When is a diphtheroid isolate clinically significant and what is the recommended management?

Clinical Significance of Diphtheroid Isolates

Diphtheroids are clinically significant when isolated as pure or predominant growth from normally sterile sites, when accompanied by inflammatory cells on Gram stain, and when clinical evidence of infection is present—otherwise, they should be considered contaminants.

When to Consider Diphtheroids Pathogenic

The determination of clinical significance depends on three key factors working together:

Site-Specific Criteria

High clinical significance (treat as pathogen):

• Blood cultures: Only when isolated from two separate blood cultures drawn on different occasions, or when a patient with an intravascular line has clinical signs (fever >38°C, chills, hypotension) and the physician institutes antimicrobial therapy 1
• Catheter-related infections: When quantitative blood cultures show ≥3-fold higher colony count from catheter versus peripheral blood, or differential time to positivity ≥2 hours 2
• Cerebrospinal fluid: 24% of isolates represent true infection, particularly Propionibacterium species associated with CNS hardware 3
• Sterile body sites (joints, pleural fluid, peritoneal fluid): 20% represent true infection 3
• Prosthetic devices: Particularly prosthetic valve endocarditis (63% occur within 60 days of surgery) 4

Moderate clinical significance (evaluate carefully):

• Wounds: 18-30% represent true infection, particularly Actinomyces species in anaerobic wounds 3
• Urine: 22% represent true infection, notably Actinomyces neuii (21%) and Corynebacterium aurimucosum (21%) 3
• Ophthalmologic cultures: 8% represent true infection 3

Low clinical significance (usually contaminants):

• Single blood culture with clinical signs absent
• Superficial wound swabs without tissue specimens
• Respiratory specimens without clinical pneumonia

Microbiologic Criteria

Indicators of true infection:

• Pure or predominant growth in culture
• Numerous inflammatory cells visible on Gram stain alongside Gram-positive bacilli 5
• Isolation from properly collected tissue specimens (not superficial swabs) 6
• Repeated isolation of the same species from multiple specimens

Species-Specific Pathogenicity

High pathogenicity species (always consider significant):

• Corynebacterium kroppenstedtii: 100% clinically significant in breast infections 3
• Corynebacterium striatum: Significant in multiple sites, highly resistant 5, 7
• Propionibacterium species: In joints and CNS with hardware 3
• Corynebacterium ulcerans: Can cause diphtheria-like illness requiring antitoxin 8

Moderate pathogenicity species:

• Corynebacterium pseudotuberculosis, C. renale, C. minutissimum: Variable significance depending on site 5
• Actinomyces neuii: Significant in urinary tract infections 3
• Corynebacterium amycolatum: Most common isolate (35.9%) but often contaminant 7

Management Algorithm

Step 1: Assess Clinical Context

• Is the patient symptomatic with fever, local inflammation, or sepsis?
• Is the isolate from a normally sterile site?
• Does the patient have prosthetic material or indwelling devices?

Step 2: Review Microbiologic Data

• Was the organism isolated in pure/predominant growth?
• Were inflammatory cells present on Gram stain?
• Was proper specimen collection technique used (tissue vs. swab)?

Step 3: Species Identification

• Request MALDI-TOF MS identification to species level 3
• High-risk species warrant treatment regardless of other factors

Step 4: Treatment Decision

Treat if ANY of the following:

• Two positive blood cultures with clinical signs 1
• Prosthetic device infection 4
• CNS infection with hardware 3
• High-pathogenicity species identified
• Pure growth from sterile site with clinical infection

Do NOT treat if ALL of the following:

• Single positive culture only
• No clinical signs of infection
• Superficial specimen collection
• Low-pathogenicity species
• No prosthetic material present

Antimicrobial Therapy

Empirical Coverage

For suspected clinically significant diphtheroid infection pending susceptibility:

• First-line: Vancomycin (covers MRSA and most diphtheroids) 2
• Alternative: Linezolid or daptomycin (if vancomycin MIC >2 μg/mL) 2

Definitive Therapy Based on Susceptibility

High activity (>80% susceptible) 7, 4:

• Vancomycin (100% susceptible)
• Linezolid (100% susceptible)
• Tigecycline (100% susceptible)
• Chloramphenicol (>80% susceptible)
• Tetracycline (>80% susceptible)

Variable activity 7, 4:

• Gentamicin (88% susceptible for prosthetic valve endocarditis strains)
• Erythromycin (88% susceptible but >50% resistance in general isolates)
• Penicillins (>50% resistance common)

For prosthetic valve endocarditis: Vancomycin is highly bactericidal; penicillin-gentamicin combinations show synergy for gentamicin-susceptible strains 4

Duration

• Catheter-related bloodstream infection: 4-6 weeks if persistent bacteremia >72 hours after catheter removal 2
• Endocarditis: 4-6 weeks minimum 2
• Other infections: Based on clinical response and source control

Critical Pitfalls to Avoid

1. Do not dismiss diphtheroids from blood cultures as contaminants when isolated from two separate cultures or when the patient has an intravascular catheter and clinical signs of infection 1

2. Do not assume all diphtheroids are susceptible to penicillin—over 50% show resistance; always obtain susceptibilities 7

3. Do not use superficial swabs for wound cultures when diphtheroids are suspected—obtain tissue specimens 6

4. Do not forget biofilm production—many clinically significant diphtheroids produce biofilms, particularly in device-related infections, requiring device removal for cure 5

5. Consider toxigenic Corynebacterium ulcerans in patients with diphtheria-like illness—requires urgent diphtheria antitoxin from CDC (770-488-7100) 8

6. Remember that multiply drug-resistant strains are common—all species in one study exhibited multidrug resistance 7