Immunization with the ferric iron-binding periplasmic protein HitA provides protection against Pseudomonas aeruginosa in the murine infection model

Pseudomonas aeruginosa responds to altered membrane phospholipid composition by adjusting the production of two-component systems, proteases and iron uptake proteins

Membrane protein and phospholipid (PL) composition changes in response to environmental cues and during infections. To achieve these, bacteria use adaptation mechanisms involving covalent modification and remodelling of the acyl chain length of PLs. However, little is known about bacterial pathways regulated by PLs. Here, we investigated proteomic changes in the biofilm of P. aeruginosa phospholipase mutant (∆plaF) with altered membrane PL composition. The results revealed profound alterations in the abundance of many biofilm-related two-component systems (TCSs), including accumulation of PprAB, a key regulator of the transition to biofilm. Furthermore, a unique phosphorylation pattern of transcriptional regulators, transporters and metabolic enzymes, as well as differential production of several proteases, in ∆plaF, indicate that PlaF-mediated virulence adaptation involves complex transcriptional and posttranscriptional response. Moreover, proteomics and biochemical assays revealed the depletion of pyoverdine-mediated iron uptake pathway proteins in ∆plaF, while proteins from alternative iron-uptake systems were accumulated. These suggest that PlaF may function as a switch between different iron-acquisition pathways. The observation that PL-acyl chain modifying and PL synthesis enzymes were overproduced in ∆plaF reveals the interconnection of degradation, synthesis and modification of PLs for proper membrane homeostasis. Although the precise mechanism by which PlaF simultaneously affects multiple pathways remains to be elucidated, we suggest that alteration of PL composition in ∆plaF plays a role for the global adaptive response in P. aeruginosa mediated by TCSs and proteases. Our study revealed the global regulation of virulence and biofilm by PlaF and suggests that targeting this enzyme may have therapeutic potential.

Iron Acquisition Proteins of Pseudomonas aeruginosa as Potential Vaccine Targets: In Silico Analysis and In Vivo Evaluation of Protective Efficacy of the Hemophore HasAp

BACKGROUND

Pseudomonas aeruginosa (PA) is a Gram-negative pathogen responsible for fatal nosocomial infections worldwide. Iron is essential for Gram-negative bacteria to establish an infection. Therefore, iron acquisition proteins (IAPs) of bacteria are attractive vaccine targets.

METHODOLOGY

A "Reverse Vaccinology" approach was employed in the current study. Expression levels of 37 IAPs in various types of PA infections were analyzed in seven previously published studies. The IAP vaccine candidate was selected based on multiple criteria, including a high level of expression, high antigenicity, solubility, and conservation among PA strains, utilizing suitable bioinformatics analysis tools. The selected IAP candidate was recombinantly expressed in Escherichia coli and purified using metal affinity chromatography. It was further evaluated in vivo for protection efficacy. The novel immune adjuvant, naloxone (NAL), was used.

RESULTS AND DISCUSSION

HasAp antigen met all the in silico selection criteria, being highly antigenic, soluble, and conserved. In addition, it was the most highly expressed IAP in terms of average fold change compared to control. Although HasAp did excel in the in silico evaluation, subcutaneous immunization with recombinant HasAp alone or recombinant HasAp plus NAL (HasAP-NAL) did not provide the expected protection compared to controls. Immunized mice showed a low IgG2a/IgG1 ratio, indicating a T-helper type 2 (Th2)-oriented immune response that is suboptimal for protection against PA infections. Surprisingly, the bacterial count in livers of both NAL- and HasAp-NAL-immunized mice was significantly lower than the count in the HasAp and saline groups. The same trend was observed in kidneys and lungs obtained from these groups, although the difference was not significant. Such protection could be attributed to the enhancement of innate immunity by NAL.

CONCLUSIONS

We provided a detailed in silico analysis of IAPs of PA followed by in vivo evaluation of the best IAP, HasAp. Despite the promising in silico results, HasAp did not provide the anticipated vaccine efficacy. HasAp should be further evaluated as a vaccine candidate through varying the immunization regimens, models of infection, and immunoadjuvants. Combination with other IAPs might also improve vaccination efficacy. We also shed light on several highly expressed promising IAPs whose efficacy as vaccine candidates is worthy of further investigation.

Functional Annotation of Hypothetical Proteins From the Enterobacter cloacae B13 Strain and Its Association With Pathogenicity

Enterobacter cloacae B13 strain is a rod-shaped gram-negative bacterium that belongs to the Enterobacteriaceae family. It can cause respiratory and urinary tract infections, and is responsible for several outbreaks in hospitals. E. cloacae has become an important pathogen and an emerging global threat because of its opportunistic and multidrug resistant ability. However, little knowledge is present about a large portion of its proteins and functions. Therefore, functional annotation of the hypothetical proteins (HPs) can provide an improved understanding of this organism and its virulence activity. The workflow in the study included several bioinformatic tools which were utilized to characterize functions, family and domains, subcellular localization, physiochemical properties, and protein-protein interactions. The E. cloacae B13 strain has overall 604 HPs, among which 78 were functionally annotated with high confidence. Several proteins were identified as enzymes, regulatory, binding, and transmembrane proteins with essential functions. Furthermore, 23 HPs were predicted to be virulent factors. These virulent proteins are linked to pathogenesis with their contribution to biofilm formation, quorum sensing, 2-component signal transduction or secretion. Better knowledge about the HPs’ characteristics and functions will provide a greater overview of the proteome. Moreover, it will help against E. cloacae in neonatal intensive care unit (NICU) outbreaks and nosocomial infections.

Shigella iron-binding proteins: An insight into molecular physiology, pathogenesis, and potential target vaccine development

Shigella is a well-known etiological agent responsible for intestinal infection among children, the elderly, and immunocompromised people ranging from mild to severe cases. Shigellosis remains endemic in Malaysia and yet there is no commercial vaccine available to eradicate the disease. Iron is an essential element for the survival of Shigella within the host. Hence, it is required for regulating metabolic mechanisms and virulence determinants. Alteration of iron status in the extracellular environment directly triggers the signal in enteropathogenic bacterial, providing information that they are in a hostile environment. To survive in an iron-limited environment, molecular regulation of iron-binding proteins plays a vital role in facilitating the transportation and utilization of sufficient iron sources. Given the importance of iron molecules for bacterial survival and pathogenicity, this review summarizes the physiological role of iron-binding proteins in bacterial survival and their potential use in vaccine and therapeutic developments.

Bacterial Ghosts of Pseudomonas aeruginosa as a Promising Candidate Vaccine and Its Application in Diabetic Rats

Infections with Pseudomonas aeruginosa (PA) pose a major clinical threat worldwide especially to immunocompromised patients. As a novel vaccine network for many kinds of bacteria, bacterial ghosts (BGs) have recently been introduced. In the present research, using Sponge-Like Reduced Protocol, P. aeruginosa ghosts (PAGs) were prepared to maintain surface antigens and immunogenicity. This is the first study, to our knowledge, on the production of chemically induced well-structured bacterial ghosts for PA using concentrations of different chemicals. The research was carried out using diabetic rats who were orally immunized at two-week intervals with three doses of PAGs. Rats were subsequently challenged either by the oral route or by the model of ulcer infection with PA. In challenged rats, in addition to other immunological parameters, organ bioburden and wound healing were determined, respectively. Examination of the scanning and transmission electron microscope (EM) proved that PAGs with a proper three-dimensional structure were obtained. In contrast to control groups, oral PAGs promoted the generation of agglutinating antibodies, the development of IFN-γ, and the increase in phagocytic activity in vaccinated groups. Antibodies of the elicited PAGs were reactive to PA proteins and lipopolysaccharides. The defense against the PA challenge was observed in PAGs-immunized diabetic rats. The resulting PAGs in orally vaccinated diabetic rats were able to evoke unique humoral and cell-mediated immune responses and to defend them from the threat of skin wound infection. These results have positive implications for future studies on the PA vaccine.

Vaccination to Prevent Pseudomonas aeruginosa Bloodstream Infections

The bacterium Pseudomonas aeruginosa (Pa) is ubiquitous in the environment and causes opportunistic infections in humans. Pa is increasingly becoming one of the most difficult to treat microorganisms due to its intrinsic and acquired resistance to multiple antibiotics. The World Health Organization estimates that at least 700,000 people die each year from drug resistant microbial infections and have listed Pa as one of three bacterial species for which there is the most critical need for the development of novel therapeutics. Pa is a common cause of bloodstream infections (BSI) and bacterial sepsis. With nearly 49 million sepsis cases and 11 million deaths worldwide, an effective vaccine against Pa could prevent the morbidity and mortality resulting from Pa BSI and lessen our dependence on antibiotics. We reviewed the current landscape of Pa vaccines in pre-clinical and clinical stages over the last two decades. It is readily apparent that Pa vaccine development efforts have been largely directed at the prevention of pulmonary infections, likely due to Pa's devastating impact on individuals with cystic fibrosis. However, the increase in nosocomial infections, BSI-related sepsis, and the emergence of widespread antibiotic resistance have converged as a major threat to global public health. In this perspective, we draw attention to potential Pa vaccine candidates and encourage a renewed effort for prophylactic vaccine development to prevent drug-resistant Pa BSI.

Outer Membrane Vesicles Displaying a Heterologous PcrV-HitA Fusion Antigen Promote Protection against Pulmonary Pseudomonas aeruginosa Infection

Along with surging threats and antibiotic resistance of Pseudomonas aeruginosa in health care settings, it is imperative to develop effective vaccines against P. aeruginosa infection. In this study, we used an Asd (aspartate-semialdehyde dehydrogenase)-based balanced-lethal host-vector system of a recombinant Yersinia pseudotuberculosis mutant to produce self-adjuvanting outer membrane vesicles (OMVs). The OMVs were used as a carrier to deliver the heterologous PcrV-HitA_T (PH) fusion antigen of P. aeruginosa for vaccine evaluation. Intramuscular vaccination with OMVs carrying the PH antigen (referred to rOMV-PH) afforded 73% protection against intranasal challenge with 5 × 10⁶ (25 50% lethal doses) of the cytotoxic PA103 strain and complete protection against a noncytotoxic PAO1 strain. In contrast, vaccination with the PH-deficient OMVs or PH antigen alone failed to offer effective protection against the same challenge. Immune analysis showed that the rOMV-PH vaccination induced potent humoral and Th1/Th17 responses compared to the PH vaccination. The rOMV-PH vaccination rapidly cleared P. aeruginosa burdens with coordinated production of proinflammatory cytokines in mice. Moreover, antigen-specific CD4⁺ and CD8⁺ T cells and their producing cytokines (tumor necrosis factor alpha and interleukin-17A), rather than antibodies, were essential for protection against pneumonic P. aeruginosa infection. Our studies demonstrated that the recombinant Y. pseudotuberculosis OMVs delivering heterologous P. aeruginosa antigens could be a new promising vaccine candidate for preventing the spread of drug-resistant P. aeruginosa.

IMPORTANCE Hospital- and community-acquired infections with Pseudomonas aeruginosa cause a high rate of morbidity and mortality in patients who have underlying medical conditions. The spread of multidrug-resistant P. aeruginosa strains is becoming a great challenge for treatment using antibiotics. Thus, a vaccine as one of the alternative strategies is urgently required to prevent P. aeruginosa infection.

Recombinant Pseudomonas bio-nanoparticles induce protection against pneumonic Pseudomonas aeruginosa infection

To develop an effective Pseudomonas aeruginosa (PA) outer-membrane-vesicles (OMVs) vaccine, we eliminated multiple virulence factors from a wild-type P. aeruginosa PA103 strain (PA103) to generate a recombinant strain, PA-m14. The PA-m14 strain was tailored with a pSMV83 plasmid encoding the pcrV-hitA_T fusion gene to produce OMVs. The recombinant OMVs enclosed increased amounts of PcrV-HitA_T bivalent antigen (PH) (termed OMV-PH) and exhibited reduced toxicity compared to the OMVs from PA103. Intramuscular vaccination with OMV-PH from PA-m14(pSMV83) afforded 70% protection against intranasal challenge with 6.5 × 10⁶ CFU (∼30 LD₅₀) of PA103, while immunization using OMVs without the PH antigen (termed OMV-NA) or the PH antigen alone failed to offer effective protection against the same challenge. Further immune analysis showed that the OMV-PH immunization significantly stimulated potent antigen-specific humoral and T-cell (Th1/Th17) responses in comparison to the PH or OMV-NA immunization in mice, which can effectively hinder PA infection. Undiluted anti-sera from OMV-PH-immunized mice displayed significant opsonophagocytic killing of WT PA103 compared to antisera from PH antigen- or OMV-NA-immunized mice. Moreover, the OMV-PH immunization afforded significant antibody-indentpednet cross-protection to mice against PAO1 and a clinical isolate AMC-PA10 strains. Collectively, the recombinant PA OMV delivering the PH bivalent antigen exhibits high immunogenicity and would be a promising next-generation vaccine candidate against PA infection.

Recombinant Ax21 protein is a promising subunit vaccine candidate against Stenotrophomonas maltophilia in a murine infection model

Stenotrophomonas maltophilia is an emerging pathogen that can cause several disease manifestations such as bacteremia, meningitis, respiratory tract infections and others. More seriously, this pathogen has a highly evolving antibiotic resistance profile. Antibiotic misuse is further aggravating the situation by inducing the development of multi- and even pan-resistance. Thus, employing diverse strategies to overcome this increasing antibiotic resistance is of paramount importance. In general, vaccination is one of these strategies that prevents the onset of infection, provides long term protection against infection, and most importantly diminishes the antibiotic consumption, thus, resulting in controlling resistance. Unfortunately, vaccine research concerning S. maltophilia is very scarce in the literature. Ax21 protein is an outer membrane protein implicated in several virulence mechanisms of S. maltophilia such as quorum sensing, biofilm formation, and antibiotic resistance. Our computational analysis of Ax21 revealed its potential immunogenicity. In the current study, Ax21 protein of S. maltophilia was cloned and heterologously expressed in Escherichia coli. Mice were immunized with the purified recombinant antigen using Bacillus Calmette-Guérin(BCG) and incomplete Freund's adjuvant (IFA) as immune-adjuvants. Enzyme-linked immunosorbent assay (ELISA) revealed significant antigen-specific IgG1, IgG2a and total IgG levels in immunized mice which reflected successful immune stimulation. Immunized mice that were challenged with S. maltophilia showed a substantialreduction in bacterial bioburden in lungs, liver, kidneys, and heart. In addition, liver histological examination demonstrated a remarkable decrease in pathological signs such as necrosis, vacuolation, bile duct fibrosis and necrosis, infiltration of inflammatory cells, and hemorrhage. Whole cell ELISA and opsonophagocytic assay confirmed the ability of serum antibodies from immunized mice to bind and facilitate phagocytosis of S. maltophilia, respectively. To our knowledge, this is the first report to demonstrate the vaccine protective efficacy of Ax21 outer membrane protein against S. maltophilia infection.

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.

Understanding Pseudomonas aeruginosa-Host Interactions: The Ongoing Quest for an Efficacious Vaccine

Pseudomonas aeruginosa is a leading cause of chronic respiratory infections in people with cystic fibrosis (CF), bronchiectasis or chronic obstructive pulmonary disease (COPD), and acute infections in immunocompromised individuals. The adaptability of this opportunistic pathogen has hampered the development of antimicrobial therapies, and consequently, it remains a major threat to public health. Due to its antimicrobial resistance, vaccines represent an alternative strategy to tackle the pathogen, yet despite over 50 years of research on anti-Pseudomonas vaccines, no vaccine has been licensed. Nevertheless, there have been many advances in this field, including a better understanding of the host immune response and the biology of P. aeruginosa. Multiple antigens and adjuvants have been investigated with varying results. Although the most effective protective response remains to be established, it is clear that a polarised Th2 response is sub-optimal, and a mixed Th1/Th2 or Th1/Th17 response appears beneficial. This comprehensive review collates the current understanding of the complexities of P. aeruginosa-host interactions and its implication in vaccine design, with a view to understanding the current state of Pseudomonal vaccine development and the direction of future efforts. It highlights the importance of the incorporation of appropriate adjuvants to the protective antigen to yield optimal protection.

Iron metabolism in Pseudomonas aeruginosa biofilm and the involved iron-targeted anti-biofilm strategies

Pseudomonas aeruginosa is a gram-negative bacterium that exists in various ecosystems, causing severe infections in patients with AIDS or cystic fibrosis. P. aeruginosa can form biofilm on a variety of surfaces, whereby the bacteria produce defensive substances and enhance antibiotic-resistance, making themselves more adaptable to hostile environments. P. aeruginosa resistance represents one of the main causes of infection-related morbidity and mortality at a global level. Iron is required for the growth of P. aeruginosa biofilm. This review summarises how the iron metabolism contributes to develop biofilm, and more importantly, it may provide some references for the clinic to achieve novel anti-biofilm therapeutics by targeting iron activities.

Emerging therapies against infections with Pseudomonas aeruginosa

Infections with Pseudomonas aeruginosa have been marked with the highest priority for surveillance and epidemiological research on the basis of parameters such as incidence, case fatality rates, chronicity of illness, available options for prevention and treatment, health-care utilization, and societal impact. P. aeruginosa is one of the six ESKAPE pathogens that are the major cause of nosocomial infections and are a global threat because of their capacity to become increasingly resistant to all available antibiotics. This review reports on current pre-clinical and clinical advances of anti-pseudomonal therapies in the fields of drug development, antimicrobial chemotherapy, vaccines, phage therapy, non-bactericidal pathoblockers, outer membrane sensitizers, and host defense reinforcement.