A Closer Look At Diabetic Foot Infections

Author(s): 
By Warren S. Joseph, DPM, Thomas Zgonis, DPM, and Thomas S. Roukis, DPM

   Diabetic foot infections arising from ulcerations are the largest non-traumatic cause of lower extremity amputations. Contributing factors include peripheral neuropathy and vascular disease, rigid pedal deformities, local trauma and pressure, extensive soft tissue loss, multi-system failure, non-compliance and severe infection.

   Over the decades, there have been a number of shifts in the way clinicians approach diabetic foot infections (DFIs). Throughout the ‘60s and into the ‘70s, clinicians felt most DFIs were, like other skin and skin structure infections, caused primarily by the gram-positive aerobic cocci, Staphylococcus aureus and Streptococcus.

   In the early ‘80s, some of the emerging literature used more sophisticated culturing modalities and laboratory techniques.1 Researchers were finding multiple organisms, primarily anaerobic gram negative rods, that had not been isolated from these infections in the past. Soon, all DFI were being called “polymicrobial” with mixed flora containing aerobic gram-positive cocci, gram-negative rods and anaerobic cocci and rods. There was an emphasis by clinicians to ensure that all of these isolates were covered by an overly broad-spectrum empiric antibiotic regimen, pending deep culture reports.

   A critical look at these studies shows many of them did not represent everyday clinical experience. There was little stratification based on the severity of the infections. Most of the studies examined end-stage, severe, chronic, malodorous, necrotic infections. In fact, the literature actually referred to just this type of severe process as the “diabetic foot.” In some cases, the researchers actually obtained cultures from amputation specimens in the morgue or pathology laboratory. Clinicians did not consider the wide range of clinical presentation of infections in these patients. Unfortunately, this concept of the polymicrobial DFI is still alive today not only in podiatric circles but throughout most of medicine.

   Around 1990, the thinking on DFI started changing again. Lipsky and Pecoraro looked at “uncomplicated” infections in the diabetic lower extremity and compared the efficacy of cephalexin with clindamycin.2 The study showed both drugs had similar effects. This was particularly interesting for a number of reasons. It was one of the first times DFI were not all grouped together as a severe, limb threatening process. There could be “uncomplicated” presentations.

   More importantly, when they compared the spectrum of activity of each drug, Lipsky and Pecoraro noted that cephalexin has activity against gram positive cocci and some gram negatives but no coverage against anaerobic organisms. Clindamycin shares the activity against the gram positive cocci but adds anaerobic coverage and has no activity against the gram negatives. In terms of coverage, the only organisms these drugs had in common were Staphylococcus and Streptococcus. One could conclude that these gram positive organisms were the primary pathogens despite what other bacteria may have been isolated from these uncomplicated DFIs.

Gram-Positive Cocci: Why It Is An Essential Consideration

   Understanding the importance of gram-positive cocci, in particular Staphylococcus aureus and Group B Streptococcus, is critical in the current antibiotic approach to DFIs. The recent Infectious Diseases Society of America (IDSA) Diabetic Foot Infection Guidelines emphasize the following point:

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