What are the common organisms involved in severe infections?

Common Organisms in Severe Infections

The most common organisms in severe infections include Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Enterococcus species, and Klebsiella species, with significant variation based on infection site and patient risk factors. 1

Skin and Soft Tissue Infections (cSSTIs)

• Staphylococcus aureus is the predominant pathogen in cSSTIs (39.9-45.9%), with methicillin-resistant S. aureus (MRSA) accounting for approximately 27-29% of S. aureus isolates 1
• Pseudomonas aeruginosa is the second most common pathogen (10.8-12.1%) 1, 2
• Other common organisms include:
  • Escherichia coli (7.0-9.7%) 1
  • Enterococcus species (7.7-8.2%) 1
  • Klebsiella species (5.1-5.8%) 1
  • Enterobacter species (5.6-5.8%) 1
  • Coagulase-negative staphylococci (3.4-4.2%) 1
  • Proteus species (3.2-3.7%) 1
  • Streptococcus species (2.6-2.7%) 1
  • Acinetobacter species (1.6-2.2%) 1
  • Serratia species (2%) 1

Intra-abdominal Infections (cIAIs)

• cIAIs are typically polymicrobial, involving both aerobes and anaerobes 1
• Most common pathogens include:
  • Enterobacteriaceae (particularly E. coli and Klebsiella pneumoniae) 1
  • Enterococcus species 1
  • Bacteroides fragilis (anaerobe) 1
  • Clostridium species (anaerobe) 3
  • Peptostreptococcus species (anaerobe) 3
• Increasing prevalence of extended-spectrum β-lactamase (ESBL) producing E. coli and K. pneumoniae is concerning 1

Severe Community-Acquired Pneumonia (SCAP)

• Streptococcus pneumoniae remains the most common bacterial pathogen in severe CAP (present in up to one-third of ICU patients) 1
• Other common pathogens include:
  • Legionella species 1
  • Haemophilus influenzae 1
  • Staphylococcus aureus 1
  • Atypical pathogens (Mycoplasma pneumoniae, Chlamydia pneumoniae) 1
  • Respiratory viruses (particularly influenza) 1
• In ICU patients, gram-negative organisms are increasingly common:
  • Enterobacteriaceae (up to 22% of ICU patients) 1
  • Pseudomonas aeruginosa (10-15% of ICU patients), particularly in those with bronchiectasis 1

Neutropenic Patients with Severe Infections

• Common gram-positive pathogens in neutropenic patients include:
  • Staphylococcus aureus (including MRSA) 1
  • Coagulase-negative staphylococci 1
  • Enterococcus species (including VRE) 1
  • Viridans group streptococci 1
  • Streptococcus pneumoniae 1
• Common gram-negative pathogens include:
  • Escherichia coli 1
  • Klebsiella species 1
  • Enterobacter species 1
  • Pseudomonas aeruginosa 1
  • Citrobacter species 1
  • Acinetobacter species 1
  • Stenotrophomonas maltophilia 1
• Fungi, particularly Candida species, become more common after the first week of prolonged neutropenia 1

COVID-19 and Bacterial Co-infections

• In patients with severe COVID-19, bacterial co-infections at ICU admission occur in approximately 28% of cases 4
• Most common co-infecting bacteria include:
  • Staphylococcus aureus (31%) 4
  • Haemophilus influenzae (22%) 4
  • Streptococcus pneumoniae (19%) 4
  • Enterobacteriaceae (16%) 4
  • Pseudomonas aeruginosa (6%) 4
• Healthcare-associated infections in COVID-19 patients often involve resistant organisms including MRSA, carbapenem-resistant Enterobacterales, and carbapenem-resistant Acinetobacter baumannii 5

Antimicrobial Resistance Considerations

• Methicillin-resistant S. aureus (MRSA) accounts for approximately 27-29% of S. aureus isolates in North America 1
• Extended-spectrum β-lactamase (ESBL) production is observed in 7.1% of E. coli and 11.3% of Klebsiella species 2, 6
• Drug-resistant Streptococcus pneumoniae (DRSP) is increasingly common, with more than 40% of pneumococci showing some level of resistance 1
• The prevalence of resistant organisms varies significantly by geographic region and healthcare setting 7

Clinical Implications

• Empiric antibiotic selection should consider the most likely pathogens based on infection site, patient risk factors, and local resistance patterns 7
• Both inadequate and unnecessarily broad empiric antibiotics are associated with higher mortality in sepsis 7
• For severe infections, early appropriate antimicrobial therapy is crucial to improve outcomes 1, 7

Understanding the common organisms in severe infections helps guide appropriate empiric antimicrobial therapy while awaiting culture results, balancing the need for adequate coverage against the risks of antimicrobial resistance.