assay. These results showcase a novel A. baumannii TCS involved in metabolism, with effects on motility, biofilm-forming ability, virulence, and antibiotic response.

Virulence Factors and Pathogenicity Mechanisms of Acinetobacter baumannii in Respiratory Infectious Diseases

Acinetobacter baumannii (A. baumannii) has become a notorious pathogen causing nosocomial and community-acquired infections, especially ventilator-associated pneumonia. This opportunistic pathogen is found to possess powerful genomic plasticity and numerous virulence factors that facilitate its success in the infectious process. Although the interactions between A. baumannii and the pulmonary epitheliums have been extensively studied, a complete and specific description of its overall pathogenic process is lacking. In this review, we summarize the current knowledge of the antibiotic resistance and virulence factors of A. baumannii, specifically focusing on the pathogenic mechanisms of this detrimental pathogen in respiratory infectious diseases. An expansion of the knowledge regarding A. baumannii pathogenesis will contribute to the development of effective therapies based on immunopathology or intracellular signaling pathways to eliminate this harmful pathogen during infections.

A novel vaccine candidate against A. baumannii based on a new OmpW family protein (OmpW2); structural characterization, antigenicity and epitope investigation, and in-vivo analysis

A. baumannii is an MDR pathogen whose SARS-CoV-2 has recently increased its mortality rate in hospitalized patients. So, the virulence factors investigation and design of a vaccine against this bacterium seem to be critical. In this regard, the OmpW2 protein was structurally characterized by this study, and its B-T cell epitopes were mapped by bioinformatic tools. In-vivo analyses were employed to verify the immunogenicity of this protein and its selected epitopes. The results indicated that OmpW2 is a conserved virulent antigen, not toxic for the host, and not similar to the human or mouse proteome. A putative interaction between OmpW2 and a Fe-S-cluster redox enzyme was detected. Based on the results, OmpW2 belongs to the OmpW superfamily and eight beta sheets have been predicted in its tight beta-barrel structure. Several exposed epitopes were detected in the OmpW2 sequence and structure, and a sub-unit potential vaccine was generated based on the epitopes. The ELISA results indicated that after the second booster vaccination of BALB/c mice with the whole OmpW2 protein or its sub-unit fragment, the IgG titer significantly raised (p < 0.05). The mortality rate and the bacterial burden in the lung, liver, kidney, and spleen in both passive and active immunized mice were significantly decreased (p ≤ 0.001). In-vivo experiments confirmed that the OmpW2 whole protein and its sub-unit fragment induce the host immune system and can be applied to design a commercial vaccine or diagnostic kit.

Acinetobacter baumannii outer membrane protein A induces autophagy in bone marrow-derived dendritic cells involving the PI3K/mTOR pathway

BACKGROUND

Outer membrane protein A (OmpA) is the major virulence factor of Acinetobacter baumannii and plays a wide role in the pathogenesis and antimicrobial resistance of A. baumannii. Dendritic cells (DCs) are the most effective antigen-presenting cells and play a crucial role in regulating the immune response to multiple antigens and immune sentries. We aimed to study the role and molecular mechanisms of OmpA-induced mouse bone marrow-derived dendritic cells (BMDCs) autophagy in the immune response of A. baumannii.

METHODS

First, purified A. baumannii OmpA was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and western blot. OmpA effect on BMDCs viability was evaluated by MTT assay. BMDCs were pretreated with autophagy inhibitor chloroquine or transfected with overexpression plasmids (oe-NC or oe-PI3K). Then BMDCs apoptosis, inflammatory cytokines, protein kinase B (PI3K)/mammalian target of rapamycin (mTOR) pathway, and autophagy-related factors levels were evaluated.

RESULTS

SDS-PAGE and western blot verified the successful purification of OmpA. BMDCs viability repressed gradually with the increase of OmpA concentration. OmpA treatment of BMDCs led to apoptosis and inflammation in BMDCs. OmpA caused incomplete autophagy in BMDCs, and light chain 3 (LC3), Beclin1, P62, and LC3II/I levels were significantly elevated with the increase of the time and concentration of OmpA treatment. Chloroquine reversed OmpA effects on autophagy in BMDCs, that was, LC3, Beclin1, and LC3II/I levels were reduced, while P62 level was elevated. Furthermore, chloroquine reversed OmpA effects on apoptosis and inflammation in BMDCs. PI3K/mTOR pathway-related factor expression was affected by OmpA treatment of BMDCs. After overexpression of PI3K, these effects were reversed.

CONCLUSIONS

A. baumannii OmpA induced autophagy in BMDCs involving the PI3K/mTOR pathway. Our study may provide a novel therapeutic target and theoretical basis for treating infections caused by A. baumannii.

A comprehensive genomic analysis provides insights on the high environmental adaptability of Acinetobacter strains

Acinetobacter is ubiquitous, and it has a high species diversity and a complex evolutionary pattern. To elucidate the mechanism of its high ability to adapt to various environment, 312 genomes of Acinetobacter strains were analyzed using the phylogenomic and comparative genomics methods. It was revealed that the Acinetobacter genus has an open pan-genome and strong genome plasticity. The pan-genome consists of 47,500 genes, with 818 shared by all the genomes of Acinetobacter, while 22,291 are unique genes. Although Acinetobacter strains do not have a complete glycolytic pathway to directly utilize glucose as carbon source, most of them harbored the n-alkane-degrading genes alkB/alkM (97.1% of tested strains) and almA (96.7% of tested strains), which were responsible for medium-and long-chain n-alkane terminal oxidation reaction, respectively. Most Acinetobacter strains also have catA (93.3% of tested strains) and benAB (92.0% of tested strains) genes that can degrade the aromatic compounds catechol and benzoic acid, respectively. These abilities enable the Acinetobacter strains to easily obtain carbon and energy sources from their environment for survival. The Acinetobacter strains can manage osmotic pressure by accumulating potassium and compatible solutes, including betaine, mannitol, trehalose, glutamic acid, and proline. They respond to oxidative stress by synthesizing superoxide dismutase, catalase, disulfide isomerase, and methionine sulfoxide reductase that repair the damage caused by reactive oxygen species. In addition, most Acinetobacter strains contain many efflux pump genes and resistance genes to manage antibiotic stress and can synthesize a variety of secondary metabolites, including arylpolyene, β-lactone and siderophores among others, to adapt to their environment. These genes enable Acinetobacter strains to survive extreme stresses. The genome of each Acinetobacter strain contained different numbers of prophages (0-12) and genomic islands (GIs) (6-70), and genes related to antibiotic resistance were found in the GIs. The phylogenetic analysis revealed that the alkM and almA genes have a similar evolutionary position with the core genome, indicating that they may have been acquired by vertical gene transfer from their ancestor, while catA, benA, benB and the antibiotic resistance genes could have been acquired by horizontal gene transfer from the other organisms.

A fresh pH-responsive imipenem-loaded nanocarrier against Acinetobacter baumannii with a synergetic effect

In recent years, the treatment of Acinetobacter baumannii infections has become a pressing clinical challenge due to its increasing incidence and its serious pathogenic risk. The research and development of new antibacterial agents for A. baumannii have attracted the attention of the scientific community. Therefore, we have constructed a new pH-responsive antibacterial nano-delivery system (Imi@ZIF-8) for the antibacterial treatment of A. baumannii. Due to its pH-sensitive characteristics, the nano-delivery system offers an improved release of the loaded imipenem antibiotic at the acidic infection site. Based on the high loading capacity and positive charge of the modified ZIF-8 nanoparticles, they are excellent carriers and are suitable for imipenem loading. The Imi@ZIF-8 nanosystem features synergistic antibacterial effects, combining ZIF-8 and imipenem to eliminate A. baumannii through different antibacterial mechanisms. When the loaded imipenem concentration reaches 20 µg/mL, Imi@ZIF-8 is highly effective against A. baumannii in vitro. Imi@ZIF-8 not only inhibits the biofilm formation of A. baumannii but also has a potent killing effect. Furthermore, in mice with celiac disease, the Imi@ZIF-8 nanosystem demonstrates excellent therapeutic efficacy against A. baumannii at imipenem concentrations of 10 mg/kg, and it can inhibit inflammatory reaction and local leukocyte infiltration. Due to its biocompatibility and biosafety, this nano-delivery system is a promising therapeutic strategy in the clinical treatment of A. baumannii infections, providing a new direction for the treatment of antibacterial infections.

Succession and change of potential pathogens in the co-composting of rural sewage sludge and food waste

Composting is an effective way to prevent and control the spread of pathogenic microorganisms which could put potential risk to humans and environment, from rural solid waste, especially sewage sludge and food waste. In the study, we aim to analyze the changes of pathogenic bacteria during the co-composting of rural sewage sludge and food waste. The results showed that only 27 pathogenic bacteria were detected after composting, compared to 50 pathogenic bacteria in the raw mixed pile. About 74% of pathogen concentrations dropped below 1000 copies/g after composting. Lactobacillus, Bacillus, Paenibacillus and Comamonas were the core pathogenic bacteria in the compost, of which concentrations were all significantly lower than that in the raw mixed pile at the end of composting. The concentration of Lactobacillus decreased to 3.03 × 10³ copies/g compared to 0 d with 1.25 × 10⁹ copies/g by the end of the composting, while that of Bacillus, Paenibacillus and Comamonas decreased to 2.77 × 10⁴ copies/g, 2.13 × 10⁴ copies/g and 3.38 × 10² copies/g, respectively, with 1.26 × 10⁷ copies/g, 4.71 × 10⁶ copies/g, 1.69 × 10⁸ copies/g on 0 d. Redundancy analysis (RDA) indicated that physicochemical factors and substances could affect the changes of pathogenic bacteria during composting, while temperature was the key influencing factor. In addition, certain potential pathogenic bacteria, such as Bacteroides-Bifidobacterium, show statistically strong and significant co-occurrence during composting, which may increase the risk of multiple infections and also influence their distribution. These findings provide a theoretical reference for biosafety prevention and control in the treatment and disposal of rural solid waste.