Most Common Organism Isolated from Bile
Escherichia coli is the most common organism isolated from bile. 1
Microbiology of Bile Infections
According to the most recent evidence from the 2022 EASL clinical practice guidelines on sclerosing cholangitis, the most frequently encountered organisms in biliary infections are Gram-negative bacteria, with Escherichia coli being the predominant isolate 1. This is consistently supported by multiple guidelines and research studies.
Common Organisms in Bile (in order of frequency):
Gram-negative bacteria:
- Escherichia coli (most common)
- Klebsiella pneumoniae
- Pseudomonas species
- Bacteroides species
Gram-positive bacteria:
- Enterococci
- Streptococci
Evidence Supporting E. coli as Most Common
The 2022 EASL guidelines explicitly state that "the most frequently encountered organisms are Gram-negative bacteria such as Escherichia coli, Klebsiella, Pseudomonas and Bacteroides species, as well as Gram-positive Enterococci, or Streptococci" 1. In this list, E. coli is mentioned first, indicating its predominance.
This is further supported by the 2020 World Society of Emergency Surgery guidelines which note that "In biliary infections, Gram-negative aerobes, such as Escherichia coli and Klebsiella pneumonia, and anaerobes, especially Bacteroides fragilis are the most commonly isolated bacteria" 1.
Research studies also confirm this finding. A 2011 study examining bile cultures found that Gram-negative bacilli were the most common isolates (94-95%), with E. coli (36%) being the most prevalent organism 2.
Clinical Implications
Understanding that E. coli is the most common organism in bile has important implications for empiric antibiotic selection in biliary infections. Initial antibiotic regimens should provide adequate coverage against E. coli and other common Gram-negative organisms.
For biliary infections, guidelines recommend:
- Aminopenicillin/beta-lactamase inhibitor combinations for mild cases
- Piperacillin/tazobactam or third-generation cephalosporins with anaerobic coverage for more severe cases 1
Special Considerations
- Biliary infections are often polymicrobial
- Healthcare-associated infections may involve more resistant strains
- Fungal infections (particularly Candida) in bile are associated with poor prognosis and are more common in advanced disease 1
Therefore, among the options presented (A. Escherichia coli, B. Klebsiella, C. Staphylococcus aureus, D. Bacteroides), the correct answer is A. Escherichia coli.