Type I pili, CsuA/B and FimA induce a protective immune response against Acinetobacter baumannii

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.

Vaccination with a trivalent Klebsiella pneumoniae vaccine confers protection in a murine model of pneumonia

Klebsiella pneumoniae (K. pneumoniae) is an opportunistic pathogen and the major cause of healthcare-associated infections, which are increasingly complicated by the prevalence of highly invasive and hyper-virulent K. pneumoniae strains, necessitating the development of alternative strategies for combatting infections caused by this bacterium. In this study, we successfully constructed a fusion antigen called KP-Ag1, comprising three antigens (GlnH, FimA, and KPN_00466) that were previously identified through reverse vaccinology. Immunization with KP-Ag1 formulated with Al(OH)3 adjuvant elicited robust humoral and cellular immune response in mice, and conferred protective immunity in a murine model of K. pneumoniae lung infection. Further analysis of serum IgG subtypes from mice immunized with KP-Ag1 revealed a predominant IgG1 response, indicating that KP-Ag1 predominantly induces a Th2-biased immune response. Additionally, opsonophagocytic killing assay suggested that humoral immune responses play a pivotal role in mediating protection conferred by KP-Ag1. Moreover, KP-Ag1 was found to promote the activation and maturation of BMDCs in vitro, which is essential for subsequent efficient antigen presentation. More importantly, vaccination with KP-Ag1 demonstrated cross-protective efficacy against clinical isolates of K. pneumoniae varying in serotypes, antibiotic resistance, and virulence profiles. Therefore, KP-Ag1 holds promise as a candidate for K. pneumoniae vaccine development.

Acinetobacter baumannii subunit vaccines: recent progress and challenges

Acinetobacter baumannii is a Gram-negative, opportunistic pathogen that causes nosocomial infection with a high mortality rate in immunocompromised individuals. With the frequent emergence of multidrug-resistant A. baumannii strains that have rapidly gained resistance to most antibiotics, an extensive search for an effective A. baumannii vaccine is ongoing. Over the decade, many subunit vaccine candidates were identified using reverse vaccinology and in vivo animal studies for validation. Nineteen subunit vaccine candidates with a wide range of efficacy, from 14% to 100% preclinical survival rates, were included in this review. This article provides an updated review of several outer membrane proteins (Omp) that emerged as vaccine candidates with great potential, including OmpA, Omp34, Omp22 and BamA, based on their high conservancy, antigenicity, and immune protection against A. baumannii infection. However, there is still no licenced A. baumannii vaccine currently due to several practical issues that have yet to be resolved, such as inconsistencies between validation studies, antigen variability and insolubility. Moving forward, much investigation and innovation are still required to tackle these challenges for the regulatory approval of an A. baumannii subunit vaccine, including standardisation of immunisation study parameters, improving antigen solubility and the incorporation of nucleic acid vaccine technology.

Immunoprotective efficacy of 3 Klebsiella pneumoniae type I fimbriae proteins in a murine model

Klebsiella pneumoniae is a primary cause of clinical mastitis in dairy cows, with prevention being crucial, as treatments often fail due to antimicrobial resistance. Recent studies identified type I fimbrial antigens of K. pneumoniae as promising vaccine candidates, but there are limited research data. In this study, 3 fimbriae genes (fimA, fimC and fimG) were cloned and recombinantly expressed in Escherichia coli and their protective efficacy against K. pneumoniae evaluated in a mouse model. All 3 recombinant fimbriae proteins elicited strong humoral immune responses in mice, significantly increasing IgG, IgG1 and IgG2a. Notably, using a model of mice challenged with an intraperitoneal injection of bacteria, FimG significantly reduced bacterial loads in the spleen and lung, whereas FimA and FimC had limited protection for these organs. Either active or passive immunization with FimG produced substantial protective effects in mice challenged with K. pneumoniae LD100; in contrast, the mortality rate in the FimA-immunized group was similar to that of the control group, whereas FimC had weak protection. We concluded that the FimG recombinant protein vaccine had a favorable protective effect, with potential for immunization against K. pneumoniae mastitis.

Current treatment strategies for targeting virulence factors and biofilm formation in Acinetobacter baumannii

A higher prevalence of Acinetobacter baumannii infections and mortality rate has been reported recently in hospital-acquired infections (HAI). The biofilm-forming capability of A. baumannii makes it an extremely dangerous pathogen, especially in device-associated hospital-acquired infections (DA-HAI), thereby it resists the penetration of antibiotics. Further, the transmission of the SARS-CoV-2 virus was exacerbated in DA-HAI during the epidemic. This review specifically examines the complex interconnections between several components and genes that play a role in the biofilm formation and the development of infections. The current review provides insights into innovative treatments and therapeutic approaches to combat A. baumannii biofilm-related infections, thereby ultimately improving patient outcomes and reducing the burden of HAI.

Pan-Genome Plasticity and Virulence Factors: A Natural Treasure Trove for Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative pathogen responsible for a variety of community- and hospital-acquired infections. It is recognized as a life-threatening pathogen among hospitalized individuals and, in particular, immunocompromised patients in many countries. A. baumannii, as a member of the ESKAPE group, encompasses high genomic plasticity and simultaneously is predisposed to receive and exchange the mobile genetic elements (MGEs) through horizontal genetic transfer (HGT). Indeed, A. baumannii is a treasure trove that contains a high number of virulence factors. In accordance with these unique pathogenic characteristics of A. baumannii, the authors aim to discuss the natural treasure trove of pan-genome and virulence factors pertaining to this bacterial monster and try to highlight the reasons why this bacterium is a great concern in the global public health system.

Csu pili dependent biofilm formation and virulence of Acinetobacter baumannii

Acinetobacter baumannii has emerged as one of the most common extensive drug-resistant nosocomial bacterial pathogens. Not only can the bacteria survive in hospital settings for long periods, but they are also able to resist adverse conditions. However, underlying regulatory mechanisms that allow A. baumannii to cope with these conditions and mediate its virulence are poorly understood. Here, we show that bi-stable expression of the Csu pili, along with the production of poly-N-acetyl glucosamine, regulates the formation of Mountain-like biofilm-patches on glass surfaces to protect bacteria from the bactericidal effect of colistin. Csu pilus assembly is found to be an essential component of mature biofilms formed on glass surfaces and of pellicles. By using several microscopic techniques, we show that clinical isolates of A. baumannii carrying abundant Csu pili mediate adherence to epithelial cells. In addition, Csu pili suppressed surface-associated motility but enhanced colonization of bacteria into the lungs, spleen, and liver in a mouse model of systemic infection. The screening of c-di-GMP metabolizing protein mutants of A. baumannii 17978 for the capability to adhere to epithelial cells led us to identify GGDEF/EAL protein AIS_2337, here denoted PdeB, as a major regulator of Csu pili-mediated virulence and biofilm formation. Moreover, PdeB was found to be involved in the type IV pili-regulated robustness of surface-associated motility. Our findings suggest that the Csu pilus is not only a functional component of mature A. baumannii biofilms but also a major virulence factor promoting the initiation of disease progression by mediating bacterial adherence to epithelial cells.

Co-regulation of biofilm formation and antimicrobial resistance in Acinetobacter baumannii: from mechanisms to therapeutic strategies

In recent years, multidrug-resistant Acinetobacter baumannii has emerged globally as a major threat to the healthcare system. It is now listed by the World Health Organization as a priority one for the need of new therapeutic agents. A. baumannii has the capacity to develop robust biofilms on biotic and abiotic surfaces. Biofilm development allows these bacteria to resist various environmental stressors, including antibiotics and lack of nutrients or water, which in turn allows the persistence of A. baumannii in the hospital environment and further outbreaks. Investigation into therapeutic alternatives that will act on both biofilm formation and antimicrobial resistance (AMR) is sorely needed. The aim of the present review is to critically discuss the various mechanisms by which AMR and biofilm formation may be co-regulated in A. baumannii in an attempt to shed light on paths towards novel therapeutic opportunities. After discussing the clinical importance of A. baumannii, this critical review highlights biofilm-formation genes that may be associated with the co-regulation of AMR. Particularly worthy of consideration are genes regulating the quorum sensing system AbaI/AbaR, AbOmpA (OmpA protein), Bap (biofilm-associated protein), the two-component regulatory system BfmRS, the PER-1 β-lactamase, EpsA, and PTK. Finally, this review discusses ongoing experimental therapeutic strategies to fight A. baumannii infections, namely vaccine development, quorum sensing interference, nanoparticles, metal ions, natural products, antimicrobial peptides, and phage therapy. A better understanding of the mechanisms that co-regulate biofilm formation and AMR will help identify new therapeutic targets, as combined approaches may confer synergistic benefits for effective and safer treatments.

Passive immunization with anti-FimA egg yolk antibodies (IgYs) mitigate Acinetobacter baumannii pneumonia in mice

Acinetobacter baumannii is a formidable pathogen, characterized by high mortality rates and pan-drug-resistant strains. Current commercial antibiotics lack efficacy against drug-resistant variants, necessitating the search for alternative treatments. This study investigates the potential of egg yolk immunoglobulin (IgY) as a cost-effective biomolecule for passive protection against A. baumannii pneumonia. FimA (ABAYE2132), a key virulence factor involved in biofilm development and lung cell adherence, emerges as a promising antigen for triggering protective IgY production. Recombinant FimA was expressed, purified, and used for intramuscular immunization of laying White Leghorn hens. IgY antibodies were subsequently extracted from egg yolks, with their reactivity assessed through indirect ELISA. Neutropenic mice received intranasal administration of IgYs one hour prior to the challenge with a clinical A. baumannii isolate (10 ×LD50). The specific anti-FimA IgYs detected recombinant FimA and provided 100% protection against bacterial infection, while non-specific IgYs prolonged survival for up to 72 h. In contrast, control mice succumbed to infection within 24 h. Analysis of bacterial loads in lungs and spleens after 16 h reveals the following order: control > non-specific IgY > anti-FimA IgY. These findings highlight FimA as a suitable antigen for the development of protective IgYs against A. baumannii.

Synergistic immunoprotection by Oma87 and Bap against Acinetobacter baumannii sepsis model

Acinetobacter baumannii is the leading cause of nosocomial infection. A surface protein commonly known as biofilm associate protein (Bap) has been identified in a bloodstream isolate of A. baumannii. Bap of A. baumannii is involved in intercellular adhesion within the mature biofilm. Outer membrane protein Acinetobacter 87 kDa (Oma87) or β-barrel assembly machinery A (BamA) has been introduced as an immunogenic outer membrane protein via in silico reverse vaccinology. Current research examines the synergistic effect of immunization of mice with both recombinant proteins viz., Oma87 and Bap. Antibodies were raised to the proteins. The mice were challenged with A. baumannii ATCC 19606 and the bacterial burden was enumerated in the mice's livers, spleens, and lungs followed by histological examination. IgG levels significantly increased, and a significant (p < 0.0001) difference was observed between bacterial burdens in the internal organs of the actively and passively immunized groups. Female BALB/c mice weighing 20-25 g, were divided into 4 groups of 14 mice each viz., control, Oma87, Bap, Oma87-Bap groups. The proteins were individually immunogenic, but the combination of both proteins had a synergistic protection property. This is further supported by the histological examination. Based on the results, the combination of Oma87 and Bap may be considered a promising vaccine candidate against A. baumannii .

Mutation in the two-component regulator BaeSR mediates cefiderocol resistance and enhances virulence in Acinetobacter baumannii

Acinetobacter baumannii has become one of the most challenging pathogens in many countries with limited treatment options available. Cefiderocol, a novel siderophore-conjugated cephalosporin, shows potent in vitro activity against A. baumannii, including isolates resistant to carbapenems. To date, few reports on the mechanisms of cefiderocol resistance are available. In order to investigate potential mechanisms of cefiderocol resistance in A. baumannii, we performed in vitro evolution experiments at sub-lethal concentrations of the antibiotic. All four cefiderocol-resistant strains obtained harbored mutations in two-component system BaeS-BaeR. When we engineered the mutations of BaeS (D89V) and BaeR (S104N) into the genome of ATCC 17978, these mutations increased cefiderocol minimum inhibitory concentrations (MICs) by 8-fold to 16-fold. Transcriptome analyses showed that the expression of MacAB-TolC and MFS transporters was up-regulated in BaeSR mutants. Strains over-expressing MFS transporter and MacAB-TolC displayed higher MICs and higher median inhibition concentration (IC50) values, while MICs and IC50 decreased when efflux pump genes were knocked out. In a BaeR mutant with up-regulated csu operon, we observed a higher number of pili, enhanced surface motility, and increased biofilm formation compared to wild-type ATCC 17978. Using the Galleria mellonella infection model, we found that the BaeS mutant in which paa operon was up-regulated exhibited increased virulence. In conclusion, the mutations in BaeSR decreased cefiderocol susceptibility of A. baumannii through up-regulating efflux pumps gene expression. BaeS or BaeR also controls the expression of csu and paa, influencing biofilm formation, surface motility, and virulence in A. baumannii. IMPORTANCE The widespread prevalence of multi-drug-resistant A. baumannii (MDRAB) poses a significant therapeutic challenge. Cefiderocol is considered a promising antibiotic for the treatment of MDRAB infections. Therefore, it is necessary to study the potential resistance mechanisms of cefiderocol to delay the development of bacterial resistance. Here, we demonstrated that mutations in baeS and baeR reduced the susceptibility of A. baumannii to cefiderocol by up-regulating the expression of the MFS family efflux pump and MacAB-TolC efflux pump. We propose that BaeS mutants increase bacterial virulence by up-regulating the expression of the paa operon. This also reports the regulatory effect of BaeSR on csu operon for the first time. This study provides further insights into the role of BaeSR in developing cefiderocol resistance and virulence in A. baumannii.

Architecture of the biofilm-associated archaic Chaperone-Usher pilus CupE from Pseudomonas aeruginosa

Chaperone-Usher Pathway (CUP) pili are major adhesins in Gram-negative bacteria, mediating bacterial adherence to biotic and abiotic surfaces. While classical CUP pili have been extensively characterized, little is known about so-called archaic CUP pili, which are phylogenetically widespread and promote biofilm formation by several human pathogens. In this study, we present the electron cryomicroscopy structure of the archaic CupE pilus from the opportunistic human pathogen Pseudomonas aeruginosa. We show that CupE1 subunits within the pilus are arranged in a zigzag architecture, containing an N-terminal donor β-strand extending from each subunit into the next, where it is anchored by hydrophobic interactions, with comparatively weaker interactions at the rest of the inter-subunit interface. Imaging CupE pili on the surface of P. aeruginosa cells using electron cryotomography shows that CupE pili adopt variable curvatures in response to their environment, which might facilitate their role in promoting cellular attachment. Finally, bioinformatic analysis shows the widespread abundance of cupE genes in isolates of P. aeruginosa and the co-occurrence of cupE with other cup clusters, suggesting interdependence of cup pili in regulating bacterial adherence within biofilms. Taken together, our study provides insights into the architecture of archaic CUP pili, providing a structural basis for understanding their role in promoting cellular adhesion and biofilm formation in P. aeruginosa.

Using an Aluminum Hydroxide–Chitosan Matrix Increased the Vaccine Potential and Immune Response of Mice against Multi-Drug-Resistant Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative, immobile, aerobic nosocomial opportunistic coccobacillus that causes pneumonia, septicemia, and urinary tract infections in immunosuppressed patients. There are no commercially available alternative antimicrobials, and multi-drug resistance is an urgent concern that requires emergency measures and new therapeutic strategies. This study evaluated a multi-drug-resistant A. baumannii whole-cell vaccine, inactivated and adsorbed on an aluminum hydroxide–chitosan (mAhC) matrix, in an A. baumannii sepsis model in immunosuppressed mice by cyclophosphamide (CY). CY-treated mice were divided into immunized, non-immunized, and adjuvant-inoculated groups. Three vaccine doses were given at 0D, 14D, and 28D, followed by a lethal dose of 4.0 × 108 CFU/mL of A. baumannii. Immunized CY-treated mice underwent a significant humoral response, with the highest IgG levels and a higher survival rate (85%); this differed from the non-immunized CY-treated mice, none of whom survived (p < 0.001), and from the adjuvant group, with 45% survival (p < 0.05). Histological data revealed the evident expansion of white spleen pulp from immunized CY-treated mice, whereas, in non-immunized and adjuvanted CY-treated mice, there was more significant organ tissue damage. Our results confirmed the proof-of-concept of the immune response and vaccine protection in a sepsis model in CY-treated mice, contributing to the advancement of new alternatives for protection against A. baumannii infections.

Deciphering the virulence factors, regulation, and immune response to Acinetobacter baumannii infection

Deciphering the virulence factors, regulation, and immune response to Acinetobacter baumannii infectionAcinetobacter baumannii is a gram-negative multidrug-resistant nosocomial pathogen and a major cause of hospital acquired infetions. Carbapenem resistant A. baumannii has been categorised as a Priority1 critial pathogen by the World Health Organisation. A. baumannii is responsible for infections in hospital settings, clinical sectors, ventilator-associated pneumonia, and bloodstream infections with a mortality rates up to 35%. With the development of advanced genome sequencing, molecular mechanisms of manipulating bacterial genomes, and animal infection studies, it has become more convenient to identify the factors that play a major role in A. baumannii infection and its persistence. In the present review, we have explored the mechanism of infection, virulence factors, and various other factors associated with the pathogenesis of this organism. Additionally, the role of the innate and adaptive immune response, and the current progress in the development of innovative strategies to combat this multidrug-resistant pathogen is also discussed.

Non-antibiotic prevention and treatment against Acinetobacter baumannii infection: Are vaccines and adjuvants effective strategies?

Acinetobacter baumannii (A. baumannii) is a Gram-negative opportunistic pathogen widely attached to the surface of medical instruments, making it one of the most common pathogens of nosocomial infection, and often leading to cross-infection and co-infection. Due to the extensive antibiotic and pan-resistance, A. baumannii infection is facing fewer treatment options in the clinic. Therefore, the prevention and treatment of A. baumannii infection have become a tricky global problem. The requirement for research and development of the new strategy is urgent. Now, non-antibiotic treatment strategies are urgently needed. This review describes the research on A. baumannii vaccines and antibacterial adjuvants, discusses the advantages and disadvantages of different candidate vaccines tested in vitro and in vivo, especially subunit protein vaccines, and shows the antibacterial efficacy of adjuvant drugs in monotherapy.

Subunit vaccines for Acinetobacter baumannii

Acinetobacter baumannii is a gram-negative bacterium and a crucial opportunistic pathogen in hospitals. A. baumannii infection has become a challenging problem in clinical practice due to the increasing number of multidrug-resistant strains and their prevalence worldwide. Vaccines are effective tools to prevent and control A. baumannii infection. Many researchers are studying subunit vaccines against A. baumannii. Subunit vaccines have the advantages of high purity, safety, and stability, ease of production, and highly targeted induced immune responses. To date, no A. baumannii subunit vaccine candidate has entered clinical trials. This may be related to the easy degradation of subunit vaccines in vivo and weak immunogenicity. Using adjuvants or delivery vehicles to prepare subunit vaccines can slow down degradation and improve immunogenicity. The common immunization routes include intramuscular injection, subcutaneous injection, intraperitoneal injection and mucosal vaccination. The appropriate immunization method can also enhance the immune effect of subunit vaccines. Therefore, selecting an appropriate adjuvant and immunization method is essential for subunit vaccine research. This review summarizes the past exploration of A. baumannii subunit vaccines, hoping to guide current and future research on these vaccines.

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 ST2^Pas 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.

Antimicrobial resistance and biofilm formation capacity among Acinetobacter baumannii strains isolated from patients with burns and ventilator-associated pneumonia

BACKGROUND

Acinetobacter baumannii is a pathogen responsible for nosocomial infections, especially in patients with burns and ventilator-associated pneumonia (VAP). The aims of this study was to compare the biofilm formation capacity, antimicrobial resistance patterns and molecular typing based on PFGE (Pulsed-Field Gel Electrophoresis) in A. baumannii isolated from burn and VAP patients.

MATERIALS AND METHODS

A total of 50 A. baumannii isolates were obtained from burn and VAP patients. In this study, we assessed antimicrobial susceptibility, biofilm formation capacity, PFGE fingerprinting, and the distribution of biofilm-related genes (csuD, csuE, ptk, ataA, and ompA).

RESULTS

Overall, 74% of the strains were multidrug resistant (MDR), and 26% were extensively drug-resistant (XDR). Regarding biofilm formation capacity, 52%, 36%, and 12% of the isolates were strong, moderate, and weak biofilm producers. Strong biofilm formation capacity significantly correlated with XDR phenotype (12/13, 92.3%). All the isolates harbored at least one biofilm-related gene. The most prevalent gene was csuD (98%), followed by ptk (90%), ataA (88%), ompA (86%), and csuE (86%). Harboring all the biofilm-related genes was significantly associated with XDR phenotype. Finally, PFGE clustering revealed 6 clusters, among which cluster No. 2 showed a significant correlation with strong biofilm formation and XDR phenotype.

CONCLUSION

Our findings revealed the variable distribution of biofilm-related genes among MDR and XDR A. baumannii isolates from burn and VAP patients. A significant correlation was found between strong biofilm formation capacity and XDR phenotype. Finally, our results suggested that XDR phenotype was predominant among strong-biofilm producer A. baumannii in our region.

KbvR mutant of Klebsiella pneumoniae affects the synthesis of type 1 fimbriae and provides protection to mice as a live attenuated vaccine

Klebsiella pneumoniae is a leading cause of severe infections in humans and animals, and the emergence of multidrug-resistant strains highlights the need to develop effective vaccines for preventing such infections. Live attenuated vaccines are attractive vaccine candidates available in the veterinary field. We recently characterized that the K. pneumoniae kbvR (Klebsiella biofilm and virulence regulator) mutant was a highly attenuated strain in the mice model. In the present study, the characterization, safety, and protective efficacy of ΔkbvR strain as a live attenuated vaccine were evaluated. The synthesis and activity of type 1 fimbriae were increased in the ΔkbvR strain. All mice inoculated by the subcutaneous route with 105, 106, and 107 colony-forming units (CFU) doses of the ΔkbvR strain survived. Subcutaneous immunization with two doses of 105 or 107 CFU ΔkbvR elicited a robust humoral immune response, and provided protection against the following K. pneumoniae intraperitoneal infection. The antisera of mice immunized with 105 CFU dose improved the opsonophagocytic ability and complement-mediated lysis not only to the same serotype strain but also to the different serotype strain. The passive transfer of antisera from 105 CFU dose-immunized mice provided protection against K. pneumoniae infection. Overall, our results suggest the great potential of the ΔkbvR strain as a novel vaccine candidate against K. pneumoniae infections in herds or humans.

Vaccination with a combination of planktonic and biofilm virulence factors confers protection against carbapenem-resistant Acinetobacter baumannii strains

Acinetobacter baumannii is a multi-drug resistant pathogen with the ability to switch between planktonic and biofilm phenotypes. Although there is no vaccine against A. baumannii infections, many attempts have been made to develop vaccines using planktonic or biofilm antigens. To cover the different phenotypes of A. baumannii during growth and attachment, we combined planktonic upregulated antigens of iron receptors with biofilm upregulated antigens of pilus rods and evaluated immune responses and protective efficacies of the combined vaccine using lethal and sub-lethal murine sepsis models. The results showed that the combined vaccine elicited high IgG antibody titers and conferred protection against lethal doses of two Carbapenem-resistant high adherent A. baumannii strains. Complete bacterial clearance from all the affected tissues of the mice challenged with A. baumannii was an excellent achievement with our quadrivalent immunogen. These results demonstrate both planktonic and biofilm antigens are important during antigen selection for vaccine design.

Myrtenol Inhibits Biofilm Formation and Virulence in the Drug-Resistant Acinetobacter baumannii: Insights into the Molecular Mechanisms

Background

blaNDM-1-producing Acinetobacter baumannii (BP-AB) remains a critical problem in nosocomial infections, because of its resistance mediated by the biofilm formation and virulence factors. No studies have confirmed myrtenol’s efficacy in inhibiting the biofilm formation and virulence associated with biofilm of BP-AB.

Methods

The tested concentrations of myrtenol were wild type (A), 50 μg/mL (B), 100 μg/mL (C), 200 μg/mL (D), 250 μg/mL (E), and 300 μg/mL (F).

Results

The BP-AB biofilm inhibition was significantly higher in the D, E, and F groups than in the A, B, and C groups. Myrtenol significantly reduced the air-liquid interface ring formation in glass tubes. It also effectively inhibited the attachment of BP-AB strains on polystyrene surfaces as shown by crystal violet staining. Microscopy showed a significant reduction in biofilm formation with dispersed BP-AB strains. The confocal laser scanning microscopy analysis showed a significant reduction in the biofilm’s biomass, covered surface area, and thickness. The scanning electron microscopy analysis revealed significantly fewer BP-AB aggregates on the coverslip surface. In the CompStat analysis, the biofilm’s biomass, maximum thickness, and surface-to-volume ratio were significantly reduced. The qPCR analysis revealed a significant down-regulation of bfmR, bap, csuA/B, and ompA expression, which positively correlated with the biofilm’s biomass, maximum thickness, and surface-to-volume ratio in BP-AB strains. Myrtenol significantly improved the susceptibility of BP-AB to the antibiotics amikacin, piperacillin/tazobactam, cefoperazone/sulbactam, and ceftazidime.

Conclusion

Myrtenol attenuates the BP-AB biofilm formation and virulence by suppressing the expression of bfmR, bap, csuA/B, and ompA.

Modern vaccine development via reverse vaccinology to combat antimicrobial resistance

With the continuous evolution of bacteria, the global antimicrobial resistance health threat is causing millions of deaths yearly. While depending on antibiotics as a primary treatment has its merits, there are no effective alternatives thus far in the pharmaceutical market against some drug-resistant bacteria. In recent years, vaccinology has become a key topic in scientific research. Combining with the growth of technology, vaccine research is seeing a new light where the process is made faster and more efficient. Although less discussed, bacterial vaccine is a feasible strategy to combat antimicrobial resistance. Some vaccines have shown promising results with good efficacy against numerous multidrug-resistant strains of bacteria. In this review, we aim to discuss the findings from studies utilizing reverse vaccinology for vaccine development against some multidrug-resistant bacteria, as well as provide a summary of multi-year bacterial vaccine studies in clinical trials. The advantages of reverse vaccinology in the generation of new bacterial vaccines are also highlighted. Meanwhile, the limitations and future prospects of bacterial vaccine concludes this review.

Promising Acinetobacter baumannii Vaccine Candidates and Drug Targets in Recent Years

In parallel to the uncontrolled use of antibiotics, the emergence of multidrug-resistant bacteria, like Acinetobacter baumannii, has posed a severe threat. A. baumannii predominates in the nosocomial setting due to its ability to persist in hospitals and survive antibiotic treatment, thereby eventually leading to an increasing prevalence and mortality due to its infection. With the increasing spectra of drug resistance and the incessant collapse of newly discovered antibiotics, new therapeutic countermeasures have been in high demand. Hence, recent research has shown favouritism towards the long-term solution of designing vaccines. Therefore, being a realistic alternative strategy to combat this pathogen, anti-A. Baumannii vaccines research has continued unearthing various antigens with variable results over the last decade. Again, other approaches, including pan-genomics, subtractive proteomics, and reverse vaccination strategies, have shown promise for identifying promiscuous core vaccine candidates that resulted in chimeric vaccine constructs. In addition, the integration of basic knowledge of the pathobiology of this drug-resistant bacteria has also facilitated the development of effective multiantigen vaccines. As opposed to the conventional trial-and-error approach, incorporating the in silico methods in recent studies, particularly network analysis, has manifested a great promise in unearthing novel vaccine candidates from the A. baumannii proteome. Some studies have used multiple A. baumannii data sources to build the co-functional networks and analyze them by k-shell decomposition. Additionally, Whole Genomic Protein Interactome (GPIN) analysis has utilized a rational approach for identifying essential proteins and presenting them as vaccines effective enough to combat the deadly pathogenic threats posed by A. baumannii. Others have identified multiple immune nodes using network-based centrality measurements for synergistic antigen combinations for different vaccination strategies. Protein-protein interactions have also been inferenced utilizing structural approaches, such as molecular docking and molecular dynamics simulation. Similar workflows and technologies were employed to unveil novel A. baumannii drug targets, with a similar trend in the increasing influx of in silico techniques. This review integrates the latest knowledge on the development of A. baumannii vaccines while highlighting the in silico methods as the future of such exploratory research. In parallel, we also briefly summarize recent advancements in A. baumannii drug target research.

Immunity induced by valine-glycine repeat protein G imparts histoprotection of vital body organs against Acinetobacter baumannii

Background

Efforts toward the development of an effective vaccine against Acinetobacter baumannii, one of the most notorious nosocomial pathogens, are still ongoing. In this regard, virulence factors are interesting targets. Type VI secretion system (T6SS) participates in the pathogenicity of A. baumannii. VgrG is a crucial component of T6SS prevalent among A. baumannii strains. This study was conducted to evaluate the immunoprotectivity of recombinant VgrG (rVgrG) cloned and over-expressed in Escherichia coli BL21 (DE3). BALB/c mice were immunized with the purified rVgrG. Specific anti-VgrG IgG titers were assessed by ELISA. Actively and passively immunized mice were challenged with lethal doses of A. baumannii ATCC 19606. The survival rate, the bacterial burden, and histopathology of tissues in infected mice were examined.

Results

Anti-VgrG IgG (p < 0.0001) was significantly increased in immunized mice. No death was seen in actively immunized mice infected with the lethal dose (LD) of 1.9 × 10⁸ CFU of A. baumannii ATCC 19606 within 72 h. Challenge with 2.4 × 10⁸ CFU of the pathogen showed a 75% survival rate. All immunized mice infected with 3.2 × 10⁸ CFU of the pathogen died within 12 h. In passive immunization, no death was observed in mice that received LD of the bacteria incubated with the 1:250 dilution of the immune sera. An increased number of neutrophils around the peribronchial and perivascular areas were seen in unimmunized mouse lungs while passively immunized mice revealed moderate inflammation with infiltration of mixed mononuclear cells and neutrophils. The livers of the unimmunized mice showed inflammation and necrosis in contrast to the livers from immunized mice. Hyperplasia of the white pulp and higher neutrophils were evident in the spleen of unimmunized mice as against the normal histology of the immunized group.

Conclusions

VgrG is a protective antigen that could be topologically accessible to the host antibodies. Although VgrG is not sufficient to be assigned as a stand-alone antigen for conferring full protection, it could participate in multivalent vaccine developments for elevated efficacy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-022-00325-4.

A novel pentavalent vaccine candidate completely protects against Acinetobacter baumannii in a mouse model of peritonitis

Acinetobacter baumannii is considered as one of the most virulent and infectious organisms that have an increased ability to both evade host immune response and resist various classes of antibiotics, leading to life-threatening infections. Multiple virulence factors have been implicated in the high prevalence rate of A. baumannii in hospitalized and immunocompromised patients. Moreover, improper use of antibiotics has led to the emergence of extensive drug-resistant strains that urgently require alternative strategies to control this superbug. Unfortunately, the availability of a licensed vaccine against A. baumannii infections is still challenged by the vast diversity among A. baumannii strains. Here, we report the development of a novel pentavalent vaccine candidate composed of two recombinant proteins (Wza and YiaD) and a pool of capsular polysaccharides isolated from 3 clinical isolates. We tested this new vaccine in vivo in a mouse model of peritonitis against the standard strain ATCC 19606 in addition to 3 clinical isolates of A. baumannii. Immunization with this vaccine completely protected the challenged mice with 100% survival rate in the case of all the tested bacteria. Further clinical studies are urgently needed to evaluate the efficacy and safety of this proprietary vaccine to protect patients from A. baumannii lethal infections. KEY POINTS: • Recombinant proteins pool (Wza and YiaD) immunization led to a synergistic immune response. • Capsular polysaccharides pool induced up to 90% protection of tested clinical isolates. • The pentavalent pool showed superiority with 100% survival of immunized mice.

Putative novel B-cell vaccine candidates identified by reverse vaccinology and genomics approaches to control Acinetobacter baumannii serotypes

In the last decade, Multi-drug resistance (MDR)-associated infections of Acinetobacter baumannii have grown worldwide. A cost-effective preventative strategy against this bacterium is vaccination. This study has presented five novel vaccine candidates against A. baumannii produced using the reverse vaccinology method. BLASTn was done to identify the most conserved antigens. PSORTb 3.0.2 was run to predict the subcellular localization of the proteins. The initial screening and antigenicity evaluation were performed using Vaxign. The ccSOL omics was also employed to predict protein solubility. The cross-membrane localization of the protein was predicted using PRED-TMBB. B cell epitope prediction was made for immunogenicity using the IEDB and BepiPred-2.0 database. Eventually, BLASTp was done to verify the extent of similarity to the human proteome to exclude the possibility of autoimmunity. Proteins failing to comply with the set parameters were filtered at each step. In silico, potential vaccines against 21 A. baumannii strains were identified using reverse vaccinology and subtractive genomic techniques. Based on the above criteria, out of the initial 15 A. baumannii proteins selected for screening, nine exposed/secreted/membrane proteins, i.e., Pfsr, LptE, OmpH, CarO, CsuB, CdiB, MlaA, FhuE, and were the most promising candidates. Their solubility and antigenicity were also examined and found to be more than 0.45 and 0.6, respectively. Based on the results, LptE was selected with the highest average antigenic score of 1.043 as the best protein, followed by FimF and Pfsr with scores of 1.022 and 1.014, respectively. In the end, five proteins were verified as promising candidates. Overall, the targets identified herein may be utilized in future strategies to control A. baumannii worldwide.

Sequential Vaccination With Heterologous Acinetobacter baumannii Strains Induces Broadly Reactive Antibody Responses

Antibody therapy may be an alternative treatment option for infections caused by the multi-drug resistant (MDR) bacterium Acinetobacter baumannii. As A. baumannii has multiple capsular serotypes, a universal antibody therapy would need to target conserved protein antigens rather than the capsular polysaccharides. We have immunized mice with single or multiple A. baumannii strains to induce antibody responses to protein antigens, and then assessed whether these responses provide cross-protection against a collection of genetically diverse clinical A. baumannii isolates. Immunized mice developed antibody responses to multiple protein antigens. Flow cytometry IgG binding assays and immunoblots demonstrated improved recognition of both homologous and heterologous clinical strains in sera from mice immunized with multiple strains compared to a single strain. The capsule partially inhibited bacterial recognition by IgG and the promotion of phagocytosis by human neutrophils. However, after immunization with multiple strains, serum antibodies to protein antigens promoted neutrophil phagocytosis of heterologous A. baumannii strains. In an infection model, mice immunized with multiple strains had lower bacterial counts in the spleen and liver following challenge with a heterologous strain. These data demonstrate that antibodies targeting protein antigens can improve immune recognition and protection against diverse A. baumannii strains, providing support for their use as an antibody therapy.

Evaluation of the immunogenicity and protective ability of a pili subunit, SBP2', of Streptococcus suis serotype 2

Streptococcus suis is an important zoonotic pathogen that leads to huge economic losses in the swine industry. Because of the enormous genetic and phenotypic diversity within S. suis, it is necessary to develop effective vaccines to control this zoonotic pathogen. SBP2' is a major pili subunit in S. suis that belongs to an srtBCD pili cluster and has already been reported to be associated with the pathogenesis of this bacterium. In this study, we aimed to evaluate the immunogenicity and protective ability of SBP2'. The rSBP2' protein was expressed by an Escherichia coli expression system and emulsified with Montanide ISA 201 adjuvant to prepare the subunit vaccine. Through active immune assays, the results showed that rSBP2' exhibited good immunogenicity and could protect mice from a lethal dose challenge. Additionally, the qRT-PCR data showed that the transcription levels of cytokines associated with systemic symptoms caused by S. suis were decreased, indicating that immunization with rSBP2' could protect the host from cytokine storms caused by S. suis. Furthermore, the passive immune assay showed that the humoral immunity induced by rSBP2' played an important role against S. suis infection. Taken together, SBP2' could provide proper immune protection against S. suis challenge and could be a candidate for S. suis subunit vaccine. The results of this study could provide new ideas for the development of effective vaccines against S. suis.

Vaccines for multidrug resistant Gram negative bacteria: lessons from the past for guiding future success

Antimicrobial resistance is a major threat to global public health. Vaccination is an effective approach for preventing bacterial infections, however it has not been successfully applied to infections caused by some of the most problematic multidrug resistant pathogens. In this review, the potential for vaccines to contribute to reducing the burden of disease of infections caused by multidrug resistant Gram negative bacteria is presented. Technical, logistical and societal hurdles that have limited successful vaccine development for these infections in the past are identified, and recent advances that can contribute to overcoming these challenges are assessed. A synthesis of vaccine technologies that have been employed in the development of vaccines for key multidrug resistant Gram negative bacteria is included, and emerging technologies that may contribute to future successes are discussed. Finally, a comprehensive review of vaccine development efforts over the last 40 years for three of the most worrisome multidrug resistant Gram negative pathogens, Acinetobacter baumannii, Klebsiella pneumoniae and Pseudomonas aeruginosa is presented, with a focus on recent and ongoing studies. Finally, future directions for the vaccine development field are highlighted.

Recent Advances in the Pursuit of an Effective Acinetobacter baumannii Vaccine

Acinetobacter baumannii has been a major cause of nosocomial infections for decades. The absence of an available vaccine coupled with emerging multidrug resistance has prevented the medical community from effectively controlling this human pathogen. Furthermore, the ongoing pandemic caused by SARS-CoV-2 has increased the risk of hospitalized patients developing ventilator-associated pneumonia caused by bacterial opportunists including A. baumannii. The shortage of antibiotics in the development pipeline prompted the World Health Organization to designate A. baumannii a top priority for the development of new medical countermeasures, such as a vaccine. There are a number of important considerations associated with the development of an A. baumannii vaccine, including strain characteristics, diverse disease manifestations, and target population. In the past decade, research efforts have revealed a number of promising new immunization strategies that could culminate in a safe and protective vaccine against A. baumannii. In this review, we highlight the recent progress in the development of A. baumannii vaccines, discuss potential challenges, and propose future directions to achieve an effective intervention against this human pathogen.