Can Staphylococcus aureus (Staph aureus) cause a true urinary tract infection (UTI)?

Is Staphylococcus aureus a True UTI Pathogen?

Yes, Staphylococcus aureus can cause true urinary tract infections, particularly in patients with urinary catheters, recent instrumentation, or healthcare-associated infections, though it is uncommon in the general population.

Clinical Context and Epidemiology

S. aureus is not listed among the typical uropathogens in major UTI guidelines. The European Association of Urology identifies E. coli, Proteus spp., Klebsiella spp., Pseudomonas spp., Serratia spp., and Enterococcus spp. as the most common species in complicated UTIs, with no mention of S. aureus 1. Similarly, the ACR guidelines cite E. coli (75% of cases), Enterococcus faecalis, Proteus mirabilis, Klebsiella, and Staphylococcus saprophyticus—but not S. aureus—as typical pathogens 1.

However, S. aureus bacteriuria represents a true pathogen in specific clinical scenarios:

When S. aureus is a True Pathogen

Catheter-associated and healthcare settings:

• S. aureus causes genuine UTI in 82% of patients with recent urinary catheterization 2
• In long-term care facilities, 33% of patients with S. aureus bacteriuria had symptomatic UTI at initial isolation 2
• Catheter-associated UTIs carry approximately 10% mortality when complicated by bacteremia 1

Risk for invasive complications:

• 13% of patients with S. aureus bacteriuria were bacteremic at initial presentation 2
• 58% maintained persistent bacteriuria for median 4.3 months, with subsequent invasive infections occurring up to 12 months later 2
• In 5 of 8 patients with late bacteremia, pulsed-field gel electrophoresis confirmed the blood isolate matched the original urine isolate 2

Critical Distinction: True Infection vs. Contamination

The key clinical challenge is distinguishing true S. aureus UTI from:

1. Asymptomatic bacteriuria (colonization) - which should not be treated in most patients 1
2. Hematogenous seeding - S. aureus bacteremia can seed the kidneys, making urine a secondary site rather than primary infection source
3. Specimen contamination - particularly with coagulase-negative staphylococci, which are not clinically relevant 1

Diagnostic criteria for true S. aureus UTI require:

• Urinary symptoms (dysuria, frequency, urgency, suprapubic pain, fever) 1
• ≥50,000 CFU/mL in properly collected specimens 1
• Absence of alternative infection source explaining bacteremia
• Clinical context supporting urinary origin (catheterization, instrumentation, obstruction) 1

Pathophysiology Supporting S. aureus as Uropathogen

S. aureus possesses specific virulence mechanisms for urinary survival:

• Urease production (especially ST1 strains) enables persistence in the nutrient-limiting urinary environment by alkalinizing urine 3
• Biofilm formation on catheters and uroepithelium, with 69.2% of UTI isolates being biofilm producers 4
• Adhesion factors including clumping factor B, elastin and laminin binding proteins detected in 80% of UTI isolates 5
• Human urine rapidly alters MRSA gene expression, enhancing survival and virulence in the urinary tract 6

Treatment Implications

When S. aureus UTI is confirmed, treatment differs from typical UTI management:

• 86% of S. aureus UTI isolates are methicillin-resistant (MRSA) 2
• Linezolid, quinupristin/dalfopristin, and chloramphenicol show highest efficacy 4
• Trimethoprim-sulfamethoxazole and doxycycline may be effective for biofilm-producing strains 4
• Standard empiric UTI regimens (fluoroquinolones, cephalosporins) are not designed for S. aureus coverage 1
• Treatment duration should be 7-14 days, with consideration for longer courses given persistence risk 1

Common Pitfalls to Avoid

Do not automatically treat S. aureus bacteriuria without symptoms - asymptomatic bacteriuria should not be screened for or treated in most populations 1. The exception is before urological procedures breaching the mucosa 1.

Always investigate for concurrent bacteremia - given the 13% rate of concomitant bacteremia and potential for hematogenous seeding rather than ascending infection 2.

Consider removing or replacing urinary catheters when feasible, as catheter duration is the most important risk factor and biofilm formation on catheters promotes persistence 1, 4.

Obtain urine culture and susceptibility testing before initiating therapy, as resistance patterns differ markedly from typical uropathogens and MRSA predominates 1, 2.