What are the common organisms that cause infections and how are they treated?

Common Infectious Organisms and Their Treatment

The most common organisms causing infections are Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and other gram-positive and gram-negative bacteria, with treatment requiring targeted antibiotics based on infection site, suspected pathogens, and local resistance patterns.

Skin and Soft Tissue Infections

• Staphylococcus aureus (including MRSA) and streptococcal species are the most common pathogens in skin and soft tissue infections 1
• For surgical site infections following clean procedures, empiric coverage should target S. aureus and streptococci 1
• For surgical site infections after operations involving intestinal or genital tracts, mixed gram-positive and gram-negative flora with both facultative and anaerobic organisms are common 1
• Vancomycin is recommended for suspected or proven MRSA infections 1
• Incisions in the axilla have higher incidence of gram-negative organisms, while perineal incisions have higher rates of gram-negative organisms and anaerobes 1

Intra-abdominal Infections

• Intra-abdominal infections typically involve multiple organisms from intestinal flora 1:
  • Enterobacteriaceae (E. coli, Klebsiella pneumoniae)
  • Enterococci
  • Bacteroides fragilis and other anaerobes
• For community-acquired intra-abdominal infections, antibiotics should be active against enteric gram-negative aerobic and facultative bacilli and β-lactam-susceptible gram-positive cocci 1
• Coverage against obligate anaerobic bacilli should be provided for distal small-bowel and colon-derived infections 1
• For health care-associated infections, more resistant flora may include P. aeruginosa, Acinetobacter species, extended-spectrum β-lactamase-producing Klebsiella and E. coli, Enterobacter species, Proteus species, MRSA, enterococci, and Candida species 1

Respiratory Infections

• Common pathogens include Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Klebsiella pneumoniae, and Pseudomonas aeruginosa 2
• In cystic fibrosis, Pseudomonas aeruginosa is the most frequent pathogen, followed by Staphylococcus aureus 3
• Combination antibiotic therapy with two different drugs is preferred over monotherapy in cystic fibrosis to delay development of antibiotic resistance 3

Spontaneous Bacterial Peritonitis

• Typically monobacterial, with gram-negative bacteria (~60%) being most common 1
• Specific microorganisms are mostly enteric (E. coli, Klebsiella pneumoniae, Staphylococcus aureus, Enterococcus faecalis, and Enterococcus faecium) 1
• Third-generation cephalosporins (ceftriaxone, cefotaxime) are recommended as first-line antibiotics in settings where multidrug-resistant organisms are not prevalent 1

Treatment Principles

• Empiric antibiotic selection should be based on:
  • Likely pathogens for the specific infection site 1
  • Community-acquired vs. healthcare-associated infection 1
  • Local resistance patterns 1
  • Patient risk factors for resistant organisms 1
• For community-acquired infections, narrower spectrum agents are preferable (e.g., ampicillin/sulbactam, cefazolin or cefuroxime/metronidazole, ticarcillin/clavulanate, and ertapenem) 1
• For healthcare-associated infections, broader coverage is often needed due to higher risk of resistant organisms 1
• Ceftriaxone is indicated for various infections including skin and skin structure infections, intra-abdominal infections, and respiratory infections when caused by susceptible organisms 2

Special Populations

• In neutropenic patients, common pathogens include E. coli, Klebsiella species, and P. aeruginosa 1
• For fungal infections, fluconazole is appropriate for C. albicans, while echinocandins (caspofungin, micafungin, or anidulafungin) are recommended for fluconazole-resistant Candida species 1
• For critically ill patients with suspected fungal infection, initial therapy with an echinocandin instead of a triazole is recommended 1

Diagnostic Considerations

• Appropriate specimens should be obtained for isolation of causative organisms and determination of susceptibility before initiating treatment 2
• Blood cultures, tissue cultures, and site-specific samples should be collected to increase the likelihood of identifying the causative organism 4, 5
• Therapy may be instituted prior to obtaining results of susceptibility testing but should be adjusted based on culture results 2

Antibiotic Stewardship

• Use antibiotics only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria 2
• Consider local epidemiology and susceptibility patterns when selecting empiric therapy 2
• Adjust therapy based on culture and susceptibility results when available 2