How do you classify soft tissue infections based on the bacteria involved, such as Streptococcus (Strep), Staphylococcus (Staph), including methicillin-resistant Staphylococcus aureus (MRSA), gram-negative bacteria, including Pseudomonas or Enterobacteriaceae, in a patient with potential underlying health conditions like diabetes or impaired immune status?

Classification of Soft Tissue Infections by Bacterial Etiology

Soft tissue infections are classified microbiologically into three main types based on bacterial pathogens: Type 1 (polymicrobial), Type 2 (monomicrobial β-hemolytic streptococci or community-acquired MRSA), and Type 3 (monomicrobial gram-negative bacilli), though this bacterial classification is less clinically useful than classification by necrotizing character, depth, purulence, and severity. 1

Primary Bacterial Classification System

Type 1: Polymicrobial Infections

• These infections involve mixed aerobic and anaerobic bacteria, typically including gram-negative organisms (Enterobacteriaceae, Pseudomonas), gram-positive cocci (Staphylococcus, Streptococcus), and anaerobes (Bacteroides species). 1
• Commonly occur in patients with diabetes, peripheral vascular disease, or following abdominal/perineal surgery. 1
• Most subcutaneous abscesses and severe cellulitis extending to fascia are polymicrobial. 2

Type 2: Monomicrobial Streptococcal or Staphylococcal Infections

• β-hemolytic streptococci (primarily Streptococcus pyogenes) cause classic erysipelas and non-purulent cellulitis, characterized by bright red erythema with tongue-like extensions, early fever/chills, and excellent response to penicillin. 3, 1
• Staphylococcus aureus (both methicillin-susceptible and MRSA) causes purulent infections including abscesses, furuncles, carbuncles, and limited cellulitis with darker red hue and less prominent systemic symptoms initially. 3, 1
• Community-acquired MRSA strains produce exotoxins and are epidemiologically distinct from healthcare-acquired strains, increasingly prevalent in purulent skin infections. 4, 1

Type 3: Monomicrobial Gram-Negative Infections

• Pseudomonas aeruginosa causes infections in immunocompromised patients, burn wounds, and following water exposure, requiring coverage with ceftazidime or antipseudomonal agents. 5, 1
• Enterobacteriaceae (E. coli, Klebsiella, Proteus, Enterobacter) cause infections associated with diabetes, chronic wounds, and genitourinary/abdominal sources. 5, 6
• Aeromonas hydrophila and Vibrio vulnificus cause rapidly progressive infections following freshwater or saltwater exposure respectively. 1

Clinically Relevant Bacterial Patterns by Infection Type

Non-Purulent Infections

• Predominantly caused by β-hemolytic streptococci (Group A Streptococcus), requiring penicillin, cephalexin, or clindamycin. 1, 3
• Erysipelas is definitively streptococcal and always responds to penicillin. 3

Purulent Infections

• Staphylococcus aureus is the predominant pathogen (both MSSA and MRSA), requiring coverage with agents active against resistant strains in areas with high MRSA prevalence. 1
• Empiric therapy should include clindamycin, trimethoprim-sulfamethoxazole, doxycycline, or cephalexin depending on local resistance patterns. 1

Necrotizing Infections by Bacterial Type

• Type 1 (polymicrobial): Mixed aerobic-anaerobic flora including streptococci, staphylococci, gram-negatives, and anaerobes—requires broad-spectrum coverage with vancomycin or linezolid PLUS piperacillin-tazobactam or carbapenem. 1
• Type 2 (Group A Streptococcus or CA-MRSA): Toxin-producing strains causing rapidly progressive disease—requires penicillin plus clindamycin (for streptococcal toxic shock) or vancomycin/linezolid (for MRSA). 1
• Type 3 (Clostridium species, Aeromonas, Vibrio): Specific environmental or traumatic exposures—Clostridium requires clindamycin plus penicillin; Aeromonas requires doxycycline plus ceftriaxone; Vibrio requires doxycycline plus ceftriaxone. 1

Special Bacterial Considerations by Patient Population

Diabetic Patients

• Infections are typically polymicrobial with mixed gram-positive cocci, gram-negative bacilli, and anaerobes, particularly in chronic wounds. 4, 7
• Require broader empiric coverage including gram-negative and anaerobic activity. 1

Immunocompromised Hosts

• Higher risk for unusual pathogens including Pseudomonas, gram-negative bacilli, and fungal organisms. 4, 8
• Any infection automatically classified as severe requiring aggressive empiric broad-spectrum therapy. 9

Bite Wounds

• Animal bites: Polymicrobial with Pasteurella multocida, Staphylococcus, Streptococcus, and anaerobes—require amoxicillin-clavulanate or ampicillin-sulbactam. 1
• Human bites: Eikenella corrodens, Streptococcus, Staphylococcus, and anaerobes—require amoxicillin-clavulanate. 1

Critical Pitfalls in Bacterial Classification

• Emerging resistance patterns: 50% of MRSA strains have inducible or constitutive clindamycin resistance; treatment failure rates of 21% reported with doxycycline/minocycline for MRSA. 1
• Erythromycin resistance in Streptococcus pyogenes is increasingly problematic, limiting macrolide use. 1
• Clinical presentation does not reliably predict bacterial etiology: Gram stain and culture from needle aspiration or punch biopsy are essential in severe infections, those failing empiric therapy, or in immunocompromised patients. 1
• Delay in recognizing polymicrobial necrotizing infections increases mortality—any patient with systemic toxicity (fever, tachycardia >100 bpm, hypotension), elevated creatinine, low bicarbonate, elevated CPK (2-3× normal), or CRP >13 mg/L requires blood cultures and aggressive diagnostic workup. 1