More than 415 million people throughout the world are diagnosed with diabetes, and that number is expected to climb to 640 million (1 in 10) by the year 2040, according to the International Diabetes Federation’s 2015 study (IDF 2015). A further 12 percent of global health budgets are allocated to the treatment of people with diabetes (USD 673 billion) [1,2]^[1][2]. People with diabetes are more likely to suffer from skin wounds, particularly chronic ulcers, due to neuropathy (nerve damage) and arterial (blood vessel) disease or trauma. Peripheral neuropathy (nerve dysfunction in the feet) and peripheral artery disease (both) are common in persons with diabetes. People with diabetes have immune system impairments that have yet to be discovered, limiting their ability to avoid or treat illnesses. Foot ulcers are a common complication in people with diabetes because they are more likely to develop in persons with the disease ^[3]. It is estimated that a person with diabetes’ lifetime risk of developing a foot ulcer is 25%, with an uninfected ulcer costing EUR 10,000 and an untreated ischemic ulcer costing EUR 17,000 in 2008 ^[4]. When these wounds become clinically infected, they cause a large amount of morbidity. A person with diabetes is amputated of a lower limb every 20 s, on average, according to worldwide statistics. When at least two typical signs or symptoms of inflammation (pain or tenderness, warmth, redness, and swelling) or purulent discharges appear in a diabetic foot ulcer, an infection has occurred (pus) ^[5]. Patients with diabetes now spend more time in the hospital due to foot issues than any other diabetic complication. In patients with diabetes, diabetic foot infections, particularly those that extend to the bone, are the primary cause of lower-extremity amputation, which results in an increased risk of mortality and a higher cost burden ^[6]. To avoid these bad outcomes, it is essential to prevent foot infections or, if that is not possible, to take care of wounds that have not been treated. There are a lot of methods to provide antimicrobial therapy: intravenous injections, injections into muscles, and other means. One of the most popular kinds of antibiotic treatment is to administer the drugs topically, in other words, locally. Even if the patient has neuropathy or vascular diseases, it is frequently difficult to tell whether a diabetic foot ulcer is infected. Furthermore, even in clinically uninfected wounds, the sheer presence of microorganisms might delay wound healing, especially if they are pathogenic or present in huge numbers ^[7]. Some doctors believe that antibiotics (especially topical ones) may effectively treat high-risk wounds that are clinically uninfected [8,9]^[8][9].

DFU treatments should follow a multidisciplinary approach that uses various diagnostic tools, is performed by various specialists, and requires years of experience in treating the condition. Patients must be educated to prevent amputations, and new categories must be used to guide treatment [10,11]^[10][11]. To learn more about DFU microbiota, it will be required to apply cutting-edge diagnostic tools such as the 16S ribosomal DNA sequence in bacteria. In addition to wound characteristics, local epidemiology-based antibiograms, personalized treatment, regular debridement, periodic wound assessment, and dressing changes, DFU is said to have a range of distinctive properties ^[12]. Infection prevention, local inflammation management, and cicatrizing efficiency may all be improved by bio-molecular therapy and many other characteristics of the human body.

2. Diabetic Foot Ulcers (DFUs)

DFUs, which are usually skin ulcers that progress across the entire lower limb with various degrees of peripheral vasculopathy and neuropathy, morbidity, disease, death, and psychosocial distress. Osteomyelitis and gangrene also accompany DFU. With extreme DFU, amputation of a significant leg is often used to manage long-term recurrence [14,15]^[13][14]. That is why there are so many various categorization systems for when a foot ulcer responds to therapy. As of yet, it has not been proven to be a commercial success. Therapeutic data-recording devices are mostly a matter of convenience, rather than clinical or theoretical usefulness, for most people with diabetes [16]^[15]. In order to determine the severity of an ulcer, the presence of osteomyelitis or gangrene, and the need for an amputation, the Wagner ulcer classification system uses the following criteria: Wagner grade 0: intact skin; Wagner grade I: superficial ulcer of skin or subcutaneous tissue; Wagner grade II: ulcers extend into tendon, bone, or capsule; Wagner grade III: deep ulcer with osteomyelitis or abscess; Wagner grade IV: partial foot gangrene; and Wagner grade V: whole foot gangrene [17]^[16]. An amputation is now required in 90 percent of patients with diabetic foot ulcers with Wagner grade III or above. Approximately 45 percent of patients with diabetic foot ulcers in China have a Wagner grade of III or above, with amputation rates from 18 to 28 percent, according to a nationwide study. Patients with DFU had mortality rates of 11% or higher. The 5-year mortality rate of DFU in Tianjin, China, was found to be 32.7 percent. In the United States, the cost of treating DFU in 2017 was USD 727 billion, while in China, it was USD 110 billion [17,18]^[16][17]. Including the fact that endovascular operations and vascular bypass surgery are the recommended treatments for ischemia foot ulcers, 40% of patients with DFU and serious limb ischemia may not follow the criteria. Consequently, amputation is often considered the safest choice for many patients with DFU. In the five years after amputation, the death rate was around 25–50 percent. Traditional therapy has recurrence rates of 40 percent after one year, 60 percent after three years, and 65 percent after five years. Because of this, new treatments are urgently needed to improve DFU healing and limb preservation rates [19]^[18]. All patients with osteomyelitis must have their DFUs discarded. If a bone sample is indicated in the case of a suspected fracture, C-reactive protein (CRP), ankle–brachial index (ABI), and X-ray/MRI imaging should all be performed. Primary care settings are constrained in their ability to conduct regular health evaluations due to the lack of time available for foot inspections. Neuropathy; peripheral artery disease (PAD); immune system variables; and in certain instances, recurring external or mild damage are among the risk factors for diabetes (which lead to skin breakdown and ultimately to the development of infection). Toe deformities (such bunions and hammertoes) are also considered risk factors since they may produce trigger points on the foot (potential locations for ulceration). Patients with neuropathy are thought to have more mechanical pain than people with diabetes without the disease. Inflammation is the most common cause of amputation, which occurs in people with severe diseases, further tissue loss, and organ failure across the body. Patients with anemia (a hemoglobin level below 11 μg/dL), those who are older, and those who suffer from PAD are at greater risk of infection and, as a result, of amputations [20,21]^[19][20]. Diabetic neuropathy and PAD are the major causes of DFUs, with trauma acting as a starting trigger. At various points in the healing process, both of these factors contribute to the development of ulcers.

3.1. Diabetic Neuropathy

Neuropathy in the sensitive, motor, and autonomous nerves is caused by oxidative stress in the nerve cells caused by hyperglycemia. When the hexosamine metabolic route is activated, it reduces the amount of aldose reductase and sorbitol dehydrogenase produced by the polyol metabolic pathway, which absorbs nicotinamide adenine dinucleotide phosphate (NADPH). These enzymes are responsible for the transformation of glucose into sorbitol and fructose [22]^[21]. Myoinositol synthesis in nerve cells reduces as these sugar products pile up, resulting in decreased neuronal conduction, increased levels of antioxidants such glutathione, and an increase in reactive oxygen species (ROS) formation [23]^[22]. In addition to the increased flow of hexosamine and polyol pathway, the altered development of substance P, nerve growth factor, and calcitonin gene-related peptide all lead to additional nerve damage and ischemia [24]^[23]. For example, when there is damage to motor neurons in the foot muscles, an imbalance in flexor and extender muscles might occur, resulting in anatomical deformity and skin ulcers. Skin breakdown may occur as a consequence of damage to the autonomic nervous system because of a decrease in sweat gland activity and an inability to moisturize the feet [25]^[24]. If peripheral sensation in the skin is reduced, it is possible that patients will be more cautious about acquiring foot wounds because the skin is less likely to contain intra-epidermal nerve fiber endings of the afferent A-delta and C-fibers, the majority of which are nociceptor nerve endings that are only stimulated by pain. Diabetes-related neuropathic illnesses, such as vitamin B12 insufficiency, alcohol toxicity, and renal failure towards the end of life, might exacerbate this condition. Epidemiological studies suggest that fat lipoproteins, high blood pressure, and smoking all have roles in the development of PAD. Charcot’s foot, the most well-known sign of motor neuropathy, is only one of several. It is crucial to keep in mind that the foot’s skin sheaths, tendons, and soft tissues make it vulnerable to infection (such as plantar aponeurosis and fascia) [26,27,28]^[25][26][27].

3.2. DFUs Pathogenesis: Immunological Involvement

The immune system of individuals with diabetes is characterized by a reduced healing response in DFUs. There are many examples, including T-lymphocyte apoptosis; proinflammatory cytokines; degradation of polymorphonuclear cell functions such as chemotaxis, adhesion, and intracellular killing; inhibition of fibrocyte proliferation; and impaired basal layer of keratinocytes with reduced migration of epidermal cells [27,28]^[26][27]. Bacteria, particularly aerobic Gram-positive cocci, such as Staphylococcus aureus (S. aureus) and hemolytic streptococci, flourish at high blood glucose levels. Carbohydrates, fibroblasts, and collagen synthesis are all affected by diabetes’ metabolic insufficiency as well as other structural inadequacies. Serum glucose concentrations more than or equal to 150 mL/dL were also considered indicative of immune system dysfunction. These traits are likely to lead to a long-term inflammatory disease [22,23]^[21][22].

3.3. PAD

Almost 80% of individuals with DFU already suffer from PAD [29]^[28]. When blood sugar levels are too high, they cause changes in the foot’s peripheral arteries, which begin at the cell level. The malfunction of endothelial cells is the most important aspect of microcirculation dysfunction. This is because endothelial cell dysfunction causes a decrease in the generation of vasodilators, most notably nitric oxide. Persistent vasoconstriction and hypercoagulation increase plasma thromboxane A2 levels, increasing the risk of ischemia and ulceration [30]^[29]. It is possible that the endothelium will show signs of reduced local angiogenesis, endocrine cell proliferation, basement membrane thickness, blood viscosity, changes in microvascular sound, and antioxidant potential. It might also show signs of reduced smooth muscle cell proliferation [31]^[30].