Is diphtheroid a pathogen in the urine of immunocompromised patients or those with a history of recurrent urinary tract infections (UTIs) or catheter use?

Is Diphtheroid a Pathogen in Urine?

Diphtheroids in urine are typically contaminants from skin flora, but they can be true pathogens in specific high-risk populations, particularly immunocompromised patients, those with indwelling catheters, and patients with recurrent UTIs or structural urinary tract abnormalities.

Understanding Diphtheroids in Urine Cultures

Diphtheroids (Corynebacterium species) are aerobic, non-sporulating, pleomorphic Gram-positive bacilli that are usually commensals of the skin and mucous membranes 1. They are often dismissed as laboratory contaminants when isolated from clinical samples because of their ubiquitous presence on skin 1.

However, the key to determining pathogenicity is clinical context, not just the organism's identity 1.

When to Consider Diphtheroids as True Pathogens

Clinical Criteria for Pathogenicity

Diphtheroids should be considered true pathogens when ALL of the following are present:

• Pure growth on culture (not mixed flora) 1
• Numerous inflammatory cells (pus cells) on Gram stain along with Gram-positive bacilli 1
• Clinical evidence of infection with UTI symptoms 1
• High-risk patient population (see below) 2

High-Risk Populations

According to the American College of Radiology and European Association of Urology guidelines, diphtheroids are more likely to be pathogenic in complicated UTI settings 2:

• Immunocompromised patients (including diabetes mellitus) 2, 3
• Indwelling catheter use (current or within 48 hours) 2
• Structural urinary tract abnormalities (bladder diverticula, fistulae, obstruction) 2
• Recurrent UTIs (≥3 episodes in 12 months or ≥2 in 6 months) 2, 4
• Prior urinary tract surgery or instrumentation 2
• Incomplete bladder emptying or voiding dysfunction 2

Specific Pathogenic Species

Research has identified specific Corynebacterium species associated with urinary tract infections 1:

• C. pseudotuberculosis, C. renale, C. ulcerans, C. striatum, C. minutissimum, and C. haemolyticum have been isolated from catheter tips, wound infections, and blood cultures with clinical significance 1
• C. jeikeium has been documented as a cause of persistent urinary tract infection, particularly in patients with prior urologic procedures 5

Antibiotic Resistance and Biofilm Formation

A critical pitfall is assuming diphtheroids will be easily treated if they are pathogens 1:

• Diphtheroids isolated from catheter tips, sputum, and wound infections show high resistance to many antibiotics 1
• These organisms demonstrate increased biofilm production, allowing them to persist in hospital environments and on indwelling devices 1
• Blood isolates tend to be more antibiotic-sensitive than those from other sites 1

Diagnostic Approach

When Diphtheroid Growth is Detected:

1. Review the clinical context immediately 2:

   • Does the patient have UTI symptoms (dysuria, urgency, frequency)? 2
   • Is there fever or systemic symptoms suggesting upper tract involvement? 2
   • Does the patient have any risk factors for complicated UTI? 2

2. Examine the urine microscopy 1:

   • Are there numerous inflammatory cells (pyuria)? 1
   • Is the organism present in pure growth? 1

3. If contamination is suspected, obtain a repeat specimen 2:

   • Consider catheterized specimen if initial clean-catch is suspect 2
   • For catheterized patients, obtain culture after changing the catheter and allowing urine accumulation 2

4. If pathogenicity is confirmed 1:

   • Perform antibiotic susceptibility testing (do not assume sensitivity) 1
   • Consider imaging if patient has risk factors for complicated UTI 2

Common Organisms in UTIs (For Comparison)

The most common uropathogens remain 2:

• E. coli (approximately 75% of recurrent UTIs) 2
• Enterococcus faecalis, Proteus mirabilis, Klebsiella, Staphylococcus saprophyticus 2

In complicated UTIs, the microbial spectrum expands to include Pseudomonas spp., Serratia spp., and Enterococcus spp. 2. Diphtheroids fall into this expanded spectrum of complicated UTI pathogens 1.

Critical Pitfalls to Avoid

• Never dismiss diphtheroids automatically as contaminants in high-risk patients 1
• Never assume antibiotic susceptibility without testing - these organisms can be multidrug-resistant 1
• Never ignore pure growth with pyuria, even if the organism is typically a commensal 1
• Never treat empirically without culture in complicated UTI patients - obtain culture and susceptibility first 2