The emergence of antibiotic-resistant bacteria is a pressing public health concern, highlighting the need for alternative approaches to control bacterial infections. Promising approaches include the development of therapeutic vaccines and the utilization of innate immune activation techniques, which may prove useful in conjunction with antibiotics, as well as other antibacterial modalities. However, innate activation should be fast and self- or actively- contained to prevent detrimental consequences. TLR (Toll-like receptor) ligand adjuvants are effective at rapidly activating, within minutes to hours, the innate immune system by inducing cytokine production and other signaling molecules that bolster the host’s immune response. Neutrophils serve as the first line of defense against invading pathogens by capturing and destroying them through various mechanisms, such as phagocytosis, intracellular degradation, and the formation of NETs (Neutrophil Extracellular Traps). Nutritional immunity is another host defense mechanism that limits the availability of essential metals, such as iron, from invading bacterial pathogens. Thus, iron starvation has been proposed as a potential antibacterial strategy.
Adjuvants are substances that can enhance the immune response to vaccine components, potentially strengthening both innate and adaptive immune responses. By activating cellular and humoral responses, adjuvants can reduce the amount of an antigen required for efficient vaccination, improving immunogenicity in populations that typically respond poorly, such as the elderly or immunocompromised. Furthermore, adjuvants, such as AS03 (GSK) 
, MF59 (Novartis Vaccines and Diagnostics) 
, Matrix-M (Novavax) 
, and monophosphoryl lipid A (MPL) (GlaxoSmithKline), have encouraging safety profiles.
Traditional vaccine adjuvants, including various Toll-like receptor (TLR) ligands, can activate the innate immune system within minutes to hours, resulting in the production of cytokines and other signaling molecules that further enhance the immune response 
. This TLR-mediated rapid immune activation, which can last for several days, is part of the natural host’s first line of defense, which is critical for preventing the further spread of the infection. Moreover, it provides the necessary groundwork and time for subsequent activation of the adaptive immune response.
Adjuvants, such as AS02, AS03, AS04, MF59, MPL, and Matrix-M, are used in some vaccines to enhance the immune response against viral and bacterial infections. AS03 and MPL activate Toll-like receptor 4 (TLR4), while MF59 activates TLR4 + TLR8, and Matrix-M activates TLR4 + TLR3. These adjuvants have been shown to increase the activation and function of various immune cells, including dendritic cells, neutrophils, natural killer (NK), and T and B cells 
Moreover, these adjuvants stimulate the production of cytokines, such as interferon γ (IFN-γ), interleukin 1α (IL-1α), IL-1β, IL-6, tumor necrosis factor (TNF)-α, IL-8, and IL-12. These cytokines activate and enhance the ability of immune cells, such as neutrophils, macrophages, dendritic cells, and NK cells, to engulf and destroy invading viruses and bacteria 
Additionally, these adjuvants stimulate the production of other components, such as chemokines, that are crucial for the immune response. For example, AS03 activates the production of the chemokines C-C motif CCL2, CCL3, and CCL5, which attract immune cells to the vaccine site, further enhancing the host’s immune response 
. Thus, it could be that these adjuvants may also be used outside the context of a vaccine, namely, as immune stimulants to be concomitantly administered with post-exposure antibacterial treatments.
Indeed, although most clinically approved adjuvants were developed as anti-viral tools, some have also been found effective against bacteria, such as Streptococcus pneumoniae
, Haemophilus influenzae
type b 
, and Mycobacterium tuberculosis 
. In animal studies, MF59 has been found to enhance the immunogenicity and protective immunity against Acinetobacter baumannii 
, while a combination of a recombinant protein and MPL was found to be immunogenic against Neisseria meningitis 
To conclude, promising new adjuvants might contribute to the closure of the gap by efficient immune responses for bacterial clearance when administered immediately post exposure, even in the absence of a specific antigen. To note, some adjuvants may only be efficient in the presence of an additional antibacterial agent. It is important to keep tracking progress in these fields, including drug delivery systems (nanoparticle-based adjuvants, virus-like particles), novel TLR agonists, and combinational adjuvant treatments.
2. Rapid Induction of Protective Immunity by Therapeutic Vaccines
Most vaccines, which are prophylactic in nature, focus on enhancing the adaptive immune response to generate a high and fast antimicrobial effect when encountered in the future with the pathogen. However, vaccines may be also used therapeutically against viruses and bacteria if activation of the innate immune system takes place fast enough to close the gap until the adaptive immune response joins the efforts to eradicate the pathogen.
Studies have shown that the Yersinia pestis
EV76 live vaccine protected mice against an immediate subcutaneous lethal challenge. Strikingly, immunization two days prior to pulmonary infection also provided protection 
. Subcutaneous immunization with a Y. pestis
strain (Kim53ΔJ+P) that over-expresses Y. enterocolitica
YopP also elicited a fast and effective protective immune response in models of bubonic, pneumonic, and septicemic plague through the induction of a prompt protective innate immune response that was interferon-γ dependent. Moreover, cross-protection to other bacterial pathogens, such as the enteropathogen Y. enterocolitica
that causes Yersinosis and Francisella tularensis
, the causative agent of Tularemia, was attained 
. Immunization, pre (adjacent) or, post-exposure to a lethal Y. pestis
infection, with another anti-Y. pestis
vaccine, based on the F1 -recombinant protein adsorbed on alum hydroxide, also provided rapid protection in the bubonic plague mice model through anti-F1 IgM and IgG antibodies that developed within a few days post-vaccination. This line of protection was attributed to the activation of innate-like B cell subsets 
. In a following study, it was shown that effective protection against subsequent lethal intranasal exposure to a fully virulent Y. pestis
strain is obtained within a week following immunization with F1 adsorbed on alum hydroxide and that the addition of the LcrV antigen reduced the time to generate protective immunity to four to five days after vaccination 
. It is intriguing to believe that therapeutic vaccines could be an add-on post-exposure treatment if administered with antibiotics or other antimicrobial treatments.
Intranasal immunization with a single dose of inactivated whole-cell Acinetobacter baumannii
vaccination provided protection from a lethal dose of Acinetobacter baumannii
as early as two days after immunization 
. Cross protection was also seen in Klebsiella pneumonia
and Pseudomonas aeruginosa
pneumonia models. Protection of the immunized mice was correlated with elevated levels of IL-6 and TNF-α, which decreased by day five. However, rapid recall responses to infection were observed after a challenge on day seven post-vaccination. TNF-α secretion and chemokine production was noticed two hours post-challenge followed by neutrophil infiltration, as early as 4 h post infection. This response was attributed to immunization-trained alveolar macrophages, posing upregulated TLR4 expression, which mediated the rapid protection through enhanced TNF-α production 
3. Neutrophil Activation
Neutrophils play a crucial role as the first line of defense against invading pathogens, utilizing phagocytosis and intracellular degradation, as well as the release of granules, reactive oxygen species (ROS), and neutrophil extracellular traps to capture and destroy them 
. In contrast to macrophages, which have a long half-life of weeks 
or even years 
, neutrophils have a significantly shorter half-life (hours-days) 
, making them more suitable for promptly controlled activation against bacteria. However, although critical for bacterial killing, neutrophils may induce collateral damage and tissue injury by amplifying inflammation 
. Therefore, when inducing protection via an innate neutrophil-mediated immune response, there should always be a delicate balance between prompt bactericidal effect (especially in the case of bacteria that silence primary neutrophil effects, as in granule release inhibition following Yersinia pestis
or recruitment inhibition following Staphylococcus aureus
) and timely resolution of neutrophils’ potentially damaging effect. Strategies for neutrophil activation and resolution are discussed below.
4. Nutritional Immunity
Various metals, such as iron, manganese, zinc, and copper, are essential for the cellular functions of bacterial pathogens. Therefore, it is not surprising that the host undergoes significant changes in response to bacterial infections to regulate metal availability as a means of defense. The host’s innate immune response, which involves the withholding of metal nutrients to prevent bacterial growth, is defined as nutritional immunity 
. For comprehensive reviews on nutritional immunity, please see