What bacteria is commonly associated with staghorn calculus?

Staghorn Calculus: Associated Bacteria

Staghorn calculi are most strongly associated with urease-producing bacteria, particularly Proteus mirabilis, though any urease-producing organism can cause these infection stones. 1

Primary Causative Organisms

Urease-producing bacteria are the definitive cause of struvite staghorn calculi. These organisms produce the enzyme urease, which splits urea into ammonia and hydroxide, creating an alkaline urinary environment (high pH) that promotes crystallization of magnesium ammonium phosphate (struvite) and calcium carbonate apatite. 1, 2

Most Common Urease-Producing Bacteria:

• Proteus mirabilis is the classic organism associated with staghorn calculi 3, 4, 5

  • In one study of staghorn calculi, Proteus mirabilis was cultured from stone cores in 7 out of 13 patients (54%) where identical species were isolated from both urine and stone specimens 3
  • Proteus-infected patients demonstrate the highest frequency of recurrent UTI episodes 6

• Other urease-producing organisms include Klebsiella, Pseudomonas, and some Staphylococcus species 3

Critical Clinical Distinction

E. coli, despite being the most common cause of urinary tract infections overall, typically does NOT produce urease and is NOT associated with struvite staghorn stone formation. 2 While E. coli was the most frequently isolated organism (43 strains) in one study of urinary calculi with infection, it was most commonly associated with other stone types, not staghorn calculi. 3 Patients infected with E. coli had higher frequencies of phosphate-containing calculi but not the classic struvite staghorn configuration. 6

Pathophysiology of Stone Formation

The urease enzyme creates a specific biochemical environment that drives staghorn calculus formation: 1, 2

• Urease splits urea → ammonia + hydroxide
• Results in alkaline urine with high ammonia concentration
• Abundant phosphate and magnesium crystallize as magnesium ammonium phosphate (struvite)
• Bacteria form an exopolysaccharide biofilm and incorporate mucoproteins into the stone matrix 1

Critically, bacteria reside WITHIN the stone itself (not just on the surface), making the stone an infected nidus that perpetuates recurrent infections. 1, 2 This is unique to struvite stones compared to other stone types where the stone remains sterile inside. 1

Clinical Implications for Morbidity and Mortality

Untreated staghorn calculi can destroy the kidney and cause life-threatening sepsis. 1, 2 The time to develop uremia from staghorn calculi averages 7.4 ± 2.0 years. 6

Complete stone removal is essential to: 1, 2

• Eradicate causative organisms residing within the stone
• Prevent recurrent infections
• Preserve kidney function
• Prevent further stone growth

Residual stone fragments will continue to harbor bacteria and serve as a source for recurrent urinary tract infections and stone growth. 1, 2

Common Pitfall to Avoid

Do not assume all UTI-associated stones are struvite staghorn calculi. The specific association requires urease-producing organisms (primarily Proteus species), not common uropathogens like E. coli. 2, 3 Culture results guide both treatment and prevention strategies, as eradication of urease-producing bacteria is essential to prevent recurrence after stone removal. 3