Is Staphylococcus (Staph) coagulase-negative bacteria part of the normal nasal flora?

Coagulase-Negative Staphylococci as Normal Nasal Flora

Yes, coagulase-negative staphylococci (CNS) are common bacteria found in the nasal cavity as part of normal flora, with studies showing they colonize the middle meatal samples of approximately 35% of normal adults. 1

Prevalence and Distribution

Coagulase-negative staphylococci represent the most prevalent bacterial species found in the nasal cavity of healthy individuals:

• They constitute approximately 35% of bacterial species recovered from middle meatal samples of normal adults 1
• They are the predominant isolates (61.6%) in healthy individuals without healthcare-associated risks or sinonasal disease 2
• CNS are normal inhabitants of human skin and mucous membranes 3

Characteristics of Nasal CNS Colonization

When found in the nasal cavity of healthy individuals, CNS typically present with these characteristics:

• Growth quantities are usually very rare (73.3%), rare (13.9%), or few (12.8%) 2
• They are considered contaminants when cultured from the middle meatus during episodes of rhinosinusitis 2
• They are part of the indigenous microflora of skin and mucous membranes 1

Clinical Significance

Understanding the role of CNS as normal flora has important clinical implications:

• When recovered from conjunctival mucosa, CNS are generally considered nonpathogenic and part of "normal flora" 1
• However, in specimens taken from the surface or interior of the eye, these same organisms are considered pathogens 1
• The frequent recovery of CNS in chronic sinusitis patients creates a diagnostic dilemma when trying to determine their pathogenic significance 1

Antibiotic Resistance Patterns

Even as normal flora, nasal CNS often display antibiotic resistance:

• CNS isolated from healthy individuals commonly show resistance to ampicillin and benzylpenicillin 4
• Many nasal CNS isolates show multiple resistances to antibiotics including erythromycin (30.2%), clindamycin (28.3%), and oxacillin/methicillin (18.9%) 2
• This resistance pattern is important to recognize when these organisms become opportunistic pathogens

Distinguishing Colonization from Infection

The distinction between CNS as normal flora versus pathogen depends on:

• Clinical context (presence of foreign bodies, immunocompromised state)
• Specimen source (surface vs. sterile site)
• Quantity of growth
• Presence of inflammatory response

Potential for Pathogenicity

While typically commensal, CNS can become opportunistic pathogens:

• They are increasingly recognized as causing clinically significant infections, particularly in immunocompromised hosts 5
• They are most commonly implicated in device-related infections 3
• In the immunocompetent host, CNS endocarditis and urinary tract infections with S. saprophyticus are the most common CNS infections 3

Understanding the dual role of CNS as both normal flora and potential pathogen is essential for proper interpretation of microbiological cultures and clinical decision-making in suspected infections.