The bacterium Staphylococcus epidermidis is a common resident of human skin, living harmlessly on the surface of most people. When this organism gains access to the bloodstream, it causes bacteremia, a serious bloodstream infection. S. epidermidis bacteremia is frequently associated with hospital settings and patients with implanted medical devices. The infection’s severity stems from the organism’s unique ability to colonize foreign materials and resist many common antibiotics, rather than high intrinsic virulence.
Defining the Bacteria and Bacteremia
Staphylococcus epidermidis is classified as Coagulase-Negative Staphylococci (CoNS), which distinguishes it from aggressive strains like Staphylococcus aureus. This Gram-positive coccus is a ubiquitous member of the normal human flora, or commensal, meaning it typically coexists with the host without causing disease. It usually lives in clusters on the skin and mucosal membranes.
Colonization is the harmless presence of the organism on the skin. Infection occurs when the bacteria breach the body’s natural barriers and invade sterile sites, such as the bloodstream, overwhelming the immune system.
Bacteremia is the presence of viable bacteria in the bloodstream. While transient bacteremia can occur briefly during routine activities, S. epidermidis bacteremia is a sustained and multiplying presence of the organism in the blood. This condition can progress rapidly to severe systemic inflammation, potentially leading to sepsis or septic shock if not controlled.
How Infection Occurs and Major Risk Factors
S. epidermidis is considered an opportunistic pathogen, meaning it only causes infection when the host’s defenses are compromised or when physical barriers are bypassed. The primary mechanism for infection is the introduction of the bacteria through a break in the skin, often related to medical procedures. Since S. epidermidis is a permanent resident of the skin, the high probability of contaminating the entry site during device insertion makes it a major cause of hospital-acquired bloodstream infections.
The most significant risk factor is the presence of indwelling foreign material, such as central venous catheters, prosthetic heart valves, shunts, or artificial joints. These devices provide a surface for the bacteria to adhere to and colonize. Once attached, S. epidermidis produces a protective, slimy matrix known as a biofilm.
This biofilm is a complex structure of extracellular polymeric substances that encases the bacteria, shielding them from the body’s immune response and antibiotic penetration. The bacteria within the biofilm can then continuously or periodically shed into the bloodstream, leading to persistent bacteremia. This unique ability to form biofilms on medical devices is the main reason S. epidermidis is a problematic pathogen in healthcare settings.
Patient populations with compromised immunity or those requiring prolonged invasive care are highly susceptible. This includes premature infants in the Neonatal Intensive Care Unit (NICU), patients undergoing chemotherapy, individuals on dialysis, and those with underlying conditions that weaken the immune system. The duration of hospitalization and the number of invasive procedures directly increase the risk of developing S. epidermidis bacteremia.
Symptoms and Confirmation of Infection
The clinical presentation of S. epidermidis bacteremia reflects a systemic inflammatory response to the bacteria in the blood. Common non-specific symptoms include the sudden onset of fever, chills, and malaise. In severe cases, the infection can progress to sepsis, characterized by low blood pressure (hypotension), a rapid heart rate, and signs of organ dysfunction.
Device-related S. epidermidis infections often have subtle or delayed symptoms. They may present as a low-grade, persistent fever rather than an acute, overwhelming illness. This delayed or indolent nature is attributed to the bacteria hiding within the protective biofilm. Localized symptoms, such as redness, pain, or pus at the site where a catheter enters the skin, may also signal a developing infection.
Confirming the diagnosis requires blood cultures, where samples of the patient’s blood are taken and incubated in a laboratory to grow any present bacteria. Because S. epidermidis is so common on the skin, a single positive blood culture is frequently dismissed as a sample contaminant introduced during the drawing process. To confirm a true bacteremia, clinicians often require multiple blood cultures drawn from different sites, or a positive culture combined with clear signs of systemic infection or evidence of device colonization.
Treatment Strategies and Antibiotic Resistance
Treating S. epidermidis bacteremia is complicated by the organism’s high rate of antibiotic resistance and its ability to form protective biofilms. A significant majority of S. epidermidis strains found in hospital settings are resistant to methicillin and other related penicillin-derived antibiotics, earning them the designation of Methicillin-Resistant S. epidermidis (MRSE). This resistance is typically due to the acquisition of the mecA gene, which alters the bacterial target, making these common drugs ineffective.
Given the high prevalence of MRSE, the initial empirical treatment for suspected S. epidermidis bacteremia is typically the intravenous antibiotic vancomycin. Vancomycin is a glycopeptide antibiotic that remains effective against most methicillin-resistant staphylococci. Alternative agents, such as daptomycin or linezolid, are considered for patients with vancomycin allergies or in cases where the bacteria show reduced sensitivity to vancomycin.
The presence of an infected medical device dictates the treatment strategy. Antibiotics alone often fail to eradicate the infection because they cannot effectively penetrate the mature biofilm. Therefore, the definitive cure often requires the physical removal or replacement of the infected indwelling device, such as the central line or prosthetic joint. Following device removal, a course of antibiotics, typically lasting 10 to 14 days, is administered to clear any remaining bacteria from the bloodstream.