Given the rapid emergence of multi drug resistant (MDR), extensively drug-resistant (XDR) and pandrug-resistant (PDR) pathogens, against which current antibiotics are no longer functioning, we are rapidly moving into a post-antibiotic era where infections will be practically untreatable [1]. According to the definition of the World Health Organization (WHO), antimicrobial resistance is a natural event that occurs when microbes become tolerant to drugs originally active, thus rendering several infections more difficult or impossible to treat ^[2][3][2,3]. Particularly, WHO has identified twelve families of bacteria to be considered as the most dangerous to human health. These families have been assigned to three priority groups, comprising critical pathogens (Acinetobacter, Pseudomonas, and Enterobacteriaceae), high priority pathogens (Enterococcus faecium, Staphylococcus aureus, Helicobacter pylori, Campylobacter, Salmonella spp., and Neisseria gonorrhoeae), and medium priority pathogens (Streptococcus pneumoniae, and Shigella spp.) ^[3][4][3,4]. Resistance in bacteria can be acquired or natural, but several mechanisms exist by which pathogens can become resistant to antibiotics (Figure 1).

As shown in Figure 1, antimicrobial resistance mechanisms include drug inactivation, decreased intracellular drug concentration, and altered drug targets [5].

Drug inactivation can occur either by enzymatic or chemical degradation, while decreased intracellular drug concentration can occur because of increasing drugs efflux and decreasing drugs influx [5]. In this regard, porin mutations in resistant strains alter the permeability of bacterial membranes, thus reducing the uptake of antibiotics into the bacterial cell. On the contrary, the hyperexpression of efflux pumps, which pump antibiotics out of the cell, dramatically reduces their concentration inside the cell [6]. Also, by the action of enzymes that chemically modify components of the bacterial outer membrane essential for antibiotic binding, some Gram-negative bacteria such as P. aeruginosa, Acinetobacter baumannii and others develop resistance to glycopeptide and polymyxin antibiotics. Furthermore, methyltransferases are a class of enzymes capable to modify the target thus promoting the resistance to antibiotics including aminoglycoside, lincosamide, macrolide, streptogramin, and oxazolidinone [7]. Another phenomenon known as “target protection” occurs when antibiotic target’s resistance proteins, such as the tetracycline ribosomal protection proteins (TRPPs), protect bacteria from the antibiotic-induced inhibition [8]. Additionally, the antibiotic resistance could be caused by the use of antibiotics in feed diet for animal production. The overuse, abuse, and misuse of β-lactams, aminoglycosides, tetracyclines, macrolides, and other antibiotics, with the purpose of promoting the development of animals, can cause the presence of residual antibiotics in the products intended for human consumption obtained from those animals, and can determine antibiotics pollution into the environment ^[9][10][11][9,10,11]. It was reported that some bacterial infections in humans are sustained by animal pathogens, namely zoonotic pathogens, thus proving that antibiotic resistance can be directly or indirectly transmitted from animal to humans [9]. A few practices, including the improvement of animal feed, waste management, and animal natural immunity, as well as the use of antibiotic alternatives such as prebiotics, probiotic vaccines, and bacteriophages can regulate and limit the antibiotic resistance, thus maintaining the potency of the available drugs [12]. However, more strategies to counteract antibiotic resistance are necessary, and currently they include the use of nanotechnology, computational methods, the use of antibiotic alternatives, drug repurposing, the synthesis of novel antibacterial agents, prodrugs, the development of efficient diagnostic agents also named rapid diagnostic tests (RDTs), the use of combination therapy, as well as the awareness, and knowledge of antibiotic prescribing (Table 1).

The activation of CB1 receptor or the concurrent activation of both receptors by ECs or PCs leads to both psychotropic, undesired effects and therapeutic outcomes. Exactly, while mind alteration, psychotropic effects, cardiovascular adverse events can occur, analgesic, sedative, antidepressant, anti-inflammatory, anti-anorexic, anti-emetic, anticancer and antibacterial desirable effects can also arise. As an example, the FDA-approved drug formulations containing the synthetic versions of Δ⁹-THC namely dronabinol (marketed as Marinol^® or Syndros^®) or nabilone (marketed as Cesamet™), as well as the extracted THC (marketed as Sativex^®), possess affinity for both CB1 and CB2 [48]. Clinically, they are primarily used to treat the chemotherapy-induced nausea, to enhance appetite in cachexic AIDS-patients, and to alleviate the spasticity and pain associated with multiple sclerosis [30]. Unfortunately, evidence of undesired psychotropic and cardiovascular adverse effects strongly limits the therapeutic efficacy of such medicines [49]. Otherwise, the selective activation of CB2 receptors, occurring for example by CBG or CBD, could provide therapeutic effects, such as immuno-modulatory properties, anti-inflammatory, anti-emetic, and anti-anorexic effects without exerting the psychotropic actions deriving from the CB1 activation. In addition, the potent analgesic effects, associated with the activation of CB2, could be helpful in alleviating chronic widespread musculoskeletal pain (CWP) disorders, such as fibromyalgia syndrome [50]. Also, the selective activation of CB2 receptors could enhance severe human diseases as osteoporosis, atherosclerosis, cancer, chronic liver injuries and neurodegeneration [48]. Collectively, CB1 and CB2 receptors together with ECs make part of the so-called EC system (ECS), which was discovered in the 1990′s by scientists researching cannabinoids, which includes also several enzymes involved in producing and recycling ECs. In humans, ECs are naturally produced by cells within the body in response to external factors, like pain or temperature. As shown in Figure 3, among other molecules, ECs include the well-known compounds 2-arachidonoylglycerol (2-AG) and anandamide (ANA), as well as the less-known ECs like virodhamine, and 2-arachidonoyl glycerol ether [51]. Particularly 2-AG and ANA activate both CB1 and CB2 receptors with affinity for CB1 higher than that for CB2 [48]. Collectively, the interaction between ECs and their corresponding receptors is pivotal in maintaining the body’s internal balance or homeostasis. ECs regulate some very important aspects of human health, as depicted in Figure 4. Researchers suggest that ECs deficiencies could cause many refractory health conditions, such as depression, arthritis, fibromyalgia, and Crohn’s disease that could ameliorate upon treatments with Cannabis, due to the activation by PCs of the same receptors activated in normal conditions by ECs. In fact, as reported above, PCs specifically produced by Cannabis plants and not by humans, when appropriately assumed can interact with CB1 and/or CB2 receptors triggering effects similar to those prompted by ECs, thus influencing the same aspects reported in Figure 4 and contributing to maintain or recover the body’s internal balance or homeostasis [41]. Anyway, if abused, the psychotropic and undesired side effects of psychoactive PCs, such as THC and CBN may overwhelm the benefits. Figure 5 shows the chemical structure of two metabolites which form in the human body after Cannabis consumption, of cannabicyclolic acid (CBLA), a degradative byproduct of cannabichromenic acid (CBCA), and of cannabicyclol (CBL) which is the product of decarboxylation of CBLA [53]. Particularly, CBL is a non-psychoactive cannabinoid, which could also derive by degradation of CBC through natural irradiation or under acid conditions [54]. Particularly, CBLA, like CBCA and CBC, is a minor cannabinoid found in low concentrations in the Cannabis plant. It is not produced by Cannabis directly, but it forms when CBCA degrades after exposure to ultraviolet (UV) light or heat. CBLA is an acidic cannabinoid, like THCA, CBCA and CBDA, which produces CBL, upon decarboxylation and release of CO₂. CBLA is not considered intoxicating, is often deemed non-psychoactive or non-psychotropic, and curiously, it does not interact with receptors CB1 and CB2. There is little research into the effects and potential therapeutic uses of CBLA and CBL. Anyway, although more research is needed to confirm these attributes, some suggestions exist, that CBLA may have anti-inflammatory, antimicrobial, and antitumoral effects, due to its structural similarity to CBCA and CBN [53]. As for the metabolite 11-NCTHC, it is a no longer active secondary metabolite of THC, which forms in the body through the oxidation of the still psychoactive metabolite of THC, 11-HTHC, by liver enzymes [55].

Structural Differences between Psychotropic and Not-Psychotropic PCs

It has been reported that the n-pentyl chain at the C-(3) position (Figure 2) works and essential role in the activity of psychotropic THC derivatives and that modification in this side chain leads to critical changes in the affinity, selectivity and pharmaco-potency of these ligands relating to the CB1 and CB2 cannabinoid receptors. Generally, while a shorter alkyl chain reduces the affinity of the compound for the cannabinoid receptor, an increase in the number of carbon atoms (hexyl, heptyl, or octyl) leads to an increase affinity for the same cannabinoid receptor ^[56][57][56,57]. Additionally, a number of other transformations in the tricyclic core of the THC cannabinoid structure have been carried out [58]. Particularly, the pyran ring-opening generally causes in the achieved compound a relative reduction in the affinity to the CB1/CB2 cannabinoid receptors, and in the psycho activity. In this regard, the absence of the tricyclic core in CBC, CBD and CBG for CB2 receptors, could be responsible for the for their higher affinity for CB2 receptors dealing with beneficial pharmacological properties, thus not exerting psychotropic effects [59].

2.2. Synthetic Cannabinoids (SCs)

The third group of cannabinoids consists of synthetic analogs of both ECs and PCs groups, appositely designed by scientists in the field, to enhance the benefits and therapeutic properties of ECs and PCs, while reducing the psychotropic and adverse effects. Among others, they include the compounds reported as examples in Figure 6 (chemical structures), and Table 3 (pharmacological properties and selectivity for receptors CB1 and CB2), which have demonstrated to be promising for treating severe humans’ chronic diseases including breast and prostate tumors, the unpleasant side-effects of chemotherapy, and chronic pain [42].

Omitting to comment on each strategy reported in Table 1, and instead focusing on the development of alternative antibiotics, it can be observed that cannabinoids, better known for many other pharmacological and psychotropic effects are included in this category. Particularly, cannabinoids are prenylated polyketides produced in Cannabis plants and particularly in Cannabis sativa, which is an herbaceous plant that has been used for millennia for both medicinal and recreational purposes. C. sativa possesses a plethora of pharmacological properties and mind-altering effects, largely due to its content in cannabinoids, more precisely phytocannabinois (PCs), given their vegetable origin [28]. Collectively, more than 1600 chemical compounds have been isolated from C. sativa, of which over 500 are phytochemicals including cannabinoids, flavonoids terpenoids and sterols [28]. Among phytochemicals, more than 180 are cannabinoids, about 125 have been isolated, that can be classified into 11 structural families ^[28][29][28,29]. The most abundant representatives of these families are Δ⁹-tetrahydrocannabinol (Δ⁹-THC, also the main psychoactive cannabinoid), cannabidiol (CBD), and cannabichromene (CBC). Additionally, other classes whose prototypes are Δ⁸-E-tetrahydrocannabinol (Δ⁸-THC), cannabigerol (CBG), cannabinodiol (CBND), cannabielsoin (CBE), cannabicyclol (CBL), cannabinol (CBN), cannabitriol (CBT), and a miscellaneous group have been identified ^[28][29][28,29]. Currently, despite its psychotropic effects, Δ⁹- THC is used as therapeutic agent in the treatment of chemotherapy-associated nausea and vomiting, AIDS related loss of appetite, as well as pain and muscle spasms in multiple sclerosis [30]. Also, its carboxylic acid precursor, THCA, not exerting psycho-active effects in humans, is currently examined for its immunomodulatory, anti-inflammatory, neuroprotective and anti-neoplastic effects as well for its effectiveness in reducing adiposity and preventing metabolic disease caused by diet-induced obesity [31]. CBD, non-psychotropic as well, is currently investigated for application in the treatment of Alzheimer’s disease, Parkinson’s disease, epilepsy, cancer and for its neuroprotective efficacy [32]. Although the most studied cannabinoids for medicinal purposes are CBD and Δ⁹-THC, nowadays the research focus moves increasingly towards other PCs, such as the not psychoactive CBC, currently investigated for its anti-inflammatory, anti-fungal, antibiotic and analgesic effects [30], CBG and cannabigerolic acid (CBGA), which is the precursor of the decarboxylated CBG and could be considered as the “mother of all cannabinoids” (see later). Particularly, CBG has many putative benefits ranging from anti-inflammatory action to pain reliever [33]. Among other more investigated therapeutic properties, PCs including Δ⁹-THC, Δ⁸-THC, CBD, CBN, CBG, and CBC and some their correspondent carboxylic acids have shown from moderate to potent antimicrobial properties mainly against Gram-positive bacteria (MICs 0.5–8 µg/mL), and especially against strains of S. aureus, including MRSA, EMRSA, as well as fluoroquinolone and tetracycline-resistant strains, [34]. Particularly, even if the precise mechanisms used by PCs remains unknown so far, recent investigations have revealed that PCs inhibits bacteria by injuring their cytoplasmic membrane ^[35][36][35,36]. Recently, Luz-Veiga et al. have reported the antibacterial activity of both CBD and CBG, being CBG the most potent compound, and their capability to inhibit Staphylococci adherence to keratinocytes without compromising skin microbiota, thus being very promising as antibacterial agents to treat skin infection by topical administration [37]. Blaskovich et al., in addition to confirm the antibacterial activity of CBD on Gram-positive pathogens, including highly resistant S. aureus, S. pneumoniae, and Clostridioides difficile, demonstrated that CBD has excellent activity against biofilms, little propensity to induce resistance, and topical in vivo efficacy [38]. Moreover, the authors reported that CBD can selectively kill a subset of Gram-negative bacteria that includes the ‘urgent threat’ pathogen Neisseria gonorrhoeae [38]. Additionally, the interaction of CBD with broad-spectrum antibiotics such as ampicillin, kanamycin, and polymyxin B was studied by Gildea et al. [39]. By disrupting membrane integrity at extremely low dosages, CBD-antibiotic co-therapy showed synergistic activity against Salmonella typhimurium, offering an intriguing alternative in the treatment of this clinically relevant bacterium. The impressively strong antibacterial activity against MRSA of CBG has been reported by Farha et al. in the year 2020 [33]. Even in comparison with standard therapy with vancomycin, CBG outcompetes classical approaches against MRSA. Additionally, CBG demonstrated to inhibit the capability of MRSA to generate de novo biofilm, showed to succeed in disaggregating the pre-formed biofilm, to kill rapidly stationary phase cells (persisters), and to effectively inhibit MRSA also in vivo, in a murine model. The authors speculated that C. sativa may produce PCs as a natural defense mechanism against pathogens and suggested PCs as a new compound class serving as novel antibiotic drug [33].

Unfortunately, since in C. sativa, CBGA is promptly and directly converted to CBDA and THCA, leaving no CBGA pool available to form CBG, the CBG levels in plants are exceptionally low. In this context, it has been suggested that a possible strategy to increase the CBG yield from hemp biomass could consist in harvesting much earlier in the ripening phase of the plants before the other cannabinoids are formed and detract the CBGA from the cannabinoid pool [40]. On the other hand, having available reliable synthetic procedures to prepare natural PCs would consent the accessibility to considerable quantities of CBG, as well as of other microbiologically promising minor cannabinoids, unlikely provided naturally by Cannabis plants, thus allowing further studies finalized to the development of novel PCs-based antibiotics.

2. Phytocannabinoids (PCs), Endocannabinoids (ECs) and Synthetic Cannabinoids (SCs)

2.1. Phytocannabinoids (PCs) and Endocannabinoids (ECs)

Generally, the term ‘cannabinoids’ refers to a heterogeneous family of compounds that exhibit activity upon particular human cannabinoid receptors, namely CB1 and CB2 ^[41][42][41,42]. They encompass the natural compounds present in the Cannabis plants, lipid mediators called ECs naturally produced by human cells, as well as by all vertebrates on planet Earth, and the synthetic analogs of both groups designed by scientist, called SCs [42]. Natural cannabinoids from Cannabis are more specifically called PCs referring to their original plant source, differently from ECs which are produced from human cells ^[43][44][43,44]. PCs and ECs could include compounds structurally very different both between the two families and inside the same class, as shown in Figure 2 and Figure 3, which report the structure of the most relevant PCs and ECs, respectively. Both PCs and ECs exert their effects by interacting with CB1 and CB2 receptors, found throughout the human body, and whose locations have been listed in Table 2. Table 2 has constructed using the valuable information contained in the relevant work by Fraguas-Sánchez et al. [45].