What is Polymicrobial Infection?
Polymicrobial infection is defined as an infection caused by the simultaneous presence of multiple microorganisms from different bacterial groups (aerobic and anaerobic), or combinations of bacteria, viruses, fungi, and parasites, where these organisms interact synergistically to cause or worsen disease. 1, 2
Key Characteristics
Polymicrobial infections are particularly common in immunocompromised patients and are associated with increased infection severity and poorer patient outcomes compared to monomicrobial infections. 2, 3
Clinical Settings Where Polymicrobial Infections Occur
The most common clinical scenarios include:
- Perianal abscesses, penetrating abdominal trauma, or surgical procedures involving the bowel 3
- Decubitus ulcers 3
- Injection sites in illicit drug users 3
- Diabetic foot infections requiring hospitalization, which often include various types of aerobes and anaerobes 3
- Necrotizing fasciitis and deep soft tissue infections, particularly those involving ischemic or necrotic wounds 3
Microbial Interactions
In polymicrobial infections, microorganisms interact through several mechanisms:
- One organism creates a niche for other pathogens to colonize 1
- One organism predisposes the host to colonization by others 1
- Multiple non-pathogenic organisms together cause disease 1
- Organisms exploit metabolites, modulate immune responses, optimize the infection niche, and induce virulence traits in each other 2
Implications for Immunocompromised Patients
Immunocompromised patients—including those with diabetes, HIV/AIDS, elderly patients, and young children—face substantially higher risk for polymicrobial infections. 3
Risk Factors in Compromised Hosts
- Malnutrition, obese BMI, comorbidities, active disease, and older age increase susceptibility 3
- Combination immunosuppressive therapies (odds ratio up to 14.5 for two or three agents) dramatically increase risk 3
- Chronic diseases including diabetes, chronic liver disease, and iron-overload states predispose to polymicrobial infections 3
Pathogen Patterns
In immunocompromised patients, polymicrobial infections typically involve:
- Gram-positive cocci (especially S. aureus, streptococci) combined with Gram-negative bacilli (Enterobacteriaceae, P. aeruginosa) 3
- Obligate anaerobes are most frequently isolated from ischemic or necrotic wounds and are rarely sole pathogens, usually appearing as part of mixed infections with aerobes 3
- Enterococci are relatively frequent isolates but usually of secondary clinical importance 3
Treatment Implications
Empirical treatment of polymicrobial infections requires broad-spectrum coverage against both anaerobic and aerobic/facultative bacteria. 4
Recommended Empiric Regimens
For critically ill or immunocompromised patients with suspected polymicrobial infection:
- Broad-spectrum carbapenems (meropenem, imipenem/cilastatin, doripenem) or extended-range penicillin/β-lactamase inhibitor combinations (piperacillin/tazobactam) are first-line 3, 4
- Multidrug therapy is often required to ensure sufficiently broad empiric coverage initially 3
- For necrotizing infections with polymicrobial etiology, piperacillin-tazobactam plus vancomycin provides coverage for mixed aerobic-anaerobic infections 3
Critical Pitfalls
Targeting antibiotic treatment against likely colonizers (coagulase-negative staphylococci, corynebacteria) may be unnecessary, though these can sometimes be true pathogens if they grow repeatedly or from reliable specimens 3
The clinical significance of additional organisms detected by molecular methods (PCR, next-generation sequencing) versus traditional culture is not yet clear, despite these methods detecting many more organisms, especially obligately anaerobic and fastidious species 3
Patients with healthcare-associated infections, recent antibiotic use, prior hospitalization, or previous colonization with multidrug-resistant organisms require even broader empiric coverage to account for resistant pathogens in polymicrobial infections 3, 5