A urinary tract infection (UTI) is a common bacterial infection, typically caused by Escherichia coli. However, a bacterium called Staphylococcus epidermidis (often shortened to S. epidermidis or S. epi) is increasingly recognized as a source of UTIs, particularly in specific patient populations. This organism commonly resides on human skin and mucous membranes but transitions into a pathogen under certain circumstances, often linked to medical intervention. Recognizing S. epidermidis as a potential uropathogen is important because it is frequently overlooked and often mistaken for harmless contamination during laboratory testing.
Staphylococcus Epidermidis in Context
Staphylococcus epidermidis is one of the most prevalent members of the normal human microbiota, residing harmlessly on the skin and mucosal surfaces. It is part of a larger group known as coagulase-negative staphylococci (CoNS), which are generally less virulent than their coagulase-positive relative, Staphylococcus aureus.
S. epidermidis acts as both a protective skin resident and a potential opportunistic pathogen. The transition to an infectious agent typically occurs when the body’s natural barriers are compromised, especially in healthcare settings. Patients with weakened immune systems or those requiring medical devices are at risk. The presence of foreign bodies, such as indwelling urinary catheters, prosthetic joints, or vascular lines, provides a surface for the bacteria to colonize and initiate infection.
When S. epidermidis causes a UTI, it is usually associated with complicated infections, such as those in catheterized patients or individuals with underlying urological abnormalities. In these scenarios, the organism bypasses the urinary tract’s natural defenses and utilizes its specific virulence factors. This context differentiates a true S. epidermidis infection from a simple, non-threatening presence of the bacteria in a urine sample.
Mechanisms of Urinary Tract Colonization
The ability of S. epidermidis to colonize the urinary tract is primarily due to its proficiency in forming biofilms. A biofilm is a complex, self-produced protective structure consisting of a sticky extracellular matrix composed of polysaccharides, proteins, and nucleic acids. This matrix allows the bacteria to adhere firmly to a surface, especially medical devices like urinary catheters.
Biofilm formation is a defining virulence factor for S. epidermidis and typically occurs in four phases: initial adherence, accumulation, maturation, and dispersal. Initial adherence involves surface proteins, known as adhesins, which allow the bacteria to latch onto the catheter surface or damaged urothelium. These adhesins facilitate the binding of the bacteria to host proteins.
Following initial attachment, the bacteria multiply and secrete the extracellular polymeric substance (EPS), often called the slime layer, responsible for the accumulation phase. The ica operon is involved in producing the polysaccharide intercellular adhesin (PIA), a major component of this protective matrix.
Once matured, this dense shield protects the embedded bacteria from the body’s immune defenses and prevents antibiotics from reaching effective concentrations. This protective structure makes S. epidermidis biofilms highly resistant to treatment, contributing to the persistence and recurrence of catheter-associated UTIs (CAUTIs).
Distinguishing Infection from Contamination
A major challenge in diagnosing a true S. epidermidis UTI is differentiating it from simple contamination, since the organism is a ubiquitous skin microbe. When a urine sample is collected, S. epidermidis from the skin can easily be introduced, leading to a false positive result. Accurate interpretation requires combining laboratory results with the patient’s clinical presentation and the method of sample collection.
One important factor is the colony count, which measures the number of bacteria present in the urine per milliliter (CFU/mL). While E. coli requires a standard threshold of 100,000 CFU/mL for a positive infection, S. epidermidis can indicate a true infection at a lower count, such as 10,000 CFU/mL. This lower threshold is especially relevant if the sample was isolated from a catheter specimen or a sterile collection method like suprapubic aspiration.
The presence of pyuria, or white blood cells (WBCs) in the urine, is another sign that the body is mounting an immune response. If a culture shows S. epidermidis but no pyuria and no clinical symptoms, it is highly likely to be contamination. Conversely, a high colony count paired with pyuria and symptoms like painful urination (dysuria) or fever strongly suggests a genuine UTI.
The method of sample collection significantly influences interpretation. Samples obtained via a clean-catch technique have a high risk of contamination from skin flora. In contrast, a sample drawn directly from a urinary catheter or obtained via a sterile aspiration procedure carries a much lower contamination risk. The significance of a positive culture must always be evaluated in the context of the patient’s underlying conditions, such as the presence of a urinary catheter or immunocompromised status.
Treatment Considerations and Resistance
Treating a true S. epidermidis UTI is often complicated by the organism’s high propensity for antibiotic resistance. A significant concern is the prevalence of Methicillin-Resistant Staphylococcus Epidermidis (MRSE), which is common in healthcare settings. MRSE strains possess the mecA gene, which confers resistance to methicillin and all other beta-lactam antibiotics, including penicillin derivatives and cephalosporins.
This high rate of methicillin resistance means that standard empiric UTI treatments, which often rely on beta-lactams, may fail against S. epidermidis. When a true infection is confirmed, treatment must be guided by susceptibility testing performed in the laboratory to determine which antibiotics are still effective. Alternative drug classes, such as vancomycin, are often reserved for confirmed MRSE infections, especially in severe cases.
The biofilm mechanism further complicates treatment, as the protective slime layer shields the bacteria from the antibiotic. Even if the drug is susceptible to the organism, it may not penetrate the biofilm effectively, leading to treatment failure. For catheter-associated infections, the most effective therapeutic measure involves the physical removal and replacement of the colonized foreign body, such as the urinary catheter, to eliminate the biofilm source.