Klebsiella pneumoniae (Kpn) was first isolated by Carl Friedlander in 1882 from the lungs of dead pneumonia patients [1]. Hence, it was initially called Friedlander’s bacillus, but in 1886, it was renamed Kpn. This Gram-negative bacillus, an encapsulated, non-motile bacterium, is a lactose-fermenting, non-spore-forming facultative anaerobe that belongs to the Enterobacteriaceae family (Figure 1) [2].

Kpn is a Klebsiella pneumoniae species complex (KpSC) member, which includes strains that have the same biochemical profile but only share 90–96% identity between their genome sequences ^[3][4][5][3,4,5]. This complex contains seven phylogroups: Klebsiella pneumoniae, Klebsiella quasipneumoniae subsp. quasipneumoniae, Klebsiella quasipneumoniae subsp. similipneumoniae, Klebsiella variicola subsp. variicola, Klebsiella variicola subsp. tropicalensis, Klebsiella quasivariicola, and Klebsiella africana (Figure 1).

Although all these strains could be associated with human diseases, Kpn is mainly related to nosocomial infections [5]. Kpn strains are classified into three categories: classical (cKpn), hypervirulent (hvKpn), and multidrug-resistant (MDR-Kpn). cKpn strains are nosocomial strains that are generally found in immunocompromised patients and produce urinary tract, acute respiratory, and bloodstream infections [5]. cKpn strains are characterised by a non-hypermucoviscosity phenotype (“string test”) and lack excessive siderophores [6]. hvKpn strains are community-acquired strains that induce pyogenic liver abscess, meningitis, endophthalmitis, and necrotising fasciitis in diabetic and healthy people. These strains are characterised by a hypermucoviscosity phenotype and harbour excessive siderophores [7]. Finally, pyogenic liver abscesses, bloodstream infections, and urinary tract infections are associated with nosocomial and community MDR-Kpn strains that usually infect immunocompromised patients [8]. MDR-Kpn strains have a hypervirulent profile but no specific hypermucoviscosity-defined pattern ^[6][9][6,9]. MDR-Kpn and hvKpn strains were previously considered different clonal groups, but now these characteristics are deemed additive [10]. The high incidence of MDR-Kpn and hvKpn strains has earned Kpn a place on the World Health Organization (WHO) list as a “priority pathogen” since 2017 [11].

The complex biology of Kpn has made it difficult to control infections caused by this pathogen due to its high antimicrobial resistance and many virulence factors. That is why designing new therapies to combat Kpn infections has been a priority. Plants are a potential source for the design of new treatments [12]. Numerous biotic and abiotic stimuli, including certain pathogenic bacteria, cause plants to produce many secondary metabolites. These metabolites are employed in medicine to treat many illnesses, including infections involving bacteria. However, very few studies have demonstrated these metabolites’ antimicrobial potential, mechanism of action, and potential for developing new antibacterial therapies.

2. Kpn Virulence Factors

Kpn has acquired many virulence factors to promote colonisation, evasion of host immune responses, and bacterial competition (Figure 2). Fimbriae, adhesins, and capsule polysaccharides (CPSs) from Kpn are the most representative virulence factors used to adhere to and colonise host cells. Fimbriae are hair-like appendages and are thin structures localised on cellular surfaces that allow Kpn adherence and colonisation. Fimbria type 1 is exclusively expressed in the urinary tract, favouring adhesion to the bladder [13]. In contrast, fimbria type 3 is expressed in kidney and lung cells for adherence and biofilm formation. Additionally, fimbriae of both types are associated with biofilm formation on abiotic surfaces such as those of catheters (Figure 3) ^[14][15][14,15]. During colonisation, capsule polysaccharides (CPSs) from Kpn are a physical barrier and promote host immune evasion (Figure 3). CPSs can block phagocytosis via IL-36 and act as a barrier against antibacterial peptides [16]. CPSs also activate the EGF–PI3K–Akt pathway, inducing cytoskeleton rearrangement in host cells and allowing the bacterium to translocate [17]. CPS types have a substantial variation, but Kpn strains possess at least two CPSs, KL106 and KL107 [18]. Outer membrane proteins (OMPs) are implicated in the modulation of immune response and antibiotic resistance as an essential factor. In Kpn infection, OmpA activates Toll-like receptor (TLR) 2, and OmpK36 contributes to resisting phagocytosis by neutrophils and macrophages ^[19][20][21][19,20,21]. On the other hand, OmpK35 and OmpK26 are associated with the efflux of antibiotics such as carbapenem [22]. Furthermore, lipopolysaccharide (LPS) in Kpn comprises an O antigen, lipid A, and an oligosaccharide core in lipid rafts. LPS protects against complement-mediated lysis and activates TLR4 [23]. Despite metal ions being critical elements required for many bacterial metabolic processes, the extraintestinal environment is deficient in ions such as iron and copper ^[24][25][24,25]. Bacteria have developed small iron-binding molecules, named siderophores, to bind and transport ions from the host cell, promoting bacterial growth ^[26][27][26,27]. The main Kpn siderophores are Enterobactin, Yersiniabactin, Salmochelin, and Aerobactin. Enterobactin is encoded in the ent cluster and is involved with MDR-Kpn strains ^[28][29][28,29]. Yersiniabactin is encoded in the mobile genetic element named ICEKp and plays a secondary role in decreasing the host immune response through evasion of lipocalin 2 [25]. Salmochelin and Aerobactin are encoded in the iucABCDiutA and iroBCDN clusters carried by the plasmid pLVPK, and their expression is associated with the hypervirulent profile in hvKpn strains [30]. Type VI Secretion System (T6SS) is another important virulence factor that Kpn has developed to translocate proteins, called effectors, into adjacent eukaryotic and prokaryotic cells. Although T6SSs are well characterised in other Gram-negative bacteria, information regarding T6SS function in Kpn remains controversial [31]. Nevertheless, Kpn T6SS is likely to provide an advantage for survival during bacterial competition, such as competition among microbiome bacteria [32]. In addition, this system has been associated with antibiotic resistance, biofilm formation, and the delivery of toxins into neighbouring cells [33]. Toxins are other virulence factors that contribute to virulence enhancement. Colibactin, encoded by the pks genomic island, is a Kpn toxin involved in DNA double-strand breaks, chromosome aberrations, and cell cycle arrest [34].

3. Klebsiella pneumoniae and Public Health

Kpn is the most frequent nosocomial opportunistic pathogen that infects critically ill and immunocompromised patients. This pathogen has become a severe public health problem due to its high prevalence and mortality rate in hospitals [35]. Many reports describing Kpn disease have focused on adults. Nevertheless, recent studies indicate that children (5.9–67.6%) are susceptible to infection by hvKpn strains ^[36][37][36,37]. As previously mentioned, the diseases produced by Kpn include bacteraemia, liver diseases, pneumonia, urinary tract infections, and septic arthritis ^[38][39][40][38,39,40]. Furthermore, it is one of the most important opportunistic pathogens to combat during solid organ transplants ^[41][42][41,42]. Haematological malignancies, antibiotic administration, mechanical ventilation, and long-term hospitalisations are the principal factors that favour Kpn dissemination in hospitalised patients ^[43][44][45][43,44,45]. Interestingly, the transmission of Kpn disease during the SARS-CoV-2 pandemic was significantly reduced, along with the transmission of Enterococcus faecium, Staphylococcus aureus, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. (called ESKAPE pathogens) ^[46][47][46,47]. Government-implemented public health measures, such as using face masks and gloves, meticulous hand cleaning, access and movement restrictions within the hospitals, and continuous surface cleaning, could be causes of the low incidence of ESKAPE pathogens [48]. Even though antibiotic therapy is ineffective against SARS-CoV-2 infection, some studies reported that 70% of hospitalised patients received broad-spectrum antibiotic therapy [49]. Furthermore, a recent mathematical-method-based study analysed the impact of health interventions on the prevalence of resistant Kpn strains; in that study, an increase in extensively drug-resistant strains was reported, with the frequency rising from 10% to 50% [50]. Genomic studies and sequence type (ST) analysis have suggested that ST307 and ST147 strains have a high number of virulence factors (antibacterial resistance genes, virulence factors, and fitness); these factors are considered a potential risk because they may contribute to the adaptation of these strains to hospital environments and the human host [51]. Kpn ST307 was described in the Netherlands in 2008; however, after five years, the clone was also reported in hospitals in the United States, Pakistan, Colombia, Italy, South Korea, and Tunisia [52]. After 2016, Kpn ST307 was dispersed worldwide, likely as a result of acquiring a second capsule cluster, a fimbrial cluster, and T6SS [53]. The Kpn ST147 clone was first reported in 2008–2009 in Hungary and Spain and is mainly resistant to the fluoroquinolones due to gyrA S83I, parC S80IQRDR, and bla_CTX-M-15 mutations [54]. From 2011 to 2013, the clone was identified in Greece, Italy, Sweden, Denmark, Canada, the United Kingdom, Finland, India, and Libya. Finally, in 2014, this clone achieved worldwide dispersion [52]. The impact of this uncontrolled antibiotic administration was a subsequent increase in resistant bacterial infections, highlighting the importance of tighter control in prophylactic practice and antibiotic administration. Thus, increasing efforts are being made to develop innovative, specific, and effective treatments to combat the severe and often deadly infections produced by Kpn strains.

4. Antibiotic Resistance

Antibiotic resistance results from bacterial specialisation across time and exposure to different drugs as a bacterial survival strategy ^[55][56][57][55,56,57]. The main antibiotic-resistance-associated mechanisms are active efflux, target protection, decreased influx, target modification, LPS modification, physical barrier, and antibiotic inactivation (Figure 4). Efflux pumps are components in bacterial membranes that can extract antibiotics from the cellular environment, a process called active efflux. It may be specific to one antibiotic or able to act against multiple antibiotics (Table 1). An example is the presence of aminoglycoside and quinolone resistance in Kpn, which are associated with the KpnO pump [58]. Meanwhile, OqxAB is expressed in quinolone-resistant Kpn. The KpnEF pump mediates the transport of aminoglycosides, cephalosporins, rifamycin, polymyxins, and tetracyclines. Worryingly, AcrAB-TolC is responsible for polymyxin resistance ^[59][60][59,60].

7. Biotechnology Applying Plant-Origin Sources to Develop New Control Strategies against Kpn

An important factor in controlling and reducing nosocomial infections, such as Kpn disease, is the implementation of new technologies that control intrahospital dispersion of the strains. Many studies have reported that hospital personnel, the environment, and medical devices are the principal routes by which these opportunistic pathogens are transferred to patients [141]. To solve this problem, nanotechnology researchers are working on a nanoparticle (NP) design that could be useful in bionic, textile, and biomedical engineering (Figure 6). NPs are solid colloidal particles with a 10–1000 nm size range that can offer many benefits [142]. In the biomedical area, NP design focuses on developing new drug delivery materials, photoablation therapy, hospital equipment, and medical clothes that can be valuable tools for controlling the spread of nosocomial diseases [143]. NPs are created by green synthesis technology, which allows the production of these materials to be non-hazardous, eco-friendly, and cost-effective [144]. NPs can be biosynthesised using plant-origin compounds such as extracts and oils (Figure 4). Metal-binding NPs such as silver (Ag) could bind to bacterial DNA and attach to ribosomes to prevent DNA duplication [145]. Phytochemical-producing plants are valuable as a source of capping agents in AgNP design. These particles have activity as an antibacterial against Kpn strains and exert anti-inflammatory and antioxidant effects in the host. AgNPs from the leaf extract of Naringi crenulata are effective against MDR-Kpn strains [146]. Other AgNPs from the leaf extract of Alternanthera sessilis have shown potential biomedical application against MDR-Kpn strains [147]. Zinc oxide (ZnSO₄) NPs produced with Brassica oleracea extract showed insufficient antibacterial activity against MDR-Kpn strains but demonstrated antibacterial and antifungal potential against other pathogens [148]. Zirconium oxide (ZrO₂) NPs are proposed to be functional in the production of medical devices, such as tissue scaffolds, microscale valves, and bone prostheses, suggesting that they possess favourable antibacterial potential. Other ZrO₂NPs produced with Phyllanthus niruri extract have antibacterial properties against several pathogens, including Kpn [149]. Non-metal-binding NP designs are in the process of being studied to identify their antimicrobial potential [150]. Administration of the nonmetal Selenium (Se) is poisonous at higher doses. Nevertheless, SeNPs are gaining increasing attention because of their biomedical effect. Furthermore, SeNPs from Olea ferruginea have antimicrobial activity against several nosocomial bacterial strains, including Kpn strains [151]. POCs represent an excellent tool in the development of synergistic therapies to combat infections caused by Kpn.

8. Special Considerations in the Application of POCs in Medicine

It is important to note that POCs are a complex mixture of secondary metabolites produced by plants, and their chemical composition can change from plant to plant, even within the same species, as a result of exposure to biotic and abiotic factors such as soil hydrology, pH, salinity, temperature, soil organisms, and pollinator insects. Additionally, postharvest treatment, extraction, and conservation methods can influence POCs’ chemical composition. These results have been the subject of numerous examinations, such as the study performed by Todorova et al. in 2023 [152]. They analysed the chemical composition of commercial and bio-cultivated lavender (Lavandula angustifolia Mill.) essential oil. While bio-cultivated lavender essential oil contains 23.13% β-linalool, commercial essential oils possess higher amounts (24.34–35.99%). Additionally, only one of the seven samples contained less than 25% linalyl acetate, the content indicated by the requirements of the European Pharmacopoeia. The pharmacological potential of POCs depends on their composition. Establishing them as an official drug could be difficult, and to solve the trouble of composition variations, standardisation of plant growth, postharvest, and extraction protocols is required. While employing POCs can be among the most effective approaches for developing novel treatments, it is crucial to acknowledge that numerous secondary metabolites synthesised by plants serve as defence mechanisms against infections, insects, and herbivores. These mechanisms function by causing harm to organisms that consume them. Consequently, evaluating the cytotoxic potential of these metabolites is imperative to guarantee their suitability for medicinal plant applications. Although POCs are frequently commended for their aromatic and therapeutic benefits, it is vital to recognise that they can also carry hazards, including the possibility of poisoning. For example, essential oils are highly concentrated plant extracts that can have adverse effects due to the intensity or high concentration of some of their compounds. Recently, many studies have been carried out to characterize this cytotoxicity in vitro and in vivo. Table 3 describes the safety and toxicity data of many POCs that are proposed to have positive effects against Kpn.