PATHOGENS

• Most diabetic foot infections (DFIs) are polymicrobial; however, if the patient hasn’t recently received abx therapy, it is often monomicrobial due to staphylococcal or streptococcal infection.
• Frequent pathogens: based on deep wound or bone cultures. Pathogens recovered from superficial swabs should be viewed with suspicion.
  • Initially, wounds usually have Gram-positive flora from the skin; as it becomes more chronic, it tilts toward Gram negatives. Following broad-spectrum abx, flora may evolve to MRSA, VRE and more resistant Gram negatives.
  • Aerobic Gram positive:
    
    • Staphylococcus aureus
      • Among the most commonly isolated pathogens in N. America and European series.
    • Streptococcal spp.
      • Especially GAS, GBS, group C or G streptococci
    • Enterococcus spp.
    • Coagulase-negative staphylococci are usually contaminants obtained from superficial swabs but should be considered genuine if obtained from deep tissue or bone.
  • Aerobic Gram negatives
    • Enterobacteriaceae
    • Pseudomonas aeruginosa (although frequently a commensal when found, studies using abx without antipseudomonal activity often yield similar results, e.g., ertapenem v. piperacillin/tazobactam[19])
  • Anaerobes, facultative anaerobes: usually when ulcers are deep, chronic and/or necrotic tissue is present.
    
    • B. fragilis
    • Clostridia spp.
    • Peptococcus and Peptostreptococcus
• Superficial, early infections (cellulitis, cellulitis involving blisters and shallow ulcers) are typically caused by S. aureus or beta-hemolytic streptococci.
• Infections of ulcers that are chronic or previously treated with antibiotics may be caused by aerobic Gram-negative bacilli, S. aureus or Streptococci.
• Deep soft tissue infections, osteomyelitis, and gangrene are more often polymicrobial, including aerobic Gram-negative bacilli and anaerobes (anaerobic streptococci, Bacteroides fragilis group, Clostridium species), but Staphylococcus aureus is also common as a single pathogen.
• Multi-drug resistant Gram-negative organisms described in DFI, especially ESBL, but most resistant organisms w/ reports from India and warmer climates.