What bacteria are commonly found in foot infections of diabetics?

Bacteria Found in Diabetic Foot Infections

The bacteria you'll encounter depend critically on the clinical presentation: acute, previously untreated infections are typically monomicrobial with Staphylococcus aureus and β-hemolytic streptococci, while chronic or previously treated infections are polymicrobial with gram-positive cocci, Enterobacteriaceae, and often anaerobes. 1

Acute/Mild Infections (Antibiotic-Naive)

For cellulitis without an open wound or newly infected ulcers in patients who haven't received antibiotics:

• Staphylococcus aureus is the predominant pathogen 1
• β-hemolytic streptococci (groups A, B, C, and G, with group B being especially common) 1
• These infections are often monomicrobial 1

Chronic or Previously Treated Infections

When ulcers are chronic or patients have received prior antibiotic therapy, expect polymicrobial infections:

• S. aureus remains common 1
• β-hemolytic streptococci persist 1
• Enterobacteriaceae (including E. coli, Klebsiella, Proteus) emerge 1, 2
• Coagulase-negative staphylococci can become pathogenic in necrotic tissue 1
• Enterococci are frequently isolated 1
• Cultures typically yield 3-5 isolates including both aerobes and anaerobes 1

Special Clinical Scenarios

Macerated/Soaked Ulcers

• Pseudomonas aeruginosa is characteristic, often in combination with other organisms 1

Fetid Foot with Extensive Necrosis/Gangrene

This malodorous presentation indicates polymicrobial infection with:

• Mixed aerobic gram-positive cocci (including enterococci) 1
• Enterobacteriaceae 1
• Nonfermentative gram-negative rods 1
• Obligate anaerobes (Bacteroides, Peptostreptococcus) 1, 2

Long-Duration Wounds with Prolonged Broad-Spectrum Antibiotics

Expect highly resistant organisms:

• Aerobic gram-positive cocci (S. aureus, coagulase-negative staphylococci, enterococci) 1
• Diphtheroids (which can be pathogenic in this context) 1
• Enterobacteriaceae 1
• Pseudomonas species 1
• Nonfermentative gram-negative rods 1
• Possibly fungi 1
• Antibiotic-resistant species are common, including MRSA, VRE, and ESBL-producing gram-negatives 1

Critical Antibiotic Resistance Patterns

MRSA prevalence is substantial and increasing:

• MRSA accounts for approximately 38% of S. aureus isolates in hospitalized diabetic foot infections 3
• Community-associated MRSA is now common and associated with worse outcomes 1
• Always consider local MRSA prevalence when selecting empirical therapy 4

Other resistance concerns:

• Extended-spectrum β-lactamase (ESBL) producers among gram-negatives 1, 5
• VRE in patients with prolonged antibiotic exposure 1
• Nearly 30% of E. coli strains resistant to amoxicillin/clavulanate and ciprofloxacin 3

Key Clinical Pitfalls

Common mistakes to avoid:

• Don't assume monomicrobial infection in chronic wounds – these are typically polymicrobial requiring broad-spectrum coverage 1
• Don't ignore anaerobes in malodorous or gangrenous infections – they require specific coverage 1, 4
• Don't rely on wound swabs – obtain tissue specimens by biopsy, curettage, or aspiration for accurate culture results 4, 6
• Don't culture before debridement – superficial colonizers will confound results; culture after appropriate wound preparation 6
• Don't treat uninfected ulcers with antibiotics – colonization does not equal infection 4

Practical Culture Approach

To obtain meaningful microbiological data:

• Send specimens for culture before starting empirical antibiotics in all cases except mild, previously untreated infections 4
• Tissue specimens are strongly preferred over swabs (biopsy, ulcer curettage, or aspiration) 4, 6
• Request both aerobic and anaerobic cultures for moderate-to-severe infections 2
• Consider bone biopsy for suspected osteomyelitis to guide definitive therapy 4