Microbiology of the Biliary Tract

An Advanced Review of the KEEPS Pathogens

Pathophysiology & Overview

The biliary tract is normally sterile, however, obstruction (choledocholithiasis, stricture) creates a favorable environment for bacterial colonization. Bacteria typically ascend from the duodenum via the Ampulla of Vater or translocate from the portal circulation.

Clinical Pearl: Infections of the biliary tree are almost always polymicrobial. While anaerobes (Bacteroides fragilis) and anaerobic cocci are present, the acute morbidity is driven by the aerobic organisms categorized under the KEEPS mnemonic, plus the resilient Pseudomonas.

The Core Pathogens: KEEPS

The following organisms constitute the primary etiologic agents of acute cholangitis and cholecystitis.

K — Klebsiella pneumoniae

• Classification: Gram-negative rod, encapsulated, lactose fermenter.
• Virulence Factors: Possesses a polysaccharide capsule that impairs phagocytosis. Hypervirulent strains (hvKP) are associated with pyogenic liver abscesses.
• Advanced Note: Klebsiella is a frequent producer of Extended-Spectrum Beta-Lactamases (ESBL), rendering it resistant to penicillins and cephalosporins. Carbapenems are often the drug of choice for severe infections.

E — Escherichia coli

• Classification: Gram-negative rod, lactose fermenter.
• Prevalence: The most common isolate in community-acquired biliary infections (approx. 60-80% of cases).
• Virulence Factors: P-fimbriae (pili) facilitate adhesion to the biliary epithelium.

E — Enterococcus spp. (E. faecalis/faecium)

• Classification: Gram-positive cocci (pairs/chains).
• Niche: The most significant Gram-positive player in biliary sepsis. Unlike many other bacteria, Enterococcus can tolerate bile salts.
• Resistance: Intrinsically resistant to cephalosporins. Must be covered specifically with Ampicillin, Piperacillin-Tazobactam, or Vancomycin (VRE). Do not rely on Ceftriaxone monotherapy if Enterococcus is suspected.

P — Proteus mirabilis

• Classification: Gram-negative rod, lactose non-fermenter (unlike E. coli/Klebsiella), swarming motility.
• Pathology: Produces Urease. This enzyme hydrolyzes urea into ammonia and CO2, increasing the pH of the bile.
• Consequence: The alkaline environment precipitates calcium and magnesium salts, leading to the formation of struvite (infection) stones.

S — Serratia marcescens

• Classification: Gram-negative rod, facultative anaerobe.
• Identification: Notable for producing a bright red pigment (prodigiosin) at room temperature, and a distinct "fishy" or "musty" odor.
• Context: Often nosocomial. It carries intrinsic resistance to Ampicillin and first-generation Cephalosporins. Can cause severe sepsis in immunocompromised hosts.

The Nosocomial Threats

While KEEPS covers the standard community profile, hospital-acquired infections (particularly post-ERCP) involve distinct resistant organisms.

Pseudomonas aeruginosa

An aerobic, non-fermenting Gram-negative rod. It thrives in moist environments and forms biofilms. It is the hallmark pathogen in hospital-acquired cholangitis or post-procedural infections. Highly resistant to many antibiotics; requires anti-pseudomonal coverage (Pip-Tazo, Cefepime, or Carbapenems).

Enterobacter cloacae

A member of the Enterobacteriaceae family. It possesses an inducible AmpC beta-lactamase. Exposure to third-generation cephalosporins can induce resistance, leading to treatment failure. It is a common cause of infection in patients with biliary stents.

Clinical Diagnosis

Charcot's Triad (Acute Cholangitis)

1. Fever (Chills/Rigors)
2. Jaundice (Obstructive pattern)
3. Right Upper Quadrant (RUQ) Pain

*If Hypotension and Altered Mental Status are present, the patient has Reynolds' Pentad, indicating Septic Shock.

Microbiological Profile Summary

Organism	Gram Stain	Key Biochemical/Feature	Clinical Note
Klebsiella	Negative Rod	Mucoid, Capsule+	Liver abscess risk; ESBL producer
E. coli	Negative Rod	Indole+, Lactose+	Most common cause
Enterococcus	Positive Coccus	Bile tolerant	Resistant to Cephalosporins
Proteus	Negative Rod	Urease+	Struvite stones
Serratia	Negative Rod	Red Pigment	Nosocomial, multi-drug resistant
Pseudomonas	Negative Rod	Oxidase+	Post-procedural; Water source

Disclaimer: This content is for educational purposes only. Clinical guidelines vary by institution; always consult local antibiograms and ID specialists.

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