Staphylococcus aureus is a bacterium commonly found on the skin and in the upper respiratory tract of humans. It is considered a commensal organism, meaning it can live on a host without causing disease, but it also has the potential to become an opportunistic pathogen. The presence of S. aureus on the body without causing illness is termed colonization or carriage. Quantifying the percentage of the population that carries this organism is complex because the rate is not a single, fixed number. Varying methodologies used in different studies, as well as the specific population being tested, lead to a range of reported percentages for this common human-microbe interaction.
The Prevalence of S. aureus Carriage
The proportion of individuals who carry S. aureus in the general population is quite high, though the exact percentage varies depending on the definition of carriage used. Approximately 20% to 30% of healthy adults are considered persistent carriers, meaning they consistently harbor the bacterium over long periods of time. This group represents a stable reservoir within the human population.
A much larger proportion, ranging between 30% and 60%, are classified as intermittent or transient carriers. These individuals carry the bacterium for short durations, appearing and disappearing over time. When considering both persistent and intermittent carriers, up to half of all people may harbor S. aureus at any given point. Only about 20% to 30% of the population are considered non-carriers, who rarely or never test positive for the organism.
Key Sites and Types of Colonization
The primary anatomical location for S. aureus colonization is the anterior nares, or the moist skin just inside the nostrils, which serves as the main reservoir for the organism. The bacteria adhere to the epithelial cells lining this area. From this central location, the organism can spread to other parts of the body.
While the nose is the most prominent site, colonization is also frequently found in other warm, moist areas of the body. These secondary sites include the axillae (armpits), the perineum (groin area), and the pharynx (throat). Colonization is distinguished by the duration of the bacteria’s presence on the host. Persistent carriage involves the continuous presence of the same strain of S. aureus for many months or years, often due to specific host factors. Conversely, intermittent carriage describes the temporary presence of the organism, often linked to environmental exposure or temporary changes in the skin’s microbial balance.
The Clinical Significance of S. aureus Carriage
Carriage of S. aureus is a major public health concern because it represents the single greatest risk factor for subsequent infection in the individual carrier. The transition from harmless colonization to active infection is often due to a process called autoinoculation, where the bacteria move from the carrier site to a vulnerable area, such as a surgical wound or a break in the skin. Carriers are estimated to have a risk of developing an S. aureus infection that is 2 to 12 times higher than non-carriers. The bacteria can cause a wide spectrum of illnesses, ranging from minor skin infections like boils and impetigo to severe, life-threatening conditions such as pneumonia, sepsis, and endocarditis. Colonization is especially relevant in healthcare settings, where carriers can transmit the organism to vulnerable patients, even if the carrier themselves remains asymptomatic.
A particular concern involves Methicillin-resistant Staphylococcus aureus (MRSA), an antibiotic-resistant strain. Nasal carriage of MRSA is a well-established risk factor for developing MRSA infections, which are more difficult to treat due to the organism’s resistance profile. The clinical risk is not limited to MRSA, as methicillin-susceptible strains can also cause severe infections, but the presence of any S. aureus strain increases the probability of an infection developing.
Factors That Influence Who Becomes a Carrier
An individual’s status as a persistent, intermittent, or non-carrier is determined by a complex interplay of host, bacterial, and environmental factors. Host characteristics, such as genetic predisposition, can affect the ability of the bacteria to adhere to nasal epithelial cells, influencing the likelihood of long-term colonization. Certain underlying health conditions also increase the risk of carriage, most notably chronic diseases like diabetes and skin disorders such as atopic dermatitis (eczema).
Environmental factors also play a significant role. Individuals with frequent exposure to healthcare environments, such as hospital workers and recently hospitalized patients, demonstrate higher rates of colonization. Lifestyle elements, including smoking and the recent use of antibiotics, can disrupt the normal microbiota and make an individual more susceptible to colonization by S. aureus. Age is also a factor, with carriage rates generally higher in infants and younger children compared to adults.