Pa is a ubiquitous gram-negative bacterial species known for its versatility and propensity to adapt to a wide range of environments. Pa can colonize moist surfaces including medical devices, hospital sinks and drains, respiratory equipment, and disinfectant solutions (Trautmann et al., 2005). Pa requires minimal nutritional thresholds for growth or survival and possesses complex regulatory and phenotypic attributes which support the bacterium’s ability to survive under a variety of harsh environmental and immunological stressors (Ramos, 2011; Remold et al., 2011). Pa is an opportunistic bacterium, yet infections caused by Pa can manifest in multiple anatomical sites leading to ophthalmic, respiratory, pulmonary, genitourinary, and bloodstream infections, among others. Pa displays intrinsic resistance to many antibiotics including, but not limited to, Ampicillin-Amoxicillin, Ampicillin-sulbactam, Amoxicillin-Clavulanate, Cefotaxime, Ceftriaxone, Ertapenem, Tetracyclines/Tigecyclines, Trimethoprim, Trimethoprim-sulfamethoxazole and Chloramphenicol (CLSI, 2017). Furthermore, its capacity to rapidly develop resistance during antibiotic treatment is one of the most common reasons for therapeutic failures (Cabassi et al., 2017). Pa is a frequent and often significant cause of healthcare-acquired infections (Ferrara, 2006; Mesaros et al., 2007; Parkins et al., 2018) which can progress to life-threatening sepsis and septic shock. For example, Pa is responsible for approximately 10%–15% of all hospital-acquired infections (Blanc et al., 1998) with reported mortality rates as high as 37.6% (Shi et al., 2019) to 39% (Kang et al., 2003) in patients with confirmed Pa bloodstream infection.

A bloodstream infection (BSI) is defined by the presence of a viable microorganism in the bloodstream that elicits an inflammatory response. In the absence of a protective host immune response or appropriate medical treatment, a BSI can ultimately lead to sepsis. Sepsis is the result of widespread inflammation throughout the body and may result in severe organ dysfunction and death. The global burden of sepsis is challenging to quantify; however, in 2017, the WHO estimated that 48.9 million cases and 11 million sepsis-related deaths occurred worldwide, which accounted for approximately 20% of all global deaths (Rudd et al., 2020). Healthcare acquired BSI and sepsis are frequent serious adverse events that occur during delivery of care, particularly in intensive care units (World Health Organization, 2017). In the last 25 years, Gram-negative bacterial (GNB) infections, including those caused by Pa, have been shown the be a main driver of sepsis (Curuţiu et al., 2013; Adamik et al., 2015). Pa was the fourth most common bacterial species causing BSI from 1997 to 2016 (Diekema et al., 2019).

With BSI, antibiotic resistance is a major factor in determining clinical unresponsiveness to treatment and progression to severe sepsis and septic shock. Studies document that Pa is one of the most resistant GNB found in clinical settings (Diekema et al., 2019), and Pa is one of the most likely GNB to be detected as PDR (Magiorakos et al., 2012; Diekema et al., 2019). AMR infections often lead to higher risk of mortality than antibiotic susceptible pathogens (World Health Organization, 2017), and despite improvements in medical care, Pa BSI-related mortality remains substantially high (Thaden et al., 2017). In fact, Pa accounts for the highest mortality rates among common GNB BSI (Vidal et al., 1996; Lodise et al., 2007).

Rapid empirical administration of antibiotics has long been a cornerstone of sepsis management (Kang et al., 2003; Micek et al., 2005; Lodise et al., 2007). However, there is some controversy over whether early administration of antibiotics improves patient outcomes (Asner et al., 2021). Empirical treatment can result in a sub-therapeutic dose or incorrect antibiotic choice. Under dosing of antibiotics can give rise to AMR. The historical reliance and continued practice of rapid empirical treatment is controversial and not sustainable for sepsis management. The lack of evidence for empirical antibiotic treatment efficacy and the risk empirical treatment may pose to worsening AMR highlight the need for other therapeutic strategies to aid in sepsis prevention.

Vaccination could offer dual benefits by preventing sepsis caused by AMR bacteria and by curtailing the rise and selection of AMR due to excessive antibiotic use. Indeed, vaccination has already been shown to be a valuable tool in combating AMR. Immunization against Streptococcus pneumoniae resulted in a 59% reduction in multidrug-resistant strains between 1999 and 2004 (Kyaw et al., 2006). Furthermore, the use of a universal pneumococcal vaccine decreased the mean number of days on antibiotics for pneumonia caused by Streptococcus pneumoniae in children under the age of 5 years by about 11.4 million days per year (Laxminarayan et al., 2016).

Vaccination presents an attractive option to prevent or reduce AMR infections caused by Pa. Historically, vaccine development for Pa has focused on prevention of lung infection in individuals with cystic fibrosis due to the devastating morbidity and mortality caused by Pa in this population. With the escalation in AMR, the aging population in the United States. And elsewhere, and the increased use of immunosuppressive drugs, the potential target population for a Pa vaccine has grown tremendously. Furthermore, individuals with increased risk of infection, such as those undergoing surgery or chemotherapy and active military personnel, represent additional potential beneficiaries of a prophylactic vaccine. Currently there is no Pa vaccine approved for human use, and the paucity of vaccine research dedicated strictly to prevention of Pa BSI is notable.

Over the past two decades, there have been less than a dozen studies of prophylactic vaccines tested against Pa BSI or sepsis in animal models (Table 1). Nonetheless, several candidate antigens including secreted proteins (Farajnia et al., 2015; Wang et al., 2018; Yang et al., 2018), surface and outer membrane components (Yu et al., 2016; Ryu et al., 2017; Elhosary et al., 2019; Bahey-El-Din et al., 2020), and other virulence factors (Kao et al., 2007; Moriyama et al., 2009; Aguilera-Herce et al., 2019; Elhosary et al., 2019; Afshari et al., 2021) combined with different delivery methods and adjuvants have shown promising results.

OprF is a highly expressed, antigenically conserved, and immunogenic surface protein of Pa (Mutharia and Hancock, 1983). It has shown great promise as a vaccine candidate as immunization with OprF elicited protection in murine models as well as promoted high levels of functional, antigen-specific IgG humans (Stanislavsky and Lam, 1997; Price et al., 2001; Sharma et al., 2011; Westritschnig et al., 2014). Mice immunized with a DNA vaccine encoding a chimeric OprF and a Herpes Simplex Virus type 1 protein VP22 (PVAX1-OprF-VP22) induced significant increases in OprF-specific total IgG, IgG2a, IFN-y production, and T-cell proliferation (Yu et al., 2016). Vaccinated mice displayed 75% and 40% survival at days 8 and 10, respectively compared to 0% survival in control mice after lethal intraperitoneal (i.p.) challenge (Yu et al., 2016). The potential of the under-investigated N-terminal porin domain of OprF as a stand-alone recombinant protein vaccine was also investigated against Pa (Bahey-El-Din et al., 2020). Immunization with N-terminal OprF elicited antigen-specific IgG1 and IgG2a and significantly reduced bacterial burdens in the kidney and lung tissues of vaccinated mice as compared to the controls (Bahey-El-Din et al., 2020). Immunization with N-terminal OprF also significantly reduced liver pathology at 48 h post i.p challenge with Pa (Bahey-El-Din et al., 2020). Collectively, these results demonstrate that the immune response of OprF-immunized mice limited systemic disease in a Pa sepsis infection model.

IC43 is a recombinant outer membrane protein-based vaccine consisting of OprF and OprI subunits that advanced to human clinical trials. In the randomized placebo-controlled phase 2 trial, Pa bacteremia and invasive infection were assessed as a measure of vaccine efficacy and were not significantly different between vaccine and placebo groups (Rello et al., 2017). The lack of clinical benefit may be attributed to the fact that the vaccinated population were mechanically ventilated ICU patients. It is possible that the onset of infection occurred before or very early in the vaccination regimen prior to the induction of vaccine-specific immunity (Rello et al., 2017).

Other OMP-based vaccines have been shown to protect against systemic infection in mice and have demonstrated safety and immunogenicity in healthy human volunteers (Kim et al., 1994; Park et al., 1997; Jang et al., 1999). Mice immunized with Pa OMP coupled with the adjuvant CIA06 elicited significantly higher antigen-specific IgG, IgG1 and IgG2 compared to control mouse sera (Ryu et al., 2017). Furthermore, antibodies from OMP-immunized mice displayed cross reactivity to heterologous Pa serotypes (Ryu et al., 2017). OMP-immunized mice displayed 100% survival at day 8 compared to 0% survival of controls after lethal i.p. challenge (Ryu et al., 2017).

Like OMPs, outer membrane vesicles (OMVs) have garnered attention as vaccines and adjuvants (Prior et al., 2021). OMVs carrying homologous or heterologous antigens induce protective immune responses to many different pathogens in mouse models (van der Pol et al., 2015). PA0833 is a component of Pa OMVs (Yang et al., 2018), and immunization with PA0833 induced a Th-2 biased immune response by promoting significantly higher IgG1 than IgG2a (Yang et al., 2018). Vaccinated mice displayed 70% survival at day 14 compared to 20% survival in controls after intravenous (i.v.) challenge (Yang et al., 2018). Immunization with PA0833 also significantly decreased bacterial burdens in the blood, liver, and spleen at days 1 and 3 in a sublethal i.v. challenge model (Yang et al., 2018).

Immunization with Pa double layered OMVs (DMVs) significantly increased the production of IL-1β, IL-6, IL-2, and IL-12p70 cytokines, promoted maturation of CD11c + dendritic cells, and increased serum IgG titers (Wang et al., 2018). Vaccinated mice displayed 50% survival at 24 h compared to 0% survival of controls after lethal challenge (Wang et al., 2018).

Pa secreted toxins play an important role in infection and may be promising vaccine targets. Exotoxin A (ExoA) is one of the most toxic virulence factors of Pa (Liu, 1974) and is considered a promising antigenic target for toxin-producing strains (Chen and Shang, 1999). Like ExoA, Pa flagellum (Fla) has also been shown to be highly immunogenic and protective in animal models (Campodónico et al., 2010). Immunization with a fusion of ExoA-Fla generated antibodies with significantly increased opsonophagocytic activity (Farajnia et al., 2015). ExoA-Fla vaccinated mice were protected following i.p. challenge with survival rates of 80% for ExoA-Fla immunized mice compared to 20% survival in PBS-immunized control mice (Farajnia et al., 2015).

Targeting proteins in bacterial iron-acquisition systems also represents a promising vaccination strategy (Elhosary et al., 2019). Immunization with a periplasmic iron-acquisition protein, HitA, coupled with Bacillus Calmette-Guerin (BCG) significantly increased IgG1, IgG2, and total IgG (Elhosary et al., 2019). Vaccination of mice with HitA significantly decreased lung burdens and liver histopathology after sublethal i.p challenge (Elhosary et al., 2019).

Alginate capsule antigen (Alg) of Pa is a virulence factor that binds to host cells, facilitates biofilm formation, and aids Pa in antibiotic resistance and host immune evasion. Immunization of mice with Alg conjugated to solid lipid nanoparticles (SLN) elicited significant increases in IgG, IgA, and IgM and promoted higher opsonophagocytic killing activity as compared to control-immunized mice (Afshari et al., 2021). Alg-immunized mice displayed significantly decreased bacterial burdens in the spleen 4 weeks after i.p. challenge (Afshari et al., 2021).

PaV antigen (PcrV) is the tip protein of the type III secretion system. These secretion systems are present in many GNB and play critical roles in pathogenesis. Several studies have shown that immunization with recombinant PcrV provides protection against Pa in murine models (Sawa et al., 1999; Moriyama et al., 2009; Aguilera-Herce et al., 2019). Immunization with a live Salmonella vaccine displaying PcrV antigen elicited significantly increased levels of antigen-specific IgG at day 21 post vaccination (Aguilera-Herce et al., 2019). Vaccinated mice displayed up to 65% survival by day 3 compared to 0% survival in controls following lethal i.p. challenge (Aguilera-Herce et al., 2019). Immunization significantly reduced lung and spleen bacterial burdens and decreased serum proinflammatory cytokines at 12 h post challenge (Aguilera-Herce et al., 2019). A repeat i.p. challenge study resulted in 100% survival of vaccinated mice compared to <20% survival of control mice at day 2 post-challenge (Aguilera-Herce et al., 2019). Immunization of mice with PcrV antigen adjuvanted with CFA elicited significantly higher serum IgG antibodies 60 days post initial vaccination (Moriyama et al., 2009). Immunocompromised vaccinated mice displayed 65% survival 48 h after lethal i.p challenge as compared to 15% survival in control groups (Moriyama et al., 2009).

Pa type IV pili play an important role in early bacterial attachment and have been examined as a vaccine target (Kao et al., 2007). Vaccination with a consensus sequence immunogen (Cs1) of Pa pili conjugated to tetanus toxin significantly protected mice from lethal i.p challenge with 4 heterogenous strains of Pa (Kao et al., 2007). Vaccinated mice displayed a survival rate of 40% vs. 10% for controls after lethal challenge with strain KB7. After challenge with strain P1, the survival rate was 80% for vaccinated mice vs. 20% for controls. Upon challenge with strain PAK, vaccinated mice demonstrated 60% survival vs. 0% for controls. Finally, mice challenged with PAO demonstrated 100% survival vs. 0% for controls at 50 h post i.p. infection (Kao et al., 2007).