Diabetes is a global disease. The number of patients with diabetes in 2000 was higher than 130 million and the number is expected to exceed 360 million in 2030¹. One of each four patients with diabetes is at risk of having diabetic foot ulcer at sometime in his life².Diabetic foot infection (DFI) can be defined as a clinical disease manifested as local inflammation or purulence with or without systemic manifestations of sepsis that take place in a site below the malleoli in a patient with diabetes ³.

Received on 08.03.2015 Modified on 30.03.2015

Research J. Pharm. and Tech. 8(5): May, 2015; Page 575-579

DOI: 10.5958/0974-360X.2015.00096.7

Diabetic foot ulcers may lead to gangrene and amputation ⁴. Diabetes-related amputation is the most common among other non-traumatic amputation, representing about 60% of amputations. Moreover, it dramatically increases the morbidity, mortality and cost of treatment ^{5, 6}.

Causes of diabetic foot ulcers

The factors that can lead to diabetic foot ulcers include peripheral neuropathy and reduced peripheral blood supply ⁷. The neuropathy has autonomic, motor and sensory manifestations ⁷. Foot deformities may result from damage of the innervations of the foot muscles and lead to abnormal bone structure with a final outcome of skin breakdown and ulceration⁷. Autonomic neuropathy interferes with sweat and sebaceous glands secretions that may lead to dryness of skin and susceptibility to infection⁷.Sensory neuropathy allows minor wounds to be unnoticed, and as a result these wounds may progress into infected ulcers and even to gangrene⁷.Neuropathy was found to be due to hyperglycemia and its resulting metabolic abnormalities ^8-10. Hyperglycemia augments aldosereductase and sorbitol dehydrogenase enzymes that convert glucose into sorbitol and fructose. When these sugars increase in concentration, the synthesis of nerve cell myoinositol, a substance needed for normal neuronal conductivity is reduced^8-10. Moreover, nicotinamide adenine dinucleotide phosphate (NADP) stores are exhausted due to chemical conversion of glucose. NADP is essential for removal of reactive species and the formation of nitric oxide that acts as a vasodilator. As a consequence of NADP exhaustion, reactive oxygen species accumulate and vasoconstriction occurs leading to ischemia, nerve cell injury and death. Other reasons for interference with nerve supply and ischemia are the glycation of nerve cell proteins and abnormal activation of protein kinase C^8-10.

Another factor that plays a vital role in development of diabetic foot ulcers is peripheral arterial disease^{11, 12}.The tibial and peroneal arteries of the calf are often affected. Hyperglycemia leads to interference with the function of endothelial cells and abnormalities of smooth cells of peripheral arteries⁸. As a result, endothelial vasodilators decrease leading to vasoconstriction. Moreover, thromboxane A2 vasoconstrictor and platelet aggregation agonist increase and this enhances the risk of plasma hypercoagulability¹³. These factors, altogether, lead to occlusive arterial disease and ischemia in the lower extremity. Micro- and macrovascular ischemia would hinder the delivery of oxygen, nutrients, and antibiotics to the ulceration site. As a result, tissue breakdown and ulceration occurs, giving access to pathogenic bacteria^14-16.

Classification systems of diabetic foot ulcers

Diabetic foot ulcers can be classified according to its characteristics such as the size of the ulcer in addition to its depth, location and appearance¹⁷. Two common classification systems are the Wagner Ulcer Classification System and The University of Texas system (Tables 1 and 2). The Wagner Ulcer Classification System depends on the depth of the wound and the degree of tissue necrosis, while The University of Texas System relies on the ulcer depth in addition to the presence of infection and ischemia, giving it an advantage over the Wagner Ulcer Classification^18-20.

Table 1. Wagner Ulcer Classification System

Grade of ulcer	Lesion description
1	Superficial ulcer
2	Deep ulcer affecting ligament, tendon, joint capsule or fascia (no abscess or osteomyelitis)
3	Deep ulcer with abscess or osteomyelitis
4	Localized gangrene (part of the forefoot)
5	Disseminated gangrene

Table 2. The University of Texas Wound Classification System

Stage of ulcer	Wound description
A	No infection or ischemia
B	Infection present
C	Ischemia present
D	Both infection and ischemia are present
Grade of ulcer	Wound description
0	Epithelized
1	Superficial
2	Deep to tendon or capsule
3	Deep to bone or joint

Microbial etiology of diabetic foot ulcer

The microbial etiology of diabetic foot infection is complex. In acute infections, especially in patients who have no recent antibiotic therapy, the common pathogens are aerobic Gram-positive cocci, mainly Staphylococcus aureus. β-hemolytic streptococci (usually group B) or coagulase-negative staphylococci are also present but with lower frequency ²¹. On the other hand, the infection in chronic wounds is polymicrobial, especially in patients who received antibiotic therapy. The predominant pathogens are aerobic Gram-negative (e.g. Escherichia coli, Proteus, Klebsiella), and obligate anaerobic bacteria (e.g. Finegoldia, Bacteroides). In subtropical, less-developed countries S. aureus was found to be less common than reported in developed countries, while Gram-negative rods, especially Pseudomonas aeruginosa exhibited higher prevalence ^22-24.

The emergence of multidrug resistant organisms (MDROs) is another complicated problem in case of DFIs. Methicillin-resistant S. aureus (MRSA) and Gram-negative organisms that produce extended-spectrum β-lactamases-(ESBLs) or carbapenemases are common ^25-27.

Biofilm formation further complicates the treatment of chronic wounds such as diabetic foot infections as it delays wound healing ^28-30. Biofilms are communities of sessile microbial cells that are anchored to a biological or a nonbiological surface and enclosed within a hydrated matrix composed of extracellular polymeric substances. The existence of bacteria as biofilm communities in chronic wounds complicates the treatment of chronic wounds such as diabetic foot infections due to their extraordinary resistance to antimicrobial therapy ^29,
31-33.

The biofilm resistance to antimicrobials is multifactorial. One factor is the increased rate of genetic transfer due to the close proximity of cells which enables plasmid transfer. Plasmids can carry genes of resistance of many antimicrobial agents ³⁴. Quorum sensing is another resistance mechanism. Quorum sensing is a mechanism of cell-to-cell communication. The bacterial cells secrete signaling molecules or autoinducers, whose concentration reflexes the number of bacterial cells. When the number of cells reaches the quorum, they begin to form biofilm ^35-37. Quorum sensing makes the bacteria to live into a slow-growing state when faced by adverse growth conditions. The slow growing state makes bacteria less sensitive to antimicrobials ³⁷. Moreover, the biofilm matrix is a permeability barrier that delays the access of antimicrobial agents into the biofilm-embedded cells ³⁴.

The biofilm also confers resistance to the immune system. They exert an antiphagocytic activity ³⁸. The polysaccharide matrix sequesters and interferes with the activity of complement and hostantibodies ³¹.

Treatment strategies of diabetic foot ulcers

Both medical and surgical treatments are necessary for management of diabetic foot infections. Surgical intervention is required to deal with necrotic or unhealthy tissues¹⁶. The benefits of surgical debridement include decreasing pressure points at foot sites with calluses. Removal of the calluses surrounding the wounds can reduce the plantar pressure and also the colonizing bacteria can be eradicated¹⁶. Moreover, it is of value in specimen collection for culture and investigating if the deep tissues are affected by the ulcer or not ¹⁶. Offloading aims to relieve pressure at ulcer locations and points at risk by use of half shoes, wheelchairs and crutches ³⁹.

Another important factor is the underlying ischemia. Proper wound healing and combating infections needs sufficient arterial blood supply. In cases of reduced distal blood supply or ulcer unresponsive to healing, arterial revascularization may be performed ⁴⁰.

Empirical antimicrobial therapy is the initial therapy used. It should be selected according to the clinical features, the local antibiotic resistance profiles and also the infection severity ⁴¹. S. aureus, especially methicillin resistant staphylococcus aureus (MRSA), as the predominant bacteria infecting diabetic foot ulcers, should be targeted. In severe and most moderate infections, Gram-negative bacteria should be covered. This is also the case for patients who do not respond to narrow spectrum antibiotic therapy. Obligate anaerobes should be covered by antibiotics in case of gangrenous or foul smelling wounds ⁴¹.

The peripheral arterial diseases accompanying most DFIs would reduce the penetration of antibiotics to the affected tissues. This was reported in most β-lactam antibiotics. On the other hand, clindamycin, fluoroquinolones, linezolid and rifampin were found to have good oral bioavailability and can penetrate well in bone, synovia, biofilm and necrotic tissue ^{42, 43}.

A wound culture is necessary to choose the antimicrobial treatment of the diabetic foot ulcer if infection is present or suspected. A superficial wound swab is not preferred and tissue curettage from the base of the ulcer following debridement is more accurate. To have ideal culture results in case of deep tissue infections, the specimens should be obtained during surgery under aseptic conditions ⁴⁴.

The severity of infections controls the choice of antimicrobial treatment and the clinical signs such as suppurative drainage, inflammation manifested as redness, hotness, edema and pain or systemic signs including fever and leukocytosis are to be considered. Inpatient care is recommended when the systemic signs appear, where supportive care and intravenous antibiotic therapy are to be provided ⁴⁵. Also, inpatient care is required when the patients are not capable of good self-care or do not respond to antibiotic therapy or when strict monitoring is needed ⁴⁴. On the other hand, outpatient therapy can be applied in the absence of serious systemic signs, but medical follow-up should be frequent in such a case ⁴⁶. In case of osteomyelitis, surgical excision of the affected bone can be performed. Instead, an extensive antibiotic course can be used⁴⁷.

The choice of wound dressing is another important factor in treating diabetic foot ulcers. There is no wound dressing of choice⁴⁸ and the wound type affects the selection of the dressing. However, a good wound dressing should provide a moist wound environment and has the ability to absorb excessive exudates ⁴⁹.

Some new treatments are now under trials. One of them is human skin equivalent that enhance tissue growth and wound healing through the employment of cytokines and dermal matrix components^{50, 51}. Other famous adjunctive treatments that are currently used are hyperbaric oxygen therapy (HBOT) and granulocyte colony stimulating factors (G-CSF). HBOT is a therapeutic strategy that uses oxygen at pressure higher than normal atmospheric pressures to increase the oxygen concentration in the blood and its diffusion capacity to the tissues. As a result, the oxygen pressure in the tissues increases and this leads to stimulation of vascularization and fibroblast replication. Moreover, phagocytosis and killing of wound pathogens are enhanced^{52, 53}. Furthermore, HBOT lowers the risk of amputation and decreases the bacterial load by interfering with the growth of anaerobes ⁵⁴, ⁵⁵. G-CSF activates neutrophils in patients with diabetes and decreases the need for surgical intervention⁵⁶.

The strategies that can be employed to treat biofilm-based diabetic foot ulcers include inhibition of bacterial adhesion, disruption of biofilm matrix and quorum sensing inhibition⁵⁷.

Lactoferrin, an iron transporter agent, is a glycoprotein present in milk and exocrine secretions and secreted from degranulated neutrophils. It possesses antimicrobial and anti-inflammatory properties⁵⁸. It was reported to inhibit biofilms of Pseudomonas aeruginosa, S. aureus and S. epidermidis⁵⁹. It acts by chelating iron. Low iron concentration will promote twitching of bacterial cells preventing their adhesion⁶⁰. Lactoferrin may be effective as a topical treatment of diabetic foot infections.

Debridement of necrotic tissues in diabetic foot ulcers can remove the colonizing bacteria and biofilm. Electrical stimulation is another physical treatment that enhances the access of topical agents. Electroporation could promote the penetration of a photosensitizer. Electrical currents can disrupt biofilms and deliver topical agents through the biofilm matrix⁶¹.

Inhibition of quorum sensing is beneficial in treatment of biofilm infections because quorum sensing controls the virulence of bacteria. The interference with quorum sensing will reduce the pathogenicity of the bacteria. The agents that can disperse biofilm cells will facilitate the eradication of these cells which will revert to the planktonic state ^{62, 63}.

CONFLICT OF INTERESTS:

The authors declare that there is no conflict of interests regarding the publication of this paper.

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