**4. Microbiological therapy**

#### **4.1 Pathogens of DFO**

Beside possible surgical intervention there is need for an antibacterial treatment. Choosing an active agent is always first empirical, and subsequently targeted to the microbiological culture results. Knowledge of the possible microorganisms is a precondition to an empirical therapy [35]. Dependent on the country, most isolates of DFO are *Staphylococcus aureus*, β-hemolytic streptococci, coagulasenegative staphylococci and Gram-negative pathogens such as *Pseudomonas aeruginosa* [36, 37]. Interestingly, the location of the infection is critical as well, so are calcaneal infections associated with *P. aeruginosa* in diabetic patients [19]. Unfortunately, despite the advocated greenish color of superficial *Pseudomonas* infections and a presumed characteristic smell of the infected wound, even longstanding clinicians cannot predict the presence of *P. aeruginosa* by visual and olfactory means alone. The microbiological laboratory assessment is still necessary [38]. Multi-resistant pathogens in DFO are increasing in frequency worldwide [39] such as extended-spectrum beta-lactamase-producing Gram-negative rods (ESBL). Compared to DFI without involvement of the bone a meta-analysis found a three times higher chance in DFO for isolating a multi-resistant pathogen [40]. Fungi are rarity. Enterococci are equally rare but relatively more prevalent in the infected diabetic foot compared to other osteoarticular infections in the body such as steptococci and staphylococci [41, 42]. It is important to recognize that in the DFO patient, we may retrieve any bacteria, including avirulent coagulase-negative staphylococci and corynebacteria. Unlike to other infections such as pneumonia or endocarditis, the causative pathogens can also change during the current therapy of DFO, by selection of new (more resistant) pathogens by the therapeutic antibiotics and iterative surgeries during treatment. Therefore, if ever there is surgery during ongoing systemic antibiotic treatment, we recommend to re-sample again. The incidence of such a new microbiological finding can be as high as 10% [43].

#### **4.2 Antibiotic therapy**

The systemic antibiotic therapy is – next to a possible surgery, iterative professional debridement, (podiatric) wound care, enhancement of the patient's compliance, and off-loading – always required, if the goal is the healing of DFO. A clear recommendation for a specific antimicrobial agent, or the general administration route, cannot be made. A lot of studies and meta-analyses failed to show a superiority of one specific drug against the others [44]. During the initial empirical treatment, we recommend to cover *S. aureus*. If the therapy fails to achieve a proper reduction of local inflammatory symptoms, then the therapy should be broadened to include (aerobic and anaerobic) Gram-negative bacteria [45]. In severe infections, (sub)tropical regions, or sepsis, a relatively broad empirical coverage targeting the local epidemiology of Gram-negative pathogens could be chosen [46] from the start. Further there is few data supporting parenteral therapy [47]. Of note, the microbiological culture results can lead to necessity of parenteral agents due to resistant pathogens.

Ideally, the DFO therapy is accompanied by professional debridement, or the resection of necrotic and infected bone (total amputation). A study of 50 patients with chronic toe DFO showed that patients with surgical resections had a significantly lower relapse rate. This was also witnessed in single-center survey with partial amputations [48]. In well-selected patients and neuropathic DFO cases without progressive ischemia, other studies report successful treatment

without surgery, with selected remission rates of 60–70% [49, 50]. When surgery is not necessary for various reasons, a strictly conservative antibiotic therapy is very reasonable. Of note, the proportion of antibiotic-related side effects in randomized-controlled DFO trials may compromise up to 20–30% of all systemic antibiotic DFO regimens [49–55].
