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Infection inDiabetes: Epidemiology,
https://t.me/med1917
Immune Dysfunctions, andTherapeutics
RuchiRoy, RajSingh, andSashaH.Shakhani
17
Abstract
Diabetes is a chronic metabolic disorder that affects approximately 10% of the global population. Unfortunately, diabetic individuals are also at a high risk of developing diabetic foot ulcers (DFUs), with an esti­mated 19–34% of individuals affected at some point in their lives. These DFUs are a leading cause of lower extremity amputations (LEAs), responsible for 60–70% of all cases, and have a 5-year mortality rate of approxi­mately 49%. The mortality rate associated with DFUs is even more alarming when compared to that of all cancers, which is approximately 31%. The economic burden of DFUs is also staggering, with a global estimated cost of $78.2 billion USD. The incidence of DFUs is further compounded by the fact that 50–60% of cases develop infections, increasing the risk of amputation by 50% com­pared to patients with uninfected DFUs. This chapter aims to provide a comprehensive overview of infections in diabetes, with a particular focus on infections in DFUs. We will explore the microbiome shift toward pathogenic bacteria in DFU, and how this shift impacts healing out­comes. Additionally, we will examine various factors that make diabetic patients prone to infections, including dys­regulations and dysfunctions in the innate immune sys-
Ruchi Roy and Raj Singh contributed equally with all other contributors.
R. Roy UICentre for Drug Discovery, College of Pharmacy, University of Illinois at Chicago, Chicago, IL, USA
R. Singh Department of Medicine, Rush University Medical Center, Chicago, IL, USA
S. H. Shakhani (*) Department of Medicine, Rush University Medical Center, Chicago, IL, USA
Cancer Center, Rush University Medical Center, Chicago, IL, USA e-mail: Sasha_Shakhani@rush.edu
tem. Finally, we will review the conventional, unconventional, and emerging therapeutic options avail­able to address infections in DFUs. By providing an in­depth understanding of the challenges associated with infections in diabetes, this chapter aims to contribute to the development of more effective treatment strategies that can help reduce the burden of DFUs on individuals and society as a whole.
Abbreviations
ADCC Antibody-dependent cell-mediated
cytotoxicity AGEs Advanced glycation end products AgNPs Silver nanoparticles AGP α1-Acid glycoprotein AMPs Antimicrobial peptides BJIs Bone and joint infections CAMs Cell adhesion molecules CCL2 Chemokine (C-C motif) ligand 2 CCL3 Chemokine (C-C motif) ligand 3 CCR1 C-C motif chemokine receptor 1 CD14 Cluster of differentiation 14 CDI Clostridium difcile infection CFU Colony-forming unit CI Condence intervals COVID-19 Coronavirus disease 2019 CXCR2 CXC chemokine receptor 2 DAMPs Damage-associated molecular patterns DEFB4A Defensin Beta 4A DFUs Diabetic foot ulcers DM Diabetes mellitus ECM Extracellular matrix eDNA Extracellular DNA ENT Ear, nose, and throat EPO Erythropoietin fMLF fMet-Leu-Phe FOXM1 Forkhead box protein M1
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 A. Veves et al. (eds.), The Diabetic Foot, Contemporary Diabetes, https://doi.org/10.1007/978-3-031-55715-6_17
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FPR Formyl peptide receptor G-CSF Granulocyte colony-stimulating factor GI Gastrointestinal GM-CSF Granulocyte-macrophage colony- stimulating
factor GPRD General Practice Research Database HbA1c Hemoglobin A1c HBD Human β-defensin HBOT Hyperbaric oxygen therapy HCV Hepatitis C virus HICs High-income countries HMGB1 High mobility group box 1 HNP Human neutrophil peptide HSP Heat shock protein ICAM-1 Intercellular adhesion molecule 1 ICU Intensive care unit IDF International Diabetes Federation IFN-γ Interferon gamma IL-10 Interleukin-10 IL-1β Interleukin-1β IRAK-1 Interleukin 1 receptor-associated kinase 1 IRRs Incidence rate ratios LEAs Lower-extremity amputations LPS Lipopolysaccharide MARCO Macrophage receptor with collagenous
structure MDT Maggot debridement therapy MMPs Matrix metalloproteinases MPO Myeloperoxidase MyD88 Myeloid differentiation factor 88 NETs Neutrophil extracellular traps NF-κB Nuclear factor kappa B NIDDK National Institute of Diabetes and Kidney
Disease NK Natural killer NLRP3 Nucleotide-binding domain, leucine-rich-
containing family, pyrin domain-containing-3 NLRs Nucleotide oligomerization domain (NOD)-
like receptors NO Nitric oxide OR Odds ratio PAMPs Pathogen-associated molecular pattern
molecules PARP1 Poly [ADP-ribose] polymerase 1 Phox Phagocyte NADPH oxidase PI Predictive interval PMN Polymorphonuclear leukocytes PRRs Pattern recognition receptors RNASE7 Ribonuclease 7 RNS Reactive nitrogen species ROS Reactive oxygen species RR Relative risk SSIs Surgical site infections
SSTI Skin and soft tissue infections STAT1 Signal transducer and activator of transcription
STAT3 Signal transducer and activator of transcription
STZ Streptozotocin T1D Type 1 diabetes T2D Type 2 diabetes TDM Trehalose 6,6-dimycolate TLRs Toll-like receptors TNF-α Tumor necrosis factor-α TRAF6 Tumor necrosis factor receptor-associated fac-
U/LMICs Upper-middle and lower-middle income
UTIs Urinary tract infections VCAM-1 Vascular cell adhesion molecule 1 VEGFR2 Vascular endothelial growth factor receptor 2 WHO World Health Organization
Introduction
Diabetes is a group of metabolic disorders characterized by chronic hyperglycemia resulting from insufcient insulin production, impaired cellular response to extracellular insu­lin, and/or impaired glucose metabolism [1, 2]. Diabetes has negative impacts on various organs that can lead to severe and life-threatening complications, such as cardiovascular diseases, neuropathy, retinopathy, nephropathy, and hearing impairment [3, 4]. Diabetes also increases the risk of certain cancers and is a known risk factor for mental health disor­ders, such as Alzheimer’s disease [5, 6]. Moreover, diabetes signicantly increases the susceptibility to infections of any origin, including bacterial, fungal, and viral infections, and infections, in turn, cause considerable morbidity and mortal­ity among diabetic patients [712].
(DFUs) and they can lead to severe complications, including the risk of amputation and even death. Approximately 50–60% of DFUs become infected, and this risk increases with the duration of the ulcer, severity of the neuropathy, and the presence of peripheral vascular disease [712]. Infection in DFUs also increases the risk of hospitalization and con­tributes to the signicant healthcare burden associated with managing these wounds [712].
bone infection that requires aggressive treatment, including surgical debridement and prolonged antibiotic therapy. Osteomyelitis can progress to limb amputation in severe cases, which is a devastating outcome for patients and has signicant social and economic implications [13].
1
3
tor 6
countries
Infections have a signicant impact on diabetic foot ulcers
Infection in DFUs can lead to osteomyelitis, a severe
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The presence of bacteria in DFUs leads to the activation of the host immune response, which results in the release of cytokines, inammatory mediators, and enzymes that impair tissue healing [14, 15]. Moreover, the presence of bacteria in DFUs can lead to biolm formation, a complex microbial community that forms a protective barrier against the host’s immune system and antimicrobial therapies [16, 17].
Effective management of DFUinfections is critical in preventing the progression of these wounds and reducing the risk of amputation [18]. Proper wound care, including appro­priate wound dressings, wound debridement, and antimicro­bial therapy, is necessary for managing DFU-related infections [18]. Additionally, addressing underlying risk fac­tors, such as controlling blood glucose levels, managing co­morbidities, and promoting healthy lifestyles, can reduce the risk of infections in DFUs [18].
In this chapter, we provide an overview of the incidence of diabetic foot ulcer infections. We next discuss different types of infections in diabetic patients while focusing on DFU infections and their impact on healing outcomes in DFUs. We then review the dysregulations and dysfunctions in innate immune system that render DFUs vulnerable to infections. We conclude the chapter by discussing the con­ventional, unconventional, and emerging therapeutics to address infection in DFUs.
Infection inDiabetes
Diabetes signicantly increasesthe risk to infection, regard­less of the origin of infective microorganism (bacterial, fun­gal, or viral); and infection in turn causes considerable morbidity and mortality in diabetic patients [712]. A recent large study conducted in South Korea assessed diabetes as a risk factor for infection, involving 66,426 diabetes patients and 132,852 age-sex-region-matched non-diabetes controls from the general population. The study found that people with diabetes had signicantly higher incidence rates of infection-related hospitalizations, intensive care unit (ICU) admissions, and deaths than the general population [19]. Another large retrospective study of primary care patients found that type 1 and type 2 diabetes accounted for nearly 6% of infection-associated hospitalizations and 12% of infection-associated mortalities [20].
It is worth noting that no organ in a diabetic patient is spared from infection. The most common sites of infection in diabetic patients are the skin, soft tissues, urinary tract, and respiratory tract, but the infections with highest mortality rates in diabetic patients include bone and joint infections, bloodstream infections and sepsis, central nervous system infections, diabetic foot ulcers, and cellulitis [7, 2023].
Interestingly, infection has also been identied as a risk factor for development of insulin resistance and diabetes. For
example, Norovirus infection has been linked to the develop­ment of type 1 diabetes by increasing the Firmicutes/ Bacteroidetes ratio and promoting the α-diversity in the gut microbiome [24]. Similarly, a meta-analysis involving 34 studies found a signicantly higher risk for type 2 diabetes in hepatitis C virus (HCV) patients as compared to matched controls, and patients with other forms of chronic liver dis­ease [25]. Another study found an estimated 33% of non­cirrhotic chronic HCV patients to also be diabetic as compared to only 5.6% in the control group without HCV and liver disease [26], although another study disputed these ndings and found a negative association between HCV and diabetes in patients with liver cirrhosis [27]. A possible mechanism by which viral infections can increase the risk of diabetes development is through virally-induced insulin resistance due to down-regulation of insulin receptor by virally induced IFN-γ productionin skeletal muscle [28].
Below, we review the common infections in diabetic patients before discussing infections in diabetic foot ulcers. A summary of these infections can be found in Table17.1.
Table 17.1 Important infections in diabetes
Infection type Prevalent causes of infection References Ear, nose, and
throat (ENT) infections Respiratory tract infections
Urinary tract infections (UTI) and pyelonephritis
Surgical site infections Bone and joint infections
Bloodstream infections Oral infections Candida albicans [7175] Gastrointestinal infections
Pseudomonas aeruginosa, fungal species (Absidia, Mucor, Rhizomucor, and Rhizopus genera)
Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis, Pseudomonas aeruginosa, fungi,
inuenza virus, COVID-19 Patients are more prone to have resistant pathogens: extended­spectrum β-lactamase-positive Enterobacteriaceae, e.g., Klebsiella spp., Proteus spp., Enterobacter spp., and Enterococci; uoroquinolone-resistant uropathogens, carbapenem-resistant Enterobacteriaceae, and vancomycin-resistant Enterococci,
Candida
Pyelonephritis causes further insulin resistance
Pseudomonas aeruginosa, Staphylococcus aureus Staphylococcus spp., aerobic Gram-negative rods, Neisseria gonorrhoeae, Pseudomonas aeruginosa, fungal, Klebsiella liver abscesses, malignant otitis externa, and emphysematous cholecystitis Escherichia coli, Klebsiella pneumoniae
Clostridium difcile, Clostridia perfringens, Escherichia coli
[3234,
36]
[10, 37,
38]
[46,
8691]
[5257,
59, 92]
[6165,
93]
[68]
[77, 78,
82, 84]
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Ear, Nose, andThroat Infections
Ear, nose, and throat (ENT) infections are more common in individuals with diabetes [2931]. Malignant otitis externa and Rhinocerebral mucormycosis are head-and-neck infec­tions seen almost exclusively in immunocompromised patients, particularly in diabetic individuals [32, 33]. Malignant or necrotizing otitis externa is a severe infection of the external auditory canal and skull base, which occurs in individuals with diabetes older than 35years, and it is almost always due to Pseudomonas aeruginosa [32, 33]. Infection starts in the external auditory canal and spreads to adjacent soft tissue, cartilage, and bone. Patients typically present with severe ear pain and otorrhea (ear discharge). Rhinocerebral mucormycosis is another serious infection caused by various fungal species (belonging to the Absidia, Mucor, Rhizomucor, and Rhizopus genera), which also occurs in poorly controlled diabetic patients [33, 34]. After initial colonization in the nose or paranasal sinuses, the organisms spread to adjacent tissues via the blood vessels, causing necrotic lesions in the soft tissues and bone erosion. Despite aggressive surgical treatment and antifungal therapy, the mortality rates associated with Rhinocerebral mucormy- cosis infections can be as high as 25–80% [35]. ENT infec- tions also tend to be more severe in individuals with diabetes. In a retrospective study involving patients with deep neck infections, abscess formation was found to occur at a signi­cantly higher rate in diabetic patients (89.3%) than in non­diabetic individuals (71.3%) [36]. Diabetic patients also required surgical drainage more frequently (86%) than non­diabetic patients (65.2%).
Respiratory Tract Infections
Respiratory tract infections are among the most severe infec­tions associated with diabetes [10]. The most frequent respi­ratory infections associated with diabetes are caused by Streptococcus pneumoniae and inuenza virus [37]. In some cases, diabetes increases the incidence rates (e.g., pulmonary infections caused by Mycobacterium tuberculosis, Staphylococcus aureus, P. aeruginosa, and fungi); whereas in other cases (e.g., COVID-19, S. pneumoniae, or inu­enza), diabetes also increases the severity of disease, mani­fested by higher morbidity and mortality rates [10, 38]. For example, diabetic patients with COVID-19 infection have a twofold increase in the risk for severe disease and related death when compared to non-diabetic patients with COVID- 19 [39]. Epidemiological evidence demonstrate an increase in the risk for hospitalization with pneumonia asso­ciated with diabetes [40]. In a population-based cohort study involving 29,900 patients, the 30- and 90-day mortality rates associated with pneumonia were found to be higher in dia-
betic patients than in non-diabetic patients, and admission hyperglycemia was found to be a predictive risk factor for pneumonia-associated death [41]. In another study, the risk of pneumonia-associated death correlated with the level of glycemic control with relative risk (RR) for those with HbA1c <7% being 1.22, versus an RR of 1.6 when HbA1c was 9% [42]. Interestingly, a recent meta-analysis study involving pooled analysis of 5,787,027 humansubjects from four observational studies reported a 59% higher risk of developing incident diabetes in post-acute COVID-19 phase as compared to healthy controls, suggesting a possible link between infection and prolonged diabetes [43].
Urinary Tract Infections
Diabetic patients are also at a signicantly elevated risk for urinary tract infections (UTIs), including asymptomatic and symptomatic pyuria and cystitis (signs of UTI), bacteriuria, and more serious upper urinary tract and kidney infections [44, 45]. In an observational study of all patients with type 2 diabetes in the UK General Practice Research Database (GPRD), the incidence rate of UTI was reported to be 56.9% higher among diabetic patients (46.9 per 1000 person-years) when compared to non-diabetic patients (29.9 per 1000 person- years) [46]. Diabetic patients also experience more severe UTIs [22]. Life-threatening complications of UTIs, such as emphysematous cystitis and pyelonephritis, renal papillary necrosis, and renal abscesses, occur more fre­quently in diabetic patients than in the non-diabetic general population [47, 48]. Pyelonephritis (kidney infection) makes control of diabetes more difcult as it may lead to further insulin resistance in diabetic patients [22, 49, 50]. It is rec­ommended that renal infection be considered in the differen­tial diagnosis of any patient with diabetes who presents with ank or abdominal pain [51].
Surgical Site Infections
Surgical site infections (SSIs) include supercial incisional infections, deep incision space infections, and organ space infections [5254]. SSIs are rampant in diabetic patients, and diabetes is associated with elevated risks for morbidity and mortality following SSI [55]. A recent systematic review and meta-analysis involving 866,427 procedures and 32,067 SSIs identied diabetes as an independent risk factor for SSIs in multiple surgical procedure types with odds ratio (OR) = 1.53 (95% predictive interval [PI], 1.11–2.12; I2,
57.2%) [55]. The association was higher for cardiac surgery
2.03 (95% PI, 1.13–4.05). Consistent with this report, type 1 and type 2 diabetic animals are also highly vulnerable to SSIs [5659].
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Bone andJoint Infections
Bone and joint infections (BJIs) include osteomyelitis, pros­thetic joint infections, septic arthritis, and spinal infections [60]. The incidence rates of BJIs are signicantly higher in diabetic patients than in the general population [6165]. Osteomyelitis is a common DFU infection, occurring in 10–15% of moderate and in 50% of severe infections [66]. The ulcers complicated by osteomyelitis often require surgi­cal treatments and long-term antibiotic therapy [66, 67]. In a meta-analysis study involving 119,244 patients from 18 studies, diabetic patients were found to have 1.84 times higher risk of prosthetic joint infections and 1.96 times higher risk of developing deep tissue infection following total knee arthroplasty [63].
Bloodstream Infections
Patients with diabetes also have a higher incidence of blood­stream infections (BSIs) compared to non-diabetic patients. A clinical study reported that diabetics have a 4.4-fold higher risk of BSI, and they are also more susceptible to sepsis of unknown origin with a greater incidence of septic complica­tions than non-diabetic patients [68]. Although urinary tract infections were the primary source of BSIs in both diabetic and non-diabetic patients, Escherichia coli was the most common pathogen responsible for BSIs. However, diabetics were found to have a signicantly higher frequency of Klebsiella pneumoniae BSIs than non-diabetics (18% vs. 5%) [68]. In another observational cohort study involving 2551 patients at two teaching hospitals in Boston, Massachusetts, diabetes was determined to be an indepen­dent risk factor for community-acquired bloodstream infec­tions (OR, 1.42; 95%; CI, 1.10–1.82; p=0.006) relative to non-diabetic patients [69]. HbA1c6.5% was also found to be associated with the risk of community-acquired blood­stream infections in the critically ill patients. In a recent ret­rospective case-controlled analysis involving 151 subjects, patients with type 2 diabetes had signicantly higher inci­dence of BSIs (odds ratio [OR]: 2.27, p< 0.001), but the 90-day mortality rates were similar. This study also found urinary tract infections as the primary source and E. coli as the primary bacterial pathogen in BSIs in both diabetic and non-diabetic patients.
Oral Infections
the non-diabetic general population (59.6% vs. 39% respec­tively) [71, 72]. If left untreated, periodontitis can lead to pain, bad breath, chewing difculties, and even tooth loss [72, 73]. Diabetes also negatively impacts healing, which in turn can interfere with the treatment of periodontal disease [70]. Similarly, the prevalence of oral candidiasis in diabetic patients has been reported to be as much as 64%, with Candida albicans accounting for 75–86.5% of oral fungal infections in this cohort [74]. In another study, 90% of uncontrolled diabetic patients and 63.3% of controlled dia­betic patients cultured positive for oral (saliva) Candida, compared to only 20% of non-diabetic patients [75]. Reduction in antimicrobial factors due to decreased salivary ow rate, poor glycemic control, and impaired innate immune defenses are among the factors contributing to the increased risk for oral infection in diabetic patients [70, 72,
76].
Gastrointestinal Infections
Diabetes is a major risk factor for various bacterial, viral, and fungal gastrointestinal (GI) infections [7782]. In a retro­spectivestudy involving 7670 patients, diabetes was reported to be an independent risk factor for Clostridium difcile infection (CDI), while metformin appeared to be protective against CDI, by likely altering the microbiota toward an increased Bacteroidetes and decreased Firmicutes popula­tions in the gut [77, 78]. In another report, diabetes was found to be an independent risk factor for recurrent CDI and CDI-associated diarrhea (adjusted OR ranged from 3.79 to
5.46, minimum lower 95% condence level: 2.01, all p- values <0.0001) [77]. Diabetes is also a risk factor for Emphysematous cholecystitis which is a rare and deadly form of GI infection, caused primarily by Clostridia perfrin- gens and E. coli [82]. It is characterized by the presence of the infection-generated gas in the gallbladder wall and has a mortality rate of 15–20% in diabetic patients [82]. Diabetes has also been reported as a predisposing factor for hepatitis C virus (HCV) infection [83, 84]. A meta-analysis results showed that patients with T2DM were at a signicantly higher risk of HCV infection than non-diabetic patients (summary OR=3.50, 95% CI=2.54–4.82, I2=82.3%) [84]. These results were corroborated by another study which also found T2DM patients to be more susceptible to HCV infec­tions [85]. Similarly, T1DM patients have also been found to be at elevated risk for HCV infections [80, 83].
Diabeticpatients have a higher risk of developing various oral bacterial and fungal infections, such as periodontitis and thrush [70]. Prevalence of severe periodontitis has been reported to be over 20% higher in diabetic patients than in
Skin andSoft Tissue Infections
Skin and soft tissue infections (SSTI) include cellulitis, osteomyelitis, and postoperative wound infections [20]. A
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Table 17.2 Skin and soft tissue infections in diabetes
Infection Prevalent pathogens References Cellulitis Staphylococcus aureus [103] Osteomyelitis Staphylococcus aureus, S.
Wound infections
Postoperative wound infections
Diabetic foot ulcer infections
epidermidis, Streptococci, Enterobacteriaceae, Escherichia coli, Klebsiella pneumoniae, Proteus, Pseudomonas aeruginosa
Methicillin-resistant Staph aureus (MRSA) and vancomycin-resistant
Enterococci (VRE), Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Enterococcus faecalis, Staphylococcus epidermidis, and Enterobacter cloacae S. aureus, Escherichia coli, P. aeruginosa, Proteus spp., Klebsiella
spp., coagulase-negative
Staphylococci, Enterococcus spp., and Streptococcus spp., Acinetobacter spp., Coryneform spp., β-hemolytic Streptococcus spp., coagulase negative Staphylococcus spp., Proteus mirabilis, and M. morganii
[61, 104,
105]
[106, 107]
[95, 100,
101, 108, 109]
summary of SSTI infections and the responsible microor­ganismscan be found in Table17.2. Hyperglycemia, sensory neuropathy, and vascular disease all predispose patients with diabetes to skin and soft tissue infections [94]. A large retro­spective cohort study compared the infection rates in type 1 (T1) and type 2 (T2) diabetics (n = 5863 T1DM and n=96,630 T2DM) with 203,518 age-sex-practice–matched control subjects without diabetes [20]. They found diabetic patients to have higher rates for all infections, with bone and joint infections (BJIs) and skin cellulitis being amongst the highest disparities. The incidence rate ratios (IRRs) for BJIs in T1D and T2D patients were IRR 22.34 [95% CI, 12.12–
41.20] and IRR 4.93 [95% CI, 4.34–5.61], respectively; and the IRRs for skin cellulitis were 2.84 [95% CI, 2.48–3.25] and 2.03 [95% CI, 1.97–2.08], respectively.
Diabetic Foot Ulcer Infections
Statistics
A recent meta-analysis (involving 16,159 diabetic patients and 22,198 microbial isolates) assessed the global microbiol­ogy of diabetic foot infections [95]. Of the 112 studies included in this meta-analysis, 55 studies tested for both aerobic and anaerobic microorganisms, while 57 studies only tested for aerobic microorganisms. In this meta-analysis report, 89.4% of diabetic ulcers were found to be infected of which 58.9% were polymicrobial and 41.1% were monomi­crobial infections. Importantly, of the 22,198 microbial iso­lates from DFUs that were examined, the overwhelming majority (~98%) were found to be of bacterial origin, 258 (1.62%) isolates were of fungal origin, and one isolate was of archaeal origin. Interestingly, when they stratied the analy­sis based on high-income countries (HICs) versus upper­middle and lower-middle income countries (U/LMICs), a different pattern in infection microbial structures emerged. In HICs, the Gram-positive bacteria were predominant accounting for 62.4% of all isolates, whereas in U/LMICs, Gram-negative bacteria were predominant accounting for
59.6% of all bacteria. The differences in the proportions of Gram-negative vs. Gram-positive infections between HICs and U/LMICs were highly signicant (p<0.0001). Although the underlying reasons for these differences remain poorly understood, sanitation, diet, and antibiotic stewardship should be considered as possible contributing factors.
Another prospective study, involving 261 diabetic patients with DFUs in a reference tertiary hospital in India, found
82.4% to be infected of which 44.3% were monomicrobial and 55.7% were polymicrobial [100]. In this report, Gram­negative pathogens were predominant accounting for 58.5% of all wound isolates. Another notable nding in this report was the high percentage (55%) of diabetic patients undergo­ing amputation of the affected areas due to DFU infections.
A different prospective study, involving 162 diabetic patients with infected foot ulcers in a diabetes and endocri­nology center in India, found 68.5% of DFU infections to be biolm-associated [101]. They also determined male sex, necrotic ulcer, previous antibiotic use, subcutaneous infec­tion and polymicrobial infection, duration of diabetes (>10 years), duration of ulcer (>1 month), size of ulcer (>4cm2), and ulcer grade (Grade II), as signicant risk fac­tors for biolm-producing infection (p<0.001in all cases).
Infection in diabetic foot ulcers (DFUs) is very common and can lead to serious comorbidities and life-threatening com­plications. Nearly 50–89% of DFUs develop infection which in turn, increases the chance of amputation by approximately 50% as compared with diabetic patients with uninfected DFUs [9599]. Majority of infections in diabetic ulcers are polymicrobial and biolm-associated, although 33–44.3% of infections are non-biolm and planktonic and 23–44.3% are monomicrobial in nature [95, 100102].
Microbiome Composition inDFU
The microbial burden and the microbiome compositions of skin varies based on factors such as distribution of sweat glands, skin pH, the availability of oxygen, anatomical fea­tures such as skin thickness and folds, hygiene practices such as feet-washing, and physical activities [110112]. Overall, the number of bacteria on healthy foot has been reported to range between 7.6×103CFU/cm2 and 1.2×105CFU/cm2 in
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men, with younger men having more bacteria on their feet [110]. The overall bacteria load on women’s feet tends to be approximately an order of magnitude higher than on men [110]. Moreover, healthy foot is predominantly colonized by commensal bacteria belonging to Firmicutes, Actinobacteria, and Proteobacteria families, although pathogenic bacteria have also been detected on healthy foot [110, 111]. Of note, pathogenic bacteria (e.g., S. aureus or P. aeruginosa) are rarely detected on healthy foot [110112]. In contrast, dia­betic foot skin harbors increased populations of pathogenic bacteria (e.g., S. aureus) and increased bacterial diversity [108, 109]. However, the microbiome in diabetic ulcers are nearly all pathogenic bacteria.
In a recent meta-analysis study involving 112 publica­tions and 16,159 diabetic patients, the isolated bacteria in DFUs were all pathogenic variety [95]. The most prevalent bacterial pathogens were S. aureus which accounted for
23.4% of all isolates, followed by Escherichia coli (11. 5%), P. aeruginosa (11.1%), Proteus spp. (8.3%), Klebsiella spp. (6.9%), coagulase-negative Staphylococci (5.8%), Enterococcus spp. (5.4%), and Streptococcus spp. (5.2%). Gram-negative pathogens were more predominant in DFUs overall, but interestingly, when the data were stratied based on country income (high-income countries (HICs) vs. upper­middle and lower-middle income countries (U/LMICs)), a different pattern emerged. In HICs, the Gram-positive bacte­ria were predominant accounting for 62.4% of all isolates, whereas in U/LMICs, Gram-negative bacteria were predom­inant accounting for 59.6% of all bacteria. The differences in the proportions of Gram-negative vs. Gram-positive infec­tions between HICs and U/LMICs were highly signicant (p<0.0001). Although the underlying reasons for these dif­ferences remain unknown; sanitation, diet, and antibiotic stewardship should be considered as possible factors contrib­uting to the observed differences in the microbiome compo­sitions in HICs vs. U/LMICs.
In another prospective study involving infected footulcers in 261 diabetic patients [100], Gram-negative pathogens were predominant accounting for 58.5%, of which P. aerugi- nosa was the most prevalent isolate found in 20.9% samples, while S. aureus was the most prevalent Gram-positive bacte- rium accounting for 26.9% all samples. Other bacterial pathogens detected in DFUs in high frequencies included
Enterococcus faecalis (12.7%), E. coli (12%), K. pneu­moniae (9.5%), and Proteus mirabilis (9%). In a different
prospective study involving 162 diabetic patients with infected foot ulcers [101], E. coli was found to be the most predominant Gram-negative pathogen (27.8%), followed by P. aeruginosa which accounted for 15.6% of all isolates. This study also found S. aureus to be the most predominant Gram-positive pathogen (23.5%) in the ulcers. Other bacte­ria detected in these ulcers included K. oxytoca (7%), K.
pneumonia (5.8%), P. vulgaris and E. faecalis (3.5% each),
Acinetobacter spp. (3.1%), Coryneform spp. (2.7%), β-hemolytic Streptococcus spp. (2.3%), coagulase-negative Staphylococcus spp. (2.3%), P. mirabilis (1.5%), and M. morganii (0.7%).
Consequences ofMicrobiome Shift Toward Pathogenic Bacteria inDFU
What is clear from these studies is that pathogenic bacteria make up almost the entire microbial structure in DFUs, indi­cating a shift toward pathogenic bacteria in diabetic ulcers. This shift has been observed in longitudinal studies of human diabetic patients and diabetic animal models [113115]. The consequences of microbiome shift toward pathogens are severe and negatively impact healing outcomes. In fact, strain-level variations in S. aureus sub-species and genetic signatures of biolm formation have been identied as inde­pendent risk factors for poor outcomes in DFU in a prospec­tive study of patients with neuropathic diabetic foot ulcers [115]. Moreover, numerous studies have demonstrated that S. aureus and P. aeruginosa-infected chronic wounds, including DFUs, exhibit signicant healing impairments compared to other DFUs [116125]. Why is the presence of pathogenic bacteria harmful to the healing of diabetic wounds? The reason may bethat these bacteria produce vari­ous virulence factors that can weaken the immune response and make diabetic wounds more susceptible to infection and delayed healing. In the following section, we will give some examples of important bacterial virulence factors that have been found to have a negative impact on immune responses in diabetic wounds.
Biolm
As discussed above, majority of infections in DFU are biolm- associated. Biolm is perhaps one of the most impor­tant, if not the most important, virulence factors that is asso­ciated with impaired healing in DFU [16, 126]. Biolm is a collection of single or multi-species microbial communities that adhere to biotic (e.g., wound) or abiotic (e.g., implants) surfaces, embedded within a complex structure composed of exopolysaccharides, extracellular DNA (eDNA), proteins, and lipids [127, 128]. Approximately 80% of lower-limb amputations in diabetic patients occur following biolm­infected foot ulceration, highlighting the devastating impact of biolm on healing outcomes [16, 129]. There are multiple reasons why biolms can be harmful to healing processes in diabetic ulcers. Biolms can trigger sustained inammatory responses which are harmful to healing processes; they can protect pathogens from antibiotics, and from detection and phagocytosis-mediated killing by immune leukocytes, thus, prolonging infection; and they can function as physical bar­rier to cellular migration of cells that play pivotal role infec-