Microevolution in the major outer membrane protein OmpA of Acinetobacter baumannii

AbOmpA in Acinetobacter baumannii: exploring virulence mechanisms of outer membrane-integrated and outer membrane vesicle-associated AbOmpA and developing anti-infective agents targeting AbOmpA

Acinetobacter baumannii is notorious for its antimicrobial resistance and its potential to cause epidemics in hospital settings, which pose a global health threat. Although this microorganism is traditionally considered a low-virulence pathogen, extensive research has been conducted on its virulence and pathogenesis in recent years. Advances in understanding the virulence mechanisms of A. baumannii have prompted a shift in the development of anti-infective agents. The outer membrane protein A (AbOmpA) of A. baumannii is a key virulence factor both in vitro and in vivo. AbOmpA exists in three forms: outer membrane-integrated AbOmpA, outer membrane vesicle (OMV)-associated AbOmpA, and free proteins. Given that outer membrane-integrated AbOmpA has been implicated in the virulence and antimicrobial resistance of A. baumannii, many studies have focused on outer membrane-integrated AbOmpA as a therapeutic target for combating drug-resistant A. baumannii, and have led to the discovery of small molecules, polypeptides, and antimicrobial peptides targeting AbOmpA. However, the pathophysiological role of OMV-associated AbOmpA and its impact on AbOmpA-targeting agents remain unclear. This review summarizes the current knowledge of AbOmpA and critically discusses OMV-associated AbOmpA in relation to virulence and its potential impact on AbOmpA-targeted therapies to provide a better understanding of AbOmpA for the development of novel therapeutics against A. baumannii.

K. pneumoniae ghosts serve as a novel vaccine formulation to enhance immune responses of A. baumannii subunit vaccine in mice

Acinetobacter baumannii (A. baumannii) is a prominent nosocomial pathogen, posing a significant threat to public health. Urgent efforts are required to develop a safe and effective vaccine. Bacterial ghosts (BGs), comprising empty bacterial cell envelopes, offer a promising platform for vaccine adjuvant development. In the present study, Klebsiella pneumoniae (K. pneumoniae, KP) ghosts were generated via PhiX-174 lysis gene E-mediated inactivation. The present study results demonstrated that KP ghosts greatly promoted maturation and activation of BMDCs by upregulating the expression of surface molecules (CD40, CD80, CD86 and MHCII) and improving the secretion of cytokines (IL-1β, TNF-α and IL-12p70). In addition, to assess the immunogenicity and protective efficacy of the vaccine candidate, C57BL/6 mice were immunized with either A. baumannii OmpA or A. baumannii OmpA plus KP ghosts. The results showed that OmpA plus KP ghosts elicited higher levels of specific IgG antibody responses compared to OmpA alone. Furthermore, OmpA plus KP ghosts also increased lymphocyte proliferation and expression of the early activation marker CD69 on T cells, augmented frequency of central memory T cells (TCM) and IFN-γ+CD4+ T cells with production of increased IFN-γ in response to OmpA stimulation, as compared to OmpA alone. Furthermore, post-challenge with A. baumannii, mice immunized with OmpA plus KP ghosts exhibit a higher survival rate and lower bacterial loads in the spleen and lungs compared to those immunized with OmpA alone. In conclusion, these findings underscore the potential of KP ghosts as a candidate vaccine formulation or immunomodulators for designing a novel vaccine against A. baumannii infection.

A novel mRNA multi-epitope vaccine of Acinetobacter baumannii based on multi-target protein design in immunoinformatic approach

Acinetobacter baumannii is a gram-negative bacillus prevalent in nature, capable of thriving under various environmental conditions. As an opportunistic pathogen, it frequently causes nosocomial infections such as urinary tract infections, bacteremia, and pneumonia, contributing to increased morbidity and mortality in clinical settings. Consequently, developing novel vaccines against Acinetobacter baumannii is of utmost importance. In our study, we identified 10 highly conserved antigenic proteins from the NCBI and UniProt databases for epitope mapping. We subsequently screened and selected 8 CTL, HTL, and LBL epitopes, integrating them into three distinct vaccines constructed with adjuvants. Following comprehensive evaluations of immunological and physicochemical parameters, we conducted molecular docking and molecular dynamics simulations to assess the efficacy and stability of these vaccines. Our findings indicate that all three multi-epitope mRNA vaccines designed against Acinetobacter baumannii are promising; however, further animal studies are required to confirm their reliability and effectiveness.

Acinetobacter baumannii OmpA-like porins: functional characterization of bacterial physiology, antibiotic-resistance, and virulence

Acinetobacter baumannii is a critical opportunistic pathogen associated with nosocomial infections. The high rates of antibiotic-resistance acquisition make most antibiotics ineffective. Thus, new medical countermeasures are urgently needed. Outer membrane proteins (OMPs) are prime candidates for developing novel drug targets and antibacterial strategies. However, there are substantial gaps in our knowledge of A. baumannii OMPs. This study reports the impact of OmpA-like protein on bacterial physiology and virulence in A. baumannii strain AB5075. We found that PsaB (ABUW_0505) negatively correlates to stress tolerance, while ArfA (ABUW_2730) significantly affects bacterial stiffness, cell shape, and cell envelope thickness. Furthermore, we expand our knowledge on YiaD (ABUW_3045), demonstrating structural and virulence roles of this porin, in addition to meropenem resistance. This study provides solid foundations for understanding how uncharacterized OMPs contribute to A. baumannii's physiological and pathological processes, aiding the development of innovative therapeutic strategies against A. baumannii infections.

Exploring the evolutionary and pathogenic role of Acinetobacter baumannii biofilm-associated protein (Bap) through in silico structural modeling

Acinetobacter species encode for extracellularly secreted Biofilm-associated protein (Bap), a multi-domain protein with variable molecular weights reaching several hundred kilodaltons. Bap is crucial for the development of multi-dimensional structures of mature biofilms. In our investigation, we analyzed 7338 sequences of A. baumannii from the NCBI database and found that Bap or Bap-like protein (BLP) was present in 6422 (87.52%) isolates. Further classification revealed that 12.12% carried Type-1 Bap, 68.44% had Type-2, 6.91% had Type-3, 0.05% had Type-6 or SDF-Type, and 12.51% lacked Bap or BLP. The majority of isolates with Type-1, Type-2, and Type-3 Bap belonged to ST1, ST2, and ST25, respectively. Phylogenetic analysis suggested that Type-1 Bap is the most ancient, while Type-3 and SDF-Type have evolved recently. Studying the interaction of predicted Bap structures with human CEACAM-1 and PIgR showed that Bap with its BIg13 and BIg6 domains interact with the N-terminal domain of CEACAM-1, involving Arg43 and Glu40, involved in CEACAM-1 dimerization. Also, we found that recently evolved Type-3 and SDF-Type Bap showed greater interaction with CEACAM-1 and PIgR. It can be asserted that the evolution of Bap has conferred enhanced virulence characteristics to A. baumannii with increased interaction with CEACAM-1 and PIgR. Using in silico approaches, this study explores the evolutionary, physicochemical, and structural features of A. baumannii Bap and unravels its crucial role in mediating interaction with human CEACAM-1 and PIgR through detailed structure modelling. These findings advance our understanding of A. baumannii Bap and highlight its role in pathogenesis.

Selection and validation of reference genes suitable for gene expression analysis by Reverse Transcription Quantitative real-time PCR in Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative bacterium considered an emerging multi-drug-resistant pathogen. Furthermore, this bacterium can survive in extreme environmental conditions, which makes it a frequent cause of nosocomial infection outbreaks. Gene expression analyses by Reverse Transcription Quantitative real-time PCR (RT-qPCR) depend on a reference gene, also called an endogenous gene, which is used to normalize the generated data and thus ensure an accurate analysis with minimal errors. Currently, gene expression analyses in A. baumannii are compromised, as there are no reports in the literature describing the identification of validated reference genes for use in RT-qPCR analyses. For this reason, we selected twelve candidate reference genes of A. baumannii and assessed their expression profile under different experimental and culture conditions. The expression stability of the candidate genes was evaluated by using statistical algorithms such as BestKeeper, geNorm, NormFinder, Delta CT, and RefFinder, in order to identify the most suitable candidate reference genes for RT-qPCR analyses. The statistical analyses indicated rpoB, rpoD, and fabD genes as the most adequate to ensure accurate normalization of RT-qPCR data in A. baumannii. The accuracy of the proposed reference genes was validated by using them to normalize the expression of the ompA gene, encoding the outer membrane protein A, in A. baumannii sensible and resistant to the antibiotic polymyxin. The present work provides suitable reference genes for precise RT-qPCR data normalization on future gene expression studies with A. baumannii.

Increased ompW and ompA expression and higher virulence of Acinetobacter baumannii persister cells

BACKGROUND

Acinetobacter baumannii is one of the main causes of healthcare-associated infections that threaten public health, and carbapenems, such as meropenem, have been a therapeutic option for these infections. Therapeutic failure is mainly due to the antimicrobial resistance of A. baumannii, as well as the presence of persister cells. Persisters constitute a fraction of the bacterial population that present a transient phenotype capable of tolerating supra-lethal concentrations of antibiotics. Some proteins have been suggested to be involved in the onset and/or maintenance of this phenotype. Thus, we investigated the mRNA levels of the adeB (AdeABC efflux pump component), ompA, and ompW (outer membrane proteins) in A. baumannii cells before and after exposure to meropenem.

RESULTS

We found a significant increase (p-value < 0.05) in the expression of ompA (> 5.5-fold) and ompW (> 10.5-fold) in persisters. However, adeB did not show significantly different expression levels when comparing treated and untreated cells. Therefore, we suggest that these outer membrane proteins, especially OmpW, could be part of the mechanism of A. baumannii persisters to deal with the presence of high doses of meropenem. We also observed in the Galleria mellonella larvae model that persister cells are more virulent than regular ones, as evidenced by their LD₅₀ values.

CONCLUSIONS

Taken together, these data contribute to the understanding of the phenotypic features of A. baumannii persisters and their relation to virulence, as well as highlight OmpW and OmpA as potential targets for drug development against A. baumannii persisters.

Evaluation the reactivity of a peptide-based monoclonal antibody derived from OmpA with drug resistant pulsotypes of Acinetobacter baumannii as a potential therapeutic approach

BACKGROUND

Acinetobacter baumannii is an opportunistic and antibiotic-resistant pathogen that predominantly causes nosocomial infections. There is urgent need for development nonantibiotic-based treatment strategies. We developed a novel monoclonal antibody (mAb) against a peptide of conserved outer membrane protein A (OmpA) and evaluated its reactivity with different pulsotypes of A. baumannii.

METHODS

Peptide derived from A. baumannii OmpA was conjugated to keyhole limpet hemocyanin and injected into BALB/c mice. Splenocytes of immunized mice were fused with SP2/0 myeloma cells followed by selection of antibody-producing hybridoma cells. After screening of different hybridoma colonies by ELISA, one monoclone was selected as 3F10-C9 and the antibody was tested for reaction with five different Acinetobacter pulsotypes that were resistant to carbapenem antibiotics. The affinity constant was measured by ELISA. The ELISA, western blotting, indirect immunofluorescence (IFA), and in vitro opsonophagocytosis assays were used to evaluate the reactivity of generated mAb.

RESULTS

The anti-OmpA antibody reacted with the immunizing peptide and had a high affinity (1.94 × 10-9 M) for its antigen in the ELISA. Specific binding of mAb to OmpA was confirmed in Western blot. IFA assays revealed that mAb recognized specific OmpA on the pulsotypes. Opsonophagocytosis assays showed that the mAb increased the bactericidal activity of macrophage cells. The antibody function was higher in the presence of serum complement.

CONCLUSIONS

The peptide-based mAb demonstrated optimal performance in laboratory experiments which may be appropriate in investigation on OmpA in Acinetobacter pathogenesis and development of passive immunization as a novel therapeutic approach.

In Silico Analyses of Extracellular Proteins of Acinetobacter baumannii as Immunogenic Candidates

Background: Acinetobacter baumannii is an important nosocomial pathogen causing high morbidity and mortality in immunocompromised patients with prolonged hospitalization. Multidrug-resistant A. baumannii infections are on the rise worldwide. Therefore, the discovery of an effective vaccine against this bacterium seems necessary as a cost-effective and preventive strategy. Methods: In this present study, 35 genomes of A. baumannii strains were considered, and the extracellular proteins were selected, maximally having one transmembrane helix with high adhesion probability and no similarity to host proteins, as immunogenic candidates using the web tool Vaxign. Subsequently, the role of these selected proteins in bacterial pathogenesis was investigated using VICMpred. Then, the major histocompatibility complex class II, linear B-cell epitopes, and conservation of epitopes were identified using the Immune Epitope Database, BepiPred, and Epitope Conservancy Analysis, respectively. Finally, the B-cell discontinuous epitopes of each protein were predicted using ElliPro and plotted on the three-dimensional structure (3D) of the proteins. The role of the unknown proteins was predicted using the STRING database. Results: In this study, eight acceptable immunogenic candidates, including FilF, FimA, putative acid phosphatase, putative exported protein, subtilisin-like serine protease, and three uncharacterized proteins, were identified in A. baumannii. Conclusions: The results of the STRING database showed that these three uncharacterized proteins play a role in nutrition (heme utilization), peptide bond cleavage (serine peptidases), and cellular processes (MlaD protein). Extracellular proteins might play a catalyst role in the outer membrane protein-based vaccine of A. baumannii. Furthermore, this study proposed a list of potent immunogenic candidates of extracellular proteins.

Diversity of carbapenem-resistant Acinetobacter baumannii and bacteriophage-mediated spread of the Oxa23 carbapenemase

Carbapenem-resistant Acinetobacter baumannii are prevalent in low- and middle-income countries such as Egypt, but little is known about the molecular epidemiology and mechanisms of resistance in these settings. Here, we characterize carbapenem-resistant A. baumannii from Alexandria, Egypt, and place it in a regional context. Fifty-four carbapenem-resistant isolates from Alexandria Main University Hospital (AMUH), Alexandria, Egypt, collected between 2010 and 2015 were genome sequenced using Illumina technology. Genomes were de novo assembled and annotated. Genomes for 36 isolates from the Middle East region were downloaded from GenBank. The core-gene compliment was determined using Roary, and analyses of recombination were performed in Gubbins. Multilocus sequence typing (MLST) sequence type (ST) and antibiotic-resistance genes were identified. The majority of Egyptian isolates belonged to one of three major clades, corresponding to Pasteur MLST clonal complex (CC^PAS) 1, CC^PAS2 and ST^PAS158. Strains belonging to ST^PAS158 have been reported almost exclusively from North Africa, the Middle East and Pakistan, and may represent a region-specific lineage. All isolates carried an oxa23 gene, six carried bla _NDM-1 and one carried bla _NDM-2. The oxa23 gene was located on a variety of different mobile elements, with Tn2006 predominant in CC^PAS2 strains, and Tn2008 predominant in other lineages. Of particular concern, in 8 of the 13 CC^PAS1 strains, the oxa23 gene was located in a temperate bacteriophage phiOXA, previously identified only once before in a CC^PAS1 clone from the USA military. The carbapenem-resistant A. baumannii population in AMUH is very diverse, and indicates an endemic circulating population, including a region-specific lineage. A major mechanism for oxa23 dissemination in CC^PAS1 isolates appears to be a bacteriophage, presenting new concerns about the ability of these carbapenemases to spread throughout the bacterial population.

Iron-Rich Conditions Induce OmpA and Virulence Changes of Acinetobacter baumannii

Background: Iron ions affect the expression of outer membrane protein A (OmpA), a major pathogenic protein in Acinetobacter baumannii.

Objective: To analyze the effect of iron ions on the expression of the OmpA protein of A. baumannii and explore its association with the virulence of OmpA.

Methods: Site-directed mutagenesis was used to construct ompA gene deletion strains and gene repair strains. The OmpA protein expression of A. baumannii under culture with different contents of iron ions was detected. The virulence of A. baumannii with different OmpA protein expression levels were evaluated in macrophages and mice.

Results: OmpA protein levels of the three strains were enhanced under iron-rich conditions. They were reduced in the presence of the iron-chelating agent 2,2′-bipyridine. A. baumannii wild type and + ompA had a remarkable toxic effect on RAW246.7 macrophages (P < 0.05). In contrast, the ΔompA had a significantly reduced toxic effect on RAW246.7 macrophages (P < 0.05). The levels of the inflammatory factors IL-1β, IL-6, IL-8, and TNFα in the mice spleen were significantly increased in the + ompA strain treatment group compared with the ΔompA strain group (all P < 0.05). In addition, the levels were higher in the presence of iron ions than in the presence of the chelating agent.

Conclusion: Iron-rich conditions increase the OmpA protein expression of A. baumannii. Strains with high OmpA protein expression were more invasive, which may be a key determinant of A. baumannii infection and pathogenicity. Iron control strategies might be used for the management of A. baumannii.

Production and characterization of novel monoclonal antibodies against outer membrane protein A (OmpA) of Acinetobacter baumannii

BACKGROUND

Infection caused by Acinetobacter baumannii has emerged as a significant clinical problem with unacceptably high mortality rate due to the increase in antibiotic-resistant strains. Producing novel monoclonal antibody (MAb) against outer membrane protein A (OmpA) could be considered as a potential tool to improve treatment of A. baumannii infections.

OBJECTIVES

In this study, we aimed to produce murine MAbs against OmpA peptide of A. baumannii.

MATERIALS AND METHODS

BALB/c mice were immunized with 18-mer amino acid peptide as a part of the OmpA protein. Four antibody-secreting hybridomas were obtained using hybridoma technology and then characterized according to isotypes, affinity constant, reactivity in ELISA, flow cytometry, indirect immunofluorescence (IFA) and opsonophagocytic killing assays.

RESULTS

All four produced MAbs (1A1-D10, 1G1-E7, 2C11-F10, and 4H2-H9) had IgG1 isotype with Kappa light chain. One of these MAbs, 1G1-E7 was purified and selected for further characterizations. 1G1-E7 showed a high reactivity with both immunogenic peptide and A. baumannii in ELISA. Our results indicated that 1G1-E7 MAb reacted with 95.3% of A. baumannii in flow cytometry as well as IFA. Moreover, the affinity of the 1G1-E7 MAb was measured 1.37 × 10⁸ M^-1. The 1G1-E7 significantly improved opsonophagocytic killing of a clinical isolate of A. baumannii.

CONCLUSION

Our findings showed that the OmpA can be identified by produced MAbs. The efficacy of novel anti-OmpA antibodies in A. baumannii targeting needs to be further investigated in challenging models, and then could be subjected for genetic engineering to produce therapeutic antibody against A. baumannii.

Facing the challenges of multidrug-resistant Acinetobacter baumannii: progress and prospects in the vaccine development

In 2017, the World Health Organization (WHO) named A. baumannii as one of the three antibiotic-resistant bacterial species on its list of global priority pathogens in dire need of novel and effective treatment. With only polymyxin and tigecycline antibiotics left as last-resort treatments, the need for novel alternative approaches to the control of this bacterium becomes imperative. Vaccines against numerous bacteria have had impressive records in reducing the burden of the respective diseases and addressing antimicrobial resistance; as in the case of Haemophilus influenzae type b . A similar approach could be appropriate for A. baumannii. Toward this end, several potentially protective antigens against A. baumannii were identified and evaluated as vaccine antigen candidates. A licensed vaccine for the bacteria, however, is still not in sight. Here we explore and discuss challenges in vaccine development against A. baumannii and the promising approaches for improving the vaccine development process.

Roles of ompA of Citrobacter werkmanii in bacterial growth, biocide resistance, biofilm formation and swimming motility

The genus Citrobacter is commonly found in environmental and industrial settings, some members of which have been used for bioremediation of heavy metals owing to the absorption ability of their biofilms. Although our previous studies have found that the outer membrane protein A (OmpA) contributes to the process of Citrobacter werkmanii biofilm formation, the underlying mechanisms remain elusive. Therefore, we deleted ompA from the genome of C. werkmanii and investigated its phenotypes in comparison to the wild type strain (WT) and the complementary strain using biochemical and molecular techniques including RNA-Seq. Our results demonstrated that the deletion of ompA led to an increase in biofilm formation on both polystyrene and glass surfaces due to upregulation of some biofilm formation related genes. Meanwhile, swimming ability, which is mediated by activation of flagellar assembly genes, was increased on semi-solid plates in the ∆ompA strain when compared with WT. Additionally, inactivation of ompA also caused increased 1,2-benzisothiazolin-3-one (BIT) resistance, differential responses to Ca²⁺ stress, curli protein expression and cellulose production. Finally, ∆ompA caused differential expression of a total of 1470 genes when compared with WT, of which 146 were upregulated and 1324 were downregulated. These genes were classified into different Gene Ontology (GO) and KEGG pathways. In summary, ompA in C. werkmanii contributes to a variety of biological functions and may act as a target site to modulate biofilm formation. KEY POINTS: • ompA is a negative regulator for biofilm formation by C. werkmanii. • ompA inhibits swimming motility of C. werkmanii. • ompA deletion causes different expression profiles in C. werkmanii.

Acinetobacter baumannii: An Ancient Commensal with Weapons of a Pathogen

Acinetobacter baumannii is regarded as a life-threatening pathogen associated with community-acquired and nosocomial infections, mainly pneumonia. The rise in the number of A. baumannii antibiotic-resistant strains reduces effective therapies and increases mortality. Bacterial comparative genomic studies have unraveled the innate and acquired virulence factors of A. baumannii. These virulence factors are involved in antibiotic resistance, environmental persistence, host-pathogen interactions, and immune evasion. Studies on host–pathogen interactions revealed that A. baumannii evolved different mechanisms to adhere to in order to invade host respiratory cells as well as evade the host immune system. In this review, we discuss current data on A. baumannii genetic features and virulence factors. An emphasis is given to the players in host–pathogen interaction in the respiratory tract. In addition, we report recent investigations into host defense systems using in vitro and in vivo models, providing new insights into the innate immune response to A. baumannii infections. Increasing our knowledge of A. baumannii pathogenesis may help the development of novel therapeutic strategies based on anti-adhesive, anti-virulence, and anti-cell to cell signaling pathways drugs.

New Provisional Function of OmpA from Acinetobacter sp. Strain SA01 Based on Environmental Challenges

Acinetobacter OmpA is known as a multifaceted protein with multiple functions, including emulsifying properties. Bioemulsifiers are surface-active compounds that can disperse hydrophobic compounds in water and help increase the bioavailability of hydrophobic hydrocarbons to be used by degrading microorganisms.

An outer membrane protein A (OmpA) from Acinetobacter sp. strain SA01 was identified and characterized in-depth based on the structural and functional characteristics already known of its homologues. In silico structural studies showed that this protein can be a slow porin, binds to peptidoglycan, and exhibits emulsifying properties. Characterization of the recombinant SA01-OmpA, based on its emulsifying properties, represented its promising potentials in biotechnology. Also, the presence of SA01-OmpA in outer membrane vesicles (OMV) and biofilm showed that this protein, like its homologues in Acinetobacter baumannii, can be secreted into the extracellular environment through OMVs and play a role in the formation of biofilm. After ensuring the correct selection of the protein of interest, the role of oxidative stress induced by cell nutritional parameters (utilization of specific carbon sources) on the expression level of OmpA was carefully studied. For this purpose, the oxidative stress level of SA01 cell cultures in the presence of three nonrelevant carbon sources (sodium acetate, ethanol, and phenol) was examined under each condition. High expression of SA01-OmpA in ethanol- and phenol-fed cells with higher levels of oxidative stress than acetate suggested that oxidative stress could be a substantial factor in the regulation of SA01-OmpA expression. The significant association of SA01-OmpA expression with the levels of oxidative stress induced by cadmium and H₂O₂, with oxidative stress-inducing properties and lack of nutritional value, confirmed that the cells tend to harness their capacities with a possible increase in OmpA production. Collectively, this study suggests a homeostasis role for OmpA in Acinetobacter sp. SA01 under oxidative stress besides assuming many other roles hitherto attributed to this protein.

IMPORTANCEAcinetobacter OmpA is known as a multifaceted protein with multiple functions, including emulsifying properties. Bioemulsifiers are surface-active compounds that can disperse hydrophobic compounds in water and help increase the bioavailability of hydrophobic hydrocarbons to be used by degrading microorganisms. In this study, an OmpA from Acinetobacter sp. SA01 was identified and introduced as an emulsifier with a higher emulsifying capacity than Pseudomonas aeruginosa rhamnolipid. We also showed that the expression of this protein is not dependent on the nutritional requirements but is more influenced by the oxidative stress caused by stressors. This finding, along with the structural role of this protein as a slow porin or its role in OMV biogenesis and biofilm formation, suggests that this protein can play an important role in maintaining cellular homeostasis under oxidative stress conditions. Altogether, the present study provides a new perspective on the functional performance of Acinetobacter OmpA, which can be used both to optimize its production as an emulsifier and a target in the treatment of multidrug-resistant strains.

Virulence role of the outer membrane protein CarO in carbapenem-resistant Acinetobacter baumannii

Novel approaches to treat carbapenem-resistant Acinetobacter baumannii (CRAB) infections are urgently needed and anti-virulence drugs represent promising alternatives, but our knowledge on potential targets is scarce. We searched for potential A. baumannii virulence factors by whole-genome sequencing-based comparisons of CRAB clinical isolates causing bloodstream infections secondary to ventilator-associated pneumonia from demographics and clinically homogeneous patients, who received optimal treatment but with different clinical outcomes. Thus, the carO gene was interrupted in CRAB isolates from surviving patients, while it was intact in isolates from non-surviving patients, and proteomic/immunoblot techniques corroborated it. When the virulence role of A. baumannii CarO was analyzed in model systems, isogenic ΔcarO mutants and a CRAB clinical isolate with truncated CarO, showed lower ability to adhere and invade A549 cells and in vivo virulence. This unnoticed virulence role for CarO postulate this A. baumannii outer membrane protein as a potential target for new therapies against CRAB infections.

The Outer Membrane Proteins OmpA, CarO, and OprD of Acinetobacter baumannii Confer a Two-Pronged Defense in Facilitating Its Success as a Potent Human Pathogen

Of all the ESKAPE pathogens, carbapenem-resistant and multidrug-resistant Acinetobacter baumannii is the leading cause of hospital-acquired and ventilator-associated pneumonia. A. baumannii infections are notoriously hard to eradicate due to its propensity to rapidly acquire multitude of resistance determinants and the virulence factor cornucopia elucidated by the bacterium that help it fend off a wide range of adverse conditions imposed upon by host and environment. One such weapon in the arsenal of A. baumannii is the outer membrane protein (OMP) compendium. OMPs in A. baumannii play distinctive roles in facilitating the bacterial acclimatization to antibiotic- and host-induced stresses, albeit following entirely different mechanisms. OMPs are major immunogenic proteins in bacteria conferring bacteria host-fitness advantages including immune evasion, stress tolerance, and resistance to antibiotics and antibacterials. In this review, we summarize the current knowledge of major A. baumannii OMPs and discuss their versatile role in antibiotic resistance and virulence. Specifically, we explore how OmpA, CarO, and OprD-like porins mediate antibiotic and amino acid shuttle and host virulence.