Staphylococcus epidermidis (S. epidermidis) is a bacterium that lives naturally on the skin of virtually every person, generally without causing harm. It is a common member of the human microbiota, yet it has become a significant concern within modern healthcare settings. This organism is capable of transforming from a harmless resident into an opportunistic pathogen, particularly in hospitalized patients. S. epidermidis is a leading cause of hospital-acquired infections, creating complex diagnostic and treatment dilemmas for clinicians.
The Organism’s Identity: Normal Flora Turned Threat
S. epidermidis is classified as a Coagulase-Negative Staphylococci (CoNS), distinguishing it from the more aggressive Staphylococcus aureus which is coagulase-positive. Its natural role is that of a commensal organism, existing on the skin and mucous membranes where it offers protective benefits by outcompeting more harmful bacteria. This bacterium is ubiquitous; a healthy person typically carries between 10 and 24 different strains of S. epidermidis.
The transition from benign resident to pathogen occurs when the natural skin barrier is breached, allowing the bacteria to enter the body. This breach often happens in high-risk patients, such as those who are immunocompromised, premature neonates, or individuals with implanted medical devices. Once inside, S. epidermidis acts as an opportunistic pathogen, causing infections that range from localized issues to life-threatening systemic bacteremia. It is now one of the most common causes of healthcare-associated infections globally.
Mechanism of Infection in Healthcare Settings
The primary reason S. epidermidis causes hospital-acquired infections (HAIs) is its remarkable ability to adhere to and colonize foreign materials. This makes infections associated with indwelling medical devices like central venous catheters, prosthetic heart valves, cerebrospinal fluid shunts, and artificial joints particularly common. S. epidermidis is the most common cause of infections associated with these devices, sometimes accounting for up to 50% of late-developing orthopedic device-related infections.
The mechanism that allows the bacteria to persist on medical devices is the formation of a biofilm. Biofilms are high-density, multi-layered communities of bacteria embedded in a self-produced matrix (e.g., Polysaccharide Intercellular Adhesin and extracellular DNA). This matrix acts as a protective shield, strongly adhering the bacteria to the surface of the implant and making the colony resistant to the host’s immune system and traditional antibiotics.
The formation process involves initial attachment to the device surface (often coated with host proteins like fibrinogen), followed by accumulation and maturation into a dense, protective layer. This structure creates an environment where the infection is persistent and difficult to eradicate. Because the bacteria are sheltered within the biofilm, the infection often becomes chronic and sub-acute, meaning it progresses slowly and can be challenging to manage.
Diagnostic Challenge: Interpreting Blood Culture Results
The ubiquitous nature of S. epidermidis on the skin presents a major obstacle in diagnosing bloodstream infections, especially when blood cultures are performed. The procedure involves drawing blood through the skin, but the organism can easily be introduced into the sample during the skin puncture. This results in a positive culture that is merely contamination from the skin flora, not a true infection within the patient’s bloodstream, known as bacteremia.
This ambiguity forces clinicians to differentiate between a harmless contaminant and a life-threatening systemic infection. The incidence of S. epidermidis as a contaminant is high, with some studies suggesting that over 90% of positive coagulase-negative staphylococci cultures may be false positives. Treating a contaminant unnecessarily exposes the patient to antibiotics, which can promote resistance, while failing to treat a true bacteremia can be fatal.
To differentiate true infection from contamination, clinicians rely on specific criteria, often requiring multiple positive blood cultures. The Centers for Disease Control and Prevention criteria suggest that Coagulase-Negative Staphylococci must be isolated from two or more blood culture sets within 48 hours, accompanied by clinical signs of infection (e.g., fever or hypotension). The time it takes for the culture bottle to signal positive, known as time-to-positivity (TTP), is another measurable factor, as a TTP of less than 16 hours often indicates a high bacterial density suggestive of true bacteremia. Furthermore, the presence of an indwelling medical device or the patient’s immunocompromised status significantly increases the likelihood that a positive culture represents a genuine infection.
Treatment and the Problem of Antibiotic Resistance
Treating S. epidermidis infections is challenging due to the organism’s antibiotic resistance, particularly to methicillin. Methicillin-Resistant S. epidermidis (MRSE) strains are common in hospital environments, with resistance rates exceeding 80% in many healthcare settings. This resistance is mediated by the acquisition of the mecA gene, which makes the organism resistant to all beta-lactam antibiotics, including methicillin, oxacillin, and cephalosporins.
Because of this resistance rate, the standard initial treatment for suspected systemic S. epidermidis infection is intravenous vancomycin, a glycopeptide antibiotic. Vancomycin is used empirically until antibiotic susceptibility testing confirms the strain’s specific profile. In cases where vancomycin is not appropriate or if the organism shows reduced susceptibility, alternative antibiotics like daptomycin or linezolid may be used.
Treatment is compounded by the biofilm, which shields the bacteria from effective antibiotic penetration. Consequently, the most effective therapeutic strategy often requires not only drug therapy but also the physical removal of the infected medical device, such as a central venous catheter or prosthetic joint. For serious infections like prosthetic valve endocarditis, treatment with a combination of antibiotics, sometimes including rifampin and an aminoglycoside, often necessitates a prolonged course of four to six weeks.